Transformation of postischemic perineuronal glial cells. I. Electron microscopic studies

The effects of cerebral ischemia on perineuronal glia were studied in the rat model of transient four-vessel occlusion. Striatum containing irreversibly injured neurons and paramedian cerebral cortex containing reversibly injured neurons were prepared for electron microscopy at intervals of 3 min up to 24 h following ischemia. Perineuronal astrocytes showed cytoplasmic swelling and configurational changes in and pleomorphism of mitochondria similar to those described previously in parenchymal astrocytes in this model. Dark oligodendroglia showed only transient swelling of cisterns of Golgi apparatus and endoplasmic reticulum. However, medium-light oligodendrocytes significantly increased in size and accumulated microtubules and tubovesicular profiles in the cytoplasm. Reactive glia with features of both oligodendrocytes and astrocytes appeared at 15 min. A sharp drop in the number of perineuronal medium-light oligodendrocytes occurred at 3 h after ischemia and was accompanied by increased numbers of astrocytes and intermediate glia. Cortical glia showed similar changes that were milder and reversible. These studies suggest that certain perineuronal glia are transformed into reactive astrocytes in areas of ischemic neuronal necrosis, although current data are insufficient to determine if the transforming cells are astrocytes, light oligodendrocytes, or intermediate glia. Possible stimuli for these glial reactions include loss of or changes in neuronal trophic factors upon CNS glia or alterations in the interstitial fluid composition.