Integral to successful reproduction is a normally functioning genital tract, both anatomically and physiologically. A number of developmental abnormalities can lead to infertility, subfertility, spontaneous abortion, or midpregnancy and preterm delivery. To care for affected women, it is imperative that the clinician have a working knowledge of genitourinary system development.
Genitourinary Tract Development
Embryologically 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 of these is thought to be multifactorial and can lead to sporadic anomalies. Normal genitourinary development is summarized in Figure 3-1 and also discussed in Chapter 7 (Development of Genitalia).
Embryonic development of the female genitourinary tract (A-F). (From Bradshaw, 2012, with permission.)
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. This further divides into the gonadal or genital ridge, which will become the ovary, and into the nephrogenic cord, which is subsequently described. The müllerian ducts become the fallopian tubes, uterus, and upper vagina and derive from coelomic epithelium covering the nephrogenic cord. It is because of this separate gonadal and müllerian derivation that women with müllerian defects typically have functionally normal ovaries and are phenotypic females.
The urinary tract develops from the mesonephros or wolffian ducts situated within each nephrogenic cord and connects the mesonephric kidney to the cloaca (Fig. 3-1A). 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. 3-1B). As each bud lengthens, it induces differentiation of the metanephros, which will become the final kidney (Fig. 3-1C). Each mesonephros degenerates near the end of the first trimester, and without testosterone, the mesonephric ducts regress as well.
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. 3-1D). The urogenital sinus is considered in three parts: (1) the cephalad or vesicle portion, which will form the urinary bladder; (2) the middle or pelvic portion, which creates the female urethra; and (3) the caudal or phallic part, which will give rise to the distal vagina and to the greater vestibular (Bartholin) and paraurethral (Skene) glands.
Embryology of the Genital Tract
Development of the genital tract begins as the ...