Paradox segmentation along inter- and intrasomitic borderlines is followed by dysmorphology of the axial skeleton in the open brain (opb) mouse mutant.

In open brain (opb) mutant embryos, developmental defects of the trunk spinal cord were spatially correlated with severe defects of the epaxial somite derivatives including sclerotomes, whereas hypaxial somite derivatives are much less affected. Later in development, the neural arches (epaxial sclerotome derivatives) formed but were severely disorganized, and also the distal ribs (hypaxial sclerotome derivatives) were malformed. Adjacent neural arches and vertebral bodies were often fused where joints should have formed suggesting defects of the intrasomitic borderlines. Moreover, neural arches frequently and ribs sometimes were split into halves at distinct levels along the dorso-ventral body axis. This suggests that 'resegmentation' of sclerotomes across the somite borders did not completely occur. These prominent skeletal defects were preceded by reduced expression of Pax1 along the intrasomitic borderlines, and incomplete maintenance of somite borders between central sclerotome moieties. The defects of the axial skeleton were accompanied by segmentation defects of the myotomes which were split distally, and also partly fused from adjacent segments across somite borders. The segmentation defects observed suggest that in opb mutants both segmental borderlines, the somite borders and the intrasomitic borderlines (fissures), were affected and behaved paradoxically.