[Somatotopic relations between the olfactory mucosa and the olfactory bulb in the triton]

The axons of receptors located in the olfactory neuroepithelium are known to synapse with mitral cell dendrites in the glomerular layer of the ipsilateral olfactory bulb, but few previous studies showed a clear-cut topological organization of this projection. We therefore injected horseradish peroxidase (HRP) into restricted areas of the olfactory mucosa in adult tritons, Triturus cristatus. The animals were perfused with glutaraldehyde 17 h later; the mucosa, the olfactory nerve and bulb were removed, serially cut and treated with diaminobenzidine to reveal the localization and extent of the injection and the areas in the glomerular layer of the bulb to which the injected HRP had been transported by anterograde axonal flow. Small injections of HRP tended to label a small area in the bulb, while larger injections projected to a wider zone. The results also indicated the existence of an orderly projection, with a transposition of the dorso-ventral axis of the neuroreceptor sheet into an essentially anteroposteriorly oriented axis in the bulb. Thus, the dorsally located areas of the olfactory mucosa project to the most anterior part of the glomerular layer, whereas the vomero-nasal mucosa, which is located near the most lateral part of the ventral mucosa, projections to the posterior enlargement of the glomerular layer in the bulb. The data also showed that nerve bundles within the olfactory nerve maintain are orderly position which depends upon their origin. It is postulated that this somatopic organization reflects the development undergone by the peripheral olfactory system during ontogenesis.