Figure 1.

Hypothetical mechanism for the assembly of the precise topological map of glomeruli. A gradient of molecular affinities of olfactory receptors. Approximately, 1,000 molecularly distinct glomeruli are arranged in a topologically precise map in the olfactory bulb. This map is bilaterally symmetrical, but only one side is illustrated here. There are four distinct zones of glomeruli in the bulb (47–50), illustrated here in various shades of red, yellow, green, and blue. Gradients of colors on glomeruli within each zone are used to suggest an orderly gradient of molecular affinities of the individual receptors. A stream of migrating neurons originates in a specific fate-mapped region of the subventricular zone (22). Cells migrate as streams with the growth cones of each contacting the cell ahead (21). Colors and gradients are used again to suggest that receptors on each cell differ in an orderly way so that neighboring cells have receptors that bind with the highest affinity to each other. After reaching the olfactory bulb, cells change their direction of migration and move toward the surface of the bulb where they generate periglomerular cells (22). The dendrites of these cells then form the targets for incoming growth cones of olfactory nerve axons. Hundreds of olfactory neurons bearing the same, specific, olfactory receptor converge on a single pair of bilaterally symmetrical glomeruli (10–12). Their growth cones synapse with the dendrites of the periglomerular cells presumed to express the identical receptor. These homophilic interactions occur with the highest affinity. According to this hypothesis, receptors on neighboring glomeruli have closely related but different structures, hence are bound with a slightly lower affinity. Mitral/tufted cells also synapse with glomeruli but are not illustrated here.