Organisation and chemical neuroanatomy of the African elephant (Loxodonta africana) olfactory bulb.

The olfactory system of mammals can be divided into a main and accessory olfactory system with initial processing for each system occurring in the olfactory bulb. The main and accessory olfactory bulbs have similar structural features, even though they appear to be functionally independent. In mammals the main olfactory bulb (MOB) is also one of two established sites of lifelong generation of new cells. The present study describes the histological and immunohistochemical neuroanatomy of the olfactory bulb of the African elephant (Loxodonta africana). The morphology of MOB of the elephant does not differ significantly from that described in other mammals; however, it lacks the internal plexiform layer. In addition, the glomeruli of the glomerular layer are organised in 2-4 "honey-combed" layers, a feature not commonly observed. The cell types and structures revealed with immunohistochemical stains (parvalbumin, calbindin, calretinin, tyrosine hydroxylase, orexin-A, glial fibrillary acidic protein) were similar to other mammals. Neurogenesis was examined using the neurogenic marker doublecortin. Migration of newly generated cells was observed in most layers of the MOB. No accessory olfactory bulb (AOB) was observed. Based on the general anatomy and the immunohistochemical observations, it is evident that the morphology of the African elephant MOB is, for the most part, similar to that of all mammals, although very large in absolute size.