Olfactory System, Turnover and Regeneration
The olfactory sensory epithelium lines portions of the nasal cavity, and in the adult animal three cell types, which are organized in columnar units, contribute to its formation: neurons, basal cells, and supporting cells (Fig. 1). The axons of the olfactory sensory neurons collect in fascicles of increasing diameter in the lamina propria mucosae from where they run centripetally toward the olfactory bulb. In their course they either assemble in a unique olfactory nerve (the olfactory nerve of lower vertebrates) or maintain their arrangement in several discrete bundles (the fila olfactoria of mammals) that penetrate into the cranial cavity through the lamina cribrosa. The olfactory axons form an intricate nerve plexus on the surface of the homolateral olfactory bulb before terminating in globose structures, the olfactory glomeruli, where they synapse with the dendrites of large cortical neurons, the mitral cells. The olfactory sensory pathway is, consequently, the only one that directly extends from the periphery to a cortical area without entering a commissure and without a synaptic interruption.