Molecular mechanisms of segmental patterning in the vertebrate hindbrain.

The finding that pattern formation in the vertebrate hindbrain occurs through a process of segmentation raises the question of the underlying molecular mechanisms. A number of genes have been implicated in this pattern through their segment-restricted expression, and the function and possible regulatory relationships of these are discussed. There is now strong evidence that certain Hox genes are involved in specifying hindbrain segments and neural crest derivatives that contribute to cranial ganglia and the branchial arches. It is likely that retinoic acid receptors either directly or indirectly regulate Hox gene expression in the hindbrain. The zinc finger gene Krox-20 controls the segment-restricted up-regulation of at least one Hox gene, Hox-B2. Studies of the Krox-20 gene reveal a progressive sharpening of its expression domains that reflect cellular interactions leading to segment formation. A receptor tyrosine kinase gene, Sek, has been implicated in such cellular interactions by its rhombomeric expression pattern. In addition, this receptor kinase may be involved in mesoderm segmentation.