PRIMARY DEMYELINATION AND REGENERATION OF ASCENDING AXONS IN THE DORSAL FUNICULUS OF THE RAT SPINAL-CORD FOLLOWING PHOTOCHEMICALLY INDUCED INJURY

The extent of primary demyelination and regeneration of ascending axon s in the dorsal funiculus of the rat spinal cord was investigated foll owing photochemically-induced ischaemic injury. Groups of rats were ki lled at intervals from 48 h to 1 month after injury and a combination of Light and electron microscopy and counting of axons in specific sit es was used to study the axonal changes. Unmyelinated axons were noted in the dorsal rim of the lesion at its centre and at the centre of th e gracile fasciculus at the caudal end of the lesion 7 days after inju ry. By 1 month, axons in these sites were thinly myelinated by Schwann cells or oligodendrocytes. In order to differentiate between remyelin ation of demyelinated axons and myelination of regenerated axons, axon counts were performed. The number of sub-pial axons present at the le sion centre did not change significantly from 48 h to 1 month after in jury, whereas the number of axons at the caudal end of the lesion incr eased significantly from 4 to 10 days after injury. We therefore concl ude that sub-pial axons at the lesion centre are demyelinated between 4 and 7 days after injury and subsequently remyelinated by Schwann cel ls. At the caudal end of the lesion, a specific population of small di ameter axons located at the centre of the gracile fasciculus regenerat es for a distance of approximately 1 mm between 4 and 10 days after in jury; these axons are then myelinated by oligodendrocytes or Schwann c ells. In contrast, larger diameter axons of the gracile fasciculus do not show a regenerative response, demonstrating the variability of axo nal responses to injury.