MICROENVIRONMENTAL CHANGES DURING AXONAL REGROWTH IN THE OPTIC-NERVE OF THE MYELIN-DEFICIENT RAT - IMMUNOCYTOCHEMICAL AND ULTRASTRUCTURAL OBSERVATIONS

Lesion-induced regenerative sprouting of CNS axons is accompanied by r eactions of the supporting glia and vascular and connective tissue whi ch may influence the extent of regeneration. In a previous report, it was shown that after crush injury, the amyelinated optic nerve of the myelin deficient (md) mutant rat contains greater numbers of regrowing axons proximal to the site of crush than that of normally myelinated littermates. The present study was designed to compare the response of the microenvironment, i.e. glial cells and vascular and connective ti ssue, in md and normally myelinated optic nerves 2, 4 and 6 days after crush injury. In unoperated normal optic nerves monoclonal antibodies to the HNK-1 carbohydrate labelled astrocytic processes at the ultras tructural level whereas in unoperated md mutants HNK-1 staining was re stricted to axonal surfaces. Immunoreactivity with monoclonal antibodi es to stage-specific embryonic antigen-1 (SSEA-1) was confined to astr ocytic surfaces in both md and wildtype animals. After axotomy of md o ptic nerves regrowing axons were more numerous in the proximal site of the crush and extended further into the lesion than in wildtype anima ls. In both md and wildtype rats regrowing axons were HNK-1-positive. In md rats strong reaction with antibodies to laminin and fibronectin was only seen in 6-day-old lesions of md rats whereas immunoreactivity was less distinct in operated littermate controls. Immunolabelling wa s obviously associated with blood vessels, since crush lesions in both md and wildtype rats were Schwann cell-free as assessed by electron m icroscopy and immunocytochemistry. In both operated md and normal litt ermates crush lesions contained degenerating astrocytes as well as rea ctive astrocytes in which the intermediate filaments of the perikarya failed to stain immunocytochemically for GFAP, vimentin, desmin, and a common determinant of intermediate filaments. In contrast, reactive a strocytes in the lesion site of normally myelinated rats expressed the SSEA-1 antigen intracytoplasmically whereas in md mutants astrocytes were completely SSEA-1-negative. Infiltration of crush lesions by macr ophages was less extensive in md rats than in normal littermates. Howe ver the overall content of macrophages in the peritoneal cavity was al so reduced. The present study demonstrates that (1) md optic nerves la ck HNK-1-reactive astrocytes; (2) in the axotomized wildtype optic ner ve impaired axonal regrowth may be associated with distinct immuno-phe notypes of the supporting glial cells, i.e. SSEA-1-positive astrocytes ; (3) laminin and fibronectin seem not to be essential for improved ax onal regrowth in md rats.