Figure 1. Regulation of germ cell migration. A: 4-week embryo. Differentiation of primordial germ cells (PGC) occurs from epiblast-derived cells present in the yolk sac near the base of the allantois. PGCs express alkaline phosphatase (AP), Oct4 and c-kit. Stem cell proteins, like fibronectin and laminin, are expressed along the PGC pathway. B: 5-week embryo. PGCs migrate along the dorsal mesentery of the hind gut to the gonadal ridges.