Abstract
During gastrulation pluripotent epiblast cells specify and spatially arrange the three germ layers. Human pluripotent stem cells (PSCs) have been shown to undergo spatially-organized fate specification on micropatterned surfaces. Since in vivo validation is not possible for the human, we developed a mouse PSC micropattern system and, by making direct comparisons to mouse embryos, reveal the robust specification of distinct regional identities. BMP, WNT, ACTIVIN and FGF directed mouse epiblast-like cells to undergo an epithelial-to-mesenchymal transition and radially pattern posterior mesoderm fates. Conversely, WNT, ACTIVIN and FGF patterned anterior mesoderm and endoderm identities. By contrast, epiblast stem cells, a more developmentally advanced state, only specified anterior identities but without patterning. The mouse micropattern system offers a robust scalable method to generate regionalized cell types present in vivo, resolve how signals promote distinct cellular identities and generate patterns, and compare mechanisms operating in vivo and in vitro and across species.