Morphologic characteristics of the skin of bald mutant rats

The skin of a new hairless mutation in the rat termed "bald" was examined histologically and enzyme histochemically with animals from three weeks to 18 months of age. The loss of hair in homozygous (bald) rats proved to occur as follows: a club hair rising within the hair follicle in the first catagen phase was not anchored and fell out due to dilatation of the follicular lumen. In the skin of bald rats from two to three months of age on, two types of cyst developed, one from the infundibulum of the hair follicle and the other from a lower follicular portion left in the dermis. Each had histologic patterns different from each other. The wall of the former cyst contained various-sized keratohyaline granules in a large number, while the latter was keratinized without granules. In addition to cyst formation, foreign-body granulomas frequently appeared from three months of age on, originating from degenerated follicular portions in the dermis. In advanced cases after 12 months of age, the granulomatous lesions were sharply demarcated from the other tissue. Histochemically, acid phosphatase activity was observed in the skin of bald rats, in the wall of the dilated hair follicles and the cystic wall where progressive keratinization with age occurred. This enzymatic activity tended to heighten as keratinization proceeded.