Projection of septal organ receptor neurons to the main olfactory bulb in rats.

The septal organ of Masera is a distinct patch of olfactory epithelium on the septal wall of the nasal cavity. It lies ventral to the main olfactory receptor sheet. Central projections of the septal organ to the main and/or accessory olfactory bulb were studied with punctate HRP injections into the posterior dorsomedial olfactory bulb of rat pups. Injection sites encompassed 2-8% of the main olfactory bulb glomeruli and some injection sites were confined to the accessory olfactory bulb glomeruli. The distribution of receptor neurons in the septal organ that were labeled as a result of retrograde transport from the injection sites was determined. Olfactory receptor neurons were labeled in septal organs of those animals with injection sites confined to the main olfactory bulb. Subsequent to the posterior dorsomedial bulbar injections of this study, labeled receptor neurons were preferentially located in the posterior portion of the septal organ and were clustered about a plane dorsal to the plane bisecting the organ. Injections confined to the accessory olfactory bulb only labeled vomeronasal organ receptor neurons. Thus it is likely that the main olfactory bulb and not the accessory olfactory bulb is the bulbar terminus of septal organ receptor neurons. To date, experimental evidence suggests that the septal organ system may comprise an anatomically and functionally unique chemosensory pathway for odor detection.