Mammary gland hyperplastic alveolar noduligenesis in the LEW/Mai rat in relation to mitotic activity, estrous cycle status, and age.

Spontaneous mammary hyperplastic alveolar noduligenesis was studied in nulliparous female LEW/Mai rats in relation to age, estrous cycle status, and mammary gland mitotic activity. The morphometric composition of the mammary gland and the incidence of hyperplastic alveolar nodules (HAN) and dysplasias were determined by examination of stained wholemount preparations of one of the inguinal mammary glands. Mitotic activity in the contralateral mammary gland rose almost linearly from 30 to 108 days of age and then fell progressively thereafter, although mitotic activity remained significantly elevated compared to that in mammary glands of rats less than 100 days old. Mitotic activity was nonrandomly distributed in the mammary glands of young pubertal rats in contrast to that in glands of prepubertal rats and in rats in persistent estrus, in which it tended to be randomly distributed. HAN were present in some rats by 150 days of age and were frequent in the mammary glands of most rats older than 200 days. All animals with HAN were in persistent vaginal estrus and had significantly increased pituitary weights. This association suggested that hormone imbalance may be an important etiologic factor in spontaneous hyperplastic alveolar noduligenesis in the rat.