Histogenesis of demyelinating lesions in the spinal cord of guinea pigs with chronic relapsing experimental allergic encephalomyelitis.

Spinal cord lesions in Hartley guinea pigs with chronic relapsing experimental allergic encephalomyelitis (EAE) were studied by light, transmission and scanning electron microscopy at different stages of the formation of demyelinated plaques. In addition the inflammatory response in the meninges was studied in isolated pia mater preparations separated from the spinal cord surface. In initial chronic lesions in the spinal cord, inflammation was restricted to penetrating parenchymal veins of the spinal cord and meninges. With the formation of large demyelinated plaques in the spinal cord, massive fibrosis of the meninges with infiltration by inflammatory cells was noted in an area covering the surface of the lesion. In plaques which reach the spinal cord surface, inflammatory cells could be seen passing between the pia and the spinal cord substance. In chronic remyelinated lesions, adhesions between meningeal fibroblasts and the astroglial limiting membrane were seen. In addition a topographical correlation between the distribution of spinal cord veins and venules and demyelinated plaques was found. These observations indicate that spinal cord lesions in chronic relapsing EAE are initiated by perivenous inflammation in the parenchyma and the meninges. Plaque formation, especially in spinal cord surface lesions, is additionally enhanced by the entrapment of inflammatory cells in the fibrosed meninges. The exchange of macrophages through the glia-limiting membrane may be responsible for the more rapid debris removal in the spinal cord in comparison with brain lesions in chronic relapsing EAE.