Gastruloid development competence discriminates different states of pluripotency between naïve and primed

Floating spheroidal aggregates (aggregomes) of mouse embryonic stem cells (mESCs) can develop into polarized/elongated organoids, namely gastruloids. Here we report a high-performing assay to measure gastruloids formation efficiency (GFE), the fraction of gastruloid-developing aggregomes. By exploiting this procedure, we provide morphological and molecular evidence that gastruloid development relies on . We also demonstrate that GFE decreases as pluripotency progresses from naïve to primed state. Indeed, naïve ESC-derived aggregomes efficiently elongate (GFE≥95%), while primed EpiSCs fail to aggregate and consequently to generate gastruloids (GFE=0%). Conversely, while early-primed EpiLCs properly aggregate, EpiLC-derived aggregomes are mostly abortive (GFE=0%). Unlike EpiLCs, L-Proline-treated ESCs (PiCs) generate productive aggregomes (GFE≥50%), which however begin to elongate earlier and generate smaller gastruloids that appear more differentiated. Like EpiLCs, PiCs are competent to differentiate into primordial germ cell-like cells (PGCLCs), suggesting that PiCs capture an EpiLC-like state with unique competence for both gastruloid formation and differentiation into PGCLCs. Thus we propose GFE assay as a simple and robust method to discriminate different phenotypic/functional states of the pluripotency continuum.