Primordial germ cells migration: morphological and molecular aspects

Primordial germ cells (PGCs) are germline stem cells that give rise to gametes in vertebrates. They originate outside the embryo very early in development and migrate by a well-defined route into the genital ridges. During early embryogenesis in mammals, the PGCs are observed in an extra-embryonic region near the yolk sac, translocate to the endodermal epithelium of the hindgut as embryogenesis advances, and then separate from the gut epithelium to enter the dorsal mesentery, through which they finally migrate to form gonadal anlage. It is accepted that PGC migration occurs in three phases: separation, migration, and colonization. The PGCs move actively by amoeboid movements to cross the migratory pathway. These cells are oval or round in shape with irregular contours and large nuclei containing prominent nucleoli. An identifying characteristic of PGCs is their high alkaline phosphatase activity. Recently, interest has been focused on the mechanism of PGC migration. At least four mechanisms have been hypothesized to explain PGC migration: attraction by chemotactic factors, PGCPGC interactions, substrate-guidance, and interaction with extracellular matrix molecules. We have demonstrated that a repertory of extracellular matrix molecules, proteoglycans in particular, are temporospatially expressed in the migratory pathways of PGCs according to the phase of the migration process. It is of note that PGCs are pluripotent cells, from which two types of equally pluripotent stem cells are derived. In vivo, PGCs engender embryonic carcinoma cells, the stem cells of teratomas and benign tumors. In vitro, mouse PGCs give rise to embryonic germ cells, stem cells capable of producing a variety of different cell types, including hematopoietic cells and myogenic cells. Greater knowledge of the mechanisms that control embryonic carcinoma cell formation and the signaling pathways that control embryonic germ cell derivation could help us understand the molecular controls of developmental potency in mammals.