Differentiation of the fetal gonad.

Gonadal differentiation may be divided into four stages: pregonadal, indifferent, primary sex differentiation, and secondary sex differentiation. Sertoli cells appear at 6-7 weeks and Leydig cells differentiate at 8 weeks, but in ovaries, primary sex differentiation occurs much later. Testosterone secretion peaks at 12-16 weeks causing male secondary sex development together with the appearance of anti-Müllerian hormone. Fetal testis is able to synthesize and secrete inhibins. Lower circulating luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels in male than in female fetuses at midgestation suggest that the fetal pituitary is already responsive to the gonadal hormones. Placental human chorionic gonadotrophin may regulate testosterone synthesis at midgestation, and both LH and FSH are likely to have some regulatory effect on fetal gonads during the last third of gestation.