Paul Hunt

The structures of the face in vertebrates are largely derived from neural crest. There is some evidence to suggest that the form of the facial pattern is determined by the crest, and that it is specified before migration as to the structures that is is able to form. The neural crest is able to control the form of surrounding, non-neural crest tissues by an instructive interaction. Some of this cranial crest is derived from a region of the hindbrain that expresses Hox 2 homeobox genes in an overlapping and segment-restricted pattern. We have found that neurogenic and mesenchymal neural crest expresses Hox 2 genes from its point of origin beside the neural plate, during migration and after migration has ceased and that rhombomeres 3 and 5 do not have any expressing neural crest beside them. Each branchial arch expresses a different combination or code of Hox genes in a segment-restricted way. The surface ectoderm over the arches initially does not express Hox genes, and later adopts an expression pattern that reflects that of neural crest that has come to underlie it. We suggest that initially the neural plate and neural crest are spatially specified, while the surface ectoderm is unpatterned. Subsequently some positional information could be transferred to the surface ectoderm as a result of an interaction with the neural crest. Given that the role of the homologous genes in insects is position specification, and that neural crest is imprinted before migration, we suggest that Hox 2 genes are providing part of this positional information to the neural crest and hence are involved in patterning the structures of the branchial arches.

We have compared the ways in which vertebrate Hox genes are used in the patterning of three distinct embryonic contexts, the branchial region, the somites, and the limb. We have identified common features of the three systems, but have suggested on the basis of their differences (in both embryological properties and use of Hox genes) that it is better to consider each as an independent system for regional specification. Nevertheless, there are sufficient common features to expect that exploitation of the distinct experimental advantages of each system will provide important insights to the mode of operation of the others.

Antennapedia class homeobox genes, which in insects are involved in regional specification of the segmented central regions of the body, have been implicated in a similar role in the vertebrate hindbrain. The development of the hindbrain involves the establishment of compartments which are subsequently made distinct from each other by Hox gene expression, implying that the lineage of neural cells may be an important factor in their development. The hindbrain produces the neural crest that gives rise to the cartilages of the branchial skeleton. Lineage also seems to be important in the neural crest, as experiments have shown that the crest will form cartilages appropriate to its level of origin when grafted to a heterotopic location. We show how the Hox genes could also be involved in patterning the mesenchymal structures of the branchial skeleton. Recently it has been proposed that the rhombomere-restricted expression pattern of Hox 2 genes is the result of a tight spatially localised induction from underlying head mesoderm, in which a prepattern of Hox expression is visible. We find no evidence for this model, our data being consistent with the idea that the spatially localised expression pattern is a result of segmentation processes whose final stages are intrinsic to the neural plate. We suggest the following model for patterning in the branchial region. At first a segment-restricted code of Hox gene expression becomes established in the neuro-epithelium and adjacent presumptive neural crest. This expression is then maintained in the neural crest during migration, resulting in a Hox code in the cranial ganglia and branchial mesenchyme that reflects the crest's rhombomere of origin. The final stage is the establishment of Hox 2 expression in the surface ectoderm which is brought into contact with neural crest-derived branchial mesenchyme. The Hox code of the branchial ectoderm is established later in development than that of the neural plate and crest, and involves the same combination of genes as the underlying crest. Experimental observations suggest the idea of an instructive interaction between branchial crest and its overlying ectoderm, which would be consistent with our observations. The distribution of clusters of Antennapedia class genes within the animal kingdom suggests that the primitive chordates ancestral to vertebrates had at least one Hox cluster. The origin of the vertebrates is thought to have been intimately linked to the appearance of the neural crest, initially in the branchial region.(ABSTRACT TRUNCATED AT 400 WORDS)