Somites and axon guidance.

The somites are arrayed in a repeating pattern along the longitudinal axis of the embryo, as are the developing sensory and sympathetic ganglia and the spinal nerves. This pattern is not a coincidence: the somite imposes a segmental pattern on the cells and axons that invade it. Both neural crest cells and axons prefer the anterior portion of the sclerotome (the ventral part of the somite) for outgrowth. What differences in anterior and posterior sclerotome are responsible? I used scanning electron microscopy to ask whether these populations differed on the tissue level in chick embryos. This study shows that differences in tissue organization are of insufficient magnitude or develop too late to explain the preference of neural crest cells and axons for the anterior half of each sclerotome. For instance, the extracellular matrix does not differ dramatically in density at the dorsal sclerotome boundary and yet neural crest cells promptly enter the anterior sclerotome when they reach this boundary. These cells have access to the cell processes of somitic cells that extend through the matrix. This suggests that neural crest cells could detect important differences in anterior and posterior populations by direct cell contact. Likewise, barriers and consistent differences in cell density, shape or orientation were not obvious before or during initial axon outgrowth. The absence of significant differences in tissue organization suggests that axons and neural crest cells become segmented by responding to diffusible cues, to differences in extracellular material or to the cell surfaces of individual anterior and posterior sclerotome cells.