Peripheral origin of olfactory nerve fibers by-passing the olfactory bulb in Xenopus laevis.

After DiI injections into the diencephalon of Xenopus, two types of retrogradely labelled cells were found in the nasal area: (i) receptor cells in the olfactory epithelium and (ii) a small cell group located between the main olfactory epithelium and the vomeronasal system. These results reveal an extensive extrabulbar olfactory projection of olfactory receptor cells. Fibers of these cells do not terminate in the olfactory bulb but innervate targets in the diencephalon directly. The other type of retrogradely labelled cells, apparently, are not part of any epithelium. They resemble similar cell groups which have previously been regarded as part of the nervus terminalis system in other vertebrates.