The number of cells expressing the myelin-supporting oligodendrocyte marker PLP-exon 3b remains unchanged in Wallerian degeneration.

Following spinal cord trauma there is controversy as to whether myelin-supporting oligodendrocytes at a distance from areas of spinal cord damage undergo apoptosis. To examine the response of oligodendrocytes to axon degeneration, we counted the number of oligodendrocytes and oligodendrocyte precursors in the dorsal funiculi during the course of Wallerian degeneration. Axons were disrupted at T13 and the number of labelled cells in the dorsal funiculi counted at T12, 4 days and 2, 4, and 8 weeks after injury using riboprobes to exon-3b of the PLP gene whose expression is considered to restricted to myelin-supporting oligodendrocytes, PDGFRalpha which is regarded as a marker of oligodendrocyte precursors, and MOG a marker previously used to identify myelin-supporting oligodendrocytes. We found that the number of PLP-exon-3b labelled cells remained constant during the course of Wallerian degeneration while the number of cells labelled with the riboprobes to PDGFRalpha and MOG increased. Significantly the number of MOG-positive cells was increased at times when the number of PDGFRalpha labelled cells was highest. The number of PDGFRalpha labelled cells decreased with time while the number PLP-exon-3b labelled cells remained constant. It is therefore possible that the apoptotic oligodendrocytes identified in previous studies could represent degenerating oligodendrocyte precursors or their progeny rather than degenerating myelin-supporting oligodendrocytes.