The differing effects of occipital and trunk somites on neural development in the chick embryo

In all higher vertebrate embryos the sensory ganglia of the trunk develop adjacent to the neural tube, in the cranial halves of the somite-derived sclerotomes. It has been known for many years that ganglia do not develop in the most cranial (occipital) sclerotomes, caudal to the first somite. Here we have investigated whether this is due to craniocaudal variation in the neural tube or crest, or to an unusual property of the sclerotomes at occipital levels. Using the monoclonal antibody HNK-1 as a marker for neural crest cells in the chick embryo, we find that the crest does enter the cranial halves of the occipital sclerotomes. Furthermore, staining with zinc iodide/osmium tetroxide shows that some of these crest-derived cells sprout axons within these sclerotomes. By stage 23, however, no dorsal root ganglia are present within the five occipital sclerotomes, as assessed both by haematoxylin/eosin and zinc iodide/osmium tetroxide staining. Moreover, despite this loss of sensory cells, motor axons grow out in these segments, many of them later fasciculating to form the hypoglossal nerve. The sclerotomes remain visible until stages 27/28, when they dissociate to form the base of the skull and the atlas and axis vertebrae. After grafting occipital neural tube from quail donor embryos in place of trunk neural tube in host chick embryos, quail-derived ganglia do develop in the trunk sclerotomes. This shows that the failure of occipital ganglion development is not the result of some fixed local property of the neural crest or neural tube at occipital levels. We therefore suggest that in the chick embryo the cranial halves of the five occipital sclerotomes lack factors essential for normal sensory ganglion development, and that these factors are correspondingly present in all the more caudal sclerotomes.