Distribution of protein gene product 9.5-immunopositive and NADPH-diaphorase-positive neurons in the common marmoset (Callithrix jacchus) accessory olfactory bulb.

The principal center of the accessory olfactory system is the accessory olfactory bulb (AOB). In primates, simians are divided into two groups, New and Old World monkeys, and the AOB is present in only New World monkeys. The common marmoset (Callithrix jacchus) is a species of New World monkey. Although the morphology of the common marmoset AOB has been demonstrated, the distribution patterns of the mitral/tufted and granule cells of the AOB remain unclear. In the present study, therefore, the distribution of the mitral/tufted and granule cells in the common marmoset AOB was examined using two histochemical markers including immuno-staining for protein gene product (PGP) 9.5 and NADPH-diaphorase staining. The vomeronasal nerves, gomeruli and mitral/tufted cells showed PGP 9.5-immunoreactivity. The mitral/tufted cells were arranged in only one or two rows along the margin of the glomerular layer to form the mitral/tufted cell layer (MTL). Since the mitral/tufted cells occurred sparsely in the common marmoset, the MTL was illegible. NADPH-diaphorase reactivity was primarily detected in the rostral and caudal areas of the AOB. In these areas, granule cells showed NADPH-diaphorase reactivity. Since the granule cells were sparse, the common marmoset AOB displayed less-developed granule cell layer. Although the functional significance of the AOB remains to be solved in the common marmoset, small-sized and less-laminated AOB may show that sexual behavior of the common marmoset has lesser dependence on the accessory olfactory system.