The gonads develop in two paired ridges of mesenchyme at the medial aspect of the mesonephros during the 5th–7th weeks of gestation (Byskov and Høyer 1988; Wartenberg 1989). The primordial germ cells originate from the yolk sac and migrate to the gonadal ridges. Regardless of the karyotype, all embryos have, after 6–7 weeks of gestation, indifferent gonads consisting of germ cells, coelomic surface epithelium and mesenchyme. At this stage also internal Müllerian (paramesonephric) and Wolffian (mesonephric) ducts, a urogenital sinus and an undifferentiated external genitalia can be identified (Fig. 20.1). The subsequent sex-specific development of internal and external genitalia is primarily dependent on the karyotype, but is also influenced by the endocrine function of the gonads. It has long been known that presence of a Y chromosome is decisive for testicular differentiation. Recently, the essential DNA sequence on the short arm of the Y chromosome has been cloned and called the sex-determining region of the Y (Sinclair et al. 1990).
KeywordsSertoli Cell Congenital Adrenal Hyperplasia External Genitalia Urogenital Sinus Wolffian Duct
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