Modular engineering of embryonic-extraembryonic interactions generates advanced gastruloid morphotypes.

During mammalian embryogenesis, reciprocal interactions between the epiblast and extraembryonic endoderm are critical for germ layer specification and body plan development during gastrulation. Gastruloids recapitulate aspects of gastrulation in the absence of extraembryonic cues, resulting in a predominantly posteriorized and dorsalized phenotype with a limited lineage diversity. Here, we establish a modular co-aggregation ("aggregoid") strategy that spatially couples embryonic and extraembryonic endoderm-like cells to reconstruct key interactions in vitro. This drives self-organized anterior-ventral patterning together with the emergence of node- and notochord-like structures, enriched endodermal populations, and increased mesodermal diversity, including cardiopharyngeal lineages and vascular endothelium. Our findings demonstrate that modular engineering of lineage interactions can direct self-organized patterning in stem-cell-based embryo models and provide a versatile framework for generating defined morphotypes.