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 PMRD1, the receptors C-KIT and CXCR4, OCT3/4 and alkaline phosphatase. Fibronectin and laminin, together with KITL, SDF1 and IFITM 1 and 3 are expressed in the mesoderm along the PGC pathway. B: 5-week embryo. PGCs migrate along the dorsal mesentery of the hind gut to the gonadal ridges.