Gonadal differentiation: a review of the physiological process and influencing factors based on recent experimental evidence.

The sexual differentiation of a gonad is determined in normal embryology by the presence or absence of the TDF gene, a short segment of DNA localized near the tip of the short arm of the Y chromosome. Under the stimulus of this gene, the somatic cells of the genital ridge differentiate into Sertoli cells and secrete anti-müllerian hormone. This hormone inhibits the germ cells from entering meiosis and may well also trigger the formation of the primary sex cords. Once the Sertoli cells are formed, further differentiation of the gonad is independent of the TDF presence, although this gene may further contribute to the normal functioning of the male gonad. In true hermaphroditism the normal regulation of the gonadal differentiation is missing. Gonadal differentiation may revert to a more primitive evolutionary level at which the rate of growth of the gonad determines its sexual direction. When this exceeds a certain threshold a testis may develop despite the absence of the TDF gene.