It is perhaps inconceivable that there is not a degree of signalling between neurones and oligodendrocytes, since their axons and myelin sheaths are so intimately apposed. Adenosine, glutamate and ATP released from electrically active axons evoke Ca2+ signals in OPCs and regulate their development and axonal myelination. Stimulation of myelinated axons evokes Ca2+ signals in myelinating oligodendrocytes; oligodendrocytes express NMDA and ACh receptors along their myelin sheaths and these may play a special role in axon-myelin signalling. There is no evidence of neurotransmitter-mediated signalling from oligodendro-cytes to axons (compare with Schwann cells in Chapter 8): oligodendrocytes do not appear to express the machinery for vesicular release of neurotransmitters, but they do express hemichannels and glutamate transporters. However, oligodendrocyte-derived signals are essential for axonal integrity and survival; these are likely to involve interactions between cell surface and extracellular molecules; for example, ablation of the oligodendroglial genes PLP or CNP results in axon degeneration, without the loss of oligodendrocytes or myelin.
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