Microcirculatory Obstruction in Focal Cerebral Ischemia: An Electron Microscopic Investigation in Monkeys

Abstract

Intraluminal membrane-bound bodies were occasionally identified but they did not appear to be producing significant obstruction. The tight endothelial junctions remained intact and there was no evidence of accelerated micropinocytosis. Severe neuronal injury frequently preceded the development of the microvascular obstruction and was more widespread than the zones of impaired perfusion.PROGRESSIVE microcirculatory obstruction in areas of focal cerebral ischemia has been demonstrated with the carbon perfusion technique by Crowell and Olsson 1 and by Little and associates.2 Light microscopic examination has shown that the obstruction lies primarily at the capillary level and appears to be related to compression of the capillary channels by the developing cerebral edema.2 Severe neuronal alterations were invariably identified before the development of the obstruction and were frequently present in areas of unimpaired perfusion. These findings suggested that the obstruction of the parenchymal vessels does not play a primary role in the production of a cerebral infarct.The object of this investigation was to study the fine structure of the microvasculature in areas of focal cerebral ischemia and to define the nature of the microcirculatory obstruction. The relationship between the parenchymal changes and the developing obstruction was also studied in order to elucidate further the significance of the obstruction. MethodsThe techniques used for the production of the ischemic lesions and carbon perfusion have been described in detail. The main steps in the procedure were as follows. (1) The right middle cerebral artery (MCA) in eight squirrel monkeys was occluded with a miniature aneurysm clip through a transorbital exposure. (2) Arterial blood samples (0.3 ml) were taken hourly for the determination of pH, Pao,, and Paco,. Rectal temperature and arterial blood pressure were constantly monitored. (3) Animals in groups of two were perfused after ischemic periods of 90 minutes, three hours, six hours, and 12 hours. They were initially perfused with 100 ml normal saline followed immediately by a mixture of colloidal carbon (250 ml) and phosphate-buffered 4% paraformaldehyde (250 ml). The clip was removed immediately prior to perfusion. (4) The brains were left undisturbed for two hours following perfusion. Then they were carefully removed from the skulls and placed in jars containing 50 ml phosphate-buffered 4% paraformaldehyde at 4°C for 24 hours. (5) The brains were cut coronally into 5 mm slices and the tissue was examined macroscopically. Thin (5 y. and 25 n) coronal sections were prepared from paraffin-embedded slices of both hemispheres and stained with hematoxylin and eosin and with thionine. (6) Multiple specimens were taken from the gray and white matter of both hemispheres of each brain. Tissue was taken from the poorly stained, intermediate, and surrounding well-stained areas of those brains which had macroscopic evidence of impaired perfusion. It was postfixed in phosphate-buffered 1% osmium tetroxide for two ...

Can Restoring Incomplete Microcirculatory Reperfusion Improve Stroke Outcome after Thrombolysis?

“…7,25,26 Histologic evaluation of the ischemic tissue with light and electron microscopy showed that microvessel lumina were narrowed and obstructed with blood cells and fibrin. 9,15,27 Notwithstanding technical concerns such as improper osmolarity of perfusion fluids may cause endothelial swelling, reports agree with regard to the histologic changes observed at various segments of the microvascular bed despite some differences possibly caused by varying duration of ischemia or recirculation and by experimental conditions (see excellent reviews by del Zoppo et al 16,28,29 ). Briefly, precapillary arterioles generally remain open, 9,25 although circular constrictions have been observed after prolonged permanent ischemia.…”

“…50 It might be interesting to reevaluate whether the constricted capillaries illustrated in electron microscopy studies might have been caused by contracted pericytes, and the swollen endfeet then passively have filled the distorted spaces. 9,15 Because capillary luminal size hardly allows passage of one row of erythrocytes and, leukocytes, because of their size and inflexibility, are not easily able to negotiate capillaries, small decreases in capillary radius typically caused by pericyte contractions (by 17% in vivo 58 ) as well as by endfeet compression and endothelial swelling may lead to blood cell entrapments. 50,59 For example, half of the superficial cortical microvessels (visualized through a cranial window) were still clogged 2 hours after recanalization after 2 hours of MCA occlusion in the mouse cortex.…”

“…Pericyte cytoplasm and mitochondria were shown to swell soon after ischemia with electron microscopy. 9,62 The pericyte dysfunction may be caused by oxygen and nitrogen radicals that are intensely generated on the microvascular wall during ischemia/reperfusion 63,64 because the unregulated pericyte contraction can be reversed by inhibiting oxidative and nitrative mechanisms. 50 Reversing pericyte contraction with these agents administered during recanalization restored microcirculatory patency and promoted tissue survival, reinforcing the idea that microcirculatory obstructions may unfavorably affect recovery after recanalization as discussed above.…”

“…[4][5][6][7] This phenomenon enjoyed considerable scientific interest after its first description in the 1970s; [8][9][10] later, it lost its popularity because of the claims that it might be an experimental artifact, [11][12][13][14] in addition to the concerns that it might be an epiphenomenon of the established tissue injury. 9,10 However, substantial experimental data accumulating since the past decades [15][16][17][18][19][20] and the recent clinical evidence suggesting that tissue reperfusion (restoration of microcirculatory blood flow) is a better predictor of outcome after thrombolysis than recanalization (reopening of the occluded artery) 21,22 necessitate that this phenomenon should be reevaluated. If indeed the microcirculatory blood flow cannot be sufficiently reinstituted after brief focal cerebral ischemia despite complete recanalization as commonly observed in the coronary circulation, 5,6 it might be one of the factors decreasing the efficacy of cerebral thrombolysis done within the time window when penumbral cells are still viable but microcirculatory obstructions have emerged.…”

“…A similar microvascular impairment was subsequently reported after focal ischemia. 9,10,24 Carbon tracer techniques and fluorescent-labeled intravascular markers showed that parts of the microcirculation were not perfused after reopening of the middle cerebral artery (MCA). 7,25,26 Histologic evaluation of the ischemic tissue with light and electron microscopy showed that microvessel lumina were narrowed and obstructed with blood cells and fibrin.…”