Ultrastructural Temporal Profile of the Dying Neuron and Surrounding Astrocytes in the Ischemic Penumbra: Apoptosis or Necrosis?

We investigated the temporal profile of isolated dying neurons (disseminated selective neuronal necrosis: DSNN) and the behaviors of astrocyte surrounding these dying neurons, in the ischemic penumbra of the cerebral cortex. In the ischemic penumbra, DSNN progressed slowly until 3 weeks after the ischemic insult. Cell bodies, cell processes, and end-feet of living astrocytes became swollen, with an increase in the number and in the volume of the mitochondria and accumulation of glycogen granules. The DSNN started 15 min after the ischemic insult, and progressed with increasing numbers of dark neurons having various degrees of electron density during 5 to 24 h. The isolated dark neurons showed homogeneous condensation of their cytosol, organelles, and nucleus, in which small loosely aggregated chromatin condensates were observed in the nuclear matrix and along the margin of the nuclear membrane. These chromatin condensations were positive for TUNEL staining. The swollen astrocytic cell processes surrounded the dark neurons. Astrocytic swelling was most prominent near the dendritic synapses. Finally, the isolated dark neurons became completely shrunken with very high electron density of the entire cell containing degenerated mitochondria having swollen matrices with occasional woolly densities. The shrunken neuron was fragmented into electron-dense debris by invading astrocytic cell processes. Some of the debris was phagocytized by astrocytes, and others moved into the extracellular space and were phagocytized by the perivascular microglia. Macrophages and other inflammatory cell were not observed in the penumbra. The ultrastructural characteristics of DSNN, in the present study, suggested necrotic neuronal death instead of apoptosis. Condensation of the isolated neuron was induced by swelling of astrocytic cell processes surrounding the dark neuron.