Human female phenotypic development: role of fetal ovaries

Morphogenesis of external genitalia was studied in human fetuses obtained from cases of medical termination of pregnancies and cases of spontaneous abortions. Eighty-two fetuses, 16 mm crown rump (CR) length (6 weeks gestation) to 240 mm CR length (26 weeks gestation), were studied. Fetal sex was determined with the help of chorionic villus of fetal skin biopsies in cases of fetuses less than 70 mm CR length. Growth and differentiation up to 50 mm CR length (9 weeks gestation) was identical for both sexes. After this there was rapid growth and differentiation in males. But in females subsequent growth until 180-200 mm CR-length stage (20 weeks gestation), was restricted to growth of labia majora. After this there was rapid ventral outgrowth of the region of perineum between the clitoris and anus, which brought the urethral and vaginal orifices to the surface. There was also further growth of labia. This process of feminization of external genitalia was completed by the 26th week of gestation. Histology revealed that ovary and testes could be clearly identified at 60 mm CR length stage. Maturation of fetal testes progressed rapidly, but there was little change in the histological appearance of ovaries until 160-180 mm CR-length stage (18-20 weeks of gestation). Follicular growth started after this. Feminization of urogenital sinus started after follicular growth started in the fetal ovaries. So from the temporal profile of events, it appears that it may be initiated by fetal ovarian steroids.