Positional cues that are strictly localized in the telencephalon induce preferential growth of mitral cell axons.

In mice, mitral cells are the major efferent neurons of the main olfactory bulb and elongate axons into a very narrow part of the telencephalon to form a fiber bundle referred to as the lateral olfactory tract (LOT). To clarify the mechanisms responsible for guidance of mitral cell axons along this particular pathway, we co-cultured mouse embryo main olfactory bulbs with the telencephalons, and analyzed the pathways taken by mitral cell axons. Ingrowth of mitral cell axons into the telencephalon was observed in those co-cultures in which the olfactory bulbs had been exactly combined to their normal pathway (the LOT position) of the telencephalon. The axons grew preferentially along the LOT position, and formed a LOT-like fiber bundle. When the olfactory bulbs were grafted at positions apart from their normal pathway, however, no mitral cell axons grew into the telencephalon. Neocortical fragments combined with the telencephalon projected fibers into the telencephalon in random directions. These results suggest that the LOT position of the telencephalon offers a guiding pathway for mitral cell axons and that guiding cues for mitral cell axons are extremely localized.