Cellular reaction to an acute demyelinating/remyelinating lesion of the rat brain stem: localisation of GD3 ganglioside immunoreactivity.

We describe a simple and reproducible acute demyelinating lesion of the rat brain stem induced by injection of ethidium bromide into the cisterna magna of young adult rats. Using immunofluorescence with a panel of antibodies to cell-specific antigens we have studied the changes in cell populations that occur at various stages during lesion progression and repair. In particular we localized the expression of ganglioside GD3 immunoreactivity, a marker for oligodendroglial progenitors in developing brain. Both astroglia (GFAP+) and oligodendroglia (CNP+) were destroyed during the early response to the ethidium bromide although axons were spared. Splitting of myelin lamellae occurred as early as 4 days post-injection (DPI), with extensive demyelination of the inferior cerebellar peduncle following by 6 DPI. Large numbers of ED1+ and OX-42+ macrophages were present in the lesion site at this stage. Astrogliosis occurred around the perimeter of the lesions. Two populations of GD3+ cells appeared within and around the lesion sites during the demyelination. One population was identified by the phenotype GD3+ ED1+ and thus probably belonged to the macrophage/microglial lineage. In these cells both antigens appeared cytoplasmic. The second population of GD3+ cells exhibited cell membrane GD3 immunoreactivity but did not express the ED1 antigen. These cells are suggested to be oligodendroglial progenitors generated in response to the demyelination. No such cells were seen in control tissue. GD3+ cells were present within the lesion sites from 6 DPI until 10-12 DPI. Following the clearance of myelin debris from the lesions, remyelination was a relatively rapid event with thin MBP+ myelin sheaths first seen at 11-12 DPI. Remyelination, which was extensive by 25 DPI, was predominantly oligodendroglial in origin (MBP+P0- myelin) with only small pockets of peripheral myelin (MBP+P0+ myelin) observed. The present study, in addition to identifying putative glial progenitors within a demyelinated lesion, also demonstrates the difficulties in unambiguously identifying such cells in the normal and damaged adult CNS.