Evidence for gradients of gene expression correlating with zonal topography of the olfactory sensory map

Signals regulating diversification of olfactory sensory neurons to express odorant receptors and other genes necessary for correct assembly of the olf actory sensory map persist in the olfactory epithelium of adult mouse. We h ave screened for genes with an expression pattern correlating with the topo graphy odorant receptor-expression zones. The Msx1 homeobox gene and a sema phorin receptor (Neuropilin-2) showed graded expression patterns in the olf actory epithelium. The gradients of Msx1 and Neuropilin-2 expression in bas al cells and neurons, respectively, correlated with expression of a retinoi c acid-synthesizing enzyme (RALDH2) in lamina propria. A BMP-type I recepto r (Alk6) showed a reverse gradient of expression in the supporting cells of the epithelium. Considering known functions of identified genes in cell sp ecification and axon guidance this suggests that zonal division of the olfa ctory sensory map is maintained, during continious neurogenesis, as a conse quence of topographic counter gradients of positional information.