• dpERK
• FGF
• imaging
• segmentation clock
• somitogenesis
• zebrafish

Somitogenesis is the rhythmic and sequential formation of somites, which are tissue blocks that give rise to segmented adult body structures, including the vertebrae and associated muscles. This process is regulated by the interplay between the segmentation clock, a molecular oscillator that regulates temporal periodicity, and morphogen gradients, which influence positional information within the presomitic mesoderm (PSM), the embryonic tissue located posterior to the forming somites, where cells are progressively specified to contribute to future somite identities.

Among these morphogens, Fgf8 plays a central role in establishing the determination front, the position in the PSM where cells become committed to forming the somites. A key intracellular readout of Fgf8 signaling is dpERK, which reflects cellular responses to the Fgf8 gradient. Despite extensive theoretical modeling, the spatiotemporal dynamics of Fgf8 signaling and the mechanisms by which cells interpret this signal at different developmental stages remain poorly understood.

This thesis investigates how the Fgf8 signaling gradient defines somite boundaries over time, through direct spatiotemporal analysis of both the Fgf8 gradient and dpERK activity in the zebrafish embryos at single-cell level. Using newly developed transgenic zebrafish lines, high-resolution light-sheet microscopy of the zebrafish tail and a novel experimental and image analysis pipeline, I analyze the dynamics of the determination front. My results reveal a progressive posterior shift of the front during development, challenging static models and offering a new interpretation of somite boundary formation.

Furthermore, this work explores how cells integrate the Fgf8 gradient depending on their developing positional identity within the forming somite. context-dependent interpretation of the FGF8 signal implies additional regulatory layers, potentially involving the segmentation clock. I propose that cells integrate FGF8 spatiotemporal information based on their future somite identity, supporting a new, dynamic model of boundary formation, that still remains to be fully understood.