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The nervus terminalis (NT) is the most anterior of the vertebrate cranial nerves. In teleost fish, the NT runs across all olfactory components and shows high morphological variability within this taxon. We compare the anatomical distribution, average number and size of the FMRFamide-immunoreactive (ir) NT cells of fourteen teleost species with different positions of olfactory bulbs (OBs) with respect to the ventral telencephalic area. Based on the topology of the OBs, three different neuroanatomical organizations of the telencephalon can be defined, viz., fish having sessile (Type I), pseudosessile (short stalked; Type II) or stalked (Type III) OBs. Type III topology of OBs appears to be a feature associated with more basal species, whereas Types I and II occur in derived and in basal species. The displacement of the OBs is positively correlated with the peripheral distribution of the FMRFamide-ir NT cells. The number of cells is negatively correlated with the size of the cells. A dependence analysis related to the type of OB topology revealed a positive relationship with the number of cells and with the size of the cells, with Type I and II topologies of OBs showing significantly fewer cells and larger cells than Type III. A dendrogram based on similarities obtained by taking into account all variables under study, i.e., the number and size of the FMRFamide-ir NT cells and the topology of OBs, does not agree with the phylogenetic relationships amongst species, suggesting that divergent or convergent evolutionary phenomena produced the olfactory components studied. The nervus terminalis (NT) is the most anterior of the vertebrate cranial nerves. In teleost fish, the NT runs across all olfactory components and shows high morphological variability within this taxon. We compare the anatomical distribution, average number and size of the FMRFamide-immunoreactive (ir) NT cells of fourteen teleost species with different positions of olfactory bulbs (OBs) with respect to the ventral telencephalic area. Based on the topology of the OBs, three different neuroanatomical organizations of the telencephalon can be defined, viz., fish having sessile (Type I), pseudosessile (short stalked; Type II) or stalked (Type III) OBs. Type III topology of OBs appears to be a feature associated with more basal species, whereas Types I and II occur in derived and in basal species. The displacement of the OBs is positively correlated with the peripheral distribution of the FMRFamide-ir NT cells. The number of cells is negatively correlated with the size of the cells. A dependence analysis related to the type of OB topology revealed a positive relationship with the number of cells and with the size of the cells, with Type I and II topologies of OBs showing significantly fewer cells and larger cells than Type III. A dendrogram based on similarities obtained by taking into account all variables under study, i.e., the number and size of the FMRFamide-ir NT cells and the topology of OBs, does not agree with the phylogenetic relationships amongst species, suggesting that divergent or convergent evolutionary phenomena produced the olfactory components studied.

The extrabulbar olfactory projections (EBOP) is a collection of nerve fibers that originate from primary olfactory receptor neurons. These fibers penetrate into the brain, bypassing the olfactory bulbs (OBs). While the presence of an EBOP has been well established in teleosts, here we morphologically characterize the EBOP structure in four species each with a different morphological relationship of OB with the ventral telencephalic area. Tract-tracing methods (carbocyanine DiI/DIA and biocytin) were used. FMRFamide immunoreactive nervus terminalis (NT) components were also visualized to define any neuroanatomical relationship between the NT and EBOP. Unilateral DiI/DiA application to the olfactory chamber stained the entire olfactory epithelium, olfactory nerve fibers, and ipsilateral olfactory bulb. Labeled primary olfactory fibers running ventromedially as extrabulbar primary olfactory projections reached various regions of the secondary prosencephalon. Only in Moenkhausia sanctaefilomenae (no olfactory peduncle) did lipophilic tracer-labeled fibers reach the ipsilateral mesencephalon. The combination of tracing techniques and FMRFamide immunohistochemistry revealed a substantial overlap of the label along the olfactory pathways as well as in the anterior secondary prosencephalon. However, FMRFamide immunoreactivity was never colocalized in the same cellular or fiber component as visualized using tracer molecules. Our results showed a certain uniformity in the neuroanatomy and extension of EBOP in all four species, independent of the pedunculate feature of the OBs. The present study also provided additional evidence to support the view that EBOP and FMRFamide immunoreactive components of the NT are separate anatomical entities. Microsc. Res. Tech. 78:268-276, 2015.

The extrabulbar olfactory projections (EBOP) is a collection of nerve fibers that originate from primary olfactory receptor neurons. These fibers penetrate into the brain, bypassing the olfactory bulbs (OBs). While the presence of an EBOP has been well established in teleosts, here we morphologically characterize the EBOP structure in four species each with a different morphological relationship of OB with the ventral telencephalic area. Tract-tracing methods (carbocyanine DiI/DIA and biocytin) were used. FMRFamide immunoreactive nervus terminalis (NT) components were also visualized to define any neuroanatomical relationship between the NT and EBOP. Unilateral DiI/DiA application to the olfactory chamber stained the entire olfactory epithelium, olfactory nerve fibers, and ipsilateral olfactory bulb. Labeled primary olfactory fibers running ventromedially as extrabulbar primary olfactory projections reached various regions of the secondary prosencephalon. Only in Moenkhausia sanctaefilomenae (no olfactory peduncle) did lipophilic tracer-labeled fibers reach the ipsilateral mesencephalon. The combination of tracing techniques and FMRFamide immunohistochemistry revealed a substantial overlap of the label along the olfactory pathways as well as in the anterior secondary prosencephalon. However, FMRFamide immunoreactivity was never colocalized in the same cellular or fiber component as visualized using tracer molecules. Our results showed a certain uniformity in the neuroanatomy and extension of EBOP in all four species, independent of the pedunculate feature of the OBs. The present study also provided additional evidence to support the view that EBOP and FMRFamide immunoreactive components of the NT are separate anatomical entities.