GENETIC-CONTROL OF SEGMENTATION OF AXIAL STRUCTURES IN VERTEBRATES

The processes of segmentation of axial structures in vertebrates durin g early embryonic development are reviewed. These processes include th e formation of neuromeres, somitomeres, cranial ganglia, and branchial arches in the head and of neuromeres, somites, spinal ganglia, and mo tor nerves in the body of the embryo. The class of vertebrate homeobox genes Hox is described with respect to the arrangement of these genes in four clusters, the structural and functional similarity of paralog ues in gene subfamilies, and the type of Hox gene expression in the he ad and body. A hypothesis concerning the existence of a genetic Hox co de, determining the fate of individual segments in neuroectodermal and mesenchymal derivatives, is discussed. In the context of this hypothe sis, phenotypic expression of the mutant Hox genes, accompanied by the loss of their function and cases of excessive and ectopic expression of Hox genes, are analyzed. Only in such cases do mutant phenotypes de monstrate symptoms of actual homeotic transformation, in which specifi c segmental structures are substituted by their homologues, as has bee n described for homeotic mutations in Drosophila.