The Role of Fibroblast Growth Factor Signaling in Somitogenesis.

Fibroblast growth factor (FGF) signaling is conserved from cnidaria to mammals (Ornitz and Itoh, 2022) and it regulates several critical processes such as differentiation, proliferation, apoptosis, cell migration, and embryonic development. One pivotal process FGF signaling controls is the division of vertebrate paraxial mesoderm into repeated segmented units called somites (i.e., somitogenesis). Somite segmentation occurs periodically and sequentially in a head-to-tail manner, and lays down the plan for compartmentalized development of the vertebrate body axis (Gomez et al., 2008). These somites later give rise to vertebrae, tendons, and skeletal muscle. Somite segments form sequentially from the anterior end of the presomitic mesoderm (PSM). The periodicity of somite segmentation is conferred by the segmentation clock, comprising oscillatory expression of Hairy and enhancer-of-split (Her/Hes) genes in the PSM. The positional information for somite boundaries is instructed by the double phosphorylated extracellular signal-regulated kinase (ppERK) gradient, which is the relevant readout of FGF signaling during somitogenesis (Sawada et al., 2001; Delfini et al., 2005; Simsek and Ozbudak, 2018; Simsek et al., 2023). In this review, we summarize the crosstalk between the segmentation clock and FGF/ppERK gradient and discuss how that leads to periodic somite boundary formation. We also draw attention to outstanding questions regarding the interconnected roles of the segmentation clock and ppERK gradient, and close with suggested future directions of study.