The embryologic development of the human vagina.

Our present understanding of the sequence and mechanisms of human genital organogenesis is reviewed. Current theories about the derivation of the vaginal epithelium are examined and tested against two anomalous circumstances, congenital androgen insensitivity and agenesis of the lower vagina, which are presented as examples demonstrating the respective participation of the urogenital sinus or of the Müllerian ducts alone in the developmental process. The abnormalities recently described in the vagina and cervix of girls exposed in utero to diethylstilbestrol (DES) correspond remarkably with those encountered in lower vaginal agenesis, particularly with regard to the presence of vaginal adenosis, the deficiency of glycogen in the squamous cells (squamous metaplasia), and the abnormal response of the squamous epithelium to Schiller's iodine test. It is concluded that the development of the human vagina is best explained by the theory which holds that the Müllerian ducts in fetal life extend caudally to the level of the future hymen. After fusion of these ducts, squamous cells arising in the epithelium of the urogenital sinus invade from below, advance, and replace completely the Müllerian mucosa up to the level of the external os of the cervical canal.