Abnormalities in the development or fusion of one or both Müllerian ducts may result in malformations which sometimes possess an obstetrical significance. Pregnancy may be associated with any one of these malformations, provided an ovum be cast off from the ovaries and no serious obstacle be opposed to the upward passage of the spermatozoa and their subsequent union with it.
—J. Whitridge Williams (1903)
GENITOURINARY TRACT DEVELOPMENT
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 during this process is thought to be multifactorial and can create sporadic anomalies. Several of these can lead to infertility, subfertility, miscarriage, or preterm delivery. Thus, knowledge of genitourinary system development is essential.
Embryology of the Urinary System
Between the 3rd and 5th gestational weeks, an elevation of intermediate mesoderm on each side of the fetus—the urogenital ridge—begins development into the urogenital tract. Subsequently, the urogenital ridge divides into the genital ridge, destined to become the ovary, and into the nephrogenic ridge (Fig. 3-1). The nephrogenic ridges develop into the mesonephros (mesonephric kidney) and paired mesonephric ducts, also termed wolffian ducts, which connect to the cloaca.
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.
The early urinary tract develops from the mesonephros and its mesonephric ducts (Fig. 3-2A). Recall that evolution of the renal system passes sequentially through the pronephric and mesonephric stages to reach the permanent metanephric system. Between the 4th and 5th weeks, each mesonephric duct gives rise to a ureteric bud, which grows cephalad toward its respective mesonephros (Fig. 3-2B). As each bud lengthens, it induces differentiation of the metanephros, which will become the final kidney (Fig. 3-2C). Each mesonephros degenerates near the end of the first trimester, and without testosterone, the mesonephric ducts regress as well.
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. 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 ...