Cell proliferation in the human mammary epithelium. Differential contribution by epithelial and myoepithelial cells.

The ductal system of the human breast consists of epithelial, myoepithelial, and basal clear cells. By labeling ducts and alveoli dissected from reduction mammoplasty specimens with 3H-thymidine in vitro and labeling human breast organoids xenografted in nude mice in vivo, it was found that cellular proliferation in the human breast is virtually confined to epithelial and basal clear cells. A pulse label of 3H-thymidine in organ culture explants was followed over a period of time, and it was found that myoepithelial cells originate from a precursor cell population within the mammary epithelium after a number of cell divisions. Myoepithelial cells were not seen to divide when fully mature.