Interrogating the potency of primordial germ cells by injection into early mouse embryos

Primordial germ cells (PGCs) are the earliest precursors of the gametes. During normal development PGCs only give rise to oocytes or spermatozoa. However, PGCs can acquire pluripotency in vitro by forming embryonic germ (EG) cells and in vivo during teratocarcinogenesis. Classic embryological experiments directly assessed the potency of PGCs by injection into the pre-implantation embryo. As no contribution to embryos or adult mice was observed, PGCs have been described as unipotent. Here we demonstrate that PGCs injected into 8-cell embryos can initially survive, divide and contribute to the developing inner cell mass. Apoptosis-deficient PGCs exhibit improved survival in isolated epiblasts, and can form naïve pluripotent embryonic stem cell lines. However, contribution to the post-implantation embryo is limited, with no functional incorporation observed. In contrast, PGC-like cells show extensive contribution to mid-gestation chimaeras. We thus propose that PGC formation in vivo establishes a latent form of pluripotency that prevents chimaera contribution. Overall design: Five primordial germ cells- embryonic stem cells (PGC-ESC) Bax knockout (KO) cell lines and three PGC-ESC Puma/Noxa double knockout (DKO) cell lines. As controls we included two previously derived BaxKO ESC lines, one Bax heterozygous ESC line and four matched wildtype ESC lines derived simultaneously from the host embryos. In addition, for comparison we included epiblast-like cells (EpiLCs) derived in vitro from 2 host ESC lines and 3 PGC-ESC DKO lines.