Olfactory ensheathing glia and Schwann cells: two of a kind?

Both olfactory ensheathing cells (OECs) and Schwann cells have been shown to promote axonal regrowth and remyelination in the central (CNS) and peripheral nervous systems under experimental conditions. During development, OECs and Schwann cells emerge from the olfactory placode and the neural crest, respectively, thus sharing a common "peripheral" origin. Both cell types are known to express a number of different molecular markers in common, to display a similar morphological phenotype in culture and to respond to the same growth-promoting molecules. Contrary to Schwann cells, OECs are found in association with neuronal processes in the CNS constituting the olfactory nerve layer of the olfactory bulb. OECs maintain their growth-promoting capacity in the CNS during adult life supporting the lifelong axonal growth of olfactory receptor neurons. Thus, OECs are considered an intermediate cell type combining a "central" location with "peripheral" permissiveness. Recently, the regenerative potential of OECs has been demonstrated in a variety of studies. However, OECs are still less clearly defined than Schwann cells. On designing future therapeutical strategies for nerve injury and disease, the important question arises as to whether OECs and Schwann cells are comparable cell types or whether they, indeed, mediate specific effects, making either the one or the other suitable for special applications. The present review summarizes recent data on the in vitro and in vivo properties of OECs and critically compares the analogies and differences in the biology of both cell types relevant in the above-mentioned context.