Proliferation kinetics in the mammary gland of the mouse during postnatal development.

Radioautographic studies after incorporation of 3H-thymidine in the mammary gland of the mouse demonstrate a decrease in the rate of mitosis in parenchymal epithelial cells from 80/1000 cells in the first week of life to 15/1000 cells in the 2nd week and an increase again at the 25th day of life of 100/1000 cells. The labeled nuclei are distributed irregularly in the glandular epithelium. The highest mitotic activity is found at the bifurcation of ducts and at the base of sprouting end buds. In contrast labeled myoepithelial cells are distributed uniformly throughout all parts of the developing gland. The external shape of the nipple of the female mammary gland is a result of local differences in the rate of mitosis of the basal cell layer of the epidermis and the epithelium of the nipple anlage.