Cellular relationships of paranodal Marchi-positive bodies studied with monoclonal antibodies against partially degraded CNS myelin fragments

Abstract

Histochemical and electron microscopical studies have shown that Marchi-positive bodies in the normal mammalian CNS are associated with paranodal regions and it has been proposed that the formation of Marchi-positive bodies represents a step in the catabolic events of normal myelin turnover. After a two-step density gradient ultracentrifugation a light 'floating fraction' highly enriched in these structures can be collected and in the present study two monoclonal antibodies, FC4 and 3B5, were produced against proteins present in the floating fraction but absent from the myelin fraction. Immunohistochemical studies showed that these antibodies bound preferentially to the PNS-CNS transitional region and the glia limitans. Double-staining experiments demonstrated an extensive overlap in these regions with cells stained by antibodies against the astrocyte marker glial fibrillary acidic protein. Centrally in the white matter, FC4 and 3B5 mainly stained cells which also stained with the microglia lectin marker Bandeiraea simplicifolia isolectin B4. Three-dimensional reconstructions made from confocal microscopic scans showed that FC4/3B5-positive cells in the white matter extend processes enveloping paranodal Marchi-positive bodies and nodes of Ranvier. It is suggested that astrocyte-like and microglia-like cells both are participants in paranodal myelin turnover and that a division of labour with respect to, for example, protein degradation and immunological functions, may be present between the two cell types.