Ultrastructural Characterization of Prolonged Normothermic and Cold Cerebral Ischemia in the Adult Rat

The ultrastructural effects of prolonged normothermic and cold ischemia on the cerebral cortex of the adult rat were investigated. Complete cerebral ischemia was produced by cardiac arrest and the animals' temperature was maintained at 37°C for periods ranging from 0 to 81 hours before electron microscopy preparation. Electron micrographs of cold cerebral ischemia were generated after stabilizing the rat's temperature at 0°C after cardiac arrest for periods ranging from 0 hours to 6 months. A qualitative examination of the electron micrographs shows structural signatures of energy depletion such as vessel leaking and chromatin clumping after 1 hour at 37°C and after 24 hours at 0°C, followed by synapse degradation after 6 hours at 37°C and 1 week at 0°C. Evidence of advanced necrosis was observed after 36 hours at 37°C and 2 months at 0°C. A deep learning algorithm was introduced that demonstrates the temperature dependence of ischemia-induced ultrastructural changes and that can also successfully distinguish between early ischemic changes and advanced necrosis.