Concentration of myelin debris-like myelin basic protein-immunoreactive particles in the distal (anterior)-most part of the myelinated region in the normal rat optic nerve

In the myelinated region of the normal rat optic nerve, neuronal fibers are encircled by myelin sheaths except at nodes of Ranvier. In addition to these myelinated fibers, concentration of small particles was observed in the distal (anterior)-most part of the myelinated region. These particles were visualized by fluorescent immunohistochemistry using mouse monoclonal anti-human myelin basic protein (MBPh) antibody (clone SMI-99). Fluorescent double immunohistochemistry by using both the rat monoclonal anti-cow myelin basic protein (MBPc) antibody (clone 12) and the anti-MBPh antibody demonstrated that myelin basic protein immunoreactive-particles detected by the anti-MBPc antibody were almost completely overlapped with those immunostained by the anti-MBPh antibody. Since these antibodies have different target sites, these particles contained the real myelin basic protein. We hypothesized that the MBPh-immunoreactive particles were myelin debris-like structures in the normal rat optic nerve. Quantitative morphological analyses indicated that only 2 out of 6 differences in sizes and shape descriptors between the particles and myelin debris observed in the damaged-optic nerve were statistically significant. Glial fibrillary acidic protein-immunoreactivity and glutamine synthetase-immunoreactivity were seen in the particles. Majority of the particles were isolated from ionized calcium binding adapter molecule 1-labeled microglia. These results demonstrate that the myelin debris-like MBPh-immunoreactive particles are concentrated on the distal-most part of the myelinated region. This evidence suggests that the distal-most part is under mildly pathological condition. Furthermore, the evidence may provide clues as to the pathophysiological background that induces localized vulnerability of the myelin sheaths.