Scanning electron microscopic evidence for physical segmental boundaries in the anterior presomitic mesoderm.

The metameric pattern of the axial skeleton is established during embryogenesis by somite formation from the unsegmented paraxial mesoderm (presomitic mesoderm). Here, we have investigated the morphology of the anterior presomitic mesoderm of chick embryos using scanning electron microscopy. We found periodically arranged transverse clefts in the anterior region of the presomitic mesoderm. These gaps can be regarded as physical boundaries between prospective somites in the determined zone of the presomitic mesoderm. This study provides additional evidence suggesting that prospective somite boundaries are not only marked by defined zones of gene expression, but are also accompanied by changes in cellular morphology that give rise to identifiable morphological segments.