Somitogenesis in zebrafish

Vertebrate segmentation involves the partitioning of paraxial mesoderm into numerous metameric units, known as somites. We have analyzed the cellular mechanics underlying early somitogenesis in wild-type zebrafish and trilobite, knypek, and knypek;trilobite mutants. We show that the formation of somite boundaries in all of these embryos involves segregation, local alignment, and cell shape changes of presumptive epitheloid border cells along nascent intersomitic boundaries. knypek;trilobite mutant embryos are extreme morphological variants whose somites lack internal mesenchymal cells, and form without convergence of the presomitic mesoderm. Although knypek;trilobite somites are composed of only two cells in their anterior-posterior dimension, they still exhibit anterior posterior intrasegmental polarity. Furthermore, morphogenesis of somite boundaries in these embryos proceeds in a manner similar to wild-type embryos. These results indicate that intersomitic boundary formation in zebrafish involves short-range movements of presumptive border cells that do not require mechanical forces generated by internal cells or compaction of the presomitic mesoderm. In order to understand how cytoskeleton-extracellular matrix interactions regulate cell movements during development, we have cloned zebrafish focal adhesion kinase (Fak), and analyzed its subcellular localization. In the axial mesoderm, Fak protein is localized to the cortex of notochord cells and is frequently present in large plaques at the cortex. During somitogenesis, Fak protein becomes concentrated at the basal region of epithelial somite cells. The wild-type segmental pattern of fak mRNA expression in the paraxial mesoderm is not dependent upon Notch signaling through Suppressor of Hairless (SuH), after eight /deltaD, or upon the activity of deadly seven . Taken together, these results suggest a dual role for FAK in mesoderm morphogenesis: (1) FAK may facilitate notochord cell intercalation, and (2) FAK may play a role in the formation and stabilization of somite boundaries.