Cell segregation in the vertebrate hindbrain relies on actomyosin cables located at the interhombomeric boundaries

Synopsis image Actomyosin cables form mechanical barriers in the hindbrain of zebrafish embryos that keep rhombomeric cells segregated. Segregating cells into compartments during embryonic development is essential for growth and pattern formation, ensuring that their fates and/or positional information remain segregated as they proliferate and move. The refinement of gene expression borders in the hindbrain is achieved by cell sorting. There are actomyosin cables in the interhombomeric boundaries: F-actin and myosin II components are enriched in the apical side of the neuroepithelial cells at the boundaries. Actomyosin barriers sort cells at rhombomeric boundaries: Inhibition of myosin II function results in dismantling of cable structures, leading to rhombomeric cell mixing. Conversely, ectopic activation of RhoA pathway leads to the enrichment of actomyosin structures. EphA/ephrin signaling regulates the formation of actomyosin cables through RhoA cascade, to segregate cells from different rhombomeres. Abstract Segregating cells into compartments during embryonic development is essential for growth and pattern formation. Physical mechanisms shaping compartment boundaries were recently explored in Drosophila, where actomyosin-based barriers were revealed to be important for keeping cells apart. In vertebrates, interhombomeric boundaries are straight interfaces, which often serve as signaling centers that pattern the surrounding tissue. Here, we demonstrate that in the hindbrain of zebrafish embryos cell sorting sharpens the molecular boundaries and, once borders are straight, actomyosin barriers are key to keeping rhombomeric cells segregated. Actomyosin cytoskeletal components are enriched at interhombomeric boundaries, forming cable-like structures in the apical side of the neuroepithelial cells by the time morphological boundaries are visible. When myosin II function is inhibited, cable structures do not form, leading to rhombomeric cell mixing. Downregulation of EphA4a compromises actomyosin cables and cells with different rhombomeric identity intermingle, and the phenotype is rescued enhancing myosin II activity. Moreover, enrichment of actomyosin structures is obtained when EphA4 is ectopically expressed in even-numbered rhombomeres. These findings suggest that mechanical barriers act downstream of EphA/ephrin signaling to segregate cells from different rhombomeres.