"Lipomatous" hamartomas and choristomas in inbred laboratory mice

In a retrospective study, 37 male and 19 female inbred laboratory mice, from 1 to 36 weeks of age, were diagnosed with "lipomatous" hamartomas or choristomas from nearly 10,000 mice examined at necropsy over a 24-month period. Hamartomas and choristomas were found to be rare, noninherited tumor-like conditions that occurred spontaneously in 18 inbred strains of mice with a predominance of the conditions in the C3H/HeJ and C57BL/6J strains. Prevalence between strains ranged from 0.6 to 6.2 cases per hundred thousand mice. The 56 cases studied had soft, raised masses that arose on the dorsal midline, primarily above the sutures of the skull. The lesions were prominent on gross examination due to abnormally long hair, change in direction of the hairs, and a change in hair color compared to the normal pelage. Microscopically, the masses consisted of normal adipose tissue in the reticular dermis and subcutis that sometimes extended through the cranial sutures, entering the brain, or expanding into the ventricles. Large masses occasionally contained normal appearing thyroid, intestine, respiratory epithelium lined cysts, squamous epithelial cysts, bone and marrow, cartilage, glands, and angiomatous anomalies. In all cases, the epidermis was intact. Hair follicles were larger in the affected areas of many cases compared to those in adjacent skin. Breeding studies did not yield affected offspring, indicating this is a congenital, noninherited abnormality. This condition resembles "lipomatous" hamartomas, a congenital defect in human beings.