Expression of glycoproteins in the vomeronasal organ reveals a novel spatiotemporal pattern of sensory neurone maturation.

The main olfactory and the accessory olfactory systems are both anatomically and functionally distinct chemosensory systems. The primary sensory neurones of the accessory olfactory system are sequestered in the vomeronasal organ (VNO), where they express pheromone receptors, which are unrelated to the odorant receptors expressed in the principal nasal cavity. We have identified a 240 kDa glycoprotein (VNO(240)) that is selectively expressed by sensory neurones in the VNO but not in the main olfactory neuroepithelium of mouse. VNO(240) is first expressed at embryonic day 20.5 by a small subpopulation of sensory neurones residing within the central region of the crescent-shaped VNO. Although VNO(240) was detected in neuronal perikarya at this age, it was not observed in the axons in the accessory olfactory bulb until postnatal day 3.5. This delayed appearance in the accessory olfactory bulb suggests that VNO(240) is involved in the functional maturation of VNO neurones rather than in axon growth and targeting to the bulb. During the first 2 postnatal weeks, the population of neurones expressing VNO(240) spread peripherally, and by adulthood all primary sensory neurones in the VNO appeared to be expressing this molecule. Similar patterns of expression were also observed for NOC-1, a previously characterized glycoform of the neural cell adhesion molecule NCAM. To date, differential expression of VNO-specific molecules has only been reported along the rostrocaudal axis or at different apical-basal levels in the neuroepithelium. This is the first demonstration of a centroperipheral wave of expression of molecules in the VNO. These results indicate that mechanisms controlling the molecular differentiation of VNO neurones must involve spatial cues organised, not only about orthogonal axes, but also about a centroperipheral axis. Moreover, expression about this centroperipheral axis also involves a temporal component because the subpopulation of neurones expressing VNO(240) and NOC-1 increases during postnatal maturation.