Primordial germ-cell development and epigenetic reprogramming in mammals.

Primordial germ cells (PGCs) are the embryonic precursors of the gametes and represent the founder cells of the germline. Specification of PGCs is a critical divergent point during embryogenesis. Whereas the somatic lineages will ultimately perish, cells of the germline have the potential to form a new individual and hence progress to the next generation. It is therefore critical that the genome emerges intact and carrying the appropriate epigenetic information during its passage through the germline. To ensure this fidelity of transmission, PGC development encompasses extensive epigenetic reprogramming. The low cell numbers and relative inaccessibility of PGCs present a challenge to those seeking mechanistic understanding of the crucial developmental and epigenetic processes in this most fascinating of lineages. Here, we present an overview of PGC development in the mouse and compare this with the limited information available for other mammalian species. We believe that a comparative approach will be increasingly important to uncover the extent to which mechanisms are conserved and reveal the critical steps during PGC development in humans.