he cloaca begins as a common opening for the embryonic urinary, genital, and alimentary tracts. By the 7th week it becomes divided by the urorectal septum to create the rectum and the urogenital sinus (Fig. 3-2D). The urogenital sinus is considered in three parts: (1) the cephalad or vesicle portion, which forms the urinary bladder; (2) the middle or pelvic portion, which creates the female urethra; and (3) the caudal or phallic part, which gives rise to the distal vagina and to the greater vestibular (Bartholin) and paraurethral glands.
• Embryology of the Genital Tract he fallopian tubes, uterus, and upper vagina derive from the miillerian ducts, also termed paramesonephric ducts, which form adjacent to each mesonephros (see Fig. 3-2B). These ducts extend downward and then turn medially to meet and fuse together in the midline. he uterus is formed by this union of the two miillerian ducts at approximately the 10th week (Fig. 3-2E). Fusion to create the uterus begins in the middle and then extends both caudally and cephalad. With cellular
FIGURE 3-2 Embryonic development of the female genitourinary tract (A-F). (Reproduced with permission from Shatzkes DR, Haller JO, Velcek FT: Imaging of uterovaginal anomalies in the pediatric patient, Urol Radiol 1991;13(1):58-66.)
FIGURE 3-1 A. Cross-section of an embryo at 4 to 6 weeks. B. Large ameboid primordial germ cells migrate (arrows) from the yolk sac to the area of germinal epithelium, within the genital ridge. C. Migration of sympathetic cells from the spinal ganglia to a region above the developing kidney.
Remnant of mesonephric duct Bladder-Urethra/ F Ureter -...--Uterus ---Rectum proliferation at the upper portion, a thick wedge of tissue cre ates the characteristic piriform uterine shape. At the same time, dissolution of cells at the lower pole forms the irst uterine cav ity (Fig. 3-2F). As the upper wedge-shaped septum is slowly reabsorbed, the inal uterine cavity is usually formed by the 20th week. If the two mullerian ducts fail to fuse, then two separate uterine horns remain. In contrast, resorption failure of the common tissue between them results in various degrees of persistent uterine septum.
As the distal end of the fused mullerian ducts contacts the uro genital sinus, this induces endodermal outgrowths from the sinus termed the sinovaginal bulbs. hese bulbs proliferate and fuse to form the vaginal plate, which later resorbs to form the vaginal lumen. his vaginal canalization is generally completed by the 20th week. However, the lumen remains separated from the urogenital sinus by the hymeneal membrane. This membrane further degenerates to leave only the hymeneal ring.