ABSTRACT
An important characteristic of MS lesions, as seen by various myelin sheath stains, is their very well-defined edge, described as looking as if they have been ‘cut out with a cookie cutter’ (Figures 12-14). This clearly differentiates the lesions of MS from those of acute disseminated encephalomyelitis. Inflammatory reactions, consisting mostly of lymphocytes and edema, are usually noted around the small blood vessels, especially in acute cases. Variable amounts of fat-staining lipid material, the socalled myelin abbau, can be observed in macrophages along with myelin fragments (Figures 15 and 16). The intensity of the fat staining is a good index of the age of the lesion: younger lesions contain greater amounts of lipid. Large abnormal astrocytes, termed ‘gemistocytic astrocytes’, are often seen at the sites of lesions (Figure 17) and may form large masses mimicking astrocytic tumors, leading to misinterpretation on biopsy. Classical MS lesions are periaxile in that the axon appears unaffected even when the myelin sheath has completely disintegrated. In older, more severe lesions, however, the axon is atrophic or has disappeared entirely. Necrosis may be seen, especially in acute lesions. Although axonal, and even neuronal, involvement was known to occur in MS for 100 years, the extent and significance of the damage were seriously underestimated until recently. It is now believed that the destruction of axons accounts for most clinical disability.
