[Histologic study of the gonads in ovarian differentiation during organogenesis]

We report the results of a histologic study of the differentiating ovary carried out in human embryos and fetuses. Up to the sixth week, the anlage of the gonad is formed by the proliferation of cells originating from the coelomic mesothelium; this proliferation starts with the migration of the primordial germ cells. Between the sixth and the tenth weeks, the primary mesenchyma invades the epithelial mass and divides it into "cords"; by division, the germ cells contained within the mass become ovocytes. At the tenth week, the gonad takes shape and is separated from the mesonephros. At the twelfth week, the gonad is formed of two zones; an outer zone formed of epithelial clusters arranged in rosettes around the ovocytes; and an inner, mesenchymal, zone that becomes the medulla and separates the outer epithelial zone from the remnants of the mesonephros. At the sixteenth week, the gonad becomes the ovary that contains millions of primordial follicles; each follicle consists of an ovocyte surrounded by a ring of granulosa cells originating from the coelomic epithelium; the follicles are separated from each other by mesenchymal cells that form the cortical stroma. The remnants of the mesonephros are clearly separated from the ovary by the medulla.(ABSTRACT TRUNCATED AT 250 WORDS)