The germ cell--the mother of all stem cells

The germline, uniquely amongst the lineages of the embryo, carries the genome from generation to generation and is therefore the only lineage which retains true developmental totipotency. Paradoxically, when mouse primordial germ cells (PGCs) are introduced into a host blastocyst, they do not contribute to either the germline or the soma, suggesting that they are restricted in developmental potency. Conversely, in vivo PGCs give rise to embryonal carcinoma (EC) cells, the pluripotent stem cells of teratomas, benign tumors containing derivatives of the three primary germ layers. Similarly, PGCs can be converted in vitro into embryonic germ (EG) cells, pluripotent stem cells capable of giving rise to somatic and germline chimeras. The ability of PGCs to form EC cells in vivo and EG cells in vitro suggests that developmental potency of PGCs is regulateable. The molecular mechanisms controlling PGC growth and differentiation are gradually being elucidated through the characterization of sterile mutants and through the use of in vitro culture systems. Understanding how a PGC can give rise to a pluripotent stem cell could give significant insights into the regulation of developmental totipotency as well as having important implications for male fertility and the etiology of testicular cancer.