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 lineag e which retains true developmental totipotency. Paradoxically, when mo use primordial germ cells (PGCs) are introduced into a host blastocyst , they do not contribute to either the germline or the soma, suggestin g that they are restricted in developmental potency. Conversely, in vi vo PGCs give rise to embryonal carcinoma (EC) cells, the pluripotent s tem cells of teratomas, benign tumors containing derivatives of the th ree primary germ layers. Similarly, PGCs can be converted in vitro int o 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 potenc y of PGCs is regulateable. The molecular mechanisms controlling PGC gr owth and differentiation are gradually being elucidated through the ch aracterization of sterile mutants and through the use of in vitro cult ure systems. Understanding how a PGC can give rise to a pluripotent st em cell could give significant insights into the regulation of develop mental totipotency as well as having important implications for male f ertility and the etiology of testicular cancer.