Male and female human embryos develop identically during the first phase of gestation. The indifferent gonads then differentiate into ovaries or testes and soon begin to secrete their characteristic hormones. If ovaries (or no gonads) are present the final phenotype is female; thus no gonadal hormones are required for female development during embryogenesis. Two hormones of the fetal testis - Müllerian regression hormone and testosterone-are responsible for the formation of the male phenotype. Analysis of fibroblasts from the skin of patients with abnormalities of sexual development due to single gene defects shows that testosterone is responsible for virilization of the male internal genital tract, that its derivative dihydrotestosterone causes development of the male external genitalia, and that both hormones act in the embryo by the same receptor mechanisms operative in postnatal life.
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