Congenital anatomic disorders of the female reproductive tract occur frequently and may result from genetic mutation, developmental arrest, abnormal hormonal exposure, or exposure to environmental insults that exert their effects at critical stages of embryonic development. Disorders range from congenital absence of the vagina and uterus, to lateral or vertical fusion defects of the müllerian ducts, to external genitalia that are ambiguous in sexual differentiation. Anatomic defects of the urinary tract are commonly found in these patients due to the concurrent embryonic development of the reproductive and urinary tracts.
An understanding of the complex embryology of the female reproductive system often aids in clarifying the structure of malformations and their association with other genitourinary anomalies (Shatzkes, 1991; Yin, 2005). Like most organ systems, the female urogenital tract develops from multiple cell types that undergo important spatial growth and differentiation. Development occurs during relatively narrow time windows and is governed by time-linked patterns of gene expression (Park, 2005). Some of the molecular mechanisms underlying this process have recently been uncovered with modern molecular genetics and are discussed later.
The urogenital tract is functionally divided into the urinary system and genital system. The urinary organs include the kidneys, ureters, bladder, and urethra. The reproductive organs consist of the gonads, ductal system, and external genitalia.
Both the urinary and genital systems develop from intermediate mesoderm, which extends along the entire embryo length. During initial embryo folding, a longitudinal ridge of this intermediate mesoderm develops along each side of the primitive abdominal aorta and is called the urogenital ridge (Fig. 18-1). Primordial germ cells first appear in the outer ectodermal layer of the embryo. At approximately 40 days of gestation, these germ cells migrate along the hindgut to the urogenital ridge (Fig. 18-1B). This ridge then divides into the nephrogenic and genital ridges.
Early development of the embryonic genitourinary tract. A. In the developing embryo, the urogenital ridge forms from coelomic mesenchyme lateral to the primitive aorta. B. Cross section through the embryo shows division of the urogenital ridges into the genital ridge (future gonad) and nephrogenic ridge, which contains the mesonephros and mesonephric (Wolffian) ducts. The metanephros is the primitive kidney and is connected by the mesonephric ducts to the cloaca. Primordial germ cells migrate along the dorsal mesentery of the hindgut to reach the genital ridge. Paramesonephic (müllerian) ducts develop lateral to the mesonephric ducts. (Images contributed by Kim Hoggatt-Krumwiede, MA.)
At approximately 60 days of gestation, the nephrogenic ridges develop into the mesonephric kidneys (mesonephros) and paired mesonephric ducts, also termed Wolffian ducts (Figs. 18-1B and Fig. 18-2A). These mesonephric ducts connect the mesonephric kidneys (destined for resorption) to the cloaca, which is a common opening into which the embryonic urinary, genital, and ...