The germ layer origin of mouse vaginal epithelium restricts its responsiveness to mesenchymal inductors: uterine induction.

The epithelium of the mammalian vagina arises from two distinct germ layers, endoderm from the urogenital sinus and mesoderm from the lower fused Müllerian ducts. While previously it has been reported that neonatal vaginal epithelium can be induced to differentiate as uterus, which normally develops from the middle portion of the Müllerian ducts, it has not been determined whether this ability is shared by both mesoderm- and endoderm-derived vaginal epithelia. To test if germ layer origin influences the ability of vaginal epithelium to undergo uterine differentiation, we have isolated sinus-derived and Müllerian-derived vaginal epithelia from newborn mice, combined them with uterine mesenchyme, and grown them for 4 weeks in female mice. Mesoderm-derived Müllerian vaginal epithelium in combination with uterine mesenchyme formed the simple columnar epithelium typical of uterus. Similar results were obtained with neonatal cervical epithelium, another mesodermal Müllerian duct derivative. On the other hand, sinus vaginal epithelium combined with uterine mesenchyme formed small cysts lined by a stratified squamous vaginal-like epithelium. This epithelium never showed evidence of cycling between the cornified and mucified states as is typically seen in vaginal epithelium combined with vaginal stroma. These results indicate that the ability of epithelium to form uterus is limited to mesoderm-derived epithelia and suggest that endoderm-derived sinus vaginal epithelium cannot undergo the typical differentiative modifications in response to the hormonal fluctuations of the estrous cycle when associated with uterine stroma.