Unerupted Dentition Secondary to Congenital Osteopetrosis in the Osborne-Mendel Rat 1

Disorders of the osseous skeleton with concomitant unerupted dentition have been described in several species. The grey-lethal (gl) mouse with a 21-30 day life span and unerupted dentition was introduced in 1935 (1). This was followed in 1941 by the incisorless (ia) rat (2) with normal longevity and transitory lack of bone resorption, since after almost complete arrest of resorption, the bone was spontaneously resorbed, and then some molars may erupt (3). The microphthalmic (mi) mouse, described in 1942, had normal longevity and delayed or partial eruption of teeth (4). The osteopetrotic rabbit was introduced in 1948 and had a life span of approximately 5 weeks and variable tooth eruption (5).

This report describes a new mutant rat with a skeletal disorder, apparent normal longevity, and unerupted molar and incisor dentition. The mutation has been assigned the symbol tl for “toothless.”

The mutation occurred spontaneously in a partly inbred rat colony derived from the Osborne-Mendel colony maintained at the National Institutes of Health, Bethesda, Maryland. The affected animals have not bred, with the exception of one F₁ generation male which was backcross mated to the original dam. Additional animals have been produced by mating of heterozygous littermates. The strain/ substrain designation for the proposed strain is TLOM/Ndri (6).

The toothless rats have been maintained on dry or water mixture of powdered diet² (7)supplemented with fatty acids.³ The maximum adult male and female weights reached were 295 and 230 gm, respectively. Thus far, the longest life span for affected rats is for living F₂ generation animals born February 13, 1973.

The toothless rats can be differentiated from their littermates at 10 days by their smaller size, shorter snout, lack of incisors, and characteristic periorbital encrustation. The eye encrustation (Fig. 1) is similar to the involvement observed in the grey-lethal mouse (8) and the incisorless rat (2).