SUMMARY
Stimulation of the nipple lines and genital and pelvic regions by self-licking, found previously to increase during pregnancy in the rat, was studied for its effect on mammary gland development. Licking was prevented by fitting wide rubber collars around the necks of rats for the entire period of pregnancy or for either the first half (days 1–12) or the second half (days 12–22) of pregnancy. The percentage of secretory tissue, and ratings of lobulo-alveolar (L-A) development and secretory (S) activity were used to measure the development of the gland on either the 22nd or the 12th day of pregnancy. Notched collars, which permitted licking, were used as a control for any stress effects that resulted from the collars, and injections of formalin were used as a control for any stress produced by the collar. Mammary development was reduced to only 50% of normal, and L-A development and S-activity ratings showed a corresponding retarded development in animals wearing a collar throughout pregnancy. Notched collars did not affect gland development and formalin injections accelerated it. There was no difference in the effects of wearing a collar during the first or second half of pregnancy: both groups were equal in gland development on the 22nd day and their glands were more developed than those of females wearing a collar throughout pregnancy. In animals that wore collars from the beginning of pregnancy, gland development was already retarded (28%) by mid-pregnancy. Rats whose glands were reduced in development during the first half of pregnancy showed less nipple-line licking during the second half of pregnancy. These findings indicate that stimulation provided by self-licking contributes significantly to mammary gland development during pregnancy and, therefore, the activity of the gland before parturition, like lactation post partum, is regulated in part by external stimulation.
Perfluorooctanoic Acid Effects on Steroid Hormone and Growth Factor Levels Mediate Stimulation of Peripubertal Mammary Gland Development in C57Bl/6 Mice
