Specialized neuroglial arrangement may explain the capacity of vomeronasal axons to reinnervate central neurons.
The neurosensory cells of the primary olfactory and vomeronasal projections are in a state of continuous replacement throughout adult life. Since their axons form synaptic terminals with neurons in the olfactory and accessory olfactory bulbs, this system is an apparent exception to the rule that peripheral axons cannot grow into the central nervous system of adult mammals. Electron microscopy of sections (especially in a plane tangential to the surface of the accessory olfactory bulb) shows a unique glial arrangement. By virtue of their greater electron density and "secretory-type" organelle content (Golgi apparatus and dense-core vesicles) the glial cells of the superficial layers of the accessory olfactory bulb are distinguished both from the glia of the vomeronasal nerves and from the astrocytes of the deeper bulbar layers. The synapses between the vomeronasal axons and the postsynaptic elements are formed in glomeruli which are encapsulated by an inner layer of glial cytoplasm derived from the superficial glia, and an outer layer derived from the astrocytes. The principle of the organization is that the superficial glial processes are reflected off the axons before they reach the synaptic terminal zone. Conversely, for the postsynaptic elements, the astrocytic processes are reflected off the dendrites of the accessory olfactory bulb neurons before they enter the core of the glomeruli. In effect, the synapses are formed in a "no-man's-land" between the two glial cell types. This peculiar glial arrangement may be important for the unique regenerative capacity of this system.
D000818:Animals; D003391:Cranial Nerves / Q000166:cytology* / Q000502:physiology; D005260:Female; D008854:Microscopy, Electron; D009296:Nasal Cavity; D009416:Nerve Regeneration; D009457:Neuroglia / Q000166:cytology* / Q000502:physiology; D009830:Olfactory Bulb / Q000166:cytology*; D051381:Rats; D011919:Rats, Inbred Strains; D012679:Sense Organs / Q000294:innervation*