eLife digestÄuring animal development, numerous cells move around the embryo to form and shape the growing tissues. This is a distinctive example of Eph/Ephrin signaling acting positively to pattern migrating cells. As well, we propose that immunocytes disperse when Sp-Eph enhances adhesion, causing haptotactic movement to regions of higher ligand abundance. We conclude that Sp-Eph signaling is necessary and sufficient for epithelial insertion. In mosaic embryos, immunocytes insert preferentially in ectoderm expressing Sp-Efn. Expressing Sp-Efn throughout embryos permits immunocyte insertion in ventral ectoderm. Interfering with expression or function of Sp-Eph results in rounded immunocytes entering ectoderm but not adopting a dendritic form. Immunocytes express Sp-Eph and Sp-Efn is expressed throughout dorsal and ciliary band ectoderm. In sea urchin embryos, pigmented immunocytes are specified in vegetal epithelium, transition to mesenchyme, migrate, and re-enter ectoderm, distributing in dorsal ectoderm and ciliary band, but not ventral ectoderm. Eph receptors usually guide migrations of cells by exclusion from regions expressing Ephrin. The mechanisms that underlie directional cell migration are incompletely understood.
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