Development of pigmented scales on rat skin: relation to age, sex, strain, and hormonal effect

Development of brownish scales on the skin was examined in seven inbred strains of laboratory rats, Rattus norvegicus (F344, WM, WF, LOU, BUF, ACI, and LEJ rats) up to 104 weeks old. Pigmented scales appeared mainly on the dorsum of the body but also on the perineum and the tail. The distribution of the scales changed with age and showed sex and strain differences. In males of all seven strains except F344, the scales on the dorsal region appeared at puberty then gradually developed and spread extensively in a strain-dependent pattern, until the rats were of adult age. The coloring gradually decreased with further aging. However, in F344 rats, the coloring did not spread after puberty, and in adult rats, it was partial and very weak. Among female rats, scales appeared only in LEJ rats when they were of adult and old age. Gonadectomy in WM and WF rats caused fading of the dorsal scales in males but slightly induced coloring in females. Androgen administration to the gonadectomized rats increased the pigmented scales in both sexes of both strains. In F344 rats, however, skin color was hardly changed by gonadectomy and/or the subsequent androgen administration in either sex. In F344 male rats, the testosterone concentration in serum was not significantly lower than that of WM and WF male rats. These results indicate that the development of brownish scales on rat skin is dependent on age, sex, strain, and androgen, and it is suggested that the ability producing the scales is genetically poor in the F344 strain.