Embryonic stem cells commit to differentiation by symmetric divisions following a variable lag period

Mouse embryonic stem (ES) cells are derived from the epiblast of the preimplantation embryo and retain the capacity to give rise to all embryo lineages. ES cells can be released into differentiation from a near-homogeneous maintenance condition. Exit from the ES cell state can be accurately monitored using the Rex1-GFPd2 transgenic reporter, providing a powerful system for examining a mammalian cell fate transition. Here, we performed live-cell imaging and tracking of ES cells during entry into differentiation for 48 hours in defined conditions. We observed a greater cell surface area and a modest shortening of the cell cycle prior to exit and subsequently a reduction in cell size and increase in motility. We did not see any instance of cells regaining ES cell identity, consistent with unidirectional developmental progression. Transition occurred asynchronously across the population but genealogical tracking revealed a high correlation in cell-cycle length and Rex1-GFPd2 expression between daughter cells. A population dynamics model was consistent with symmetric divisions during exit from naive pluripotency. Collapse of ES cell identity occurred acutely in individual cells but after a variable delay. The variation in lag period can extend up to three generations, creating marked population asynchrony.