Ultrastructural organization of the accessory olfactory bulb of the lizard Podarcis hispanica

The ultrastructural organization of the accessory olfactory bulb (AOB) was studied in a squamate reptile, Podarcis hispanica. Five types of neuronal cell bodies were differentiated by their size, location, and ultrastructure. Dark and light small somata were observed in the glomerular layer and correlated with two possible subpopulations of periglomerular cells. Mitral cells, the biggest neurons in the AOB, were preferentially located in the mitral cell layer, but also observed in plexiform layers, and they could be classified into two different types mainly on the basis of the size of their somata. Finally, a fifth neuronal population, the granule cells, were observed in the deepest layers of the AOB. On the basis of their location and ultrastructure, five different types of synaptic contacts were also observed in the AOB. Vomeronasal nerve terminals made asymmetric synaptic contacts in the glomerular layer. Reciprocal synapses between mitral cell dendrites and granule cell gemmules were identified preferentially in the external plexiform layer, but also in the mitral cell and internal plexiform layers. Terminals forming symmetric synapses on mitral cells were also recognized in the external plexiform layer. In the deepest layers, two types of terminals established asymmetric and symmetric synaptic contacts, respectively, on granule cells. The basic organization of the AOB of lizards appears rather similar to that of the mammalian olfactory bulb, but some notable differences as to their neuronal composition were found.