Female: differentiation of the canal system in the genital organ
During the 7th week the canal system of the female sex organs differentiates. The mesonephric duct and its tubules atrophy and out of the paramesonephric duct (Müller) arises the future fallopian tube, the uterus and the upper part of the vagina.
Sometimes, a few embryonic remnants of the mesonephric duct remain in the form of the epoöphoron, the paroöphoron at the level of the mesovarium, and a row of small cysts of Gartner.
Out of the upper, non-fused portion of the paramesonephric duct (Müller) arises the fallopian tube and its ampulla. The lower section fuses after it crosses medially on both sides of the inferior ovarian gubernaculum and forms the utero-vaginal canal. The medial septum in between disappears at the end of the 3rd month.
To be observed is the development of the ligaments. The ovarian gubernaculum gets attached on the developing utero-vaginal canal there where it goes over into the fallopian tube. Above it forms the ovarian ligament and below the round ligament of uterus, which goes through the inguinal canal and inserts in the female genital swelling (labia majora).
If the separating wall beyond the fusion location of the two paramesonephric ducts is not resorbed, various utero-vaginal abnormalities result.
The blind end of the utero-vaginal canal forms the sinu-vaginal eminence (stage 22) and ends at the back wall of the urogenital sinus (SUG).
The sinu-vaginal eminence becomes thicker due to epithelial proliferation and retracts, while the wall of the SUG also thickens there. These epithelial layers, which form at the lower end of the utero-vaginal canal, are known as the vaginal plate. At their cranial end they form a circular protrusion, the location of the future vaginal fornix.
Through canalization of the vaginal plate the utero-vaginal canal opens itself towards the outside. The upper 3/4 of the vagina comes from the mesoderm and the lower fourth from the endoderm.
The fibromuscular walling forms from the neighboring mesenchyma. The vagina is separated from the SUG by the hymen. Its origin is not entirely clear. Discussed is a passive invagination of the back wall of the SUG.