The origin and identity of embryonic stem cells.

Embryonic stem (ES) cells are used extensively in biomedical research and as a model with which to study early mammalian development, but their exact origin has been subject to much debate. They are routinely derived from pre-implantation embryos, but it has been suggested that the cells that give rise to ES cells might arise from epiblast cells that are already predisposed to a primordial germ cell (PGC) fate, which then progress to ES cell status via the PGC lineage. Based on recent findings, we propose here that ES cells can be derived directly from early epiblast cells and that ES cells might arise via two different routes that are dictated by their culture conditions.