Significance of fluid flow for morphology of acute hypoxic-ischaemic brain cell injury

Morphological studies on cerebral cortical lesions induced by transient ischemia in Mongolian gerbil--diffuse and peripheral pallor of the neuronal perikarya

Unilateral transient cerebral ischemia was produced in Mongolian gerbils by clipping the left common carotid artery for 1 h. About 60% of the gerbils with neurological symptoms had post-ischemic seizures. The majority of those that had seizures died within a few days, and sections of their cerebral cortices contained many dark and shrunken neurons. However, the gerbils that did not have seizures survived without any severe complications. In the cerebral cortex of the latter, the neurons with diffuse or peripheral pallor of the perikarya were seen along with a small number of dark and shrunken neurons. Diffuse pallor occurred within a few hours following ischemia in layers III, V, and VI, and disappeared 1 or 2 days after recirculation. Electron microscopically, these neurons showed dispersion of ribosomes, simple and elongated profiles of rough endoplasmic reticulum (r-ER), clustered vacuoles, and mild to moderate mitochondrial swelling. Occasional net-like tubulomembranous structures, probably derived from r-ER, were observed. On the other hand, peripheral pallor became apparent after 5 days following ischemia, usually involving layer II first and gradually extending to the deeper layers. Concomitantly, the amount of neuropil decreased and the dendrites exhibited tortuosity and irregularity in layer II. Electron microscopically, these neurons showed marked swelling of peripheral perikarya and polyribosomes and organelles were located peripherally to the nuclei. In addition, numerous degenerated axon terminals and distended dendrites were observed around the neurons. These observations indicate that diffuse pallor represents damage directly induced by ischemia and subsequent recirculation, while peripheral pallor is the delayed and remote effect of ischemia, probably due to degeneration of neuronal processes.

Structural aspects of ischemic brain damage

The structural changes during cerebal ischemia are reviewed. In the acute phase the neurons may show either pale or dark type of ischemic injury. The former is usually associated with complete ischemia and the structural alterations are fairly inconspicious, while the latter is seen in incomplete ischemia or ischemia with recirculation and is characterized by shrinkage of neurons with extensive mitochondrial swelling and astrocytic edema. Both types of injury may be irreversible, but long post-ischemic period is usually necessary to see the final outcome of the insult, all the more since the neurons may not die until after a free postischemic interval (even with resumed function) of several hours to days. The delayed death is preceded by peculiar proliferation of cytoplasmic membranes before the doomed neurons become shrunken and disintegrate. This "maturation phenomenon" or "delayed neuronal death" is understandably important since it suggests that a longer postischemic interval for therapeutic interventions may exist. Several factors both during and after the ischemic insult can modify the changes and affect the severity of the damager among the most important ones are the degree of lactic acidosis during the ischemic period, as well as the characteristics of the neurons, since excitoxic damage by transmitter substances released by the ischemic insult has been suggested to be responsible for the delayed neuronal death.

Limitations of tetrazolium salts in delineating infarcted brain

Tetrazolium salts, histochemical indicators of mitochondrial respiratory enzymes, have been used by some pathologists to detect infarcts in myocardium. We explored the utility of this technique in detecting experimental brain infarcts and report our findings. Infarcts were produced in cats, gerbils, and rats by unilateral temporal and permanent cerebral vessel occlusion. After various time periods the animals were killed, and their brains were reacted with 2,3,5, triphenyl, 2H-tetrazolium chloride (TTC). The experimental and contralateral hemispheres were examined by light and electron microscopy. The TTC-stained tissue was correlated with histology. In some situations the histological condition of the tissue correlated well with the TTC staining results. Brain regions supplied by temporarily occluded vessels and judged infarcted by light and electron microscopy did not stain. In these regions less than 6% of the mitochondria were intact. In brain tissue from animals with permanent vessel occlusion (no reflow) mitochondria were intact despite the fact that other cellular organelles, such as nuclei, were destroyed. TTC stained such mitochondria and as a result could not distinguish infarcted brain in complete ischemia situations (no reflow). Another draw back to this staining procedure was 36 h after infarction macrophages with intact mitochondria would replace damage neurons and be stained. Under ideal conditions though this technique can detect irreversibly damaged brain as early as 2.5 h after artery occlusion.