摘要：SummaryThe segregation of definitive endoderm (DE) from bipotent 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 spatiotemporal window in the embryo. Here, we employ a dual Bra/Sox17 reporter cell line to study DE onset dynamics. We find Sox17 expression initiatesin vivoin isolated cells within a temporally restricted window. In 2D and 3Din vitromodels, 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 upregulation of E-cadherin but is not necessary for DE differentiation or proliferation. Ourin vivoandin vitroresults highlight basic rules governing DE onset and patterning through the commonalities and differences between these systems.Graphical abstractDisplay OmittedHighlights•Sox17 onsets in a few isolated cells within Bra-expressing population•Sox17 onset followed by expansion and self-sorting•Final number of Sox17+ cells does not depend on self-sorting or cell movement•The DE segregation pattern is similar inin vivoand in 2D, 3Din vitrosystemsDevelopmental genetics; Embryology
