Histological and morphometrical studies on the rat nipple during the reproductive cycle

Histological changes in the rat nipple during the reproductive cycle were observed. In virgin and the first half (days 5 and 10) of pregnancy, the nipple had a dull conical shape and the germinative layer of epidermis, thicker than that of the skin surrounding the nipple, deeply ingrew into the dermis in the basal region. From the second half (days 15 and 20) of pregnancy to the post-weaning period, the nipple appeared columnar in shape and many wrinkles were observed in the nipple wall especially during the lactating period. Collagen fibers longitudinally running in the nipple wall mainly comprised the dermis of the nipple and became loose during lactation. Small numbers of elastic fibers running parallel with smooth muscles were also observed in the nipple wall, and these increased in number and thickness from the second half of pregnancy, and most became frizzy structures during lactation. Around the lactiferous sinus, smooth muscle cells were arranged longitudinally but a few muscle cells were seen in a concentric layer, but during the lactating period the sinus was distended and many epithelial folds were observed. Morphometrical analysis indicated that the length of the nipple increased from the second half of pregnancy and reached the maximum on day 15 of lactation, approximately 3.7 times that in the virgin period. The outer diameter of the nipple and thickness of the nipple wall during lactation also reached approximately twice that in virgin. The size of the nipple decreased gradually after weaning. These observations suggest that the histological changes in the rat nipple during the reproductive cycle were mainly characterized by hyperplasia of the epidermis and hypertrophy of connective tissue in the dermis from the second half of pregnancy.