Radial glia interact with primary olfactory axons to regulate development of the olfactory bulb

Abstract

The developing olfactory system - merging of the peripheral and central nervous systems: The olfactory system is responsible for the sense of smell and is comprised of a complex topographic map that regenerates throughout life. In rodents each olfactory sensory neuron expresses one of ~1,300 odorant receptors with the neurons being distributed mosaically within the epithelium. The axons of the sensory neurons do not maintain near-neighbour relationships and instead project to disparate topographic targets in the olfactory bulb within the central nervous system. The development of the targets relies on the intermingling of the sensory axons with the interneurons, glia and second order neurons of the olfactory bulb. Thus the formation of the olfactory system involves the coordinated integration of the axons of the peripheral olfactory sensory neurons with the cells of the olfactory bulb. While the final topographic map exhibits high precision of axon targeting, this is not the case during its development. In the embryonic and postnatal olfactory system many olfactory sensory axons make errors and mis-project into incorrect targets or over-project past the target layer and penetrate the deeper layers of the olfactory bulb ([Figure 1]; Graziadei et al., 1980; Amaya et al., 2015). These mis-targeted axon errors need to be corrected and the mis-targeted axons removed. The glia of the olfactory system, olfactory ensheathing cells, have been shown to remove the debris arising from degenerated olfactory axons ([Figure 1]; Su et al., 2013; Nazareth et al., 2015) along the nerve fascicles. More recently it has been shown that radial glia within the deeper layers of the olfactory bulb are the principal cells that phagocytose the debris arising from axons that over-project past their target layers (Amaya et al., 2015).