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Early in life, embryos of male and female sex are indistinguishable from one another (Table 18-1). At critical stages of embryonic development, insults can lead to congenital anatomic disorders of the reproductive tract. Influences include genetic mutation, epigenetic factors, developmental arrest, or abnormal hormonal exposures. 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. Sexual differentiation is complex and requires both hormonal pathways and morphologic development to be normal and correctly integrated. Thus, it is not surprising that neonates with genital anomalies often have multiple other malformations. Associated urinary tract defects are especially frequent and are linked to the concurrent embryonic development of both reproductive and urinary tracts (Hutson, 2014).
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The urogenital tract is functionally divided into the urinary system and genital system. The urinary organs include the kidney, ureters, bladder, and urethra. The reproductive organs are the gonads, ductal system, and external genitalia. Like most organ systems, the female urogenital tract develops from multiple cell types that undergo important spatial growth and differentiation. These develop during relatively narrow time windows and are governed by time-linked patterns of gene expression (Park, 2005).
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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. Subsequently, the urogenital ridge divides into the nephrogenic ridge and the genital ridge, also called the gonadal ridge (Fig. 18-1).
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