Structural changes in endometrial basal glands during menstruation

Objective To prospectively observe the changes occurring in endometrial glandular morphology during menstrual shedding and regeneration. Design Prospective observational study. Setting The academic gynaecological endoscopy unit of a university teaching hospital. Population Thirteen patients investigated for a variety of benign, non-infective gynaecological disorders during the active bleeding phase of the menstrual cycle. Methods The morphological appearances of concurrent histological and scanning electron microscopic images of endometrium taken at different stages of the active bleeding phase of menstruation were studied and correlated with the simultaneous immunohistochemical expression of the Ki-67 proliferation marker and the CD68 marker of macrophage activity. Main outcome measure Change in morphology of endometrial glands at various stages of menstruation. Results Endometrial glands within the basalis show evidence of apoptosis and associated macrophage activity immediately before and during menstruation. There is subsequent destruction and removal of old secretory glandular epithelial elements, and rapid replacement with new narrow glands lined with small epithelial cells. There is no evidence of mitotic cell division or expression of Ki-67 in the glandular cells during this replacement process, but there is evidence of marked macrophage activity prior to glandular cell loss. Conclusions Early endometrial epithelial repair after menstruation is not a consequence of mitotic cell division. It occurs without evidence of Ki-67 expression. There is structural evidence of programmed cell death and intense macrophage activity associated with glandular remodelling. Dead epithelial cells are shed from the glands and accumulate within the endometrial cavity to be replaced by new small epithelial cells that appear to arise by differentiation of the surrounding stromal cells. We propose that these stromal cells are endometrial progenitor/stem cells.