Ultrastructure of fetal human gonad before sexual differentiation and during early testicular and ovarian development

The development of the human gonad was investigated by light and transmission electron microscopy in 20 embryos and fetuses between 4.5 and 11.5 weeks of gestation, i.e. during the stages of sex-indifferent gonad, initial testicular and ovarian development. The gonadal blastema in 4.5-week-old embryos appeared formed by poorly differentiated somatic mesothelial cells, and by specialized germ cells (PGCs) with signs of ameboidism, cellular structures suggesting active protein biosynthesis and mitotic activity. The sexual differentiation of the gonads was clearly observed in 7-week-old embryos and involved at the same time the testis and the ovary. The former contained seminiferous cords formed by palisades of poorly differentiated Sertoli cells, which were segregated from the mesothelium by a rudimentary albuginea. The interstitial tissue at this age contained mesenchymal cells. Between 8 and 11.5 weeks of gestation, there was a synchronous cytodifferentiation of both Sertoli and Leydig cells. The latter acquired features of steroidogenic elements. The ovaries of 7-week-old fetuses contained packed ovigerous cords formed by somatic and germ cells (oogonia). The former embraced the oogonia with thin overlapping cytoplasm projections, and acquired features similar to those of cells in primary follicles, already at this early fetal age. At the same time the sexual differentiation of the gonads involved somatic and germ cells. In the female, the oogonia continued to show the main features they had during migration and colonization, including a high mitotic rate, signs of ameboidism and a developed apparatus for protein synthesis. On the contrary Gonocytes enclosed in the seminiferous cords progressively entered a quiescent phase characterized by a reduced mitotic rate, a decrease of endoplasmic reticulum and nucleolar complexity. The chronological relationship between the cytodifferentiation of Sertoli and Leydig cells, and changes of germ cells, suggest that somatic components of the testis may contribute to a male type of differentiation of germ cells from the very beginning of sexual differentiation.