Cellular proliferation in the rat mammary gland during pregnancy and lactation

Cellular proliferation during different stages of pregnancy and lactation in the rat mammary gland was studied by use of autoradiography at the ultrastructural level or in combination with immunocytochemistry. The study consisted of two groups of animals, young 45-day-old rats undergoing their first pregnancy and old multiparous rats undergoing their third pregnancy. Both groups of animals were injected with [3H]thymidine, 2 hours before sacrifice. The timing of proliferative activity through pregnancy and lactation was similar in the two groups. A peak level of proliferation was observed on the 5th day of gestation and the rate of proliferation was at its lowest at the end of gestation. A second small peak was observed on the 3rd day of lactation. In virgin animals, both epithelial and myoepithelial cells showed a high level of proliferative activity. The epithelial cells continued to divide during all stages of their differentiation, throughout pregnancy and lactation. Myoepithelial cells showed active division in early pregnancy but their rate of proliferation declined as the cells acquired abundant cytoplasmic filaments for their contractile function. Intra-alveolar dendritic cells, which have been recently identified in the lactating rat breast, showed a high proliferative rate at the end of gestation.