Is there a mesonephric cell contribution to the gonadal primordium before sexual differentiation in humans?: An ultrastructural study.

Differentiation of the intermediate mesoderm involves a complex series of events that result in the formation of the rudiment of the entire adult renal system, gonads and gonoducts. This work, using light, transmission, and scanning electron microscopy describes in human embryos of different ages, the development and spatiotemporal organization of the mesonephric nephron, and the development of the gonadal primordium, with the purpose of knowing if and how these two blastemas contribute to the origin of the non germinal cell content of the gonad primordium. Our results show that between Carnegie stage 13 and 20, the mesonephric nephrons facing the gonadal area, are separated by a band of mesenchymal tissue from the gonad primordium and they retain their structural integrities at the level of epithelial wall and their basement membrane. The morphological stability of basement membranes of different nephric structures, as well as the mesonephric duct during the period studied, confirm the previous opinion that the mesonephros is functional. During the same period of time, the structural events underlying gonad development show that primordial germ cells (PGCs) first invade the gonadal area, and thereafter interact both with epithelial coelomic cells and mesenchymal cells. Both types of cells surround PGCs, initiating the growth and differentiation of the gonadal primordium. Therefore, a mesonephric cell contribution to the genesis of the somatic cell components of the gonadal primordium should be discarded in humans. The present work emphasizes the need for more research in this field.