Projections from the accessory olfactory organ into the medial region of the olfactory bulb in the sea lamprey (Petromyzon marinus): a novel vertebrate sensory structure?

Although four different primary olfactory pathways have been described in tetrapod vertebrates, polymorphic olfactory sensory neurons comingle in the olfactory epithelium and project axons into separate bulbar regions in teleost fish. However, spatially segregated neurons may exist in the peripheral olfactory organ of lampreys, extant representatives of ancestral jawless vertebrates. In lampreys, the caudoventral portion of the peripheral olfactory organ contains tubular diverticula, named the accessory olfactory organ (AOO). Short, ciliated AOO cells were retrogradely labelled following application of biocytin or carbocyanine dyes to the medial region of the olfactory bulb. Tracer application to eight radial locations within the layer of glomeruli with mitral cells, of the olfactory bulb, showed that AOO projections were restricted to the medial region of the olfactory bulb. The outer boundary of the AOO projection extended to the ventromedial region of glomerular neuropil in 43% of the specimens. The olfactory sensory neurons in the main olfactory epithelium projected to glomerular neuropil throughout the olfactory bulb, including sparse projections to the medial region of the olfactory bulb. This study shows that these AOO neurons and their projections in the medial region of the olfactory bulb are anatomically distinct regions of the primary olfactory pathway in the sea lamprey.