Growth of mouse mammary glands after neonatal sex hormone treatment.

Mammary gland growth was examined in female mice immediately and during 1 month after neonatal treatment with sex hormones. BALB/c mice were given daily injections of hormones for the first 5 days after birth. As a criterion of growth, the number of ductal branchings was counted in stained wholemounts of the fourth (inguinal) pair of glands. At day 6, the growth of the glands was significantly inhibited in animals treated with 17 beta-estradiol (E) or diethylstilbestrol (DES); after treatment with progesterone (P) or androgens, no immediate effect was evident. At day 33, neonatal treatment with E stimulated growth, as did neonatal treatment with testosterone (T) or 5 alpha-dihydrotestosterone (5 alpha-DHT); treatment with a high dose of DES inhibited growth. Treatment with P or 5 beta-dihydrotestosterone had no effect. In conclusion, neonatal treatment with sex hormones has two distinct effects on the growth of mouse mammary gland: 1) inhibition by estrogens, observed immediately after treatment, and 2) stimulation by E, T, and 5 alpha-DHT, observed 4 weeks after treatment.