Endogenous FGFs drive ERK-dependent cell fate patterning in 2D human gastruloids.

The role of FGF is the least understood of the morphogens driving mammalian gastrulation. Here, we have investigated FGF function in a 2D gastruloid model for human gastrulation. We observed a ring of FGF-dependent ERK activity that closely follows the emergence of primitive streak (PS)-like cells but expands further inward. This ERK activity pattern depends on localized activation of basolateral FGF receptor 1 (FGFR1) by endogenous FGF gradients and is required for PS-like differentiation, with loss of PS-like cells upon FGF receptor inhibition rescued by direct ERK activation. Single cell transcriptome analysis confirmed that, among the ligands, FGF2 is broadly expressed, FGF8 is transiently expressed during PS-like differentiation and FGF4/17 are specifically expressed in PS-like cells - similar to the human and monkey embryo but different from the mouse. FGF4 knockdown greatly reduced PS-like differentiation, while FGF17 knockdown primarily affected subsequent mesoderm differentiation. FGF8 expression was spatially and temporally displaced from PS markers and FGF4 expression, while knockdown expanded PS-like cells, suggesting FGF8 may limit PS-like differentiation. Thus, we have identified a previously unreported role for FGF-dependent ERK signaling in 2D gastruloids and possibly the human embryo, where FGF4 and FGF17 signal through basolateral FGFR1 to induce PS-like cells and derivatives, potentially restricted by FGF8.