Proliferative and secretory activity in the pregnant and lactating human breast.

In this study the proliferative and secretory activity of human pregnant and lactating breast has been examined. 17 cases of pregnant tissue were obtained, comprising 9 from first trimester, 6 from the second and 2 from the third, together with 3 cases of lactating breast. Proliferation, measured by 3H thymidine autoradiography, gave a constant Thymidine labelling index of approximately 6% during the first 20 weeks, with a reduction by 50% thereafter. Levels of 0.2% were seen during lactation. Morphological assessment showed a high frequency of both mitosis and apoptosis throughout gestation. Although mitotic frequency did reduce with time, values were more variable than thymidine labelling. The reduction in thymidine uptake halfway through pregnancy coincided with generalised morphological lactational differentiation and the major immunocytochemical expression of alpha-lactalbumin. However, cytoplasmic expression of the secretory component of IgA was generalised and marked even during the first trimester and was closely related to increased proliferation and general synthetic cellular activity. These and other observations support a complex relationship of autocrine and paracrine factors in addition to the customary endocrine ones for breast regulation.