The proliferative behavior of the human adult mammary epithelium.

Normal, human, resting, breast tissue was cultured in the defined culture medium 199 with and without insulin. DNA synthesis was assessed by use of autoradiography. A labeling index (LI) showed a different sensitivity in terms of DNA synthesis in the three basic segments of the mammary tree: intralobular ductules, terminal ducts and large ducts. The LI increased significantly after six hours in the terminal ducts, both in the basic unenriched 199 medium and in the presence of 1 microgram/ml insulin. Cumulative LIs were representative of the interlobular ducts, the most active part of breast epithelium.