Figure 5.11 The embryonic development of the female genital tract. The formation of the uterus and vagina. A: The uterus and superior end of the vagina begin to form as the paramesonephric ducts fuse together near their attachment to the posterior wall of the primitive urogenital sinus. B, C: The ducts then zipper together in a superior direction between the 3rd and 5th months. As the paramesonephric ducts are pulled away from the posterior body wall, they drag a fold of peritoneal membrane with them, forming the broad ligaments of the uterus. A–C: The inferior end of the vagina forms from the sinovaginal bulbs on the posterior wall of the primitive urogenital sinus.
The cranial unfused portions of the paramesonephric ducts open into the celomic (future peritoneal) cavity and become the fallopian tubes.
The fusion of the paramesonephric ducts brings together two folds of peritoneum, which become the broad ligament and divide the pelvic cavity into a posterior rectouterine and anterior vesicouterine pouch or cul-de-sac. Between the leaves of the broad ligament, mesenchyme proliferates and differentiates into loose areolar connective tissue and smooth muscle.
Vagina The vagina forms in the 3rd month of embryonic life. While the uterovaginal canal is forming, the endodermal tissue of the sinusal tubercle begins to proliferate, forming a pair of sinovaginal bulbs, which become the inferior 20% of the vagina. The most inferior portion of the uterovaginal canal becomes occluded by a solid core of tissue (the vaginal plate), the origin of which is unclear. Over the subsequent 2 months, this tissue elongates and canalizes by a process of central desquamation, and the peripheral cells become the vaginal epithelium. The fibromuscular wall of the vagina originates from the mesoderm of the uterovaginal canal.
Accessory Genital Glands The female accessory genital glands develop as outgrowths from the urethra (paraurethral or Skene) and the definitive urogenital sinus (greater vestibular or Bartholin). Although the ovaries first develop in the thoracic region, they ultimately arrive in the pelvis by a complicated process of descent. This descent by differential growth is under the control of a ligamentous cord called the gubernaculum, which is attached to the ovary superiorly and to the fascia in the region of the future labia majora inferiorly. The gubernaculum becomes attached to the paramesonephric ducts at their point of superior fusion so that it becomes divided into two separate structures. As the ovary and its mesentery (the mesovarium) are brought into the superior portion of the broad ligament, the more proximal part of the gubernaculum becomes the ovarian ligament, and the distal gubernaculum becomes the round ligament.
Early in the 5th week of embryonic life, folds of tissue form on each side of the cloaca and meet anteriorly in the midline to form the genital tubercle (Fig. 5.12). With the division of the cloaca by the urorectal septum and consequent formation of the perineum, these cloacal folds are known anteriorly as the urogenital folds and posteriorly as the anal folds. The genital tubercle begins to enlarge. In the female embryo, its growth gradually slows to become the clitoris, and the urogenital folds form the labia minora. In the male embryo, the genital tubercle continues to grow to form the penis, and the urogenital folds are believed to fuse to enclose the penile urethra.
Figure 5.12 The comparative development of the female and male external genitalia. A: In both sexes, the development follows a uniform pattern through the 7th week and thereafter begins to differentiate. B: The male external genitalia. C: The female external genitalia.