Experimental thrombo-atherosclerosis

Do microemboli reach the brain penetrating arteries?

BACKGROUND

As they are "end arteries," microembolic obstruction of brain penetrating arteries would be expected to create ischemia. Yet the mammalian brain appears to have an impressive tolerance to experimental microembolization with ischemia occurring only after the injection of large numbers of particulates. Potential explanations could be that the majority of these particulates marginate along the pial vasculature or escape the cerebral circulation via arteriovenous (AV) fistulae.

METHODS

To test these theories, we first established the level of injury created by the injection of 20, 45, and 90 μm fluorescent microspheres in Sprague-Dawley rats. Brains were examined by immunohistochemistry for injury and for infarction. We then injected 1000 size 20 μm, 500 size 45 μm, and 150 size 90 μm and harvested the brains and lungs for assays of fluorescence. The location of microemboli within the brain was established by determining the percent of 20 and 45 μm fluorescent microspheres entering the superficial versus deeper layers of the brain. The location of larger microemboli was established by 2T-MRI after injection of 60-100 μm microthrombi labeled with supraparamagnetic iron oxide (SPIO) particles.

RESULTS

With 20 μm microspheres there were no areas of injury or infarction after injection of 500 and rare areas of injury and no infarctions after injection of 1000 microspheres. With either 250 or 500 size 45 μm microspheres there were a few (≤ 6) small areas of injury per animal with ≤ 2 areas of infarction. After injection, 93%-96% of injected microspheres remained in the brain. Approximately 40% of either fluorescent or SPIO labeled microthrombi were found on the brain surface.

CONCLUSIONS

As in humans, the rat brain has an impressive tolerance to microemboli, although this clearly varies with emboli size and number. Wash out of particulates through AV connections is not a major factor in brain tolerance in this model. Approximately 40% of microemboli remain in the larger pial vasculature where the more extensive collateralization may limit their effects on distal perfusion. However, the remaining 60% enter penetrating arteries but few create ischemia.

Cellular content and permeability of intraluminal thrombus in abdominal aortic aneurysm

PURPOSE

A pathologic feature commonly associated with abdominal aortic aneurysms is the presence of variably sized and shaped intraluminal thrombus, which may be fundamental to the disease process. However, the precise role of the intraluminal thrombus in the formation, enlargement, and rupture of abdominal aortic aneurysms is unknown. The hypothesis tested in this study was whether there were structural features of aortic thrombi to suggest that it may be involved in the pathogenesis of abdominal aortic aneurysms. We have investigated this hypothesis using a variety of structural and biochemical techniques.

METHODS

Tests performed were light, transmission, and scanning electron microscopy; fluid permeability measurements; and Western blots.

RESULTS

Intraluminal thrombus found in abdominal aortic aneurysms is structurally complex and is traversed from the luminal to abluminal surface by a continuous network of interconnected canaliculi. Quantitative microscopic analysis of the thrombus shows cellular penetration for at least 1 cm from the luminal surface of the thrombus. Macro-molecular penetration may be unrestricted throughout the entire thickness of the thrombus. Fibrin deposition occurred throughout the thrombus, whereas fibrin degradation occurred principally at the abluminal surface.

CONCLUSIONS

These principally structural studies support the hypothesis that the thrombus is a self-sustaining entity that may have significance in the pathophysiologic mechanism of abdominal aortic aneurysms.

Visualization of apo B, fibrinogen/fibrin, and fibronectin in the intima of normal human aorta and large arteries and during atherosclerosis

Apolipoprotein B (apo B), fibrinogen/fibrin, blood platelets, factor VIII-related antigen of the blood coagulation system, and smooth muscle cells (SMC) were identified in the intima of normal and atherosclerotic human aorta and large arteries by the indirect immunofluorescence technique. Fibrinogen/fibrin was revealed by a monoclonal antibody (monAb) against the C-terminal region of human fibrinogen A alpha-chain. Fibronectin was visualized by monAb to the cellular form and against an epitope shared by different fibronectin subunit variants. In normal intima, fatty streaks, small amounts of fibrinogen/fibrin together with large amounts of apo B were observed. Fibronectin detected by two types of monAb was not found in extracellular matrix (ECM), whereas cellular fibronectin encircled SMC. According to the data obtained, fibrinogen/fibrin accumulates in plaques as a result of intramural thrombus incorporation, blood insudation, intramural haemorrhage, and in or around cells, apparently macrophages.

Effect of intravenously injected collagenase on the concentration of circulating platelets in rats

An enzymatically induced irritation of the vessel wall by intravenous injection of collagenase (1.5 U per kg body weight) into rats resulted in a transient decrease of the platelet concentration in the flowing blood. This may be caused by an occurrence of inter-endothelial gaps distributed throughout the total area of the circulatory system, as could be observed by electron-microscopic studies. Subendothelial structures, at the sites of those gaps, induce a formation of multilocal microthrombi of platelets. At some gaps within the endothelial layer a deposit of fibrin could also be observed hinting at a participation of the blood coagulation system in the sealing processes.

Spontaneous aortic lesions in fallow deer (Dama dama L)

In 19 out of 22 aortas from fallow deer, 15 months to 5½ years, aortic lesions were found. Three types of lesions were observed, fatty streaks were seen in 2, fibrous plaques in 15 and fibrous plaques with complicated lesions (calcification and acid mucopolysaccharides) in 2 of the aortas. Elastic tissue degeneration of the inner two thirds of the tunica media was frequently found in the aorta of the animals > 3½–5½ years of age. There was no statistical evidence for a correlation between age and frequency (P ~ 0.10) but a trend towards age dependence was seen. The percentage of involved surface was found to significantly increase (0.05 > P > 0.01) with age. Lesions were found to start in the abdominal aorta in young animals and to extend cranially to the thoracic aorta with age. The percentage of involved surface in different affected regions, comprising all age groups were, in the posterior abdominal portion, 10.5 %, in the anterior abdominal portion, 4.3 % and in the posterior thoracic portion, 1.04 %. The influence of hemodynamic flow upon the localization of the aortic lesions, the endothelial cell population density and the endothelial nuclear patterns were discussed.

Pulmonary vascular disease in different types of congenital heart disease. Implications for interpretation of lung biopsy findings in early childhood

Pulmonary vascular structure was studied by analysing serial reconstructions of the arterial pathways and random sections of tissue in the lungs of 16 children who died with different types of congenital heart disease and pulmonary hypertension. Cases of ventricular septal defect showed an appreciable increase in muscularity of both preacinar and intra-acinar (respiratory unit) arteries, and intimal proliferation was infrequent and mild. By contrast, cases of transposition of the great arteries with ventricular septal defect and atrioventricular septal defect showed an increase in preacinar muscularity, a short heavily muscularised arterial segment containing intimal proliferation at the entrance to the acinus, whereas the intra-acinar arteries beyond showed only a moderate increase in muscularity. In these children who were less than 1 year of age an increase in pulmonary vascular resistance was due to strategically placed small areas of intimal proliferation and not to widespread obliterative pulmonary vascular disease. The study demonstrated and explained differences in the appearance of the peripheral pulmonary arteries in different types of congenital heart disease, which help interpret the findings of lung biopsies.

Vessel injury and atherosclerosis

Repeated endothelial injury causes lipid-rich lesion in animals on a normal diet. In severely thrombocytopenic animals these lesions do not form or are markedly inhibited. The occurrence of lipid in some experimental designs is related to continued or repeated deposition of thrombus. Lipid deposition occurs in areas where endothelium is repeatedly removed and regrows. Repeated deposition of thrombus may bring about changes in the metabolism of the neo-intima which favor lipid deposition.

Diet-induced aortic atherosclerosis in Malaysian long-tailed monkeys (Macaca irus)

Moderate hypercholesterolaemia has been produced in eight adult male M. irus monkeys by feeding an egg-toast preparation containing butter and 0.14 per cent. total cholesterol. At autopsy, after two to four years of persistent hypercholesterolaemia, there was massive fatty streaking of the aorta involving 21 to 78 per cent. of the intimal surface. The distribution and morphology of the fatty streaks was similar to that in man. In addition, there were fibrous plaques including "soft" lipid-rich atherosclerotic plaques identical to their human counterpart. There was no grossly detectable thromobosis, haemorrhage or ulceration. Atherosclerotic lesions were also present in the coronary, carotid, subclavian, iliac and femoral arteries. In a control group of monkeys fed a low-fat, cholesterol-free diet, arterial lesions were identical in type and extent to those in freshly captured wild monkeys.

Lipid composition and metabolism of thromboatherosclerotic lesions produced by continued endothelial damage in normal rabbits

Thromboatherosclerotic and fibrous lesions were produced by endothelial damage with polyethylene catheters inserted into the aortas in rabbits on a normal diet. Two weeks after insertion of the catheters, the concentration of both free cholesterol and cholesteryl ester in the thromboatherosclerotic lesions was significantly greater than that in the adjacent normal intima. A further increase in the concentration of free cholesterol and particularly of cholesteryl ester occurred during the remainder of the 4-month study period. Gas-liquid chromatography indicated that the raised thromboatherosclerotic lesions contained more cholesteryl oleate and less cholesteryl linoleate than did either the normal intima or the fibrous lesions. The incorporation of [1 -14 C] oleic acid into combined lipid in the aortas incubated in vitro showed that, by 2 weeks, two to three times more oleic acid had been incorporated into cholesteryl ester in the thromboatherosclerotic raised lesions than in the normal intima. A similar increase was demonstrated at 2 and 4 months. When [ 32 P] phosphate was used as a precursor, incorporation into lecithin was higher and incorporation into phosphatidyl inositol was lower in the raised lesions than they were in the normal intima. Fibrous lesions did not differ significantly from the adjacent normal intima in their incorporation of either of these precursors into lipid. Therefore, the accumulation of cholesteryl ester in the thromboatherosclerotic lesions resulted in part from synthesis in the arterial wall, which was stimulated by the prominent platelet components of the lesions.

Effect of removal of cholesterol diet upon serum sickness-cholesterol-induced atherosclerosis

In this report, the role of vascular allergy (i.e., hypersensitivity) in the potentiation of atherogenesis has been studied. In order to accomplish this, bovine serum albumin (BSA) was administered to rabbits in quantities sufficient to cause the occurrence of serum sickness (a type of hypersensitivity known to cause injury to the endothelial linings of certain blood vessels). This was immediately followed by the feeding of a special cholesterol-supplemented diet, which is known to be capable of initiating a high incidence of atheromatous disease in rabbits after prolonged feeding. Results indicated that those animals which received the combined treatment developed an incidence of pathology after only 2 wk of special diet which was not equaled in the diet-only control groups until they had been treated for 4 wk. This indicated that vascular allergy could potentiate lipemia-induced atherogenesis in the rabbit, and was in confirmation of an earlier study of a similar nature. Indeed, because of the relatively mild vascular injury caused by a single injection of BSA, it would seem as though vascular hypersensitivity was extremely effective in the potentiation of atherogenesis. In addition, these results may have given some indication of the degree of vascular injury necessary for the induction of irreversible atheromatous disease. While the incidence of lesions in serum sickness controls was seen to decrease with passage of time after BSA challenge, it appeared to increase after cessation of treatment in those animals which received the combined treatment of BSA plus 2 wk of cholesterol-supplemented diet. It would therefore appear that the atheromatous lesions seen as early as 2 wk after initiation of the experiment may already have been irreversible in terms of the resolution of established pathology.

Experimental mural thrombi in the pig aorta. The early natural history

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Role of thrombus in plaque formation in the human diseased coronary artery

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Lipidization induced by bone and glass fragments within an arterial plaque

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Focal lipid lesions in blood vessels due to erythrocytes and platelets. Experimental observations on goats and rabbits

Experiments on goats and rabbits living on a normal diet demonstrated that the lipid associated with organizing thrombi became concentrated in fibrosed focal lesions of blood vessels. The lipid that appeared during organization of lysed erythrocytes underwent changes similar to those of lipid in thrombi. From this it was concluded that what red cells there were in the thrombi contributed to the total lipid present.

These observations are pertinent to the pathogenesis of atherosclerosis, because they help to explain the thrombosis characteristically associated with this disease, and how lipid can be delivered to restricted regions of the arterial intima.

Immunity of the mature thrombo-atherosclerotic plaque to hypercholesteraemia

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INDUCTION OF HEMORRHAGES IN PRE-EXISTING ARTERIAL PLAQUES

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INDUCTION OF THROMBI UPON PRE-EXISTING ARTERIAL PLAQUES

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THE ATHEROGENIC POTENTIAL OF DIETARY DERIVED CHOLESTEROL

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ARTERIAL HYPERPLASIA IN THE PRESENCE OF VARIOUS FORMS OF CHOLESTEROL

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OBSERVATIONS CONCERNING THE EVOLUTION OF ATHEROSCLEROSIS IN THE RABBIT AFTER CESSATION OF CHOLESTEROL FEEDING

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Thromboatherosclerosis in aortic venous autografts: a comparative study

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Experimental venous and arterial thromboatherosclerosis: a comparative study

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