The onset of germ cell migration in the mouse embryo.

Mouse primordial germ cells (PGCs) are specified between embryonic day 6.5 (E6.5) and E7.5, when they have been visualized as an alkaline phosphatase-positive (AP+) cell population in the developing allantois. By E8.5, they are embedded in the hind-gut epithelium. Previous experiments have suggested different sites for PGCs' origin, and it is unclear how they reach the gut epithelium. We have used transgenic mice expressing GFP under a truncated Oct4 promoter to visualize living PGCs. We find GFP+/AP+ cells in the posterior end of the primitive streak as a dispersed population of cells actively migrating into the allantois, and directly into the adjacent embryonic endoderm. Time-lapse analysis shows these cells to be actively migratory from the time they exit the primitive streak.