Proliferative and secretory activity in human breast during natural and artificial menstrual cycles.

Events of cell proliferation have been assessed by thymidine labeling index (TLI) in morphologically normal breast lobules from women of reproductive age. TLI is higher during the second half of the menstrual cycle both in women with natural menstrual cycles and in those with artificial cycles due to oral contraceptive (OC) use. TLI varies between 0.04% and 5.7% and declines with increasing age. There is no difference in mean TLI between OC users and nonusers if correction is made for the difference in age distribution between these two groups. Immunohistochemical detection of immunoglobulin A (IgA), secretory component (SC), and alpha-lactalbumin shows more frequent positive staining during artificial than natural menstrual cycles, and positive staining tends to be associated with higher TLI values. No significant variation in staining with cycle phase is detected in natural or artificial cycles. These results show that breast tissue does not resemble endometrium in the relationships that exist between proliferation, secretion, and menstrual cycle phase.