The thymus of the hairless rhino-j (hr/hr-j) mice

The hairless (hr) gene is expressed in a large number of tissues, primarily the skin, and a mutation in the hi gene is responsible for the typical cut aneous phenotype of hairless mice. R;Mutant hr mouse strains show immune de fects involving especially T cells and macrophages, as well as an age-relat ed immunodeficiency and an accelerated atrophy of the thymus. These data su ggest that the hr mutation causes a defect of this organ. although hr trans cripts have not been detected in fetal or adult mice thymus. The present st udy analyses the thymus of young (3 mo) and adult (9 mo) homozygous hr-rh-j mice (a strain of hairless mice) by means of structural techniques and imm unohistochemistry to selectively identify thymic epithelial cells, dendriti c cells, and macrophages. There were structural alterations in the thymus o f both young and adult rh-rh-j mice, which were more severe in older animal s. These alterations consisted of relative cortical atrophy, enlargement of blood vessels, proliferation of perivascular connective tissue, and the ap pearance of cysts. hr-rh-j mice also showed a decrease in the number of epi thelial and dendritic cells, and macrophages. Taken together, present resul ts strongly suggest degeneration and accelerated age-dependent regression o f the thymus in hr-rh-j mice, which could explain at least in part the immu ne defects reported in hairless mouse strains.