Neurobiology of gender identity and sexual orientation


Pub Med Central

 
The empirical basis for hypothesising that gonadal hormones influence gender identity and sexual orientation is based on animal experiments involving manipulations of hormones during prenatal and early neonatal development. It is accepted dogma that testes develop from the embryonic gonad under the influence of a cascade of genes that begins with the expression of the sex-determining gene SRY on the Y chromosome., Before this time, the embryonic gonad is “indifferent”, meaning that it has the potential to develop into either a testis or an ovary. Likewise, the early embryo has 2 systems of ducts associated with urogenital differentiation, Wolffian and Müllerian ducts, which are capable of developing into the male and female tubular reproductive tracts, respectively. Once the testes develop, they begin producing 2 hormones, testosterone and anti-Müllerian hormone (AMH). In rats, this occurs around day 16–17 of gestation, whereas, in humans, it occurs at about 7–8 weeks of gestation. Testosterone and one of its derivatives, dihydrotestosterone, induce the differentiation of other organs in the male reproductive system, whereas AMH causes the degeneration of the Müllerian ducts. Female ovaries develop under the influence of a competing set of genes that are influenced by expression of DAX1 on the X chromosome and act antagonistically to SRY. The female reproductive tract in the embryo develops in the absence of androgens and later matures under the influence hormones produced by the ovary, in particular oestradiol.


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