Behaviour of macroglial cells, as identified by their intermediate filament complement, during optic nerve...

Assessment of glial cell behaviour during optic nerve (ON) regeneration in Xenopus tadpoles is hampered by the lack of classical cellular markers that distinguish different glial cells in mammals. We thus have characterized the intermediate filament (IF) complement of tadpole glial cells and used it to follow the fate of glial cell subsets during the first 10 days after ON crush. Glial cells synthesize a restricted number of cytokeratin (CK) species and vimentin. This pattern remains essentially unchanged during metamorphosis and regeneration. However, vimentin turnover is specifically enhanced after injury. The expression of CKs and vimentin has been followed immunocytochemically in situ and in isolated cells recovered from dissociated ON segments. In the normal nerve, 79% of ramified glial cells express both CK and vimentin, 1% CK and 4% vimentin only, whereas 16% express neither IF protein. We tentatively classified CK expressing cells as mature astrocytes and those without IF proteins as oligodendrocytes. In the regenerating ON, the relative number of oligodendrocytes is decreased, while the astrocytic subset becomes accordingly larger but is decreased by day 10 already in favour of cells expressing vimentin only. Astrocytes invade the lesion site soon after crush, arrange into a central core within the distal nerve segment and establish a peripheral scaffold that is readily crossed by axons. Unlike mammalian astrocytes that remain absent from the lesion site but form a scar at some distance to it, amphibian astrocytes appear to provide active guidance to axons growing through the lesion site.