ELECTROLYTE-INDUCED DEMYELINATION IN RATS .2. ULTRASTRUCTURAL EVOLUTION

This study presents the electron microscopic evolution of lesions in e lectrolyte-induced demyelination (EID) in rats, a lesion which bears s triking histological and clinical similarity to central pontine myelin olysis. The earliest change was observed during the hyponatremic phase and consisted of minimal intracellular edema present throughout the b rain. Following the injection of hypertonic saline, additional changes were observed which were restricted to sites previously reported to b e frequently involved in EID. Early dilatation of the inner tongue of oligodendrocyte cytoplasm in myelinated nerve fibers was observed at 3 h post hypertonic saline injection (PHS). This was followed, at 48h PH S, by the appearance of degenerative changes consistent with dying oli godendrocytes. Well-delineated, vacuolar and spongy lesions, seen by l ight microscopy, were present by 48h PHS at the same sites as above. E lectron microscopically, this appearance was found to be due to striki ng intramyelinic edema. By 96h PHS, macrophages containing myelin and other cellular debris were frequently present at these sites. Concomit antly, myelin sheaths underwent vesicular disruption and disintegratio n. This sequence of events suggests a lesion of the oligodendrocyte-my elin complex, secondary to initial blood-brain barrier damage and edem a.