The accessory olfactory bulb in the adult rat: a cytological study of its cell types, neuropil, neuronal modules, and interactions with the main olfactory system

The accessory olfactory bulb (AOB) in the adult rat is organized into external (ECL) and internal (ICL) cellular layers separated by the lateral olfactory tract (LOT). The most superficial layer, or vomeronasal nerve layer, is composed of two fiber contingents that distribute in rostral and caudal halves. The second layer, or glomerular layer, is also divided by a conspicuous invagination of the neuropil of the ECL at the junction of the rostral and caudal halves. The ECL contains eight cell types distributed in three areas: a subglomerular area containing juxtaglomerular and superficial short-axon neurons, an intermediate area harboring large principal cells (LPC), or mitral and tufted cells, and a deep area containing dwarf, external granule, polygonal, and round projecting cells. The ICL contains two neuron types: internal granule (IGC) and main accessory cells (MACs). The dendrites and axons of LPCs in the two AOB halves are organized symmetrically with respect to an anatomical plane called linea alba. The LPC axon collaterals may recruit adjacent intrinsic, possibly gamma-aminobutyric acid (GABA)-ergic, neurons that, in turn, interact with the dendrites of the adjacent LPCs. These modules may underlie the process of decoding pheromonal clues. The most rostral ICL contains another neuron group termed interstitial neurons of the bulbi (INBs) that includes both intrinsic and projecting neurons. MACs and INBs share inputs from fiber efferents arising in the main olfactory bulb (MOB) and AOB and send axons to IGCs. Because IGCs are a well-known source of modulatory inputs to LPCs, both MACs and INBs represent a site of convergence of the MOB with the AOB.