The histogenesis of the gonad in rat embryos.

Gonadogenesis was investigated using Wistar rat embryos at 12-14 days after fertilisation. In indifferent gonads, a mesonephric tubule which bifurcates from the mesonephric duct at the upper end of the gonadal anlage ramifies into six to eight branches, the distal portions of which are contiguous, in contact with, or in close proximity to, the coelomic epithelium of the gonadal ridge. After the primordial germ cells reach the gonadal ridge, the overlying epithelium proliferates and clear cells appear in the distal portion of each mesonephric tubule, proliferating and forming cord-like structures. The primordial germ cells appear to enter these cell cords by an amoeboid type of movement. The basal lamina covering the cell cord partially disappears near the germ cells. The germ cells within the cord migrate toward the proximal portion of the cell cord and proliferate in great profusion. From the present observations, it can be concluded that the gonad is mainly formed of clear cell cords originating in mesonephric tubules into which germ cells have entered. The original mesenchymal cells and blood vessels form the interstitial tissue of the gonad. The rete testis and rete ovarii are of mesonephric tubule origin.