Molecular mechanisms of pattern formation in the vertebrate hindbrain

During early stages of neural development a series of repeated bulges, termed rhombomeres, form in the vertebrate hindbrain. Studies in the chick have shown that rhombomeres are segments that underlie the patterning of nerves in the hindbrain, and this raises the question of the molecular basis of segment development. Several genes have been found with expression patterns consistent with roles in the formation or differentiation of rhombomeres. The zinc finger gene Krox-20 is expressed in two alternating rhombomeres, r3 and r5, in the mouse hindbrain; these stripes of gene expression are established prior to the morphological appearance of segments. Krox-20 is also expressed in this pattern in the chick and Xenopus, suggesting that it has a conserved role, possibly in the formation of rhombomeres. Four members of the Hox-2 homeobox gene cluster have limits of expression at rhombomere boundaries. Three genes, Hox-2.6, -2.7 and -2.8 have progressively more anterior limits of expression at two-segment intervals, whereas expression of Hox-2.9 is restricted to one rhombomere, r4. The Hox-2 genes are expressed in spatially restricted patterns in early neural crest cells. These findings suggest that the Hox genes have roles in specifying the identity of rhombomeres and of neural crest.