Characteristics of a new hairless mutation (bald) in rats.

Hereditarily hairless rats have been maintained by sibmatings since 1973. These animals were termed "bald rats", and examined for their characteristics on genetics, physiological, hematological and clinical chemistry values and histology. The bald rats began to lose their hair at approximately three weeks of age. At five weeks of age, the skin was devoid of all general body hair except for the vibrissae, and the skin became wrinkled at two months of age. Histologically, follicular cysts were observed in a large number, from three months of age onward. The bald condition was a simple recessive character (ba); homozygous animals lost hair and heterozygous ones did not. The bald females attained sexual maturity around eight weeks of age and littered normally, but failed to nurse their young. The incapability of supplying sufficient milk supposedly resulted from the mammary glands that involuted after delivery. The bald animals consumed more food but showed lower values for body weights, plasma triglyceride, and epididymal adipose tissue amount than the haired animals. In addition, the bald males had brown adipose tissue in the interscapulum which was histologically activated as in newborn animals. Aged bald animals showed spontaneous tumors of the thymus, pituitary, testis, uterus, mammary gland, and skin. The incidence of skin tumors was much higher than those of tumors of other origins.