Rebuilding pluripotency from primordial germ cells.

Mammalian primordial germ cells (PGCs) are unipotent progenitors of the gametes. Nonetheless, they can give rise directly to pluripotent stem cells in vitro or during teratocarcinogenesis. This conversion is inconsistent, however, and has been difficult to study. Here, we delineate requirements for efficient resetting of pluripotency in culture. We demonstrate that in defined conditions, routinely 20% of PGCs become EG cells. Conversion can occur from the earliest specified PGCs. The entire process can be tracked from single cells. It is driven by leukemia inhibitory factor (LIF) and the downstream transcription factor STAT3. In contrast, LIF signaling is not required during germ cell ontogeny. We surmise that ectopic LIF/STAT3 stimulation reconstructs latent pluripotency and self-renewal. Notably, STAT3 targets are significantly upregulated in germ cell tumors, suggesting that dysregulation of this pathway may underlie teratocarcinogenesis. These findings demonstrate that EG cell formation is a robust experimental system for exploring mechanisms involved in reprogramming and cancer.