Bridging cells: Potential crosstalk between the peripheral and central olfactory sensory system

The olfactory sensory system in unique is that the first synaptic contact lies within the central nervous system meaning that the same olfactory sensory neuron whose dendrites are in contact with outside world, has axons extending directly into the central nervous system. Furthermore, neurons of both the peripheral olfactory organ and central olfactory bulb undergo neurogenesis throughout life, although it is unknown whether this is a coordinated process. Here we describe a group of cells in the adult olfactory epithelia that are neuron-like in appearance, yet express the oligodendrocyte precursor marker Olig2:EGFP. These cells extend processes that bypass the olfactory bulbs to arrive at the anterior telencephalic ventricle, the site of precursors that produce interneurons of the olfactory bulbs via the rostral migratory stream. Characterization in sections from intact, wholemount adult brains revealed Sox10 positive cells in patterns consistent with potential migration between the olfactory epithelia and the olfactory bulbs. Based on the expression of Sox10 protein, these cells are assumed to be olfactory ensheathing cells that populate the peripheral and central olfactory system. These cells are a unique type of "hybrid" glia expressing markers associated with astrocytes, oligodendrocytes, and microglia, as well as with progenitor characteristics. Thus, the Olig2:EGFP positive cells link the peripheral olfactory epithelia with the anterior telencephalic vesicle, the source of rostral migratory stream progenitors. These cells provide a potential link to the Sox10+ olfactory ensheathing cells in the adult brain shown here, thus supporting previous findings showing that olfactory ensheathing cells can exert an attractive influence on neuronal migration within the rostral migratory stream.