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Human gonadal development was observed in early embryos by electron microscopy. In 5-week-old embryos, the genital ridge (GR) was forming with a single layered superficial epithelium, which consisted of two types of cells, clear and dark cells. In 6-week-old embryos, the swelling into the coelom of the GR was greatly increased. Primary sex cords (PSCs) were forming as columnar projections from the primitive cortex cells. The basal lamina appeared between the future cortex and medulla, indicating that PSCs are of cortex origin. The primordial germ cells (PGCs) completed their migration into the GR: many of them were incorporated, in PSCs. In 7-8-week-old embryos, the PSCs were well developed, and clear and dark cells were still distinguishable within the PSCs. Sex differentiation in the primitive gonad became discernible on a histologically basis. In the testis, the tunica albuginea appeared and PGCs were being incorporated in the PSCs and were associated with both clear and dark cells. In the ovary, the cortex was dominantly proliferated and in the zone demarcating the cortex and medulla, collagen fibers were observed. In male embryos at 9 weeks onwards, the PGCs located in the PSCs underwent some ultrastructural changes, and the PSCs were branching and their peripheral ends were anastomosing with each other to form the rete testis.

Electron Microscopic Study in the Gonadal Development in Early Human Embryos