Emergence and Patterning Dynamics of Mouse Definitive Endoderm

The segregation of definitive endoderm (DE) from bi-potent mesendoderm progenitors leads to the formation of two distinct germ layers. Dissecting DE commitment and onset has been challenging as it occurs within a narrow spatio-temporal window in the embryo. Here we employ a novel dual Bra/Sox17 reporter cell line to study DE onset dynamics. We find Sox17 expression initiates in a few isolated cells in vivo within a temporally restricted window. 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. A subpopulation of Bra-high cells commits to a Sox17+ fate independent of external Wnt signal. Self-sorting coincides with up-regulation of E-cadherin but is not necessary for DE differentiation or proliferation. Our in vivo and in vitro results highlight basic rules governing DE onset and patterning through the commonalities and differences between these systems.