The Skin of the Rhino Mouse

Abstract

Surely one of the oddities of nature, the rhino mouse as it matures becomes ``curiouser and curiouser.'' This recessive, single gene mutant of the house mouse was first described by Howard in 1940. During the first 2 weeks of life, the homozygous animal resembles its heterozygous and normal littermates. Born naked, it develops a full, darkly pigmented first pelage. Then, over the next 10--12 days, the hair is lost in a cephalad to caudal wave, leaving the animal once again quite naked. With the exception of a few hairs in an occasional animal, a second pelage never appears because of an aberration in the first catagen (Mann 1971). The dermal papilla fails to follow the contracting follicle and becomes isolated in the subcutaneous tissue. The two never rejoin, rendering the follicle permanently hairless. Shortly thereafter, the skin becomes progressively loose and redundant, falling increasingly into rhinoceroslike folds, flaps, and ridges (Fig. 234). This extraordinary expansion of the surface occurs because the empty follicles widen into ampulliform cavities or ``utriculi'' which become distended with retained horny cells. The horn-filled utriculi are not grossly apparent until the animal ages (Fig. 235), which it does prematurely in 6 months to 1 year. The mature rhino mouse usually weighs more than its haired littermates mainly because of the large masses of skin. Indeed, the muscles and subcutaneous tissue regress (Davies et al. 1971).