Somites in zebrafish doubly mutant for knypek and trilobite form without internal mesenchymal cells or compaction

In vertebrates, paraxial mesoderm is partitioned into repeating units calle d somites. It is thought that the mechanical forces arising from compaction of the presumptive internal cells of prospective somites cause them to det ach from the unsegmented presomitic mesoderm [1-3]. To determine how prospe ctive somites physically segregate from each other, we used time-lapse micr oscopy to analyze the mechanics underlying early somitogenesis in wild-type zebrafish and in the mutants trilobite(m209) (tri), knypek(m119) (kny), an d kny;tri, which are defective in convergent extension during gastrulation. Formation of somite boundaries in all of these embryos involved segregatio n, local alignment, and cell-shape changes of presumptive epitheloid border cells along nascent intersomitic boundaries. Although kny;tri somites form ed without convergence of the presomitic mesoderm and were composed of only two cells in their anteroposterior (AP) dimension, they still exhibited AP intrasegmental polarity. Furthermore, morphogenesis of somite boundaries i n these embryos proceeded in a manner similar to that in wild type embryos. Thus, intersomitic boundary formation in zebrafish involves short-range mo vements of presumptive border cells that do not require mechanical forces g enerated by internal cells or compaction of the presomitic mesoderm.