In the urogenital system, knowledge of the embryology is crucial in understanding the functions and interconnections between the reproductive and urologic systems. The adult genital and urinary systems are distinct in both function and anatomy, with the exception of the male urethra, where the 2 systems are interconnected. During development, these 2 systems are closely associated. The initial developmental overlap of these systems occurs 4–12 weeks after fertilization. The complexity of developmental events in these systems is evident by the incomplete separation of the 2 systems found in some congenital anomalies. For the sake of clarity, this chapter describes the embryology of each system separately, rather than following a strict developmental chronology.
In view of the complexity and duration of differentiation and development of the genital and urinary systems, it is not surprising that the incidence of malformations involving these systems is one of the highest (10%) of all body systems. Etiologies of congenital malformations are sometimes categorized on the basis of genetic, environmental, or genetic-plus-environmental (so-called polyfactorial inheritance) factors. Known genetic and inheritance factors reputedly account for about 20% of anomalies detected at birth, aberration of chromosomes for nearly 5%, and environmental factors for nearly 10%. The significance of these statistics must be viewed against reports that (1) an estimated one-third to one-half of human zygotes are lost during the first week of gestation and (2) the cause of possibly 70% of human anomalies is unknown. Even so, congenital malformations remain a matter of concern because they are detected in nearly 3% of infants, and 20% of perinatal deaths are purportedly due to congenital anomalies.
The inherent pattern of normal development of the genital system can be viewed as one directed toward somatic "femaleness," unless development is directed by factors for "maleness." The presence and expression of a Y chromosome (and its testis-determining genes) in a normal 46,XY karyotype of somatic cells directs differentiation toward a testis, and normal development of the testis makes available hormones for the selection and differentiation of the genital ducts. When male hormones are present, the mesonephric (wolffian) system persists; when male hormones are not present, the "female" paramesonephric (müllerian) ducts persist. Normal feminization or masculinization of the external genitalia is also a result of the respective timely absence or presence of androgen.
An infant usually is reared as female or male according to the appearance of the external genitalia. However, genital sex is not always immediately discernible, and the choice of sex of rearing can be an anxiety-provoking consideration. Unfortunately, even when genital sex is apparent, later clinical presentation may unmask disorders of sexual differentiation that can lead to problems in psychological adjustment. Whether a somatic disorder is detected at birth or later, investigative backtracking through the developmental process is necessary for proper diagnosis and treatment.
Overview of the First 4 Weeks of Development*
Transformation of the bilaminar embryonic disk into a trilaminar disk ...