Fig. 1. Early germ cell development in Drosophila, C. elegans and mice. (A,B) Anterior is towards the left. (A) In C. elegans, the germ plasm (pink) becomes asymmetric in the zygote and segregates specifically to the germline blastomeres P₁-P₄. Z2 and Z3, the daughters of P_4, become the primordial germ cells (PGCs), and move inside the embryo in close association with intestinal cells (yellow). Later they are joined by Z1 and Z4, the founder cells of the somatic gonad. (B) Drosophila: germ plasm (pink) is assembled during oogenesis and preformed in the posterior pole region of eggs. Pole cells incorporate germ plasm, and are carried inside the embryo during germ band extension, pass through the midgut epithelia (yellow) into the hemocoel, migrate toward mesoderm (green), and then coalesce with somatic gonadal cells to form the embryonic gonads. (C) The anterior-posterior axis of mouse embryos is established by the anterior visceral endoderm (AVE). During embryonic day (E) 6.25-6.5, extra-embryonic signals promote four to eight proximal epiblast cells to activate Blimp1 expression (pink). These cells migrate to an extra-embryonic location and by E7.25, have proliferated to form 40 alkaline phosphatase (AP)-positive PGCs that are present at the root of the allantois. These PGCs migrate back into the embryo in association with the hindgut (yellow) to eventually colonize the genital ridges (somatic gonad).