Ultrastructural epithelial zonation of the primate endometrium (rhesus monkey)

The uterine endometrium of menstruating primates (rhesus monkey, human) consists of a germinal basalis that regenerates a transient functionalis during each menstrual cycle. The endometrium is further subdivided into 4 zones that differ histologically and in epithelial mitotic rate along the longitudinal axes of the uterine glands and microvasculature (Bartelmez et al: Contrib. Embryol. Carnegie Inst., 34:99-146, 1951; Bartelmez: Am. J. Obstet. Gynecol., 74:931-955, 1957; Padykula et al.: Biol. Reprod., 32:1103-1118, 1118, 1984; Biol. Reprod., in press, 1988). The zones are defined as follows: functionalis I, luminal epithelium; functionalis II (upper straight gland segments); basalis III (middle gland segments), and basalis IV (bottoms of the glands). The surrounding stroma and microvasculature also differ zonally. Ultrastructural epithelial differences are evident among the 4 zones during 3 distinct functional states during natural menstrual cycles and after ovariectomy: 1) basal level after ovariectomy and 2) estrogen dominance and 3) progesterone dominance. Zonal structural differences persist at a minimal level of differentiation after ovariectomy and thus zonation is an inherent property. During estrogen dominance, distinctive ultrastructural differences are evident among the 4 zones, such as epithelial cell heterogeneity in functionalis I and homogeneity in functionalis II. Also a distinctive glandular cell type occurs in basalis III and IV that is recognized by a highly irregular cisternal rough endoplasmic reticulum that permeates the cytoplasm. During progesterone dominance, ultrastructural differences exist among the 4 zones except for similarity between the epithelial cells of functionalis II and basalis III. Postovulatory epithelial cells of functionalis I and II and basalis III become postmitotic via progesterone inhibition but intracellular differentiation continues progressively. Postovulatory epithelial mitotic activity in basalis IV escapes progesterone inhibition as the [3H]thymidine labeling index continues to increase from 1 to 12% during the menstrual cycle (Padykula et al.: Reprod., 30(Suppl.1):92 (Abstr. 123), 1984). This post-ovulatory proliferation coupled with progressive differentiation in basalis IV may represent a stem-progenitor set of cells for postmenstrual endometrial regeneration or alternatively for creation of the maternal placenta.