Toxicologic Pathology

Classification of rat hepatocellular proliferative lesions can vary between pathologists as the many qualitative histologic criteria have not been satisfactorily evaluated and ranked for prognostic value. Computer-assisted morphometry offers an objective method to evaluate certain cellular features. The Solt-Farber resistant hepatocyte model was used in this study to produce populations of rats with a full range of hepatocellular proliferative lesions. Cellular features within the lesions were then measured morphometrically and the data were analyzed by animal age and by subjective lesion diagnosis. The nuclear/cytoplasmic ratio followed by the cell area and nuclear area were found to be the most important parameters for separating microscopic foci and islands of cellular alteration, an early hyperplastic lesion, from other hepatocellular proliferative lesions. The coefficient of variation, as a relative measure of heterogeneity, increased in a linear manner for cell, nuclear and nucleolar areas as the animals aged and was significantly higher for cell and nuclear area in hepatocellular carcinoma compared to other hepatocellular proliferative lesions. Hepatocyte nodules (representing primarily late hyperplastic lesions) and persistent hepatocyte nodules (lesions with similarities to hepatocellular adenoma) could not be satisfactorily seaprated within the limits of this study. As these borderline lesions show a continuum of cytologic change, other features, such as architectural change, are necessary to satisfactorily classify them on a subjective basis. An alternative approach is to use discriminant functions derived from morphometric studies.

The cellular and subcellular morphology of hepatocellular carcinomas (HCs) from feral English sole (Parophrys vetulus) living in contaminated waterways of Puget Sound was compared with normal-appearing liver from English sole inhabiting reference sites in Puget Sound. The most distinctive feature of English sole HCs was the occurrence of large dilations of RER containing densely packed microfilamentous-like material that was often arranged in complex branching arrays that nearly filled the cytoplasm of affected hepatocytes. These arrays have not previously been reported in HCs of any other fishes. Several other characteristics also distinguished HCs from reference liver, including: the apparent proliferation of Golgi complexes and mitochondria; occurrence of torus-shaped mitochondria; elongation and/or proliferation of perisinusoidal cells; and inclusion of large intrahepatocellular vacuoles.

A standard infiltration of the rat tibialis anterior muscle with 0.1 ml of local anesthetics was used as a model to help choose criteria for testing the intramuscular toxicity of drugs. Anesthetics used were 0.2% carbisocaine, 1% and 2% Lidocaine®, 0.5% Marcaine®, 1% and 2% Mesocaine®, 1% and 2% Procaine®. Increases in the serum levels of creatine kinase were monitored for 24 hours, and the weight, as well as macro- and microscopic changes in the muscle for a period of 1 month. Exposure of the muscle to local anesthetics resulted in 2 types of lesions. One was characterized by selective muscle fiber damage in the injected area. The other type of lesion was a generalized one that involved a number of cell types. To assess the intramuscular toxicity of drugs we defined the type of lesion, its size, and the rate of subsequent muscle regeneration. We recommend the following criteria for the assay of myotoxicity of new drugs being developed as pharmaceutical agents: 1) serum creatine kinase level 1 hour after intramuscular injection of the drug; 2) microscopic findings at 3, 7, and 21 days; 3) the cross-sectional area of the lesion at 3 days; and 4) the weight of the muscle at 7 and 21 days.

This paper describes the review process for pathology data submitted to the Division of Pathology, Center for Food Safety and Applied Nutrition of the Food and Drug Administration. The Division of Pathology independently evaluates the pathology data submitted in support of the safety of a given compound. The submissions are examined for agreement between summarized information and data from individual animals, appropriateness of terminology applied to lesions, and adequacy of information (distribution and severity of observed lesions). Problems and concerns encountered during review sometimes require examination of microscopic slides. The slide review provides an independent characterization of the lesions and a verification of their incidence. We present some problems commonly encountered in our review of the pathology data and describe some recent pathology evaluations. Finally, we suggest some considerations for reporting pathology data that may facilitate regulatory review.

Hematopoietic and related neoplasms were morphologically studied in a total of 1,765 male and 1,765 female Sprague-Dawley rats from 6 chronic toxicity studies conducted at a large toxicology testing laboratory. The most common types of lymphoid neoplasms seen included the lymphoblastic lymphoma (0.65%) and the large granular lymphocyte lymphoma (LGL, Fischer or mononuclear cell leukemia) (0.60%). The most common type of nonlymphoid neoplasm with morphologic features similar to lymphoid tumors was histiocytic sarcoma (1.1%). A small number of cases of myelogenous leukemia were also seen. Additional work is needed in the classification of hematopoietic neoplasms in rats including refined immunomorphological studies.