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EMBRYOLOGY AND ANATOMY
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No gene has yet been identified that generates an ovary from an undifferentiated gonad. It is only in the absence of the sex-determining region of the Y gene (SRY) that the gonad develops into an ovary. (For more details, see the discussion of sexual differentiation in Chapter 14.) Primordial germ cells, which give rise to oocytes or spermatogonia, are first identifiable in the yolk sac endoderm (hindgut) at 3 to 4 weeks of gestation. Once specified, they migrate and proliferate en route through the dorsal mesentery into the gonadal ridge, which is located lateral to the dorsal mesentery of the gut and medial to the mesonephros (Figure 13–1). Studies in mice have suggested that the process of proliferation and navigation to the gonad depends on several genes, including Steel (kit ligand and receptor), β1 integrin, pog (proliferation of germ cells), and many cytokines. Failure of primordial germ cells to develop or migrate into the gonadal ridge results in failure of ovarian development. In contrast, it is suggested that male gonadal development may continue to develop into functional testis despite the absence of germ cells.
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