Chapter 105. Differentiation of The Female Genital Tracts: Uterus, Vagina, Auxiliary Glands, Mesenteries - Review of Medical Embryology Book

IN EMBRYOS WITH OVARIES the mesonephric ducts regress, and the paramesonephric (m?llerian) ducts develop to form the female genital tracts. The cranial, unfused parts of the paramesonephric ducts form the uterine tubes and the caudal fused parts form the single median uterovaginal primordium (about week 8) which gives rise to the epithelium and glands of the uterus. The endometrial stroma and myometrium form from adjacent mesenchyme

The fusion of the caudal parts of the paramesonephric ducts commences caudally and progresses up to the future uterine tubes

The two mesonephric (wolffian) ducts regress, but persist as vestigial structures, G?rtner's ducts

FUSION OF THE PARAMESONEPHRIC DUCTS brings together 2 peritoneal mesenchymal folds, which form the right and left broad ligaments (attached to the abdominal wall on each side and are a continuation of the urogenital mesentery); and 2 peritoneal compartments of pelvic cavity, the uterorectal pouch (of Douglas) and uterovesical pouch

Between the layers of the peritoneal broad ligaments, on each side of the uterus, the mesenchyme proliferates and differentiates into loose connective tissue and smooth muscle to form the parametrium
The inguinal ligaments caudal to the uterine tubes become the round ligaments of the uterus

The terminal end of the primitive uterovaginal canal touches the posterior wall of the urogenital sinus and forms M?ller's tubercle or sinus. The mesonephric ducts enter the urogenital sinus on each side of this tubercle
The posterior wall of the urogenital sinus thickens opposite the tubercle and with it forms the vaginal epithelial plate
From the plate, 2 solid evaginations, the sinovaginal bulbs, grow and encircle the caudal end of the uterovaginal primordium (uterine canal)
Canalization of the vaginal plate (begins at week 11) proceeds from caudal to cranial end, producing the lumen of the vagina, with the peripheral cells remaining as the vaginal epithelium. Canalization is completed by month 5
The sinovaginal bulbs surround the uterine cervix to form the fornices of the vagina
Thus, the vagina is of entodermal origin derived from the wall of the urogenital sinus. The paramesonephric ducts form the body and cervix of the uterus

The vaginal lumen remains separated from the urogenital sinus until late fetal life by a membrane, the hymen. The latter usually ruptures during perinatal life

BUDS FROM THE URETHRA grow into the mesenchyme and form the urethral glands and the paraurethral glands (of Skene). Both correspond to the male prostate gland
OUTGROWTHS FROM THE UROGENITAL SINUS form the greater vestibular glands (Bartholin) and are homologous to the male bulbourethral (Cowper's) glands

THE UTERINE (FALLOPIAN) TUBES, which were the cranial parts of the paramesonephric (m?llerian) ducts, are initially vertical. During development of the uterus, they move toward the interior of the abdominal cavity and become horizontal
THE OVARY is temporarily found cranial to the uterine tube on the posterior side of the regressing mesonephric body, but moves to a position posterior to the uterine tube
THE ASSOCIATED MESENTERIES follow the positional changes. Their arrangement in the pelvis forms the broad ligament of the uterus with its 3 "flanges": the round ligament of the uterus; the mesovarium, posteriorly; and the mesosalpinx, cranially