Olfactory projections in a chondrostean fish, Acipenser baeri: An experimental study

The presence of extrabulbar primary olfactory projections has been well est ablished in teleosts. In order to investigate the phylogeny of these projec tions and to compare their targets with those of the secondary olfactory pr ojections, the connections of the olfactory epithelium and olfactory bulb w ere studied by means of tract-tracing methods in a chondrostean, Acipenser baeri. Primary olfactory projections mainly extend to the glomerular layer of the olfactory bulb, but a significant number of extrabulbar efferent fib ers course to various telencephalic and diencephalic regions. Both extrabul bar primary and olfactory bulb projections course in diffuse pathways. Exte nsive overlap was observed between the targets of these extrabulbar primary olfactory fibers and those of the secondary efferent projections of the ol factory bulb, though the latter were more numerous and reached additional t argets. Tracer application to the olfactory bulb also revealed a number of bulbopetal neurons in the ipsilateral and contralateral telencephalic areas , as well as crossed interbulbar projections. The presence in a chondrostea n of important extrabulbar primary projections and their extensive overlap with secondary olfactory projections suggest that such projections are a de rived characteristic of bony fishes. (C) 2000 Wiley-Liss, Inc.