Organization of the septal organ projection to the main olfactory bulb in adult and newborn rats.

The septal organ, which is regarded as an olfactory subsystem, is a small patch of sensory epithelium located ventral to the main olfactory sheet on the septal wall of the nasal cavity. The only consensus to date regarding some proper area of projection of this subsystem is that the septal organ projects to the medial aspect of the main olfactory bulb. The purpose of our study was to analyze precisely the topographical organization of the bulbar projection of the septal organ in adult rats and in 3- to 15-day-old rats following WGA-HRP placements at the level of the septal epithelium. Results show that the septal organ projects exclusively to the posterior half of the main olfactory bulb and its projection area is mainly restricted to the ventromedial bulbar aspect. When the septal organ was fully injected, the pattern of bulbar projection was characterized by two types of glomerular labeling: 1) presence of single heavily labeled glomeruli identified as "septal" glomeruli, since they were mainly built up by afferents coming from the septal organ and (2) presence of a thin network of labeled septal fibers distributed in glomeruli which were mainly formed by afferents coming from the main olfactory epithelium. Although the pattern of mucosobulbar projection of the septal organ is already established in newborns, a significant increase in the number of "septal" glomeruli occurs during the first 15 postnatal days. Anatomical data indicate that even if the projection of the septal organ does not appear completely segregated in the olfactory bulb, this projection is not either exactly similar to that of the main olfactory epithelium.