OLFACTORY GLOMERULI IN THE ZEBRAFISH FORM AN INVARIANT PATTERN AND ARE IDENTIFIABLE ACROSS ANIMALS

Glomeruli are anatomical and possibly functional modules in the verteb rate olfactory bulb. We investigated the spatial arrangement of glomer uli in the olfactory bulbs of adult zebrafish (Brachydanio rerio). A s olution of the lipophilic tracer Dil was injected into the nasal cavit ies. Axons of sensory neurons projecting from the olfactory epithelium into the bulb were traced anterogradely, thus labeling the whole popu lation of glomeruli. The glomerular distribution was analyzed in detai l by confocal laser-scanning microscopy. We find that a typical olfact ory bulb contains a small number of about 80 glomeruli that have a ste reotyped configuration in all animals investigated. All glomeruli exhi bit bilateral symmetry. Twenty-two single glomeruli could be identifie d from animal to animal by their characteristic position and morpholog y. The remaining glomeruli either are embedded in glomerular plexus an d therefore cannot be delineated reliably, or belong to a densely clus tered subpopulation of on average 49 glomeruli in the dorsal olfactory bulb. No sexually dimorphic glomeruli were identified. To test whethe r glomerular constancy is specific for the zebrafish, we performed sim ilar tracing experiments in the goldfish and found several indications for a similar invariance of glomeruli in this species. The remarkable stereotypy of this pattern is reminiscent of the insect olfactory sys tem and has been demonstrated here for the first time in a vertebrate. It will now be possible to examine whether these identifiable glomeru li are functionally specialized in terms of odor processing. If so, ze brafish may emerge as a tractable model system for studies on olfactor y coding.