Correction of aberrant axon growth in the developing mouse olfactory bulb

During development of the primary olfactory system, sensory axons project from the nasal cavity to the glomerular layer of the olfactory bulb. In the process axons can branch inappropriately into several glomeruli and sometimes over-shoot the glomerular layer, entering the deeper external plexiform layer. However in the adult, axons are rarely observed within the external plexiform layer. While chemorepulsive cues are proposed to restrict axons to the glomerular layer in the embryonic animal, these cues are clearly insufficient for all axons in the postnatal animal. We hypothesised that the external plexiform layer is initially an environment in which axons are able to grow but becomes increasingly inhibitory to axon growth in later postnatal development. We have determined that rather than having short localised trajectories as previously assumed, many axons that enter the external plexiform layer had considerable trajectories and projected preferentially along the ventro-dorsal and rostro-caudal axes for up to 950 μm. With increasing age, fewer axons were detected within the external plexiform layer but axons continued to be present until P17. Thus the external plexiform layer is initially an environment in which axons can extensively grow. We next tested whether the external plexiform layer became increasingly inhibitory to axon growth by microdissecting various layers of the olfactory bulb and preparing protein extracts. When assayed using olfactory epithelium explants of the same embryonic age, primary olfactory axons became increasingly inhibited by extract prepared from the external plexiform layer of increasingly older animals. These results demonstrate that primary olfactory axons can initially grow extensively in the external plexiform layer, but that during postnatal development inhibitory cues are upregulated that reduce axon growth within the external plexiform layer.