International Harmonization of Nomenclature and Diagnostic Criteria (INHAND): Nonproliferative and Proliferative Lesions of the Dog

Other Term(s)

Accessory (tissue).

Pathogenesis/Cell of Origin

Pathogenesis varies, depending on the originating tissue organ, for example:

Shedding off of fragmented adrenal tissue during development

Diagnostic Features

Predominantly normal tissue in an abnormal location.

Comment

In most instances, ectopic tissues are asymptomatic and represent an incidental finding during necropsy or microscopic examination.

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Increased permeability of the vascular system leading to excessive accumulation of vascular fluid in tissues due to

increased hydrostatic pressure in venous capillaries while an unchanged colloid–osmotic pressure is maintained (stasis)

Diagnostic Features

Swelling of the affected tissue; microscopically, the structures appear less dense.

Differential Diagnoses

Comment

Edema is frequently noted in the subcutaneous tissue and the lungs but can occur in any organ and tissue.

Other Term(s)

Pathogenesis/Cell of Origin

Extravasation of red blood cells.

Diagnostic Features

Red blood cells in the parenchyma, interstitial space, or body cavity, outside the vascular system.

Differential Diagnoses

Congestion. Accumulation of blood within the vascular system.

Comment

Hemorrhage, bleeding, extravasation, or the escape of blood from the blood vessels may be observed in virtually every organ and tissue and is microscopically characterized by the extravasal presence of erythrocytes in tissues. Usually, the following etiologies are discerned:

Trauma

For detailed descriptions of the various types, the reader is referred to textbooks of general pathology. Most relevant in toxicologic pathology is hemorrhage by diapedesis, caused by, for example, allergic reactions, infections, toxic agents, thrombocytopenia, disorders of blood coagulation, and so on. Extravasation of red blood cells in areas of exsanguination as part of the euthanasia process is generally not recorded as part of a general toxicology study as this is considered an agonal and procedural finding, unrelated to the objective of the study.

Other Term(s)

Aggregates, inflammatory cell.

Modifier

Eosinophil, (histiocyte), lymphocyte, macrophage (preferred terminology over “histiocyte”), mixed cell, mononuclear cell, neutrophil, plasma cell, polymorphonuclear cell.

Diagnostic Features

Differential Diagnosis

Inflammation: In addition to the inflammatory cell infiltrates, additional features are edema, tissue damage, hemorrhage, and/or fibrosis.

Comment

Frequently used term to describe the presence of inflammatory cells without significant accompanying inflammation, tissue damage, or morphologic evidence of cellular injury, noted in a variety of organs and tissues. May be within limits of normal. The term should be used with a descriptive modifier of the cell type, for example, infiltrate, lymphohistiocytic.

Other Terms

“-itis,” specific to the organ affected, for example, encephalitis, gastritis, pneumonia, and so on.

Comment

For organ-specific, detailed characteristics, refer to the description in the respective organ section.

Various approaches to further characterize inflammation are used by pathologists, including chronicity (peracute, acute, subacute, etc), location (perivascular, peribiliary, etc), and others. To implement a descriptive terminology, the use of the term “inflammation” is preferred over the “-itis” terminology, and the indication of the predominant cell type(s) in the diagnosis is recommended over the conventional use of chronicity, for example:

Inflammation, neutrophilic

When applicable the terms are combined, for example:

Inflammation, lymphoplasmocytic

Further description by location and distribution is recommended.

Comment

For details on the diagnostic criteria of parasites, the reader is referred to textbooks on veterinary parasitology. While parasitic structures can be observed in multiple organs and tissues of domestic dogs, their occurrence in laboratory dogs used in toxicity studies is very limited due to the stringent hygiene conditions in breeding facilities and laboratories. Parasites that may be noted occasionally include the nematode Toxocara canis and the arthropods Demodex canis and Sarcoptes scabiei. When parasites are observed, they should be recorded as “parasite” and, if possible, specified further in a comment to the finding. Parasite may be recorded as “present” or graded depending on the data collection system used in the toxicology study.

Other Term(s)

Fatty change, lipidosis, lipid accumulation, phospholipidosis, and so on.

Comment

The descriptive term vacuolation is the preferred term over the etiologic terminology listed above. It is considered good practice to add modifiers such as lipid, fatty change, or phospholipidosis if the nature of the vacuoles has been confirmed by special methodology, for example, lipid stain or electron microscopy. In the case of lipid vacuolation, micro- and macrovesicular vacuolation may be also indicated by modifier or comment.

Introduction

Review of the incidence of spontaneous cardiovascular lesion in dogs has been published, ^6,7 and protocol for the collection and dissection of the heart in dogs is available. ⁸ For detailed general considerations of the cardiovascular system, refer to the INHAND publication on the rodent cardiovascular system. ⁹

In nonclinical studies, both morphological and functional end points are essential in the identification of any potential interaction between drugs and cardiovascular structures. As a minimum, the standard morphological evaluation of the cardiovascular system includes heart weight measurement and conventional light microscopic histopathology of the myocardium and valvular leaflets in the heart, aorta, and blood vessels within tissues. Evaluation of serum levels of heart-specific biomarkers, electron microscopy, and immunohistochemistry may represent essential investigative tools to better identify the pathological process caused by xenobiotics. Functional assessments, such as blood pressure and echocardiography, can provide valuable in-life information correlating with morphological alterations observed at the postmortem examination.

Additional structures of the conduction system may also be evaluated following careful sampling. ¹⁰ The microscopic evaluation of the vascular structure in the other organs is performed as part of evaluation of each specific tissue. This may reveal the presence of either organ-specific vascular changes (possibly part of an organ-specific toxicity) or widespread vascular changes visible in several organs pointing toward a systemic vascular injury.

Heart

Introduction

At necropsy, similar to other lab animal species, the heart is sampled with the root of the large vessels and fixed in buffered formalin solution. The heart is generally opened prior to immersion in the fixative in order to ensure adequate fixation and eliminate large blood clots occupying the cardiac chambers. Conventional light microscopic histopathology of the heart is based on a thorough and consistent microscopic evaluation of hematoxylin and eosin–stained section of all relevant compartments and structures of the heart, including the ventricular, atrial, and septum wall, the valves, and the coronary vessels. This allows an accurate identification of changes in the muscular cells, extracellular matrix, conduction system within the myocardium, and the vascular structures within the myocardium and the adjacent epicardial tissue. Attention should also be given to regional distribution of lesions within the heart. For example, subendocardial zones in the left ventricle and left papillary muscles, especially near insertion site of chordae tendineae, are especially vulnerable to ischemic-based myocardial lesions. ^11,12

The recommended terminology of microscopic lesions observed in the heart of dogs is presented in Table 3.1.

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Table 3.1.

Microscopic Findings of the Cardiovascular System: Heart; Dog.

Heart	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Amyloid		x
Apoptosis, cardiomyocytea,b		x
Cardiomegaly		x
Cardiomyopathy, rodent progressive				x
Degeneration, cardiomyocyte		x
Degeneration/necrosis, cardiomyocyte		x
Edema, myocardium		x
Fibrosis, myocardium	x
Hemorrhageb,c	x
Hypertrophy, cardiomyocyte		x
Infarct		x
Infiltrate [insert appropriate cell type], myocardium	x
Infiltrate/fibrosis	x
Mineralization, cardiomyocyte, or myocardium	x
Mononuclear cell infiltrate/fibrosis, myocardium	x
Necrosis, cardiomyocyte		x
Necrosis/infiltrate, cardiomyocyte		x
Pigment, cardiomyocyte, or myocardium		x
Rodent progressive cardiomyopathy				x
Thrombus, atrium		x
Vacuolation, cardiomyocyte		x
Vacuolation, conduction systemb,c	x
Proliferative (nonneoplastic)
Hyperplasia, mesothelial, epicardium, or pericardium	x
Hyperplasia, Schwann cell, subendocardium				x
Neoplasticd
Rhabdomyoma, myocardiumc		x

^a Finding more frequent as an induced change.

^b Terminology addressed in the General Pathology Section.

^c Terminology with diagnostic criteria and/or comments described in the text.

^d For a comprehensive list of neoplasms and tumor-like lesions, refer to the “International Histological Classification of Tumors of Domestic Animals,” the more recently published “Tumors of Domestic Animals”¹, and other pertinent literature.

Comment

General aspects and a description of hemorrhages are included in the General Pathology section. In the heart, subendocardial, epicardial, and valvular hemorrhages (“heart valve hematomas”) are often observed and readily noted at necropsy. In addition, myocardial hemorrhages may be noted microscopically. It is recommended to indicate the location as descriptor or modifier to the term.

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Purkinje fiber.

Diagnostic Features

Round (spherical) empty spaces (vacuoles) usually centrally located in Purkinje fibers.

Differential Diagnoses

Not applicable.

Comment

Vacuolation of the conduction system is occasionally observed in beagle dogs. In a publication on morphologic evaluation of the heart in dogs and monkeys, Keenan and Vidal ⁷ reported prominent vacuolation of Purkinje fibers in 15% of the males and 12% of the females, while Bodié and Decker ⁶ reported this change in 34% of the male and 28% of the female control dogs. Authors also noted that vacuolation varied in prominence but was more evident in fibers following vascular adventitia into the ventricles.

Other Term(s)

Rhabdomyomatosis; congenital glycogenic tumor.

Pathogenesis/Cell of Origin

Exact histogenesis is uncertain, although conducting fiber, myocardial fiber, and pluripotent embryonic cells are considered to be origin.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

The PAS reaction with or without diastase digestion is applied for the detection of glycogen in neoplastic cells. Immunohistochemically, neoplastic cells are demonstrated positively for desmin and myoglobin but negatively for smooth muscle actin or vimentin.

Comment

Cardiac rhabdomyoma has been reported in various animals including humans and guinea pigs. It is a controversial issue whether cardiac rhabdomyoma is a true neoplasm or hamartoma. Human cardiac rhabdomyomas are caused by mutation in the TSC1 and TSC2 gene. A mouse model of cardiac rhabdomyoma is associated with loss of the TSC-1 gene in ventricular myocytes. ¹³ One of the most interesting aspects of these tumors is their tendency to undergo spontaneous regression. ¹⁴ Rhabdomyoma can occur in the myocardium, skeletal muscles of the larynx, and in the head region in both human and animals.

Heart Valves

Observation of drug-induced valvulopathy in postmarketing surveillance in man has led to withdrawal of some commercialized compounds, such as appetite-suppressant drugs. This has increased the interest in the evaluation of heart valves during preclinical studies.

Histopathological examination of the cardiac valves in all laboratory animal species, including dogs, may suffer from inconsistencies and artifacts related to both the technical approach and the inherent small size of the valves. The trimming/sectioning of the heart may greatly influence the type, number, and correct orientation of valves within the histological sections, which finally affects the accuracy of the microscopic examination. Therefore, in those studies where drug-related effects are suspected to occur in the cardiac valves, the pathologist may need to adapt the preparation of the heart and trimming of valves to ensure the best representation of most or all 4 cardiac valves. Although congenital valvular anomalies (pulmonary stenosis, aortic stenosis) can be observed in dogs, ¹⁵ these are very uncommon in beagle dogs purpose-bread for biomedical research and used in toxicity studies.

The recommended terminology of microscopic lesions observed in the heart valves of dogs is presented in Table 3.2.

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Table 3.2.

Microscopic Findings of the Cardiovascular System: Heart Valve; Dog.

Heart valve	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Angiectasis, valvular	x
Degeneration, myxomatous, valve*	x
Hemorrhagea,b	x
Inflammation, valveb		x
Proliferation, stroma, valve		x
Neoplastic
Myxoma, valvea		x
Myxosarcoma, valvea		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General Pathology section.

Comment

Myxomatous valvular degeneration is a common valvular lesion in older dogs, affecting chiefly the left atrioventricular valve. ¹⁵ Macroscopically, the valve may appear irregular, shortened, thickened, opaque, with discrete to prominent nodules that may extend to chordae tendineae. Microscopically, thickening of the spongiosa by loose fibroblastic glycosaminoglycan-rich tissue and degeneration of the fibrosa with often hyalinized and fragmented collagen fibers are prominent features. The presence of myxomatous tissue at the base of the heart, near the aortic valve and aorta, is a common normal component of the heart of dogs ⁷ and should not be confused with myxomatous degeneration.

Comment

General aspects and a description of hemorrhages are included in the General Pathology section. In the heart, subendocardial, epicardial, and valvular hemorrhages (“heart valve hematomas”) are often observed and readily noted at necropsy. In addition, myocardial hemorrhages may be noted microscopically. It is recommended to indicate the location as descriptor or modifier to the term.

Species

Dog (term does not exist in rodent goRENI).

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Multipotent vasoformative cells originating in subendocardial layer.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Histochemical stain such as PAS and Alcian blue stain may be used to confirm the acid mucopolysaccharide nature of the myxoid matrix, and IHC may be used to confirm the presence of endothelial cells and mesenchymal nature (vimentin-positive) of myxoma cells.

Comments

Species

Dog (term does not exist in rodent goRENI).

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Multipotent vasoformative cells originating in subendocardial layer.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Histochemical stain such as PAS and Alcian blue stain may be used to confirm the acid mucopolysaccharide nature of the myxoid matrix, and IHC may be used to confirm the presence of endothelial cells and mesenchymal nature (vimentin-positive) of myxosarcoma cells.

Comments

This tumor is extremely rare in dogs. As for the benign form, cardiac myxosarcoma may be associated with hemodynamic consequences according to the size of the mass and anatomical location.

Blood Vessels

The microscopic evaluation of the vasculature in organs is generally performed as part of the evaluation of each specific organ. This may reveal the presence of either organ-specific vascular changes (possibly as part of an organ-specific toxicity) or widespread vascular changes visible in several organs pointing toward a systemic vascular injury. Specific collection of larger vessels may be required in particular when intravascular administration of xenobiotics is performed (eg, through bolus injection or slow infusion), with potential for local and/or systemic vascular injury. Vascular injury in dogs, as well as in other laboratory animal species, can occur as a spontaneous pathological change or be drug induced. In preclinical drug development, vascular injury can be caused by a wide array of compounds encompassing small and large molecules (such as monoclonal antibodies) and antisense oligonucleotides. Several papers have been published to better characterize drug-induced vascular injury and associated biomarkers in preclinical studies. ^{16 –19} However, the distinction between the spontaneous and drug-induced vascular injury may still be challenging and a correct identification of the histological pattern of vascular changes is crucial.

The observation of a vascular injury may reflect an organ-specific toxicity or represent a widespread change visible in several organs and, therefore, suggest the occurrence of a systemic vascular disorder.

Histopathological evaluation of the heart and the other organs according to standard guidelines for organ trimming is considered sufficient to ensure the thorough evaluation of the vascular tree.

The recommended terminology of microscopic lesions observed in the blood vessels of dogs is presented in Table 3.3.

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Table 3.3.

Microscopic Findings of the Cardiovascular System: Blood Vessels; Dog.

Blood vessels	Common	Uncommon	Not Observed but potentially relevant	Not applicable
Nonneoplastic
Amyloid, media or wall, artery		x
Aneurysm, artery or aortic		x
Angiectasis		x
Degeneration/necrosis, media, or wall	x
Embolus		x
Fibrosis, perivascular	x
Hemorrhage, media or walla	x
Hypertrophy, endothelium	x
Hypertrophy, media or wall, artery	x
Infiltrate [insert appropriate cell type], perivasculara	x
Inflammation, media or wall, arterya	x
Intimal thickening, acellular, artery		x
Intramural plaque, artery		x
Intramural plaque, fattyb		x
Mineralization, media or wall, arterya		x
Necrosis/inflammation, media or wall, arterya,b	x
Thrombus		x
Vacuolation, media or adventitia, arterya		x
Proliferative (nonneoplastic)
Hyperplasia, angiomatous		x
Hyperplasia, hemangioendothelium		x
Hyperplasia, intravascular papillary endothelialb		x
Proliferation, intima, artery or vein	x
Neoplastic
Hemangioma		x
Hemangiosarcoma		x

^a Terminology addressed in the General Pathology section.

^b Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Atheroma, fibrofatty plaque.

Pathogenesis/Cell of Origin

Intimal/medial accumulation of lipids intermixed with inflammatory cells and extracellular matrix. Lesion typically associated with atherosclerosis.

Diagnostic Features

Differential Diagnoses

Arteriosclerosis—Thickening of the intima/media by hyaline material (mucopolysaccharides, fibrin) or by laminar proliferation of smooth muscle cells and extracellular matrix, without accumulation of lipids.

Special Techniques for Diagnostics

Intramural fatty plaque may be highlighted with histochemical stains for elastic fibers, fat, and mineralized material and immunohistochemical stain for macrophages and smooth muscle cells.

Comment

Intramural fatty plaques in arteries/arterioles, characteristically associated with atherosclerosis, are rare spontaneous finding in dogs as dogs are resistant to the development of atherosclerosis. However, this lesion is seen infrequently in dogs following developing hypercholesterolemia associated with endocrinopathies.

The intramural fatty plaque in dogs will typically affect the vasculature of the heart, brain, and kidney and less often other vasculatures; deposition of lipids typically begins in the middle and outer layer of the media, eventually extends to the intima and affects more extensively the small muscular arteries. In contrast, intramural fatty plaques in humans are present primarily in the intima and affect primarily the major elastic arteries (aorta, carotid) and the large and medium caliber muscular arteries (coronary, popliteal).

Other Term(s)

Beagle pain syndrome, idiopathic canine polyarteritis, canine juvenile polyarteritis syndrome, steroid-responsive meningitis-arteritis.

Pathogenesis/Cell of Origin

Necrotizing polyarteritis; exact etiopathogenesis unknown, a Th2-mediated immune response, and upregulation of CD11a integrin and activation of metalloproteases are suspected.

Diagnostic Features

Differential Diagnoses

Comment

It is suggested to use necrosis/inflammation, medial or mural, artery to describe necrotizing inflammation in arteries. It can be difficult to differentiate idiopathic canine polyarteritis from drug-induced vascular injury, especially when an increased incidence of chronic lesion consistent with this disease is present following treatment with a vasoactive drug. Vasodilator-induced lesions are generally characterized by medial/adventitial hemorrhage and necrosis with minimal inflammation (acute) to intimal and adventitial proliferative changes (chronic); are typically limited to the coronary arteries (extramural and intramural), often associated with hemodynamic and myocardial changes; and may be associated with atrial hemorrhage. Vasoconstrictor-induced lesions usually affect small size arteries in a variety of tissues and include medial thickening and necrosis, with hyalinization. Hypersensitivity vasculitis (type IV) affects small vessels (not limited to arteries) of skin or of other tissues and is characterized by transmural nonnecrotizing vasculitis with infiltrates of mononuclear and variable number of eosinophils, generally with no fibrinoid necrosis or thrombosis.

The site predilection, dose–response, and clinical and laboratory changes can be differentiating. In drug-induced vascular injury, vascular lesions are generally restricted to coronary arteries, and clinical signs are variable, likely unrelated to vascular effects. Idiopathic canine polyarteritis will usually show prominent transmural to periarterial inflammation with little or no hemorrhage, will typically affect arteries of the heart and of many other tissues as well, myocardial changes will only rarely be present, and idiopathic canine polyarteritis is not associated with hemodynamic changes or atrial hemorrhage. Clinically, acute idiopathic canine polyarteritis is typically associated with pyrexia, anorexia, reluctance to move, cervical rigidity and pain, with elevated polymorphonuclear cell count and concentration of IgA and acute phase proteins in serum and cerebrospinal fluid. In the chronic form of the disease, progressive atrophy of temporal and cervical muscles and neurological deficits may occur.

Other Term(s)

Reactive vascular proliferation; proliferation, intravascular endothelial; angiomatosis, intravascular.

Pathogenesis/Cell of Origin

Endothelial proliferation in response to traumatic injury followed by thrombosis, inflammation, and stasis within vascular bed.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Immunostaining for vimentin, von Willebrand factor, and CD31 are applied for identification of endothelial cells.

Comment

Canine intravascular papillary endothelial hyperplasia has been surveyed by Gamlem and Nordstoga. ^20,21 This lesion is thought to represent an abnormal morphologic type of organizing thrombus. Therefore, it should not be confused with preneoplastic or neoplastic lesions. ²² Scrotal vascular hamartoma in dogs may be the same entity of this lesion.

Introduction

For detailed general considerations on the digestive system, refer to the INHAND rodent publication. ²³

This chapter provides a set of standardized terms, diagnostic criteria, and examples for the upper and lower digestive tract, as well as for the salivary glands and the exocrine pancreas of beagle dogs used in toxicologic pathology studies.

The digestive tract is the entry site into the body for orally administered test articles. An irritant test article may lead to local acute lesions at this first site of contact to the body, and the digestive tract can be affected by adverse drug reactions. On the other hand, orally administered test articles usually reach systemic exposure, sufficient to be toxic to other organs, without noticeable effect on the digestive tract.

A distinctive feature of the digestive tract is the high proliferative rate of the epithelium, making it particularly sensitive to agents interfering with cell division but resulting also in a high regenerative capacity. Because of the large surface area of these tissues, accurate assessment of potential treatment effects is almost entirely dependent on a thorough gross examination and sampling of focal lesions.

For the classification of neoplastic lesions, refer to the “International Histological Classification of Tumors of Domestic Animals: Tumors of the Alimentary System of Domestic Animals,” published by the Armed Forces Institute of Pathology in conjunction with the American Registry of Pathology and the World Health Organization, ²⁴ the more recently published “Tumors of Domestic Animals,” ²⁵ and other pertinent literature.

Upper Alimentary Tract (Oral Cavity, Tongue, Pharynx, and Esophagus)

Most commonly, microscopic lesions in the upper alimentary tract are observed in tissues that are required to be examined microscopically, based on regulatory guidelines and the preferences of the laboratories, for example, esophagus, pharynx, tongue. In most instances, other lesions in the oral cavity and pharynx are examined only when gross lesion were recorded. Table 4.1, based on the rodent manuscript, lists the nonproliferative and nonneoplastic proliferative changes of the upper alimentary tract.

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Table 4.1.

Microscopic Findings in the Digestive System: Oral Cavity and Esophagus; Dog.

Oral cavity and esophagus	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Cleft, palatea		x
Dilatation, esophagus		x
Diverticulum, esophagus		x
Ectopic tissue, sebaceous glandb		x
Malformation		x
Nonneoplastic
Amyloid		x
Apoptosis, squamous epitheliumb,c		x
Apoptosis/single-cell necrosisb,c		x
Atrophy, squamous epithelium		x
Cyst		x
Degeneration/necrosis, muscle		x
Edemab		x
Erosion/ulcerc		x
Hemorrhageb		x
Hyperkeratosis		x
Infiltrate [insert appropriate cell type]b	x
Inflammation, foreign body		x
Inflammation, mixed cell	x
Inflammation, mononuclear cell	x
Inflammation, vessel		x
Mineralizationb,d		x
Necrosis, squamous epitheliumb		x
Pigmentb		x
Proliferative (nonneoplastic)
Eosinophilic granulomad		x
Hyperplasia, basal cell		x
Hyperplasia, squamous cellc		x
Papilloma, squamous celld		x

^a This and other malformations are rarely observed in toxicity studies and generally represent gross observations; a microscopic correlate may be required, however.

^b Terminology addressed in the General/Systemic Pathology section.

^c Finding more frequent as an induced change.

^d Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Pathogenesis/Cell of Origin

The pathogenesis of calcinosis circumscripta is not known, and a variety of causes have been discussed.

Diagnostic Features

Differential Diagnoses

Not applicable.

Comment

In the dog, calcinosis circumscripta represents an entity described in text books as a primarily subcutaneous change; it is also observed periarticular and has been described in the tongue of dogs. The lesion is characterized by sharply defined, irregular areas of amorphous, calcified (Kossa positive) material, surrounded by epithelioid and multinucleated macrophages, and peripheral fibrosis. Predisposed are young dogs and large breeds, in particular German shepherd. ^24,26,27 The use of the term is not recommended, rather the term “mineralization” should be used instead, and, if considered appropriate, a comment could be added indicating that the finding is consistent with calcinosis circumscripta.

Pathogenesis/Cell of Origin

Eosinophilic granuloma (EG) is suspected to be due to an inherited defect in eosinophil regulation.

Diagnostic Features

Differential Diagnoses

Oral mast cell tumor: Anaplastic mast cells and eosinophils are diffusely present, whereas in the EG, there is a predominance of eosinophils with a few normal mast cells.

Comment

Eosinophilic granulomas are rare in dogs, the features are similar to those observed in cats. Eosinophilic granuloma may be observed at all ages, is more common in dogs younger than 3 years old and in males, and has been noted in various breeds; there is a predilection in Siberian Huskies and cavalier King Charles spaniels. ^24,25

Other Term(s)

Oral papillomatosis.

Pathogenesis/Cell of Origin

Hyperplastic response to viral infection; canine papillomavirus 1.

Diagnostic Features

Differential Diagnoses

Not applicable.

Comment

Typically, a lesion of young dogs. Outbreaks of papillomatosis can occur in experimental dog colonies. The papillomas regress spontaneously after 1 to 2 months, and the dog is protected from reinfection through antibody-mediated immunity. ^28,29

Stomach

Unlike the rodent, the stomach of the dog consists of glandular mucosa only and is devoid of a nonglandular mucosa; therefore, the terminology for the nonglandular stomach of rodents is not applicable for the dog and was omitted. As in other species, the glandular mucosa of the dog is composed of various glands that are present in specific regions of the organ. To assure a thorough examination of the stomach, examination of representative sections of all regions is mandatory. In the dog, these include the cardia, fundus, and pylorus regions with correspondingly named glands. The recommended nomenclature for nonproliferative and nonneoplastic proliferative lesions in the stomach of beagle dogs used in toxicity studies are listed Table 4.2.

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Table 4.2.

Microscopic Findings in the Digestive System: Stomach; Dog.

Stomach	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Ectopic tissuea		x
Nonneoplastic
Amyloid		x
Atrophy		x
Apoptosisa, b		x
Apoptosis/single-cell necrosisa, b		x
Cellularity increased, lymphocyteb, c		x
Cyst, glandular		x
Dilatation, glands		x
Edemaa,c		x
Eosinophilic globules			x
Erosion/ulcerb		x
Helicobacter		x
Hemorrhageb		x
Hypertrophy, mucous cell		x
Infiltrate, [insert appropriate cell type]a	x
Inflammationa,c		x
Mineralizationa	x
Necrosis, mucosaa, b		x
Secretory depletion		x
Vacuolation, epitheliuma		x
Yeast		x
Proliferative (nonneoplastic)
Diverticulum		x
Hyperplasia, focal, mucosac		x
Hyperplasia, diffuse, mucosa		x
Hyperplasia, neuroendocrine cell		x
Hypertrophy, chronic pylorusc		x
Polyp, hyperplastic (regenerative)c		x
Polyp, inflammatory (benign lymphoid)c		x

^a Terminology addressed in the General/Systemic Pathology section.

^b Finding more frequent as an induced change.

^c Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Pathogenesis/Cell of Origin

Increased number of lymphocytes and plasma cells.

Diagnostic Features

Differential Diagnoses

Not applicable.

Comment

Increased lymphoid cellularity, increased size of lymphoid nodules, or increased number of lymphoid nodules are rare findings in untreated healthy dogs but occur more frequently in diseased animals and also may be seen frequently as an induced change. It is considered a benign, nonneoplastic, reactive response to an immune stimulus. If the change pertains to lymph follicles, the sublocation may be added.

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Increased permeability of the vascular system leading to excessive accumulation of vascular fluid in tissues (see also General Pathology section).

Diagnostic Features

Swelling of the stomach wall, without increase in inflammatory cell infiltrates or fibrosis.

Differential Diagnoses

Comment

In the dog, edema of the stomach wall as the only lesion is rare and, if noticed, may be due to incidental intoxication, ³⁰ which is unlikely to occur in a laboratory setting.

Other Term(s)

Comment

Variants of inflammation include lymphoplasmacytic gastritis and scirrhous eosinophilic gastritis.

Lymphoplasmacytic gastritis is characterized by a thickening of the stomach wall by lymphocytes and plasma cells infiltrating area of fibrosis. In their investigation of accompanying gastric lesions in dogs with intestinal abnormalities, Lidbury et al ³¹ found that lymphoplasmacytic gastritis was the most prevalent entity. Scirrhous eosinophilic gastritis is a rare condition in the dog with a focal or multifocal distribution. A concomitant arteritis has been described; an allergic reaction or hypersensitivity was suspected. ³²

Other Term(s)

Stenosis, pyloric.

Comment

In old dogs, an acquired hyperplastic lesion has been described as pyloric stenosis, characterized by annular thickening of the pyloric region, due to hyperplasia of the glands, granulation tissue in the submucosa, and hypertrophy of the lamina muscularis. ²⁴

In toxicity studies in dogs, hyperplasia of the pylorus region may be observed as a treatment-related finding. ³³

Other Term(s)

Giant rugal hypertrophy, giant hypertrophic gastritis, chronic hypertrophic gastritis.

Pathogenesis/Cell of Origin

Gastric glands.

Diagnostic Features

Differential Diagnoses

Parietal cell hyperplasia, in association with Zollinger-Ellison syndrome.

Comment

Generally, the change is rare and breed dispositions have been reported, for example, Basenji and Drentse Patrijshond dog. ^24,34 There are anecdotal case reports for other breeds and species. The diffuse variant is considered to be analogous to Menetrier disease in humans.

Pathogenesis/Cell of Origin

Gastric epithelium.

Diagnostic Features

Differential Diagnoses

Comment

Most commonly, canine stomach polyps are observed in the pylorus region.

Polyp, Inflammatory—Stomach

Other Term(s)

Benign lymphoid polyp.

Diagnostic Features

Differential Diagnoses

Hyperplastic polyp: Thickened cystic epithelium with a fibrous stalk.

Comment

As per Head et al, ²⁴ examples of inflammatory polyps have been described in the dog; in a retrospective analysis, Taulescu et al ³⁵ report 3 cases of inflammatory polyps.

Small and Large Intestine (Duodenum, Jejunum, Ileum, Cecum, Colon, Rectum)

The morphology of the canine intestine is basically similar to that of rodents and humans, but relative size and length of the cecum of dogs are smaller and shorter than the rodent cecum. The ileum communicates only with the colon, and the ileocolic valve is an apparent structure when this region is opened at the gross examination.

Table 4.3 summarizes the suggested nomenclature of nonproliferative and nonneoplastic proliferative lesions in the small and large intestine of the dog.

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Table 4.3.

Microscopic Findings in the Digestive System: Small and Large Intestines; Dog.

Small and large intestines	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Cyst, squamous			x
Ectopic tissuea,b		x
Malformation		x
Nonneoplastic
Amyloid		x
Apoptosisb, c		x
Apoptosis/single-cell necrosisb, c		x
Atrophy		x
Atrophy, Brunner’s gland		x
Congestionb	x
Degeneration/necrosis		x
Degeneration/necrosis, Brunner’s glands		x
Degeneration, neuron, myenteric plexus			x
Dilatation, crypta	x
Dilatation		x
Erosion/ulcerc		x
Hemorrhageb	x
Hypertrophy		x
Hypertrophy, Paneth cell		x
Infiltrate [insert appropriate cell type]b	x
Inflammationb	x
Intussusception		x
Lymphangiectasia		x
Metaplasia, osseous			x
Metaplasia, Paneth cell		x
Mineralizationb		x
Necrosis, mucosab		x
Paneth cell, decreased		x
Parasite			x
Syncytia, epithelium		x
Vacuolation, mucosab		x
Proliferative (nonneoplastic)
Hyperplasia, mucosa		x
Hyperplasia, Brunner’s glands		x
Metaplasia, squamous cell			x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General/Systemic Pathology section.

^c Finding more frequent as an induced change.

Modifier

Fundic gland, pancreas.

Pathogenesis

Presence of pancreatic cells or fundic glands in the mucosa or submucosa.

Diagnostic Features

Pancreatic acini with or without islets of Langerhans, in the mucosa or submucosa of intestine, adjacent to the mesenteric attachment.

Comment

Presence of ectopic gastric mucosa with feature of the gastric fundic gland in the small intestine, in addition to ectopic pancreas as described in the rodent manuscript, is reported in Beagle dogs. ³⁶

Other Term(s)

Pathogenesis/Cell of Origin

Unknown.

Diagnostic Features

Differential Diagnoses

Diverticulum: Extension of glands through muscularis mucosae into submucosa.

Comment

Mucosal cysts appear more common in duodenum and jejunum than large intestine and are supposed to be acquired as part of degenerative process. ³⁷

Salivary Glands

Table 4.4 summarizes the suggested nomenclature for nonproliferative and nonneoplastic proliferative changes of the salivary glands.

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Table 4.4.

Microscopic Findings in the Digestive System: Salivary Glands; Dog.

Salivary glands	Common	Uncommon	Not observed but potentially relevant	Not Applicable
Congenital
Ectopic tissuea		x
Nonneoplastic
Accumulation, adipocytes		x
Amyloid		x
Apoptosisa,b		x
Apoptosis/single-cell necrosisa,b		x
Atrophy		x
Calculus, ductular	x
Depletion, secretory, acinar cell			x
Ectasia, duct		x
Edemaa		x
Fibrosisa		x
Focus, hypertrophic, basophilic			x
Granules increased, granular duct			x
Hemorrhagea		x
Hypertrophy, acinar cell		x
Infiltrate [insert appropriate cell type]a		x
Inflammationa		x
Metaplasia, acinar cell		x
Metaplasia, necrotizingc		x
Mineralizationa		x
Necrosisa		x
Pigmenta		x
Secretory decreased, acinar cell			x
Secretory decreased, granular duct			x
Vacuolationa		x
Proliferative (nonneoplastic)
Hyperplasia		x
Metaplasia, squamous cellc		x

^a Terminology addressed in the General/Systemic Pathology section.

^b Finding more frequent as an induced change.

^c Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Necrotizing sialometaplasia, salivary gland infarction. ^38,39

Pathogenesis/Cell of Origin

Trauma-induced compromise of blood vessels; immune-mediated vascular damage has been discussed.

Diagnostic Features

Differential Diagnoses

Not applicable.

Comment

The condition occurs in small breeds (terriers), preferably in the submandibular gland.

Other Term(s)

Squamous metaplasia; ductal squamous metaplasia.

Pathogenesis/Cell of Origin

Squamous transformation of ductal or acinar epithelium due to vitamin A deficiency.

Diagnostic Features

Squamous epithelium replaces the cubic/cylindrical epithelium of the salivary gland acini and ducts.

Differential Diagnoses

Not applicable.

Comment

The condition occurs in small breeds.

Exocrine Pancreas

Table 4.5 summarizes the suggested nomenclature for nonproliferative and nonneoplastic proliferative lesions in the exocrine pancreas of dogs.

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Table 4.5.

Microscopic Findings in the Digestive System: Pancreas; Dog.

Pancreas	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Ectopic tissuea		x
Nonneoplastic
Accumulation, adipocytes		x
Amyloida		x
Apoptosisa,b		x
Apoptosis/single-cell necrosisb		x
Atrophy, acinar cell	x
Autophagic vacuoles, acinar cell		x
Ectasia, duct		x
Edemaa		x
Eosinophilic globules		x
Fibrosisa		x
Focus, basophilic		x
Halos, peri-insular, decreased			x
Halos, peri-insular, increased			x
Hemorrhagea		x
Hypertrophy, acinar cell		x
Infiltrate [insert appropriate cell type]a		x
Inflammationa		x
Inflammation, vessel		x
Metaplasia, ductule		x
Metaplasia, hepatocytic			x
Mineralizationa		x
Necrosisa		x
Pigmenta		x
Secretion decreased, acinar cell		x
Vacuolation, acinar cell		x
Proliferative (nonneoplastic)
Hyperplasia, acinar cell		x
Hyperplasia, ductal cell		x
Proliferation, extra-islet endocrine cell (nesidioblastosis)c		x

^a Terminology addressed in the General/Systemic Pathology section.

^b Finding more frequent as an induced change.

^c Terminology with diagnostic criteria and/or comments described in the text.

Comment

Nonneoplastic proliferation of islet and ductular tissue. ⁴⁰ For a more detailed description, refer to the section on endocrine pancreas in endocrine system.

Introduction

This chapter provides a recommended nomenclature for classifying microscopic lesions observed in the endocrine system of the laboratory beagle dog in toxicity studies. The endocrine system is divided into the pituitary, pineal, thyroid, parathyroid, adrenal cortex and medulla and the endocrine pancreas (islets of Langerhans). For detailed general considerations of the endocrine system, refer to the INHAND publication on the rodent.

Pituitary Gland

The pituitary gland (hypophysis) occupies a bony recess (sella turcica) in the basisphenoid bone and is attached to the hypothalamus by the infundibular stalk. The pituitary gland has 2 major compartments including the (1) adenohypophysis (anterior lobe) composed of the pars distalis, pars tuberalis, and pars intermedia and (2) neurohypophysis (posterior lobe) composed of the pars nervosa infundibulum and lobus nervosus. Macroscopically, the adenohypophysis is vascular and soft, whereas the neurohypophysis is pale with the texture of brain tissue.

The adenohypophysis consists of 3 types of endocrine cells identified by routine H&E staining: acidophils, basophils, and chromophobic cells. The frequency of each cell type varies according to the age, sex, and physiological and pathological states. The acidophils have a generally uniform pattern of distribution with increased numbers near the central region and produce growth hormone and/or prolactin. The basophils occur in greater numbers at the periphery of the adenohypophysis and produce adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone, luteinizing hormone, or follicle-stimulating hormone. The chromophobic cells appear in moderate numbers near the central region and produce ACTH. The pituitary also contains supporting cells (folliculostellate cells) and undifferentiated stem cells. The pars tuberalis is composed of epithelioid cells, sinusoids, and occasionally small follicles lined by folliculostellate or endocrine cells. The pars intermedia is located adjacent to the neurohypophysis and is separated from it by a fine layer of vascularized pial connective tissue. The majority of the cells in the pars intermedia are chromophobic cells which produce both ACTH and melanocyte-stimulating hormone. Immunohistochemistry techniques are required to accurately identify the specific cell populations.

The neurohypophysis is joined to the hypothalamus by the infundibular stalk and is composed of nonmyelinated axons and capillaries supported by modified glial cells (pituicytes). The capillaries in the pars nervosa are termination sites for unmyelinated axons, which originate from the hypothalamic neurosecretory neurons. Axons arising from supraoptic and paraventricular nuclei terminate in the pars nervosa. Both oxytocin and vasopressin (antidiuretic hormone) are synthesized in supraoptic and paraventricular nuclei as large precursor molecules, which contain both active hormones and their associated neurophysins. As the biosynthetic precursor molecules travel along the axons in secretion granules from the neurosecretory neurons, the precursors are cleaved into the active hormones.

The recommended terminology of microscopic lesions observed in the pineal gland of dogs is presented in Table 5.1.

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Table 5.1.

Microscopic Findings in the Endocrine System: Pituitary Gland; Dog.

Pituitary gland	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Aberrant craniopharyngeal structures				x
Aplasia/hypoplasia		x
Persistent Rathke’s pouch	x
Nonneoplastic
Angiectasis		x
Atrophy		x
Cyst	x
Extramedullary hematopoiesisa,b	x
Fibrosisb		x
Gliosis, pars nervosa		x
Hemorrhageb		x
Hypertrophy, pars distalis		x
Hypertrophy, pars intermedia		x
Infiltrate [insert appropriate cell type]b		x
Inflammationb		x
Metaplasia, osseous		x
Pigmentb		x
Pseudocyst	x
Thrombusb		x
Vacuolationb		x
Proliferative (nonneoplastic)
Hyperplasia, pars distalis		x
Hyperplasia, pars intermedia		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General Pathology section.

Other Term(s)

Hematopoietic cell proliferation; erythropoiesis; extramedullary erythropoiesis.

Pathogenesis/Cell of Origin

Extramedullary hematopoietic cells.

Diagnostic features

Differential Diagnoses

Pineal Gland

The pineal gland is located on the dorsal midline of the diencephalon and is part of the epithalamus. The gland forms the caudal boundary of the roof of the third ventricle. The pineal gland is mainly composed of epithelioid pinealocytes, neurons, and supporting glial cells. Pinealocytes produce and secrete melatonin and also contain serotonin. The neurons connect to the central nervous system, and the gland is innervated by the sympathetic system. The glial cells are characterized by numerous filaments, and their processes form a glial layer bordering connective tissue spaces. Myelinated and unmyelinated nerve fibers are also present.

The recommended terminology of microscopic lesions observed in the pineal gland of dogs is presented in Table 5.2.

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Table 5.2.

Microscopic Findings in the Endocrine System: Pineal Gland; Dog.

Pineal gland	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Striated muscle fibers			x
Nonneoplastic
Fibrosisa		x
Infiltrate [insert appropriate cell type]a		x
Mineralizationa		x
Vacuolationa		x

^a Terminology addressed in the General Pathology section.

Thyroid Gland

The thyroid gland originates as a thickened plate of epithelium in the ventral oropharynx to form the thyroglossal duct, which extends along the midline to the region of the larynx in the fetus. The paired lobes of the thyroid gland develop from the thyroglossal duct on either side of the larynx and proximal trachea. The ultimobranchial bodies fuse with the thyroid and deliver the C cells (neural crest origin) to each thyroid lobe. Accessory thyroid tissue, which lacks C-cells, is common in the dog and may be located anywhere from the larynx to the diaphragm. Thyroglossal duct cysts may be present in the ventral anterior cervical region and represent a postnatal, persistent portion of the thyroglossal duct.

The major components of the thyroid gland are follicular cells and C-cells (parafollicular cells). The follicular cells form follicles, which contain liquid colloid, the stored form of thyroglobulin. These follicular cells are flattened, cuboidal, or columnar depending on the biosynthetic activity of the gland. C-cells are present in the wall of the follicle or between follicles and form large clusters in the central region of the thyroid lobes. The C-cells predominantly synthesize calcitonin and also smaller quantities of calcitonin gene-related peptide, somatostatin, and bombesin, among other minor hormones.

The recommended terminology of Microscopic lesions observed in the thyroid gland of dogs is presented in Table 5.3.

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Table 5.3.

Microscopic Findings in the Endocrine System: Thyroid Gland; Dog.

Thyroid gland	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
C-cell complexa	x
Cyst, ultimobranchial	x
Ectopic tissue, thymus	x
Ectopic tissue, thyroid	x
Persistent thyroglossal duct		x
Thyroid dysplasia				x
Nonneoplastic
Accumulation, adipocytea,b	x
Amyloid		x
Atrophy			x
Colloid alteration		x
Cystic follicle		x
Dilatation, follicle, diffuse		x
Hypertrophy, follicular cell		x
Infiltrate [insert appropriate cell type]b	x
Inflammationb	x
Mineralizationb	x
Pigmentb		x
Proliferative (nonneoplastic)
Hyperplasia, C-cella		x
Hyperplasia, follicular cell		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General Pathology section.

Other Term(s)

Special parafollicular cell complex.

Pathogenesis/Cell of Origin

Remnant of cells of ultimobranchial body origin.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Immunohistochemistry: Positive for vimentin, anti-19S-thyroglobulin, and calcitonin. The immature follicular cells and C cells are positive for vimentin and 19S-thyroglobulin. ^{41 –43}

Comment

Vimentin is expressed by the immature follicular cells derived from the ultimobranchial anlage. The vimentin filaments may participate in thyroglobulin synthesis and folliculogenesis. When the follicular cells accumulate colloid in the follicular lumens, the vimentin immunoreactivity disappears. Typical thyroid follicles have no immunoreactivity for vimentin.

In the dog, the calcitonin-producing C cells are very common and particularly prominent; these cells are regarded as normal and do not have to be recorded in toxicity studies. The C cells are frequently noted in the perithyroidal tissue near the hilus of the thyroid gland and along the main branches of the thyroid artery. C-cell complexes need to be distinguished from C-cell hyperplasia, which should be diagnosed only if there is a significant increase in C-cell numbers throughout each thyroid lobe compared to age-matched controls. To achieve this, both thyroid lobes should be sectioned longitudinally in a consistent manner. ^44,45

Other Term(s)

Fat infiltration, fat replacement, infiltration, adipocyte, lipomatosis.

Pathogenesis/Cell of Origin

Mature adipocytes.

Diagnostic Features

Comment

Slight accumulation of adipocytes is recognized as an incidental or background lesion in healthy dogs frequently observed in preclinical studies, but usually not recorded. When the parenchymal tissue is severely or almost completely replaced by adipocytes, the observation should be recorded including indicating the severity.

Comment

In the dog, the calcitonin-producing C cells are very common and particularly prominent. They are regarded as normal and, therefore, do not necessarily have to be recorded in toxicity studies. They are frequently noted in the perithyroidal tissue near the hilus of the thyroid gland and along the main branches of the thyroid artery. C-cell complexes need to be distinguished from C-cell hyperplasia, which should be diagnosed only if there is a significant increase in C-cell numbers throughout each thyroid lobe compared to age-matched controls. To achieve this, both thyroid lobes should be sectioned longitudinally in a consistent manner. ^44,45

The difference between focal C-cell hyperplasia and C-cell adenoma is subjective and arbitrary. As a general guideline, the size of an adenoma should exceed several thyroid follicles; these smaller adenomas will not compress thyroid follicles and not exhibit a capsule. ⁴⁵

Parathyroid Gland

The 4 parathyroid glands are of endodermal origin derived from the pharyngeal pouches in close association with the primordia of the thymus. The external gland is on the proximal surface of the thyroid lobe, and the internal gland is embedded within the medial lateral thyroid lobe.

The parathyroid glands contain a single type of endocrine cell, namely, the chief cells. The parenchyma of the parathyroid glands consists of densely packed, folded, branching cords of polygonal chief cells separated by a delicate stroma of reticular and collagen fibers with occasional fibrocytes. The cords are usually 1 to 2 cells thick arranged in a trabecular or rarely an acinar arrangement.

The chief cells synthesize and secrete parathyroid hormone (PTH) that regulates systemic calcium homeostasis. The amount of cytoplasm reflects the level of activity of the chief cells. Chief cells with increased synthesis and secretion of PTH are hypertrophied with increased cytoplasm and reduced eosinophilia or vacuolation of the cytoplasm.

The recommended nomenclature for microscopic lesions observed in the parathyroid gland of dogs is presented in Table 5.4.

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Table 5.4.

Microscopic Findings in the Endocrine System: Parathyroid Gland; Dog.

Parathyroid gland	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Ectopic tissue, cartilagea		x
Ectopic tissue, parathyroidb		x
Ectopic tissue, thymusb		x
Nonneoplastic
Amyloid			x
Angiectasis		x
Atrophy		x
Cyst	x
Fibrosisb		x
Hypertrophy		x
Infiltrate [insert appropriate cell type]b		x
Inflammationb		x
Multinucleated giant cells			x
Proliferative (nonneoplastic)
Hyperplasia		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General Pathology section.

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Developmental anomaly.

Diagnostic Features

Differential Diagnoses

Metaplasia: Metaplasia can occur when there is abnormal stimulation of tissue growth, generally due to aberrant wound healing from persistent toxic insult or constant mechanical disruption of the injured area.

Adrenal Gland (Cortex and Medulla)

The adrenal glands are located bilaterally craniomedial to the kidneys. The outline of the left gland is flattened dorsoventrally and oval in the cranial portion and cylindrical in the caudal projection, and that of the right gland has an acute angular bend with its cortex projecting cranially. The adrenal gland is composed of 2 different tissues forming the adrenal cortex and medulla. The cortex originates from mesenchymal cells of the coelomic mesoderm. These cells initially proliferate near the genital ridge and form the fetal cortex and differentiate to form the adult cortex while the fetal cortex regresses. The medulla arises from neural crest ectoderm cells, which migrate from their point of origin into the developing mesodermal mass. The medulla is separated from the cortex by a delicate network of reticular and loose connective tissues.

Cortical zones consist of the zona glomerulosa, zona fasciculata, and zona reticularis. The outermost cortical zone is the zona glomerulosa, and the cells are in arches and clusters that constitute approximately 25% of the adrenal cortex. This zone produces aldosterone which regulates blood pressure and extracellular fluid volume by acting at distal and collecting tubules of the kidney to promote sodium retention and potassium excretion. The zona fasciculata occupies approximately 50% of the cortex, and cells in this zone are arranged in long anastomosing cords or columns, separated by small capillaries. They are responsible for the secretion of glucocorticoid hormones (eg, cortisol), which have diverse homeostatic actions on many organ systems including an increase in glucose production, decreased lipogenesis, and immunosuppression in high concentrations. The innermost cortical zone is the zona reticularis, which has a relatively random and loose network of cells and composes 25% of the cortex and produces sex hormones (androgens and estrogens). The adrenal cortex is dependent on trophic hormones secreted from the pituitary gland.

The adrenal medulla is composed of chromaffin cells, which produce catecholamine hormones, norepinephrine (noradrenaline), and epinephrine (adrenaline). Epinephrine is involved in stress and induces the “fight or flight” response, whereas norepinephrine functions as a major sympathetic neurotransmitter.

The recommended nomenclature for microscopic lesions observed in the adrenal gland of dogs is presented in Table 5.5.

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Table 5.5.

Microscopic Findings in the Endocrine System: Adrenal Gland; Dog.

Adrenal gland	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Cyst, cortex		x
Ectopic tissue, adrenocortical	x
Nonneoplastic
Amyloid		x
Angiectasis		x
Atrophy, cortex		x
Degeneration, cystic			x
Extramedullary hematopoiesisa		x
Fibrosisa		x
Hemorrhagea		x
Hypertrophy, cortex, diffuse		x
Hypertrophy, cortex, focal		x
Infiltrate [insert appropriate cell type]a	x
Inflammationa		x
Metaplasia, osseousa		x
Mineralizationa		x
Necrosis		x
Persistent X-zone				x
Pigmenta		x
Hyperplasia/hypertrophy zona glomerulosab		x
Thrombusa		x
Vacuolation, cortex, decreased, focala		x
Vacuolation, cortex, increased, diffusea		x
Vacuolation, cortex, increased, focala		x
Proliferative (nonneoplastic)
Hyperplasia, cortex, diffuse		x
Hyperplasia, cortex, focal		x
Hyperplasia, medulla, diffuse		x
Hyperplasia, medulla, focal		x
Hyperplasia, subcapsular cell			x

^a Terminology addressed in the General Pathology section.

^b Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Hyperplasia, zona glomerulosa; hypertrophy, zona glomerulosa; thickening, zona glomerulosa.

Pathogenesis/Cell of Origin

Response to endocrine stimulation of the zona glomerulosa.

Diagnostic Features

Differential Diagnoses

Not applicable.

Special Techniques for Diagnostics

Comment

Drug-induced thickening of the zona glomerulosa can occur with increased aldosterone secretion and increased number of zona glomerulosa cells. Calcium channel blockers have been reported to induce hypertrophy/hyperplasia of the zona glomerulosa. Increased thickness of the zona glomerulosa can occur in dogs with pulmonary hypertension due to heartworm (Dirofilaria immitis) infestation. ⁴⁶

Endocrine Pancreas: Islets of Langerhans

The endocrine pancreas originates from the foregut endoderm, and the endocrine cells are arranged in small foci of periductular cells or as aggregates called the islets of Langerhans. The islets distribute throughout the pancreas including the right lobe, body, and left lobe. The body and left lobe have various sized islets, while the right lobe has small islets.

There are 3 major endocrine cell types that produce glucagon (α cells), insulin (β cells), and somatostatin (δ cells). Small number of endocrine cells produce pancreatic polypeptide (PP of F cells), ghrelin, gastrin, or substance P (enterochromaffin cells). These endocrine cells are located in specific regions of the islets. In the body and left lobe, β-cells are distributed in all parts of the islet, and α-cells are scattered in the center or periphery of the islet regardless of the size of the islet. Fewer δ-cells are present in the center or periphery of the islets. In the right lobe, β-cells occupy the majority of the islet with few scattered δ-cells. ⁴⁷

Table 5.6 contains the recommended nomenclature of microscopic lesions observed in the endocrine pancreas of dogs.

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Table 5.6.

Microscopic Findings in the Endocrine System: Endocrine Pancreas (Islets of Langerhans); Dog.

Endocrine Pancreas (Islets of Langerhans)	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Amyloid, islet		x
Angiectasis, islet		x
Apoptosis, islet cell		x
Atrophy, islet cella		x
Degranulation, islet cell		x
Fibrosis, isleta		x
Hemorrhage, isleta		x
Hypertrophy, islet cell		x
Infiltrate [insert appropriate cell type]a		x
Inflammationa		x
Metaplasia, hepatocyte				x
Pigment, isleta		x
Single-cell necrosisa		x
Vacuolation, islet cella		x
Proliferative (nonneoplastic)
Hyperplasia, islet cell		x
Proliferation, extra-islet endocrine cellb	x

^a Terminology addressed in the General Pathology section.

^b Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Nesidioblastosis.

Pathogenesis/Cell of Origin

Congenital anomaly or neogenesis of islet cells arising from ductal epithelium.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Immunohistochemical staining for endocrine cells and duct cells is useful to identify proliferating cells.

Comment

Nesidioblastosis in dogs is a nonneoplastic diffuse or disseminated hyperplasia of pancreatic islets and ductules. In the lesions, the relative numbers of alpha cells is decreased, and there is no significant change in the number of beta and delta cells. Canine nesidioblastosis is not usually demonstrated as a functional disorder of the endocrine pancreas, whereas in humans, it is characterized by persistent hyperinsulinemic hypoglycemia due to defective nonneoplastic proliferation of β-cells and usually occurs in newborns and rarely adults. ^{48 –50}

Introduction

For general considerations on the hematolymphoid system, refer to the rodent manuscript. In addition, guidance on the microscopic pathology of the hematolymphoid system is the subject of an STP Position Paper ⁵¹ and is also included in a special issue of Toxicologic Pathology: A Monograph on Histomorphologic Evaluation of Lymphoid Organs, Volume 34 (5); 2006.

This chapter includes microscopic findings of the hematolymphoid system, namely bone marrow, thymus, lymph node, spleen, mucosal associated lymphoid tissue (MALT), other lymphoid structures, that is, tertiary lymphoid structures and serosa-associated lymphoid structures (TLSs and SALSs), and general hematolymphoid changes. The tabulated list is based on the nonneoplastic changes described in the INHAND manuscript for rodents. Detailed descriptions will focus on findings that have unique features in the dog compared with the rodent, while the reader is referred to the rodent document for all other findings. Findings are tabulated with an indication of their prevalence or applicability to vehicle control beagle dogs used in nonclinical toxicology studies. While the rodent manuscript details descriptive, conventional (traditional), and enhanced nomenclature, it is the opinion of nonrodent working group for the dog that more descriptive microscopic findings are generally more appropriate in the dog in general toxicology studies.

General Terminology

General terms that can be used across organs and tissues of the hematolymphoid system are presented Table 6.1.

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Table 6.1.

Microscopic Findings in the Hematolymphoid System: General Terms; Dog.

Hematolymphoid system: General terms	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Aplasia/hypoplasia			x
Nonneoplastic
Abscess		x
Amyloid			x
Apoptosis, increased, lymphocytea		x
Erythrophagocytosisb	x
Extramedullary hematopoiesis, (EMH)a		x
Granuloma		x
Infiltrate [insert appropriate cell type]c		x
Inflammation (indicate modifier)c		x
Inflammation, granulomatous		x
Inflammation, mononuclear cell, lymphocyte, monocyte, eosinophil, mixed cell, lymphoplasmacytic, or pyogranulomatous		x
Inflammation, neutrophil
Metaplasia, osseous			x
Mineralizationa		x
Necrosis		x
Pigment, macrophage		x
Tingible body macrophage, increased		x
Vacuolation, macrophagea,b,c		x
Proliferative (nonneoplastic)
Cellularity, increased, mast cell		x

^a Finding more frequent as an induced change.

^b Finding addressed in the General Pathology section.

^c Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Erythrophagia.

Pathogenesis/Cell of Origin

Erythrophagocytosis is the main route of normal and pathophysiological clearance of damaged erythrocytes. Resident bone marrow macrophages and red pulp macrophages are essential in this process. ⁵²

Diagnostic Features

Differential Diagnoses

Comment

Erythrophagocytosis is an important physiologic process to recycle iron from senescent or damaged erythrocytes to extract heme. ⁵⁴ In diseased animals, the severity of the process may be increased.

Other Term(s)

Phospholipidosis.

Comment

The descriptive (general) term “Vacuolation, macrophages” is the preferred term over phospholipidosis. The term phospholipidosis may be used as a comment or modifier but should only be used if verified by other methods, for example, electron microscopy. ⁵⁵

Bone Marrow

In nonclinical toxicity studies, hematology data in conjunction with microscopic examination of the bone marrow provide an assessment of overall hematopoietic cellularity. If the combined data are inadequate, then cytologic evaluation of the bone marrow smears (from rib or sternum) may be considered for assessing the effect of test articles on the hematopoietic system. ^51,56 Other techniques, such as flow cytometric evaluation, clonigenic assays, and electron microscopy, may also be helpful in certain situations. ⁵⁶

In dogs, bone that contains red marrow, preferably rib, sternum, or vertebrae, is sampled for the microscopic evaluation of the bone marrow. Long bones such as proximal humerus or femur should be avoided because the diaphyseal marrow cavities of these bones may be almost completely replaced by fat. ⁵⁶

The recommended microscopic terminology for the bone marrow of dogs is presented in Table 6.2.

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Table 6.2.

Microscopic Findings in the Hematolymphoid System: Bone Marrow; Dog.

Bone marrow	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Angiectasis		x
Cellularity decreased, adipocytes		x
Cellularity decreased, bone marrow	x
Dyshematopoiesis		x
Emperipolesisa		x
Fibrosisb		x
Hypersegmentation, granulocyte		x
Serous atrophy of fatb		x
Proliferative (nonneoplastic)
Cellularity increased, adipocytes	x
Cellularity increased, bone marrow	x
Cellularity increased, macrophagec		x
Cellularity increased, mast cell		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Finding addressed in the General Pathology section.

^c Finding more frequent as an induced change.

Other Term(s)

Engulfment of another cell.

Pathogenesis/Cell of Origin

Diagnostic Features

Differential Diagnoses

Comment

Emperipolesis is often noted in megakaryocytes containing intact neutrophils or other hematopoietic cells. The significance of the finding is unclear as it can be observed in healthy animals but also associated with disease.

Thymus

The thymus should be trimmed transversely through the middle of the organ after fixation to include both lobes. The cortex and medulla should be evaluated separately considering the cortex: medulla ratio average across multiple lobules.

The thymus involutes (shrinks) with age in dogs. The thymus of beagle dogs undergoes progressive involution during the period from 6 to 23 months of age. ⁵⁷ Therefore, the thymus weights are of low if any significance. Keratinization of Hassall’s corpuscles is prominent in young dogs as in minipigs and nonhuman primates.

The recommended microscopic terminology for the thymus of dogs is presented in Table 6.3.

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Table 6.3.

Microscopic Findings in the Hematolymphoid System: Thymus; Dog.

Thymus	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Apoptosis, increased, lymphocyte		x
Cellularity decreased, lymphocyte	x
Corticomedullary ratio, decreased		x
Corticomedullary ratio, increased		x
Cyst, epithelial	x
Ectopic tissue, (specify tissue)a		x
Ectopic tissue, parathyroida		x
Ectopic tissue, thymusa		x
Hypoplasia	x
Involution, age-related	x
Loss of corticomedullary distinction		x
Necrosis, lymphocyteb		x
Proliferative (nonneoplastic)
Cellularity increased, epithelial cells			x
Cellularity increased, lymphocyte		x
Epithelium-free areas, increased		x
Thymic corpuscles, increased		x

^a Finding addressed in the General Pathology section.

^b Finding more frequent as an induced change.

Spleen

Routinely, histopathological evaluation of the spleen involves examination of a single cross-section, which is sufficient to evaluate the red pulp and includes the serosal surface, but white pulp may be not adequately present. A longitudinal section will increase the amount of lymphoid tissue (white pulp) available for assessment. ⁵⁸ The role of the spleen in dogs is primarily a storage organ, whereas in rodents, it is primarily defensive. ⁵⁹ Due to the at times massive barbiturate-related congestion in euthanatized dogs, spleen weight is of little value in nonclinical toxicologic pathology studies.

The recommended microscopic terminology for the spleen of dogs is presented in Table 6.4.

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Table 6.4.

Microscopic Findings in the Hematolymphoid System: Spleen; Dog.

Spleen	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
White pulp
Apoptosis increased, lymphocytea		x
Cellularity decreased, white pulp		x
Hyaline materialb	x
Red pulp
Angiectasis			x
Cellularity decreased, red pulp		x
Congestion	x
Contraction	x
Ectopic tissue, spleen		x
Erythrophagocytosis	x
Fibrosis			x
Hemosiderotic plaquesb	x
Proliferative (nonneoplastic)
White pulp
Cellularity increased, plasma cell, white pulp		x
Cellularity increased, white pulp		x
Red pulp
Aggregates, macrophage, increased	x
Cellularity increased, adipocyte		x
Cellularity increased, macrophage	x
Cellularity increased, mast cell		x
Cellularity increased, mesothelial cell		x
Cellularity increased, plasma cell, red pulp		x
Cellularity increased, stromal cell		x
Extramedullary hematopoiesis, increaseda		x
Hyperplasia nodulara		x

^a Finding more frequent as an induced change.

^b Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Hyaline deposits.

Diagnostic Features

Differential Diagnoses

Hemosiderotic Plaque—Spleen (Figures 6.7 and 6.8)

Other Term(s)

Siderotic plaque, siderofibrotic plaque, sidero-calcific plaque, Gamna-Gandy bodies.

Pathogenesis

Diagnostic Features

Present in the fibrous connective tissue of the capsule, trabeculae, and/or around the hilar vessels and consists chiefly of bilirubin, hemosiderin, and/or calcium deposits. May not be seen macroscopically in young dogs but are recognized macroscopically in older dogs as yellow or grey encrustations or nodules.

Comment

Senile change, no toxicologic significance.

Lymph Node

Typically examined lymph nodes in toxicologic pathology studies include a section of mandibular and mesenteric lymph node, but other nodes may be sampled as indicated by the study design or target biology. Ideally, a standardized midsagittal section of lymph node should be obtained to include all the lymph node compartments, which constitute the follicles and interfollicular cortex, the paracortex (or deep cortex), and the medulla, including medullary cords and medullary sinuses (Table 6.5). ⁶¹

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Table 6.5.

Microscopic Findings in the Hematolymphoid System: Lymph Node; Dog.

Lymph node	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Cortex, paracortex, and medullary cords
Apoptosis, increased, lymphocyte	x
Cellularity decreased lymphocyte	x
Hyaline materiala	x
Sinuses and lymphatics
Dilatation, sinus		x
Erythrocytes, intrasinusoidal	x
Fibrosis			x
Lymphangiectasia		x
Proliferative (nonneoplastic)
Cortex, paracortex, and medullary cords
Aggregates, increased, macrophage	x
Cellularity increased, interdigitating dendritic cell		x
Cellularity increased, lymphocyte	x
Cellularity increased, plasma cell	x
Cellularity increased, stromal cell		x
Hyperplasia, angiomatous		x
Hypertrophy/hyperplasia high endothelial venules		x
Sinuses and lymphatics
Cellularity increased, macrophages, intrasinusoidal	x
Cellularity increased, mast cell			x

^a Terminology with diagnostic criteria and/or comments described in the text.

Other Term(s)

Hyaline deposits.

Diagnostic Features

Hyalinized material is occasionally seen in the follicles of lymph nodes and spleen. ⁶⁰

Differential Diagnoses

Mucosa-Associated Lymphoid Tissue

The MALT is usually organized into nonencapsulated accumulations or diffuse collections of lymphocytes scattered along most mucosal surfaces. The MALT, a normal anatomical structure, is considered a secondary immune tissue, which initiates immune responses to specific antigens encountered at or via the mucosal barrier. Following antigen stimulation, lymphocytes forming MALT may proliferate, as is commonly noted with inflammation, but test articles or xenobiotics may also directly stimulate MALT. It would be inappropriate to record normal or stimulated MALT as mononuclear or lymphocytic infiltrates or inflammation. The most common examples of MALT are gastrointestinal-associated lymphoid tissue in the stomach, intestines, and gallbladder; bronchial-associated lymphoid tissue in the lung; nasal-associated lymphoid tissue in the nasal cavity; conjunctiva-associated lymphoid tissue in the eyelids. Less frequently, MALT may be encountered in sections of trachea, larynx, urinary bladder, reproductive tract, and the ducts of various exocrine glands.

Tonsils represent another secondary lymphoid tissue that may be examined in toxicity studies in dogs. In the dog, 3 tonsils are present, the lingual tonsil, the paired palatine tonsil, and the pharyngeal tonsil. ⁶² For further anatomical and histological details, please refer to anatomical textbooks or the excellent review article by Casteleyn et al. ⁶²

The recommended microscopic terminology for the mucosa-associated lymphoid tissue of dogs is presented in Table 6.6.

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Table 6.6.

Microscopic Findings in the Hematolymphoid System: Mucosa-Associated Lymphoid Tissue (MALT); Dog.

Mucosa-associated lymphoid tissue	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Cellularity decreased, lymphocytea	x
Degeneration, follicle-associated epithelium		x
Hyaline material	x
Lymphangiectasia		x
Proliferative (nonneoplastic)
Aggregates, macrophage			x
Cellularity increased, lymphocytea	x
Cellularity increased, macrophagea		x
Hyperplasia, follicle-associated epithelium			x
Hyperplasia, goblet cell, follicle-associated epithelium			x
Hypertrophy/hyperplasia high endothelial venule (HEV)		x
Metaplasia squamous, follicle-associated epithelium			x

^a Finding more frequent as an induced change.

Other Lymphoid Tissues (TLSs and SALSs)

For the definition and function of TLSs and SALSs, the reader is referred to the rodent manuscript. Usually, these structures are examined microscopically in toxicologic pathology studies when they are included in the section of the associated tissue (like MALTs).

The recommended microscopic terminology for other lymphoid tissues of dogs is presented in Table 6.7.

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Table 6.7.

Microscopic Findings in the Hematolymphoid System: Other Lymphoid Tissues (Tertiary Lymphoid Structures and Serosa-Associated Lymphoid Structures); Dog.

Other lymphoid structures	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Tertiary lymphoid structures		x
Proliferative (nonneoplastic)
Increased serosa-associated lymphoid structures		x

Introduction

The liver is a common target organ for chemical-induced toxicity in all species, including dogs, because most drug exposure occurs via the oral route and the liver receives the absorbed chemical directly via the portal vein. The liver metabolizes (biotransforms) many of these substances, termed xenobiotics, for excretion as water-soluble compounds. During biotransformation, intermediate compounds can pose a risk of cellular injury to hepatocytes and biliary epithelium.

Liver

Liver pathology has been covered in detail in the rodent INHAND document ⁶³ as well as in other publications. ^64,65 This section provides recommended terminology for the hepatobiliary system in the laboratory beagle dog during preclinical toxicity studies with diagnostic criteria and histology images included only for nomenclature where there are distinguishing features of the process that differ between the dog and the rodent.

In all organs and tissues, including those of the hepatobiliary system, tumors are very rarely observed in toxicologic pathology studies in beagle dogs and, therefore, are generally not included in this manuscript. For the nomenclature and diagnostic criteria of the neoplastic changes, refer to the chapter on hepatobiliary neoplasms in the textbook of Tumors of Domestic Animals. ^66,67

Physiology

Physiology of dog hepatobiliary system is generally similar to rodents and has been described. This functional heterogeneity across the lobule is characterized by greater drug metabolism via the cytochrome P450 enzymes in the centrilobular regions of the liver. Periportal regions, supplied with the most oxygenated blood, are generally more active in synthetic activities. The zonation develops as a result of the oxygen gradient across the acinus or the gradient of hormones and nutrients.

The recommended microscopic terminology for the liver of dogs is presented in Table 7.1.

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Table 7.1.

Microscopic Findings in the Hepatobiliary System: Liver; Dog.

Liver	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Congenital vascular anomalya		x
Ductal plate anomalya		x
Hepatodiaphragmatic nodule			x
Nonneoplastic
Amyloid		x
Angiectasis		x
Apoptosisa,b	x
Atrophy, hepatocyte		x
Bile pluga		x
Congestionb		x
Crystalsc		x
Cysts, bile duct		x
Cytoplasmic alteration		x
Degeneration, cystic		x
Degeneration, hydropic		x
Erythrophagocytosis, Kupffer celld		x
Extramedullary hematopoiesisb	x
Fatty change (see Vacuolation)	x
Fibrosisb		x
Helicobacter sp. hepatitis				x
Hypertrophy, hepatocytec		x
Hypertrophy/hyperplasia, Kupffer cellc		x
Hypertrophy/karyomegaly, endothelial cell		x
Inclusions, intranuclear and cytoplasmic		x
Inclusions, intranuclear brick inclusionsa		x
Infarct		x
Infiltrate [insert appropriate cell type]b,c,e	x
Infiltrate, mixed inflammatory cellb,c,e		x
Infiltrate, mononuclear cellb,e	x
Infiltrate, neutrophilb,c,e		x
Infiltrate, peribiliary (intrahepatic)b,c,e		x
Inflammationa,b		x
Intrahepatocellular erythrocytes		x
Intravascular hepatocytes			x
Karyocytomegaly and/or multinucleated hepatocytesc		x
Metaplasia, glandular, hepatocyte		x
Metaplasia, pancreatic acinar cell			x
Mineralization^		x
Mouse hepatitis virus hepatitis				x
Murine norovirus hepatitis				x
Necrosis, focal/multifocalc		x
Necrosis, zonal		x
Phospholipidosisc		x
Pigment		x
Rarefactiona	x
Single-cell necrosisb	x
Thrombusb		x
Tyzzer’s disease (Clostridium piliforme infection)			x
Vacuolation, hepatocytea		x
Proliferative (nonneoplastic)
Cholangiofibrosis			x
Focus of cellular alteration			x
Hyperplasia, bile ductc		x
Hyperplasia, hemangioendothelial		x
Hyperplasia, hepatocyte, nonregenerative		x
Hyperplasia, hepatocyte, regenerativec		x
Hyperplasia, Ito cell			x
Hyperplasia, oval cellc		x
Neoplastic
Adenoma, hepatocellulara		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General Pathology section.

^c Finding more frequent as an induced change.

^d For criteria and description of erythrophagocytosis, refer to the Hematolymphoid System/General Terms.

^e For the recommended use of infiltrate, refer to Infiltrate [insert appropriate cell type] in the section General/Multisystemic Pathology.

Other Term(s)

Microvascular dysplasia.

Pathogenesis/Cell of Origin

Abnormal development of portal vasculature.

Diagnostic Features

Differential Diagnoses

Portal vein thrombosis.

Comment

Congenital vascular anomalies are occasionally observed and are generally macroscopic findings made at necropsy, except for primary hypoplasia of the portal vein, which does not involve a shunt vessel and requires histologic confirmation. ⁷⁰ Affected animals have small livers, and histologic features in these dogs include portal vein hypoperfusion, small or absent portal veins, proliferated hepatic arterioles, and hepatocyte atrophy. Primary hypoplasia of the portal vein resembles portosystemic shunts histologically, but affected animals often have portal hypertension and resultant ascites. In the absence of a diagnosed shunt and particularly in the presence of portal hypertension, the most likely diagnosis is portal vein hypoplasia.

Other Term(s)

Ductal plate malformation, juvenile polycystic disease/congenital hepatic fibrosis, congenital dilatation of the large and segmental bile ducts (macroscopic observation).

Pathogenesis/Cell of Origin

Persistence or aberrant remodeling of the embryonic ductal plate.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Cytokeratin 7 or 19 to identify biliary epithelium.

Comment

This is a rare developmental anomaly in dogs. ^70,71

Other Term(s)

Programmed cell death.

Pathogenesis/Cell of Origin

Diagnostic Features

Differential Diagnoses

Single-cell necrosis: Single-cell necrosis results from acute cellular injury and is characterized by cell and nuclear swelling, karyolysis, karyorrhexis, karyopyknosis, pale eosinophilic cytoplasm, cytoplasmic vacuoles, loss of cellular detail, cellular debris, and inflammation.

Comment

A comprehensive description of the processes involved and a guidance on which terminology to use is provided by the publication of the INHAND Apoptosis/Necrosis Working Group. ⁷² In brief:

The diagnosis should be based primarily on the morphological features in H&E-stained sections. When needed, additional, special techniques to identify and characterize apoptosis can also be used.

Other Term(s)

Cholestasis, icterus, pigment, pigment deposition.

Pathogenesis/Cell of Origin

Obstruction of bile flow, hepatocellular injury.

Diagnostic Features

Differential Diagnoses

Other pigments, including hemosiderin, lipofuscin, porphyrin, hematoxylin, and formaldehyde: A comprehensive discussion of differentiation is included in the rodent manuscript under pigmentation. ⁶³

Proof of bile pigment can be achieved by special stains (eg, iodine test or Hall test).

Other Term(s)

Acidophilic crystalline intranuclear inclusions, paracrystalline intranuclear inclusions.

Pathogenesis/Cell of Origin

The genesis is unknown.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Brick inclusions are positive with acid-fast staining ^73,74 ; it can be confirmed by electron microscopy.

Comments

Brick inclusions are occasionally observed in young beagle dogs using toxicological studies, although it commonly appears in older dogs. There are no evidences of biological or pathologic significance. Similar inclusions occur in the proximal tubular epithelial cells in kidneys in dogs. There are no evidences relating to heavy metal intoxication nor viral infection.

Other Term(s)

Hepatitis.

Comment

Diagnostic term “inflammation” should be used carefully. The term inflammation is the recommended, preferred term over hepatitis or any etiologic terminology; the etiology, when determined, may be added as a comment or descriptor to the finding. The type of inflammation is usually added as a modifier and preferably should include the predominant cell type (eg, neutrophilic, lymphoplasmocytic, histiocytic, etc). Indicating the chronicity as a modifier (peracute, acute, subacute, chronic) is acceptable. Traditionally, hepatic inflammatory responses have been classified as acute, subacute, chronic, granulomatous, and so on. These terms are somewhat interpretative, lack precise definitions, vary depending upon study duration, usually do not consist of a singular cell type, and do not have exclusive pathognomonic features. A more descriptive approach is recommended and can be qualified by lesion distribution or use of subclassification and discretionary qualifiers.

Other Term(s)

Cytoplasmic rarefaction, excess glycogen accumulation, ballooning degeneration, corticosteroid hepatopathy.

Pathogenesis/Cell of Origin

Excessive endogenous or exogenous corticosteroid exposure leads to significant accumulation of cytoplasmic glycogen beyond that seen in fed dogs, most likely due to a disturbed balance between glycogenesis and glycogenolysis. ^{75 –77}

Other Term(s)

Fatty change, steatosis, lipidosis, lipid accumulation; phospholipidosis.

Pathogenesis/Cell of Origin

Excessive accumulation of vacuoles (generally lipids) in the cytoplasm

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Frozen sections of unfixed or fixed tissue can be stained for fat (oil red O, Sudan IV) and are helpful in recognizing microvesicular lipidosis and in differentiating fat from other substances that may accumulate in vacuoles.

Comment

Comment

A rare benign tumor in the liver of beagle dogs, which has been observed in toxicologic pathology studies. The neoplasm is included in the INHAND rodent manuscript of the hepatobiliary system. ⁶³ For the diagnostic criteria in dogs of this and other neoplastic findings, readers are encouraged to refer the chapter on hepatobiliary neoplasms in the textbook Tumors of Domestic Animals ⁶⁶ and/or current literature.

Gallbladder and Biliary Tree

Table 7.2 summarizes the recommended nomenclature for gall bladder changes that can be observed in dogs used in preclinical toxicological studies.

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Table 7.2.

Microscopic Findings in the Hepatobiliary System: Gallbladder and Biliary Tree; Dog.

Gallbladder and biliary tree	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Ectopic tissue, hepatocytes		x
Ectopic tissue, pancreasa		x
Nonneoplastic
Calculus		x
Hemorrhagea		x
Hyalinosis			x
Infarcta		x
Inflammation		x
Mucocelea		x
Proliferative (nonneoplastic)
Hyperplasia		x
Hyperplasia, cystic mucinousa		x
Metaplasia, glandular			x

^a Terminology with diagnostic criteria and/or comments described in the text.

Comment

For a detailed description of the ectopic pancreas tissue in the gallbladder, refer to the rodent manuscript (Ectopic Tissue, Pancreas). To the authors’ knowledge, there is only one reference available that describes this lesion in the dog, specifically in a Siberian Husky. ⁷⁹ A case of ectopic pancreatic tissue in a beagle dog has been brought to the authors’ attention; refer to the image (Figure 7.12).

Other Term(s)

Pathogenesis/Cell of Origin

Extravasation of red blood cells.

Diagnostic Features

Red blood cells outside of the vascular system in the parenchyma, interstitial space, or cavity of the hollow organ.

Differential Diagnoses

Congestion. Accumulation of blood within the vascular system.

Comment

Hemorrhage, bleeding, extravasation, or the escape of blood from the blood vessels may be observed in virtually every organ and tissue and is microscopically characterized by the extravasal presence of erythrocytes in tissues. For the types of hemorrhages observed, refer to the comment under Hemorrhage in the General Pathology section. The exact location of the hemorrhage (eg, “submucosa”) may be added as a modifier, locator, or comment, depending on the data acquisition system and the pathologist’s preference.

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Occlusion of the cystic artery.

Diagnostic Features

Transmural coagulative necrosis

Differential Diagnoses

Gall bladder mucocele may occur concurrently or infarction may occur as a single entity.

Special Techniques for Diagnostics

Phosphotungstic acid hematoxylin staining may help identify arterial thrombi.

Comment

The pathogenesis of arterial thrombosis is not known. ⁸⁰

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Etiology is unknown, may be related to decreased gallbladder motility, bile stasis, altered bile composition, and viscosity.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

The PAS may help highlight the mucus in the lumen of the gall bladder.

Comment

This condition may cause extrahepatic biliary obstruction. ^81,82

Other Term(s)

Cystic hyperplasia, cystic mucinous hypertrophy, papillary adenomatous hypertrophy.

Pathogenesis/Cell of Origin

Unknown.

Diagnostic Features

Differential Diagnoses

Special Techniques for Diagnostics

Not applicable.

Comments

Diagnostic overlap with gallbladder mucocele as some cases may have elements of cystic mucinous hyperplasia as well as excessive production of abnormal mucus. Regarded as an age-related disorder, so very uncommon in the laboratory beagles generally used in preclinical toxicology studies. This finding may be associated with exposure to progestational agents. ^83,84

Introduction

This section provides a proposed standardized nomenclature for classifying lesions observed in the skin and appendages of dogs, namely epidermis, dermis, hair follicles, sebaceous glands, apocrine (epitrichial) glands, eccrine (atrichial) glands, and nailbed using standard H&E-stained paraffin sections only. Lesions in the mammary gland are covered separately. Dermal lesions, which were covered in the soft tissue, nervous system, or cardiovascular system INHAND rodent manuscripts, are also covered separately, under those respective systems. The nomenclature of skin lesions in the dog differs from the rodent most strikingly in the classification of neoplasms. The authors recommend the use of the WHO nomenclature for domestic animals or the more recently published text titled “Tumors of Domestic Animals”¹ for reference if a skin neoplasm is identified in chemical or pharmacological toxicity studies.

In this section, only notable differences between dogs and rodents will be addressed, as the anatomy of dog skin is comparable to that of other mammals. For detailed general considerations on the integument, please refer to the INHAND publication on rodent integument, ⁸⁵ and, for a detailed description of the anatomy and physiology of canine skin, refer to Muller and Kirk’s Small Animal Dermatology. ⁸⁶ Surveys of skin lesions in beagle dogs are included in the publications of Sato et al ⁴⁹ and Scudamore. ⁶⁰ Parasitic diseases in the dog include flea infestations (Ctenocephalides canis and others), sarcoptic mange (Sarcoptes scabiei), and demodectic mange (Demodex canis). For details on the diagnostic criteria of parasites, the reader is referred to textbooks on veterinary parasitology. In laboratory settings of toxicologic pathology studies, parasitic diseases of the skin are very rare. When parasites are observed, they should be recorded as “parasite” and, if possible, specified further in a comment to the finding. Also refer to the General/Multisystemic Pathology section.

In general, the thickness of dog skin ranges from 0.5 to 5.0 mm, with the thickest areas on the head, dorsum of the neck, back, and sacrum. ⁸⁶ In a recent study, the thickness of beagle dog skin was referred with 1.3 to 1.4 mm at different measurement sites. ⁸⁷ The epidermis of normal haired skin varies from 0.1 to 0.5 mm with 2 to 3 nucleated keratinocyte layers. ⁸⁸ Rete ridges are not found in the normal haired dog skin but may be observed at the nasal planum, footpads, and scrotum. The epidermal surface varies from undulating at the densely haired skin of the back to heavily folded at the skin of the abdomen. ⁸⁸ The surface of footpads is papillated. Beside keratinocytes, the epidermis comprises several other cell types, for example, melanocytes, Langerhans cells and Merkel cells. ⁸⁹ The melanocytes are located in the basal layer of the epidermis, in the outer root sheath of hair follicles, in the hair matrix, and in ducts of sebaceous and sweat glands. Single melanocytes can also be found in the superficial dermis. Long dendritic processes of melanocytes extend between keratinocytes and transfer pigment-containing melanosomes to keratinocytes. ⁸⁶

The canine dermis does not usually have dermal papillae. The dermis is sparsely populated with cells, but there can be several mast cells around superficial dermal blood vessels. ⁸⁶

The beagle dog, most commonly used in preclinical safety studies, has a short coat composed of primary (outercoat) and secondary (undercoat) hair. Canine hair follicles are arranged in clusters of several primary hairs surrounded by groups of smaller secondary hairs. Each primary hair is accompanied by sebaceous glands, apocrine sweat glands, and arrector pili muscles, whereas secondary hairs may have sebaceous glands. ⁹⁰ Sebaceous glands and apocrine sweat glands are distributed throughout the haired skin, whereas eccrine sweat glands are limited to the footpads. Besides these adnexal glands, dogs have specialized glands including circumanal glands, glands of external ear canal, the anal sac gland, and the tail gland. Circumanal glands and tail gland (supracaudal gland) are composed of hepatoid cells, which can also be found in the prepuce. Anal sac glands are composed of abundant large sebaceous glands and apocrine sweat glands, whereas the glands of the external ear canal (ceruminous glands) are apocrine glands. Meibomian glands are modified sebaceous glands. ⁸⁶

Table 8.1 summarizes the microscopic lesions that can be observed in the skin of beagle dogs in toxicological safety studies.

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Table 8.1.

Microscopic Findings in the Integument: Skin; Dog.

Skin	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Skin: Epidermis
Apoptosisa,b,c		x
Atrophy, epidermis		x
Edema, epidermis, intercellular		x
Edema, epidermis, intracellular		x
Erosion/ulcer	x
Hyperkeratosis, epidermis	x
Infiltrate [insert appropriate cell type], epidermisb	x
Necrosis, epidermis		x
Parasiteb.d		x
Pigment, decreaseda		x
Pigment, increaseda	x
Pustule		x
Vesicle		x
Skin: Cutaneous adnexa
Adnexal dysplasia		x
Atrophy, adnexal		x
Hyperkeratosis, adnexa	x
Inflammation, adnexab	x
Necrosis, adnexac		x
Parasiteb,d		x
Skin: Dermis and subcutis
Atrophy, dermis		x
Edema, dermis		x
Elastosis			x
Parasiteb,d		x
Proliferative (nonneoplastic)
Skin: epidermis
Cyst, squamous		x
Hyperplasia, epidermisa		x
Papilloma, squamousa		x
Skin: Cutaneous adnexa
Cysta		x
Hyperplasia, adnexaa		x
Skin: Dermis and subcutis
Hyperplasia, melanocyte		x
Polyp, fibroepitheliala		x
Neoplastic
Cutaneous histiocytomaa,e		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General (Multisystemic) Pathology section.

^c Finding more frequent as an induced change.

^d For the recording of parasites, refer to the General (Multisystemic) Pathology section.

^e The cutaneous histiocytoma is also included in the soft tissues section.

Other Term(s)

Programmed cell death.

Pathogenesis/Cell of Origin

Diagnostic Features

Differential Diagnoses

Single-cell necrosis: Single-cell necrosis results from acute cellular injury and is characterized by cell and nuclear swelling, karyolysis, karyorrhexis, karyopyknosis, pale eosinophilic cytoplasm, cytoplasmic vacuoles, loss of cellular detail, cellular debris, and inflammation.

Comment

A comprehensive description of the processes involved and a guidance, which terminology to use is provided by the publication of the INHAND Apoptosis/Necrosis Working Group ⁷² . In brief:

Other Term(s)

Hypopigmentation, hypomelanosis.

Pathogenesis/Cell of Origin

Decreased pigmentation reflects lower amounts of melanin pigment in the epidermis either microscopically or macroscopically (loss of skin or hair pigmentation). While it may be associated with congenital or acquired idiopathic defects on melanization in some dog breeds, in toxicologic pathology, most often it is the result of the effects of certain chemicals (eg, monobenzyl ether of dihydroquinone) on melanocytes, inflammatory disorders that affect melanization or destroy melanocytes, hormonal disorders, or dermatoses featuring hydropic degeneration of basal keratinocytes. In dermatoses associated with hydropic degeneration of basal cells and subsequent hypopigmentation, the underlying superficial dermis usually reveals pigmentary incontinence characterized by increased size and/or number of melanosomes. The mechanism of pigmentary incontinence is unclear but could include epidermal injury and melanin release to the superficial dermis through a damaged basement membrane.

Diagnostic Features

Comments

Decreased pigmentation may be associated with pigmentary incontinence. In general, pigmentary incontinence is not recorded separately in toxicity studies, but may be described as a component of a distinct lesion with melanin release to the superficial dermis.

Other Term(s)

Hyperpigmentation, melanosis.

Pathogenesis/Cell of Origin

Increased deposition of melanin within the epidermis. Skin pigmentation is affected by local inflammation because melanocytes respond to inflammatory mediators by increasing or decreasing melanogenesis and by altering melanin transfer to keratinocytes.

Diagnostic Features

Comments

Hyperpigmentation is a common, nonspecific finding that can be observed in many skin diseases associated with chronic inflammation, numerous hormonal dermatoses, as well as some developmental and neoplastic disorders. In general, this finding is not described or recorded separately in toxicity studies but may be described as a component of a complex inflammatory finding or may be recorded when describing a chronic or pronounced change. Increased pigment may be observed in the hormone responsive skin of dogs that have been administered the dopaminergic and prolactin-inhibiting agent, bromocriptine. ^91,92

Comment

Epithelial hamartoma (pigmented epidermal nevus, linear epidermal hamartoma, verrucous epidermal nevus), Cutaneous horn (papillomatous hyperplasia), and Seborrheic keratosis are considered entities to be summarized under epidermal hyperplasia. A comment may be added to the diagnosis to further specify the lesion.

Epidermal hamartomas are congenital or acquired malformations that are commonly recognized only later in life. In the dog, association with papilloma virus has been described in cases. Inflamed linear epidermal nevus is a very rare lesion in dogs. The lesion is characterized by focal irregular epidermal papillary hyperplasia, laminated hyperkeratosis, and keratohyalin granules and may be present in the stratum granulosum and may be hyperpigmented. The inflamed linear epidermal nevus is characterized by prominent parakeratosis, neutrophilic pustules, and variable dermal inflammation.

The spontaneous cutaneous horn is a very rare lesion in beagle dogs. It may occur at any age and has been observed in association with other hyperplastic or neoplastic skin entities. Microscopically, the change is characterized by an exophytic cylindrical formation of compact keratin, focally hyperplastic epidermis, extensive, compact, laminated, ortho- or parakeratotic hyperkeratosis. On the surface, the stratum corneum forms columns.

Seborrheic keratosis is a benign, acanthotic, and often pigmented epidermal proliferation; other features include an exophytic or slightly endophytic growth pattern, monomorphic and basaloid appearance of proliferating keratinocytes, and multifocal keratin accumulations that may form small horn cysts, and hyperpigmentation due to numerous melanin granules distributed throughout proliferating keratinocytes. The lesion is rarely diagnosed in the dog. So far, no breed, sex, or site predilection was recognized. The lesion seems to occur more frequently in older dogs

Other Term(s)

Idiopathic squamous papilloma, warts.

Pathogenesis/Cell of Origin

Originates from squamous epithelium of the epidermis.

Diagnostic Features

Comment

A number of cystic lesions with hyperplastic properties have been described in the skin of dogs. Usually, their terminology reflects the origin and/or their features. Examples are:

These conditions are generally rarely observed in toxicologic pathology studies. Should these lesions occur, it is recommended to diagnose them as cyst with a descriptive modifier. For the diagnostic criteria, refer to the section Epithelial and Melanocytic Tumors of the Skin by Goldschmidt and Goldschmidt ⁹³ in the textbook “Tumors of Domestic Animals”, to “Integumentary System” by Mauldin and Kennedy ²⁷ in Jubb, Kennedy, and Palmer’s Veterinary Pathology, Volume I, the WHO fascicle Histological Classification of Epithelial and Melanocytic Tumors of the Skin of Domestic Animals, ⁹⁴ and/or pertinent literature.

Comment

Apocrine (sweat gland) hamartomas, fibroadnexal hamartomas, follicular hamartomas, and sebaceous hamartomas are considered entities to be summarized under adnexal hyperplasia. The lesions originate from apocrine, follicular, and/or sebaceous epithelial cells. Microscopically, they are characterized by an accumulation of disorganized but mature specialized cells or tissues indigenous to the particular site. The lesions represent proliferative lesions of hair follicles, sebaceous glands and ducts, apocrine glands, or pilosebaceous units. A comment may be added to the diagnosis to further specify the lesion.

Apocrine (sweat gland) hamartomas are rare but have been described in dogs; they are proliferating sweat glands in the superficial dermis and/or panniculus; apocrine glands are larger than normal, ectatic and lined by attenuated epithelium; the overlaying epidermis may be hyperplastic.

Fibroadnexal hamartomas (adnexal nevus, focal adnexal dysplasia, folliculosebaceous hamartoma) occur most commonly on the distal limbs, pressure points, and in interdigital areas. The lesions may be congenital or acquired following trauma to the area. Microscopic features include hyperplastic, distorted, and dilated hair follicles; no connection to the skin surface and no hair bulbs; surrounded by dense, mature collagen; and variable inflammation.

Follicular hamartomas (follicular nevus) are rare lesions that occur more frequently in younger dogs. Predisposed sites include the limbs and neck. Microscopically, the lesions consist of clusters of very large anagen hair follicles that extend more deeply than adjacent normal follicles. In the majority of cases, hair follicles within a cluster are uniform and have the appearance of primary follicles; associated glands may be enlarged. The lesions are surrounded by hyalinized collagen, and secondary folliculitis may occur.

Sebaceous hamartomas are regarded as an uncommon variant of the fibroadnexal hamartoma characterized by an intradermal proliferation of predominantly large sebaceous glands and ducts. They are located in the mid to deep dermis without involvement of superficial dermis; hair follicles are small and malformed; the lesions are surrounded by mature collagen.

Other Term(s)

Cutaneous tag, skin tag, acrochordon, fibrovascular papilloma.

Pathogenesis/Cell of Origin

Proliferative fibrovascular lesion

Diagnostic Features

Comment

Fibroepithelial polyps are fibrovascular lesions that rather represent a proliferative response to trauma or inflammation than actual neoplasms. They occur mostly on the trunk, sternum, and pressure points of the limbs and may be solitary or multiple.

Other Term(s)

Not applicable.

Pathogenesis/Cell of Origin

Langerhans cells.

Diagnostic Features

Differential Diagnoses

Comment

Usually a solitary lesion of young dogs, less than 3 years old, but has been observed in dogs of all ages. The lesion occurs in all breeds, predisposed are brachycephalic breeds, including boxers, but also dachshund, Doberman pinschers, and cocker spaniels. Spontaneous regression is a feature. ^96,100

Introduction

For detailed general considerations, refer to the INHAND rodent publication. ¹⁰¹ A detailed description of estrous cycle-related changes in the canine female genital tract, including the mammary gland, is provided in a review by Rehm et al ¹⁰² ; cycle-related changes of the mammary gland are also presented in Table 11.1 of the female reproductive system. The following findings may be observed as part of the normal estrus cycle in dogs: fibrosis in early diestrus, apoptosis (single-cell necrosis), and pigment accumulation in proestrus. ¹⁰³

In routine toxicologic pathology studies, 1 section of the mammary gland, usually one of the inguinal glands, is examined per female and male. The section should include the nipple, the duct, and a representative amount of glandular tissue. In male dogs, glandular tissue may be difficult to obtain; the presence of ductular structures is considered sufficient in these cases.

Substantial differences between dogs and rodents are observed in the morphology and terminology of mammary gland neoplasms; for a detailed description of the neoplastic and tumor-like lesions, refer to the “International Histological Classification of Tumors of Domestic Animals: Mammary Tumors of the Dog and Cat,” published by the Armed Forces Institute of Pathology in conjunction with the American Registry of Pathology and the World Health Organization, ¹⁰⁴ the more recently published “Tumors of Domestic Animals,” ¹⁰⁵ and other pertinent literature.

The recommended terminology of microscopic lesions in the mammary gland of dogs is presented in Table 9.1.

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Table 9.1.

Microscopic Findings in the Mammary Gland; Dog.

Mammary gland	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Amyloid		x
Angiectasis		x
Apoptosis/single-cell necrosisa,b	x
Atrophy		x
Basophilia		x
Congestionb		x
Corpora amylacea		x
Degeneration		x
Dilatation		x
Edemab		x
Fibrosisb	x
Hemorrhage ^		x
Hypertrophy/hyperplasia, alveolar and/or ductal cell		x
Infiltrate [insert appropriate cell type]b		x
Inflammationb		x
Mineralizationb		x
Necrosisb		x
Parasiteb		x
Pigmentb	x
Single-cell necrosis ^	x
Thrombosisb		x
Proliferative (nonneoplastic)
Hyperplasia, lobuloalveolar		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General Pathology section.

Other Term(s)

Programmed cell death.

Pathogenesis/Cell of Origin

Diagnostic Features

Differential Diagnoses

Single-cell necrosis: Single-cell necrosis results from acute cellular injury and is characterized by cell and nuclear swelling, karyolysis or karyorrhexis, nuclear pyknosis, pale eosinophilic cytoplasm, cytoplasmic vacuoles, loss of cellular detail, cellular debris, and inflammation.

Comment

A comprehensive description of the processes involved and guidance on which terminology to use is provided by the publication of the INHAND Apoptosis/Necrosis Working Group. ⁷² In brief:

Use necrosis and apoptosis as separate diagnostic terms.

Introduction

Species used for neurotoxicological research exhibit 3 different general models of brain architecture based on the prominence of cerebrocortical subdivisions (archicortex, paleocortex, neocortex). The rodent model, which also applies for the rabbit, is characterized by a lissencephalic cerebral surface and large archicortex. The carnivore (dog, cat) model shows prominent gyri and sulci in the cerebrum, which abuts the cerebellum but does not cover it in contrast to the primate model. The carnivore model shows far less complex cerebrocortical subdivisions than the primate. The most striking difference between carnivore and rodent brain is the presence of a well-developed neocortex in carnivores. In the spinal cord, a notable difference between rodent and carnivore is the much larger size of the dorsal funiculus in carnivores. The location and size of spinal motor tracts show notable species differences: the corticospinal tract of rats is found in the dorsal funiculus, while in carnivores and primates, it is located in the lateral funiculus; compared to carnivores, the rodent shows smaller sensory tracts in the dorsal funiculus. In the brain stem, the olivary nucleus is enlarged in carnivores and primates compared to rodents, reflecting also the greater lateral expansion of the cerebellar hemispheres. The size of the medial and lateral lemniscus is increased in carnivores and primates compared to rodents. The profile of the vermis on the caudal surface of the cerebellum is generally straight in rodents and primates but is twisted into a serpentine loop in carnivores because of its elongation in these species. The corpus striatum is unified in rodents but divided in carnivores and primates into caudate and lentiform nuclei. The information summarized here came from a review article, ¹⁰⁶ and the reader is referred to that publication for additional details. Other pertinent references include:

A comparative brain atlas of rat, dog, and monkey in Fundamental Neuropathology for Pathologists and Toxicologists, Chapter 3. ¹⁰⁷

The atlas “The Beagle brain in stereotactic coordinates” ¹⁰⁸ is a useful reference for neuroanatomy in the dog.

The large brain size of the carnivore compared to rodents necessitates a different approach to trimming and embedding the brain; a recommended trimming scheme is presented in an STP position paper by Bolon et al ¹⁰⁹ resulting in 7 sections/blocks.

The nomenclature of lesions of the nervous system in the dog differs from the rodent most strikingly in the classification of neoplasms, which is outside the scope of this document, given the very low incidence of nervous system neoplasms in the young age and well-controlled breeding programs of purpose-bred dogs commonly used in general toxicity studies. For a detailed description of the neoplastic and tumor-like lesions, refer to “Tumors of Domestic Animals” ¹¹⁰ and other pertinent literature.

Table 10.1 summarizes the recommended terminology of microscopic lesions observed in dogs used in toxicity studies.

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Table 10.1.

Microscopic Findings in the Nervous System: Brain, Spinal Cord, and Peripheral Nerve; Dog.

Brain, spinal cord, and peripheral nerve	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Neuron cell body
Decreased cellularity, neuron		x
Chromatolysis		x
Heterotopia, neuronal		x
Necrosis, neuron		x
Neuronophagia		x
Pigment, lipofuscin		x
Vacuolation, neuron		x
Neuron axon
Atrophy, axonal		x
Degeneration, axonal		x
Degeneration, nerve fiber	x
Dystrophy, axonal	x
Radiculoneuropathy			x
Glia cell body
Type II astrocytes		x
Astrocyte swelling/vacuolation		x
Astrocyte swelling		x
Astrocytosis		x
Gliosis (NOS)		x
Microgliosis		x
Pigment, lipofuscin		x
Satellitosis		x
Glia myelin
Accumulation, laminar, Schwann cell		x
Demyelination		x
Intramyelinic edema		x
Choroid plexus
Extramedullary hematopoiesisa		x
Vacuolationb		x
Vascularc
Necrosis/inflammation, media or wall, artery		x
Infarct		x
Thrombusb		x
General
Cholesterol clefts		x
Cyst, squamous		x
Dilatation, ventriclea		x
Hemorrhageb		x
Hydrocephalus		x
Hydromyelia		x
Inflammationb		x
Infiltrate, [insert appropriate cell type]b		x
Mineralizationb	x
Syringomyelia		x
Syringomyelia/hydromyelia		x
Ectopic tissue	x
Meninx
Inflammationb		x
Mineralizationb		x
Peripheral nervous system
Accumulation, laminar, Schwann cell	x
Accumulation, matrix		x
Autophagy, neuron, ganglion		x
Necrosis/inflammation, media or wall, artery		x
Atrophy, axonal		x
Decreased cellularity, neuronal		x
Chromatolysis		x
Degeneration, axonal		x
Degeneration, nerve fiber	x
Demyelination		x
Dystrophy, axonal		x
Edema, intramyelinicb		x
Heterotopia, neuronal		x
Inflammationb		x
Infiltrate [insert appropriate cell type]b		x
Necrosis, neuronal		x
Neuronophagia		x
Vacuolation, neuronalb		x
Proliferative (nonneoplastic)
Meninx
Aggregates, granular cell			x
Meningioangiomatosisa		x
Peripheral nerve
Renaut bodiesa	x
Other cell lineages
Hamartoma, lipomatous		x

^a Terminology with diagnostic criteria and/or comments described in the text.

^b Terminology addressed in the General Pathology section.

^c Refer to description in the section “Cardiovascular System.”

Other Term(s)

Hematopoietic hyperplasia; myeloid hyperplasia; erythroid hyperplasia; megakaryocytic hyperplasia; hematopoietic cell proliferation; myeloid metaplasia.

Pathogenesis/Cell of Origin

Hematopoietic progenitor cells.

Diagnostic Features

Hematopoietic cells of 3 lineages (myeloid, erythroid and megakaryocytic) may be present.

Comment

Extramedullary hematopoiesis (EMH) in the choroid plexus represents a very subtle finding in beagles; it has been noted in almost 35% of 19- to 56-week-old males and 20% of 24- to 85-week-old females. ¹¹¹ In dogs from certain breeders, increased incidences were noted, and the observation was not necessarily associated with a pathologic condition. In another publication, EMH was described in dogs euthanatized because of seizures; no other remarkable hematopoietic findings were noted and no other site of extramedullary hematopoiesis was observed. ¹¹² However, no coherent evidence for a connection between EMH and seizures was provided.

Other Term(s)

Ventriculomegaly.

Pathogenesis/Cell of Origin

Unknown.

Diagnostic Features

Differential Diagnoses

Hydrocephalus: Internal hydrocephalus is also characterized by a widening of the ventricular space; it is caused by disturbance or blockage of ventricular drainage and accompanied by degenerative changes, including atrophy and cavitation of the white matter.

Comment

Dilatation of the ventricle(s) or ventricular system is usually a diagnosis readily made at trimming. The need of histomorphologic correlation may require the repetition of the term. Vullo et al ¹¹³ reported 17 asymptomatic beagle dogs with ventricular dilation by magnetic resonance imaging and ultrastructure evaluations (normal ependymal cells, white matter, and glia/microglial cells with normal cerebrospinal fluid flow).

Biological Behavior

Meningioangiomatosis is a benign proliferative condition of the central nervous system, which is considered to be a vascular malformation or hamartoma (ie, a disorganized, benign mass composed of cells indigenous to the site) and not a neoplasm.

Other Term(s)

Not applicable.

Pathogenesis

This finding likely develops from mesenchymal, pluripotent cell with fibroblastic or meningothelial differentiation.

Diagnostic Features

Comment

Meningioangiomatosis is a rare benign proliferative condition of central nervous system reported in humans and animals including young dogs. It is generally considered to be a hamartoma; however, whether meningioangiomatosis is hamartomatous, neoplastic, or a developmental defect remains controversial. ¹¹⁴ Affected dogs exhibit ataxia, circling, rotatory nystagmus, mild tetraparesis, proprioceptive deficits, or muscle atrophy. ^115,116

Biological Behavior

Benign proliferation of endoneural collagen forming well-demarcated elliptical, lamellar structures containing a core of intensely eosinophilic fibers.

Other Term(s)

Not applicable.

Histogenesis

Mesenchymal origin endoneural collagen subtypes I, II and VI) and mucopolysaccharides

Pathogenesis

Physiologic adaptation of nerves (usually distal portion of nerves within fore and hind legs) hypothesized to be a response to mechanical stress imposed on them.

Diagnostic Features

Special diagnostic techniques:

Differential Diagnoses

Schwannoma, mesenchymal neoplasia.

Comment

Renaut bodies have been described in humans at sites of chronic nerve entrapment ¹¹⁸ as well as in control beagle dogs ^117,119,120 and horses. ^119,121 The incidence of RB can increase in subchronic and chronic studies ¹²⁰ and have been reported in hypertrophic neuropathy. ¹²² These lesions may serve to cushion adjacent nerve fibers from mechanical trauma. ¹¹⁹ This premise is supported by the experimental demonstration that RB may be induced by nerve compression, ^120,123,124 suggesting that mechanical trauma is the primary pathogenesis for the change, although other mechanisms may be involved since affected autonomic nerves near viscera are not subjected to great mechanical stress. ¹¹⁷ The number of RB increases with age ¹²⁵ and may vary by the vendor. ¹²⁶

Introduction

For general aspects on the female reproductive system, refer to the rodent publication. This chapter provides the recommended nomenclature for the canine female reproductive tract. Generally used preferred terms of systemic nonproliferative lesions across organ systems, such as hemorrhage, inflammation, or thrombus, are included in separate INHAND manuscripts and are therefore not included in this section, except where there are distinguishing features of the process relevant to the female reproductive system. In all organs and tissues, including those of the female reproductive system, tumors are very rarely observed in toxicologic pathology studies in beagle dogs and therefore are generally not included in this manuscript. For the nomenclature and diagnostic criteria of neoplastic changes, refer to the World Health Organization’s Histologic Classification Tumors of the Genital System of Domestic Animals, ¹²⁷ the more recent publication on Tumors of Domestic Animals by Meuten¹, and pertinent literature.

Anatomy and Physiology

The anatomy and histology of the canine female reproductive tract has been described extensively, and it is beyond the scope of this chapter to detail the morphology of this system and its cycle-dependent changes. The reviews by Rehm et al ¹⁰² and Chandra and Adler ¹²⁸ are excellent references for this topic.

Briefly, the cycle is composed of proestrus (lasting 1-2 weeks), estrus (lasting 1-2 weeks), and diestrus (lasting between 2 and 3 months). Metestrus in the dog is a very short phase usually not distinguished from estrus. ¹⁰² The anestrus phase of the cycle is unique to the dog. It represents a quiescent phase following diestrus and lasting between 3 and 5 months. ¹⁰² Dog-specific peculiarities in the reproductive physiology include the presence of polyovular follicles, ovulation of an immature oocyte, and a periovulatory period with particularly high concentrations of circulating progesterone. ^{129 –131}

In the female dog, the ovary, uterus, cervix, and vagina show marked cyclical changes, which are displayed in Table 11.1. For easy reference, the cycle changes in the mammary gland are included as well.

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Table 11.1.

Normal Cyclic Changes in the Female Reproductive System.

Stage of the cycle	Duration	Ovary	Uterus	Vagina	Mammary gland
Immature	Puberty around 6 months of age	No corpus luteumWaves of follicular growth	Atrophic	2-3 cells thick	No glandular structures
Proestrus	7-9 days	Medium-large follicles	Mild edema and congestion	Thickened	Inactive glands/duct dilatation
Estrus/metestrus	9-10 days	Large follicles/start of luteinization	Edema and gland development	Keratinization and submucosal edema	Stromal proliferation with edema
Diestrus	60-90 days	Large mature corpus luteumEarly: bright eosinophilicLater: vacuolated	Two zones(i) Inner compact (vacuolated cells, folding of surface)(ii) Outer spongy (dilatation of glands)	Variable thickness, +/− leukocytes (neutrophils)	Secretion +++
Anestrus	4.5-5 months	Degenerate corpus luteumApoptosis, pigment deposit	Atrophic	2-3 cells thick	Inactive remnants

Nomenclature

The recommended nomenclature for nonproliferative and nonneoplastic proliferative changes in beagle dogs used in general toxicology studies listed in Tables 11.2 to 11.6. The nomenclature is adapted from the rodent manuscript as listed on the go-RENI.org website. Diagnostic criteria for nonneoplastic changes are only described if they differ from those used in mice and rats.

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Table 11.2.

Microscopic Findings in the Female Reproductive System: Ovary, Dog.

Ovary	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Ectopic tissuea		x
Ovotestis		x
Nonneoplastic
Amyloid		x
Angiectasis		x
Atrophy		x
Atrophy, age-related		x
Atrophy, corpora lutea		x
Cyst, bursal		x
Cyst, epithelial		x
Cyst, follicular		x
Cyst, luteal		x
Cyst, NOS		x
Cyst, paraovarian		x
Cyst, rete ovarii		x
Decreased number/absent corpora lutea		x
Decreased number/absent follicles		x
Degeneration, corpora lutea		x
Degeneration, oocyte		x
Edemaa		x
Follicle, luteinizedb	x
Follicle, polyovular	x
Hypertrophy, corpora lutea		x
Hypertrophy, interstitial cell		x
Immature	x
Increased number, atretic follicles		x
Increased number, corpora lutea		x
Infiltrate [insert appropriate cell type]		x
Inflammation, ovary		x
Mineralizationa	x
Pigmenta	x
Vacuolation, corpora luteaa		x
Vacuolation, granulosa cella		x
Vacuolation, interstitial cella		x
Vacuolation, theca cella		x
Proliferative (nonneoplastic)
Hyperplasia, cystic/papillary		x
Hyperplasia, granulosa cell		x
Hyperplasia, interstitial cell		x
Hyperplasia, rete ovarii		x
Hyperplasia, Sertoli cell		x
Hyperplasia, sex cord stromal, mixed		x
Hyperplasia, smooth muscle, mesovarium		x
Hyperplasia, theca cell		x
Hyperplasia, tubulostromal		x
Neoplastic
Adenomac		x

^a Terminology addressed in the General Pathology section.

^b Preovulatory follicles are luteinized in the dog and should not be diagnosed as a finding if the dog is in late estrus.

^c Terminology with diagnostic criteria and/or comments described in the text.

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Table 11.3.

Microscopic Findings in the Female Reproductive System: Oviduct, Dog.

Oviduct	Common	Uncommon	Not observed but potentially relevant	Not applicable
Nonneoplastic
Atrophy		x
Infiltrate [insert appropriate cell type]a		x
Inflammationa		x
Salpingitis isthmica nodosa				x
Proliferative (nonneoplastic)
Hyperplasia, epithelium		x

^a Terminology addressed in the General Pathology section.

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Table 11.4.

Microscopic Findings in the Female Reproductive System: Uterus, Dog.

Uterus	Common	Uncommon	Not observed but potentially relevant	Not applicable
Congenital
Mesonephric duct remnants		x
Nonneoplastic
Abscess(es)		x
Amyloid		x
Angiectasis		x
Aplasia, segmental		x
Apoptosisa		x
Atrophy		x
Cyst, NOS		x
Decidual reaction			x
Decidualization, focal			x
Dilatation, glandular, cystic	x
Dilatation, luminal	x
Fibrosis		x
Granuloma		x
Hypertrophy, epithelium		x
Hypertrophy, myometrial		x
Hypoplasia		x
Infiltrate [insert appropriate cell type]a	x
Inflammationa		x
Inflammation, endometrium		x
Inflammation, myometrium		x
Metaplasia, squamous cell		x
Necrosisa		x
Pigmenta		x
Prolapse		x
Pyometra		x
Vacuolation, epitheliuma		x
Proliferative (nonneoplastic)
Adenomyosis		x
Hyperplasia, angiomatous		x
Hyperplasia, endometrial stromal		x
Hyperplasia, endometrial, diffuse		x
Hyperplasia, glandular, cystic		x
Hyperplasia, glandular, focal		x
Hyperplasia, granular cell		x
Hyperplasia, myometrial		x
Hyperplasia, segmental cysticb		x
Polyp, endometrial stromalb		x

^a Terminology addressed in the General Pathology section.

^b Terminology with diagnostic criteria and/or comments described in the text.