Tese de Doutoramento Biomedicine 2023 Faculdade de Ciências Médicas, Universidade NOVA de Lisboa

During vertebrate development, symmetry breaking occurs in the left-right (LR) organizer (LRO). The transfer of asymmetric molecular information to the lateral plate mesoderm is essential for the precise patterning of asymmetric internal organs, such as the heart. However, it is also necessary to maintain symmetry at the somite level for correct musculature and vertebrae specification. In this study, we investigated the effects of LR signals on the behavior of zebrafish somite cell precursors. Here, we demonstrate how LR signals affect the behavior of zebrafish somite cell precursors. We describe a population of cells in the vicinity of the LRO, named Non-KV Sox17:GFP+ Tailbud Cells (NKSTCs), which migrate anteriorly and contribute to future somites. We showed that NKSTCs originate in a cluster of cells aligned with the midline, posterior to the LRO, and leave that cluster in an LR alternating manner, primarily from the left side. Fate mapping revealed that more NKSTCs integrated into somites on the left side of the embryo. To investigate the role of asymmetric cues from the LRO, we used dand5-/- mutant embryos and found that NKSTCs no longer displayed asymmetric patterns. Cell exit from the posterior cluster became bilaterally synchronous in dand5-/- embryos, revealing a new link between somite specification and Dand5 function. Moreover, we observed compromised anterior movement and reduced myoD expression in the absence of the KV, indicating its role in tailbud cell movement and somite specification. Our findings demonstrate the asymmetric behavior of somite precursor cells in response to LR signals in vivo, which requires Dand5, and suggest an additional role for the KV in tailbud cell movement and somite specification. Additionally, using single-molecule fluorescence in situ hybridization (smFISH), we quantified the number of dand5 mRNA transcripts in 3D in the KV over time. We found that dand5 asymmetry is already established at 6 ss, which is one hour earlier than previously reported by less sensitive methods, and primarily occurs on the anterior side of the KV. Overall, our findings demonstrate the asymmetric behavior of somite precursor cells in response to LR signals in vivo, which requires Dand5, and suggest an additional role for the KV in tailbud cell movement and somite specification. This study provides new insights into dand5 asymmetric expression in zebrafish and sets the foundation for studying the involvement of other genes of the LR cascade in this context. Our results have implications for understanding the role of asymmetric cues in the formation of symmetric structures and provide insights into the mechanisms underlying embryonic development