Nerve Growth in Embryonic Mice: The Events Regulating Axonal Overextension in the Olfactory Bulb

The olfactory system has become a popular model for studying neural regeneration and the underlying mechanisms for developing neural circuits. By utilising transgenic mice (S100β-DsRed and OMP-ZsGreen) we have the ability to visualise olfactory neurons (OMP-ZsGreen) and glial cells (S100β-DsRed). During development olfactory axons travel a considerable distance to the developing olfactory bulb. In this period overextension of axons can be seen. For proper development axons need to be guided to their target glomeruli and overextending axons need to be degraded. In E13.5 embryos the olfactory bulb houses S100β-DsRed positive cells that appear to play a role in the formation of the olfactory bulb. Before the glomeruli are formed these cells do not allow axons to protrude into the olfactory bulb. When overextending axons do enter the olfactory bulb they do so in locations where these cells are minimal. As development progresses the DsRed “barrier” cells are now located deeper in the olfactory bulb (E15.5), which coincides with the establishment of the glomerular layer. As overextending axons enter the olfactory bulb via the DsRed “barrier” gaps they are met by the radial glia filament scaffold. The axons travel caudally along the filament branches until they are eventually degraded. We also show evidence to indicate that after degradation, OMP-ZsGreen positive debris is taken up by the radial glia cells lining the ventricular lumen. These findings illustrate new events for the establishment of the olfactory bulb and show crucial cellular interactions for proper topographical development.