A balance between secreted inhibitors and edge-sensing controls gastruloid self-organization

The earliest aspects of human embryogenesis remain mysterious. To model patterning events in the human embryo we used colonies of human embryonic stem cells (hESCs) grown on micropatterned substrate and differentiated with BMP4. These recapitulate the embryonic arrangement of the mammalian germ layers and provide an assay to assess the structural and signaling mechanisms patterning the human gastrula. Structurally, high-density hESCs relocalize their TGF-β receptors to their lateral side in the center of the colony, while maintaining apical localization of receptors at the edge. This relocalization insulates central cells from apically applied ligands while maintaining response to basally presented ones. Additionally, BMP4 directly induces the expression of its own inhibitor, Noggin, generating a reaction-diffusion mechanism that underlies patterning. We develop a quantitative model that integrates edge sensing and inhibitors, to predict human fate positioning in micropatterns, and potentially the human embryo. We performed RNAseq of human embryonic stem cells cultured in micropatterned colonies at different time points after the addition of BMP4.