Mammalian germ cells: Birth, sex, and immortality

The germ cell lineage in the mouse is not predetermined but is established during gastrulation, in response to signalling molecules acting on a subset of epiblast cells that move through the primitive streak together with ext ra-embryonic mesoderm precursors. After migration to the site of the future gonads, germ cell sex determination is achieved, with germ cell phenotype in male and female embryos diverging. Evidence suggests that all germ cells spontaneously take the female pathway, entering prophase of the first meio tic division five or six days after the birth of the germ cell lineage, wit h the exception of those located in the embryonic testis, which exit the ce ll cycle in response to some inhibitory signal and remain in G(o) until aft er birth, when spermatogenesis begins. In culture, germ cells respond to ce rtain growth factors by proliferating indefinitely. These immortalized embr yonic germ (EG) cell lines are chromosomally stable and pluripotent, closel y resembling the embryonic stem (ES) cell lines derived from blastocyst-sta ge embryos. Human EG and ES cell lines have recently been made, raising the hope that their differentiation could be directed to specific cell types, of value in the clinical treatment of degenerative diseases.