The final developmental pattern of multicellular organisms is extraordinarily precise, with sharp boundaries between cell types that can be as fine as a single-cell layer. Although morphogen gradients that direct cell fates can be quite defined, development operates on a background of substantial variability due to intrinsic stochasticity in cell fate specification and active migration of cells within the morphogenic field. Cell sorting is thought to enable systems with initially noisy fate specification to generate robust final patterns. In the early zebrafish embryo, an initially mixed mesendodermal population ultimately resolves into distinct mesodermal and endodermal cell layers, but how this precise patterning is achieved is not understood. Previous studies demonstrated that induced endodermal cells transplanted to ectopic locations can still sort into the endogenous endodermal domain. Here, we aim to understand how this sorting behavior is achieved and how cell sorting enables pattern refinement during endodermal morphogenesis.
We experimentally introduced positional errors by first overexpressing the constitutively activate Nodal receptor TARAM-A* to induce endoderm fate and then transplanting these cells to the animal pole of a wild type embryo, which normally gives rise primarily to ectoderm. We then used whole embryo imaging to track the transplanted cells. Consistent with previous reports, we found that these ectopically introduced endodermal cells preferentially migrate to the correct endodermal layer, but surprisingly, these cells did not follow the normal endodermal migration pattern of epiboly and involution. Instead, they appeared to take a short-cut by radially ingressing into the inner layer. We propose that this ingression ensures that cells that may arrive at the margin late can still find a path into the inner layer, thus increasing the precision of the first step of endoderm formation. Currently we are exploring the informational sources of such directionality. By examining contractility based extrusion of the surrounding cells and membrane protrusions of ectopic endodermal cells, we will be able to determine the underlying mechanisms of the ingression based sorting behavior, and thus further our understanding of the largely unexplored frontier of how cell sorting facilitates the robustness of multicellular development.