Ultrastructural and immunohistochemical analysis of axonal regrowth and myelination in membranes which form over lesion sites in the rat visual system.

Glial-connective tissue membranes which form bridges over lesion cavities in the brachial and pretectal region of the rat visual system contain regenerated myelinated and unmyelinated axons. The lesions were made between 10 and 16 days postnatal--a time at which neonatal regeneration would not be expected. A detailed ultrastructural study of these membrane bridges has been undertaken in order to describe the cellular and extracellular conditions that are associated with the regeneration, myelination and continued survival of identified retinal and other axons. The lesion-induced membrane bridges possessed a limiting surface of fibroblasts and were composed of glial cells, macrophages, endothelial cells, pericytes and collagen. There was some variability in the ultrastructural appearance of the glial cells; the majority of criteria indicate that they were astrocytes. These astrocytes formed 'glia limitans'-like surfaces beneath the fibroblasts. They contained numerous filaments and extended fine, electron-dense cytoplasmic processes, often arranged into lamellated stacks. Basal lamina was present on the outer surfaces of the astrocytes. Astrocytic processes isolated clusters of myelinated and unmyelinated axons in lacunae which may have served as conduits for axonal elongation. This suggests a role for these astrocytes in the regeneration and maintenance process which appears to recapitulate events which occur during normal development. Interestingly, regrowing retinal axons were never found adjacent to astrocytic surfaces possessing a basal lamina. We did not detect evidence of Schwann cell invasion into the lesion. By ultrastructural criteria the myelin ensheathment which occurred on the larger axons in the membrane bridge was of central rather than peripheral type. The cytoplasmic domain external to the sheath was limited to a small tongue; no basal lamina invested the fibre; and the periodicity of the myelin was equivalent to that of other CNS structures. Similarly, the CNS character of the myelin was demonstrated by intense immunostaining of myelin sheaths for myelin basic protein and proteolipid [corrected] protein and lack of staining for the PNS component PO. The oligodendrocytes responsible for this myelination may either have extended cytoplasmic processes from the adjacent neuropil, or may have differentiated from precursor cells within the membrane bridge.