Effect of estrogen as well as a combination of estrogen and proges terone on the nonlactating mammary gland of female albine rats was studied ultrastructurally by the transmission electron microscope.
The mammary lobules of control nonlactating rats is formed of solita ry ducts surrounded by abundant adipose tissue. The lining epithelium of the duct was found to be formed of dark, light and myoepithelial cells.
In estrogen treated rats, lobulo-alveolar proliferation with slight secretory activity could be found, while in rats treated with a comb ination of estrogen and progesterone, a proliferative and marked secretory activity of the milk alveoli were pronounced.