Structure and diversity in mammalian accessory olfactory bulb

The accessory olfactory bulb (AOB) is the first neural integrative center f or the olfactory-like vomeronasal sensory system. In this article, we first briefly present an overview of vomeronasal system organization and review the history of the discovery of mammalian AOB. Next, we briefly review the evolution of the vomeronasal system in vertebrates, in particular the repti les. Following these introductory aspects, the structure of the rodent AOB, as typical of the well-developed mammalian AOB, is presented, detailing la minar organization and cell types as well as aspects of the homology with t he main olfactory bulb. Then, the evolutionary origin and diversity of the AOB in mammalian orders and species is discussed, describing structural, ph ylogenetic, and species-specific variation in the AOB location, shape, and size and morphologic differentiation and development. The AOB is believed t o be absent in fishes but present in terrestrial tetrapods including amphib ians; among the reptiles AOB is absent in crocodiles, present in turtles, s nakes, and some lizards where it may be as large or larger than the main bu lb. The AOB is absent in bird and in the aquatic mammals (whales, porpoises , manatees). Among other mammals, AOB is present in the monotremes and mars upials, edentates, and in the majority of the placental mammals like carniv ores, herbivores, as well as rodents and lagomorphs. Most bat species do no t have an AOB and among those where one is found, it shows marked variation in size and morphologic development. Among insectivores and primates, AOB shows marked variation in occurrence, size, and morphologic development. It is small in shrews and moles, large in hedgehogs and prosimians; AOB conti nues to persist in New World monkeys but is not found in the adults of the higher primates such as the Old World monkeys, apes, and humans. In many sp ecies where AOB is absent in the adult, it often develops in the embryo and fetus but regresses in later stages of development. Finally, new areas in vomeronasal system research such as the diversity of receptor molecules and the regional variation in receptor neuron type as well as in the output ne urons of the AOB and their projection pathways are briefly discussed. In vi ew of the pronounced diversity of size, morphologic differentiation, and ph ylogenetic development, the need to explore new functions for the vomeronas al system in areas other than sexual and reproductive behaviors is emphasiz ed. (C) 1998 Wiley-Liss, Inc.