Retinoic acid and FGF signaling interact to control elongation and lineage specification in a mouse gastruloid model.

Axial elongation and cell fate decisions during early embryonic development are regulated by gradients of signaling molecules, such as retinoic acid (RA) and fibroblast growth factor (FGF). To assess the role of RA in vitro, studies have relied on its direct addition to the medium, which can produce teratogenic effects. Here, we examined the role of a RA precursor, retinol, on axial elongation, anteroposterior development, and FGF-dependent signaling in murine gastruloids, an in vitro model system for early embryogenesis. Rather than applying RA ectopically, we supplemented a retinoid-free medium with its precursor, retinol, prompting the cells to produce RA endogenously. Our results showed that the spatiotemporal RA and FGF signaling can be manipulated with different doses of retinol, influencing gastruloid elongation and lineage specification. Gastruloids cultured in high doses of retinol showed an enrichment of ectoderm, whereas low doses resulted in early mesoderm specification and increased FGF signaling. We further used our approach to confirm that RA signaling inhibits FGF in gastruloids. Thus, retinol can modulate cellular composition in gastruloids, allowing us to investigate fate commitment of ectoderm and mesoderm progenitors in vitro.