The effect of heat shocks, which alter somite segmentation, on Rohon-Beard neurite outgrowth from the spinal cord of Xenopus embryos.

The vertebrate spinal cord shows a segmental pattern of dorsal and ventral nerve roots. It is believed that this peripheral neural segmentation depends on the segmentation of the somitic mesoderm into somites. I have studied the relationship between somite segmentation and the pattern of Rohon-Beard sensory neurite outgrowth from the spinal cord in Xenopus embryos. Most Rohon-Beard neurites grow out in fascicles from the spinal cord at the intersomite clefts, a smaller number of neurites growing out over the dorsal somite margins. Previous work on peripheral neural segmentation has relied on transplanting or destroying somitic mesoderm. Here a non-invasive technique has been used. The pattern of somite segmentation was disrupted by heat shocks, leading to regions of somite fusion where one or two intersomite clefts are absent. At the regions of somite fusion the number of sensory fascicles is then unchanged but their distribution is different. The segmental pattern of sensory fascicle outgrowth is replaced by a fairly even distribution of outgrowth fascicles. Two interpretations of the difference in fascicle outgrowth at fused and control somites are discussed. Firstly, that heat shocks have a direct affect on fascicle outgrowth. Secondly, that heat shocks affect fascicle outgrowth indirectly by disrupting somite segmentation.