Connecting the dots: uncovering the mechanisms linking fate and shape in vertebrate gastrulation

Gastrulation is a hallmark event in embryonic development, resulting in germ layer specification and formation of the body axes. While the role of diffusible morphogen signals (e.g. BMP, Wnt, Nodal) in gastrulation is well-established, recent studies increasingly implicate mechanical forces as an additional source of morphogenetic information. Taking advantage of a minimal pluripotent stem cell (PSC)-based model of human gastrulation, we are investigating the interplay between cell fate and mechanics during gastruloid disc self-organization. These are micropatterned PSC colonies, which self-organize into concentric rings of the three germ layers and the extraembryonic amnion upon BM4 stimulation. Taking advantage of their simplified geometry, we established a computational image analysis pipeline to project, segment and automatically extract key morphometric parameters (e.g. cell shape, tissue stress) during gastruloid self-organization. We found that extraembryonic amnion cells, induced at the gastruloid edge, adopt a squamous organization while the differentiated epiblast remains columnar, consistent with their in vivo counterparts. This differential cell shape pattern arises due to gastruloid specification rather than to purely geometrical constrains, as edge cells remain columnar in confined pluripotent gastruloids. Our ongoing work supports that the squamous organization of the amnion is cell-autonomous, since it is observed even colonies homogeneously induced to amnion identity. We are now investigating the mechanical forces driving these shape changes, as well as how this biophysical mechanism is linked to amnion specification. Overall, this multiscale approach is allowing us to quantitatively characterize the mechanical forces arising during cell fate acquisition and gastruloid patterning, opening the door for a mechanistic understanding of the regulation and functional role of mechanical forces in human gastrulation.