In females, the external genitalia, gonads, and müllerian ducts each derive from different primordia and in close association with the urinary tract and hindgut. Abnormal embryogenesis can lead to reproductive organs that predispose to infertility, subfertility, miscarriage, or preterm delivery.
GENITOURINARY TRACT DEVELOPMENT
Between the 3rd and 5th gestational weeks, an elevation of intermediate mesoderm on each side of the fetus begins to develop into the urogenital tract. This urogenital ridge divides into the genital ridge, destined to become the gonads, and into the nephrogenic ridge (Fig. 3-1). Each nephrogenic ridge produces a mesonephros (mesonephric kidney). Recall that evolution of the renal system passes sequentially from the mesonephric stage to reach the permanent metanephric system (de Bakker, 2019; Upadhyay, 2014). Each nephrogenic ridge also gives rise to a mesonephric duct, also termed wolffian duct, and to a paramesonephric duct, also called müllerian duct.
A. Cross-section of an embryo at 4 to 6 weeks. B. Primordial germ cells migrate (arrows) from the yolk sac to the area of germinal epithelium, within the genital ridge. C. Development of gonad. Also, migration of sympathetic cells from the spinal ganglia to a region above the developing kidney.
The early urinary tract develops from the mesonephric ducts (Fig. 3-2A). Between the 4th and 5th weeks, each mesonephric duct gives rise to a ureteric bud, which grows cephalad (Fig. 3-2B). As each bud lengthens, it induces differentiation of the metanephros, which will become the final kidney (Fig. 3-2C) (Davidson, 2019). The metanephros ascends to its final position by the 9th week because of disproportionate growth of the embryo’s caudal region (Jain, 2018). Each ureteric bud also gives rise to an elongation that becomes the metanephric duct or future ureter.
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.)
The cloaca begins as a common opening for the embryonic urinary, genital, and alimentary tracts (Gupta, 2014). By the 7th week, it is divided by the urorectal septum to create the hindgut and the urogenital sinus (Fig. 3-2D) (Valentini, 2016). 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.
Near the end of the first trimester, each mesonephros degenerates, and without testosterone, ...