Barrios, Arantza; (2002) The role of the Eph signalling pathway in epithelialisation of the somites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Vertebrate somitogenesis involves the establishment of a segmental pattern of gene expression within the paraxial mesoderm and the subsequent translation of this pattern into physical furrows and epithelial somites. In this thesis I present first the development of an in vitro technique to culture embryo trunk explants. These explants develop and segment correctly despite the absence of overlying ectoderm, however no defects in somite formation were observed when treated with Eph signalling blocking reagents. Using mosaic analysis I provide evidence for a role for the Eph/ephrin signalling pathway in somite epithelialisation. A comparative analysis of the cellular behaviours and gene expression patterns within the segmental plate during somitogenesis was carried out in wild type zebrafish embryos and in fusedsomites (fss) mutants. In wild type embryos, the receptor EphA4 and the ligand ephrin-B2a are segmentally expressed in complementary non-overlapping domains in the anterior presomitic mesoderm and the forming somites. Somite boundaries form at the interface between cells expressing EphA4 and cells expressing ephrin-B2a. Cells at somite boundaries acquire epithelial morphology manifested by the re-localisation of adhesion molecules towards the apical pole and nuclear migration towards the basal pole. In fss embryos, expression of EphA4 is absent in the paraxial mesoderm and ephrin-B2a is expressed throughout the segmental plate. Somite boundaries fail to form and cells within the paraxial mesoderm remain mesenchymal failing to epithelialise. Restoration of the Eph/ephrin signalling in the paraxial mesoderm of fss mutants resulted in the rescue of ectopic physical boundaries (Durbin, 2000). Moreover, some aspects of epithelialisation such as the re-localisation of β-catenin to the apical pole and nuclear migration towards the basal pole were also rescued.

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