Abstract
The segregation of definitive endoderm (DE) from mesendoderm progenitors leads to the formation of two distinct germ layers. Dissecting DE onset has been challenging as it occurs within a narrow spatio-temporal window in the embryo. Here we employ a dual Bra-GFP, Sox17-RFP reporter cell line to study DE onset dynamics. We find Sox17 starts in a few isolated cells in vivo. Using 2D and 3D in vitro models, we show that DE cells emerge from mesendoderm progenitors at a temporally regular, but spatially stochastic pattern, which is subsequently arranged by self-sorting of Sox17+ cells. Self-sorting coincides with up-regulation of E-cadherin but is not necessary for DE differentiation or proliferation. A subpopulation of Bra-high cells commits to a Sox17+ fate independent of external Wnt signal. Our in vivo and in vitro results highlight basic rules governing DE onset and patterning through the commonalities and differences between these systems.
