Morphological evidence for two types of Mammalian vomeronasal system.

The vomeronasal (VN) systems of rodents and opossums are of the segregated type, i.e alpha-subtype G protein Gi2- or Go-expressing VN neurons, which are sensory cells, project discretely to the rostral or caudal region of the accessory olfactory bulb (AOB). Although this zone-specific projection is believed to be a common feature for processing pheromones in mammals, we previously found a uniform-type VN system in goat in which only Gi2-expressing VN axons terminate at the AOB. In most mammals, it remains unclear whether their VN systems are of the segregated or uniform type. Therefore, we investigated morphologically the VN systems of different mammalian species (dog, horse, musk shrew and common marmoset). Consequently, all VN axons of the examined animals were positively stained with immunohistochemistry for Gi2 in the same way as that in the goat. On the other hand, we observed immunoreactivities against Go in the olfactory axons, but not in the VN axons. These results suggest that many mammals have uniform-type VN systems, and at least two types of VN systems exist in terrestrial mammals. This morphological evidence will help us determine the processing function of VN systems.