The human gonocytes in the early phases of gonadal differentiation. Observations of one embryo at Carnegie stage 11.
With the aim of emphasize the characteristics of the migration of human primordial gonocytes through the mesenchyme of the dorsal mesentery towards the genital ridge, a very early human embryo, corresponding to Carnegie stage 11 (23-24 days of development) was studied together with other embryos from the 4th to the 10th week of pregnancy. The early embryo was embedded in resin, serially cut and the observations were made on semithin coronal sections, de-plasticized and stained with toluidine blue. Migrating primordial germ cells were identified in the thickness of the splanchnopleure, where they appeared as pale and round elements with a nucleolated nucleus and granular cytoplasm. The clusters of primordial gonocytes were constantly surrounded and in close relationship with smaller, polyhedral and intensely stained elements. These features suggest the hypothesis that gonocytes and dark cells join together during the process of migration and that the last can participate in the formation of the gonadal bud. The hypothesis is confirmed by the observation of dark cells, surrounding the gonocytes, in the primitive ovaries or testes, in later stages of development. Perhaps they do not participate directly in the formation of the sexual cords, but reach their periphery where they assume an elongate shape.