Until 7 weeks of age, human embryos remain in the indifferent stage, possessing both structures that will give rise to female sexual organs (Mullerian ducts) and those that will result in male genitalia (Wolffian ducts). At 7 weeks, male embryos express a gene located on the Y chromosome near the pseudoautosomal region, SRY, which induces formation of the testes and promotes the conversion of primordial germ cells to spermatogonia.
Inside the testes, Sertoli cells produce Mullerian Inhibiting Substance, which causes active regression of the Mullerian ducts, while Leydig cells produce testosterone, which guides the development of the ductus deferens, epididymis, and seminal vesicle. Other structures, like the penis and prostate, require an enzymatic conversion of testosterone to dihydrotestosterone (DHT) using the reaction shown below:
These early events in human sexual determination, which begin at 7 weeks and continue throughout gestation, are essential for proper embryological development; any disruption to these pathways can pose dire implications for the health of the fetus.
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