The Development of the Human Female Reproductive Tract: Part 2-Vagina.

Urogenital epithelium replaces the original paramesonephric (Müllerian) epithelium in the human fetal vagina. We re-investigated this cell replacement histologically and with three-dimensional reconstructions. In the lesser pelvis, both Müllerian ducts fuse to form the uterovaginal canal. Its large caudal "head" is flanked by the locally widened Wolffian ducts. In the 9th week, the urogenital epithelium that covers the Müllerian tubercle differentiates into small, dense-staining basal cells and columnar "clear cells" apically. Mesonephric (Wolffian-duct) outlet epithelium in contact with clear cells degenerates and is replaced by clear-cell epithelium. Meanwhile, the small cells of the urogenital-sinus epithelium form a funnel that physically contacts the underlying uterovaginal head, locally breaks down its basement membrane, and establishes a bridgehead. Here, extensive cell mixing of urogenital and Müllerian cells produces a "transformational" epithelium with regressing Müllerian and healthy urogenital cells as components. After spreading throughout the vaginal epithelium, the urogenital cells near their incursion site under the Wolffian-duct outlets begin to produce the bilateral "vaginal plates." Its cells surround the transformational epithelium as a deflated double-layered cell plate. Where the vaginal plates contact the transformational epithelium, the epithelial cell layer thickens, its surface becomes rugged, and large regressive cells become numerous. The number of regressive cells drops precipitously in the adjacent vaginal plates, implying that this band of cells functions as a "purging zone." Once the purging process reaches the midline, the vaginal epithelium consists of urogenital cells only. Their arrangement as a deflated double-layered cell plate temporally occludes the vaginal lumen.