Correlated patterns of neuron differentiation and Hox gene expression in the hindbrain: A comparative analysis

Hindbrain neurons are organized into coherent subpopulations with character istic projection patterns and functions. Many of these serve vital function s that have been conserved throughout the vertebrate radiation, but diversi fication to modified or highly specialized functions has also occurred. The differentiation of identifiable neuron groups in specific spatial domains must involve the regional expression of determinants within the hindbrain n euroepithelium. The Hox genes are involved in longitudinal regionalization of the neural tube, and their expression patterns in the hindbrain are clos ely related to the rhombomeres which partition the hindbrain into morphogen etic units. Hox gene expression also exhibits conserved patterning as well as phylogenetic variation. One plausible mechanism that may have contribute d to evolutionary diversification in hindbrain neuron populations is theref ore the emergence of species-specific differences in Hox gene expression. T his article presents a comparative overview of the regional patterning of s elected Hox genes and hindbrain neuron populations in several embryological ly important species. Although tantalizing correlations exist, the relation ship between Hox genes and neuronal patterning is complex, and complicated by dynamic features in each. Much more comparative and developmental data m ust be obtained before the link between Hox gene expression and hindbrain n euron patterning can be elucidated satisfactorily in an evolutionary contex t. (C) 2001 Elsevier Science Inc.