Experimental diffuse intimal thickening of the femoral arteries in the rabbit

The Effect of Diltiazem on Experimental Intimal Thickening Caused by Common Carotid Artery Investment in Rabbits

We studied the prophylactic effect of diltiazem on experimental arteriosclerosis caused by investing the common carotid artery of rabbit with a polyethylene tube for three weeks. The arteriosclerotic lesions consisted mainly of fibrocellular intimal thickening, which is similar to spontaneous arteriosclerosis in human beings.

Diltiazem (100 mg/kg/day orally) suppressed the intimal thickening and the destruction of the internal elastic lamina but not significantly.

This model might be suitable for simulation of human fibrocellular intimal thickening without hyperlipidemia and be useful for the estimation of antiarteriosclerotic effects of various drugs.

Cuff-Induced Neointimal Formation in Mouse Models

Ischemic heart failure caused by atherosclerosis is a major cause of death worldwide. Although remarkable technological advances have been made in the treatment of coronary heart disease, there is as yet no treatment that can sufficiently suppress the progression of atherosclerosis, including neointimal thickening. Therefore, a precise understanding of the mechanism of neointimal hyperplasia will provide the development of new technologies. Both ApoE-KO and LDLR-KO mice have been employed to generate other relevant mouse models of cardiovascular disease through breeding strategies. Although these mice are effective tools for the investigation of atherosclerosis, development of a progressive atherosclerotic lesion takes a long time, resulting in increase of both the costs and the space needed for the research. Thus, it is necessary to develop simpler tools that would allow easy evaluation of atherosclerosis in mouse models. In this review, we discuss our experience in generating mouse models of cuff-induced injury of the femoral artery and attempt to provide a better understanding of cuff-induced neointimal formation.

Expression of parathyroid hormone-related protein in human and experimental atherosclerotic lesions: functional role in arterial intimal thickening

We investigated the expression of parathyroid hormone-related protein (PTHrP) in atherosclerotic lesions and the role of PTHrP in the development of arterial neointima formation. Immunohistochemical staining of PTHrP in the neointima of rat aorta produced by balloon injury and of rat femoral artery produced by non-obstructive polyethylene cuff placement, and in the atherosclerotic lesion of human coronary artery was performed using anti-human PTHrP-(1-34) antibody. Anti-muscle actin antibody, HHF-35, and anti-macrophage antibody, HAM-56, were used to identify smooth muscle cells and macrophages, respectively. Immunoreactivity of PTHrP was detected in the thickened intima of rat and human lesions where the predominant cell types were smooth muscle cells or macrophages dependently on the lesion type. In the next series of experiments, we examined the effect of PTHrP on the development of cuff-induced intimal thickening of rat femoral artery. Either PTHrP-(1-34) or PTHrP-(7-34), a PTH/PTHrP receptor antagonist, suspended in pluronic F-127 gel was locally applied around the rat femoral artery. Intimal thickening induced by cuff placement was evaluated 2 weeks later. PTHrP-(1-34) dose-dependently inhibited intimal thickening determined as intima/media ratio and % stenosis whereas PTHrP-(7-34) dose-dependently enhanced that. These results suggest that PTHrP, which is expressed in atherosclerotic lesions, inhibits the development of neointimal formation.

von Willebrand factor does not influence atherogenesis in arteries subjected to altered shear stress

The role of von Willebrand factor (vWF) in arterial neointimal formation that develops in arteries with altered shear stress was investigated using normal, heterozygous, and homozygous von Willebrand disease pigs (ie, vWD, or lacking vWF) that were fed normal pig chow. Shear stress was applied to carotid and femoral arteries with a Goldblatt clamp for 14 days, producing a > or = 80% stenosis. Neointimal lesion size was measured by computer-assisted morphometry. Expression of proliferative cell nuclear antigen (PCNA) by neointimial and medial cells was used as a relative index of proliferative activity. For shear-stressed arteries, there was no significant difference in the number of smooth muscle cell layers in the lesion, lesion size, and percent of PCNA-positive neointimal or medial cells among normal, heterozygous, and homozygous vWD pigs (P> or =.1, ANOVA). Lesions in pigs that expressed vWF (normals and heterozygotes) contained large amounts of vWF in the neointima, whereas lesions in vWD pigs had no detectable vWF. Moreover, no foam cells were detected in the lesions. Thus, the absence of vWF apparently does not alter the size of lesions in shear-stressed arteries in vWD pigs or the number of neointimal or medial cells expressing PCNA. Mechanism(s) involved with shear-induced modulation of smooth muscle cell proliferation, then, can operate independently of vWF in normolipemic pigs.

Effects of ethyl-all-cis-5,8,11,14,17-icosapentaenoate (EPA-E), pravastatin and their combination on serum lipids and intimal thickening of cuff-sheathed carotid artery in rabbits

The anti-arteriosclerotic effects of ethyl all-cis-5, 8, 11, 14, 17-icosapentaenoate (EPA-E), pravastatin and their combination in cuff-treated rabbits were investigated. EPA-E at 600 mg/kg, pravastatin at 50 mg/kg or their combination was orally administered once daily for 5 weeks, and each of the animals was sheathed with a cuff on the carotid artery 2 weeks after the beginning of drug administration. EPA-E, pravastatin and their combination significantly reduced serum total cholesterol compared to the control group. EPA-E also potently reduced serum triglyceride, while pravastatin only slightly reduced it. The combination of these two agents had the most potent effect on the level of serum triglyceride. Serum phospholipids were also reduced by these treatments in a similar fashion. At the end of treatment, diffuse intimal thickening was observed in the cuff-covered region in all animals in the control group, and the intima/media area ratio in this group was 0.293 +/- 0.038. Treatment with EPA-E alone tended to prevent the intimal thickening, and the intima/media area ratio was 0.209 +/- 0.058 (p = 0.094). This ratio was 0.287 +/- 0.048 (p = 0.902) when pravastatin was administered alone, indicating that it had no significant effect on intimal thickening. The ratio was 0.175 +/- 0.041 (p = 0.042) when both EPA-E and pravastatin were administered, indicating that this combination had a significant inhibitory effect on intimal thickening in the cuff-sheathed region. These findings suggest that combined treatment with EPA-E and pravastatin is more effective than respective monotherapies in lowering serum lipids and/or preventing an intimal thickening as events of atherogenesis.

Estrogen inhibits cuff-induced intimal thickening of rat femoral artery: effects on migration and proliferation of vascular smooth muscle cells

The present study was performed to elucidate the mechanism underlying the anti-atherogenic action of estrogen. We investigated the effect of estrogen on intimal thickening of the rat femoral artery induced by cuff placement and further examined the effect of estrogen on migration and proliferation of vascular smooth muscle cells (VSMCs) in culture. Intimal thickening was significantly greater in males than in control females. Intimal thickening in females was increased to the level in males by ovariectomy. Estrogen replacement to ovariectomized rats reversed this effect. Proliferating cell nuclear antigen immunohistochemistry showed that in vivo proliferation of VSMCs contributed to the difference in intimal thickening. There was no difference in blood pressure and serum lipids, suggesting that estrogen directly acted on artery and inhibited intimal thickening. 17 beta-Estradiol (E2, 1-100 nmol/l) inhibited migration of cultured rat VSMCs, assayed using a microchemotaxis chamber, in a concentration-dependent manner. E2 (0.01-100 nmol/l), but not progesterone or testosterone, also inhibited [3H]thymidine incorporation in rat VSMCs in a concentration-dependent manner. Indomethacin, NG-monomethyl-L-arginine and methylene blue did not influence the inhibitory action of E2 on [3H]thymidine incorporation, suggesting that prostanoids and nitric oxide are not involved in the action of E2. E2 did not provoke VSMC injury, as measured by the release of incorporated [3H]2-deoxy-D-glucose. These results suggest that the inhibition of migration and proliferation of VSMCs contributes to the inhibitory effect of estrogen on intimal thickening.

Local delivery of dexamethasone for prevention of neointimal proliferation in a rat model of balloon angioplasty

A periadventitial polymer system is an alternative local drug delivery technique to obtain and maintain high tissue levels of the drug at the site of vascular injury. To determine if local periadventitial delivery of dexamethasone decreases neointimal proliferation after balloon vascular injury, in three groups of Sprague-Dawley rats, 5% dexamethasone, 0.5% dexamethasone, and placebo silicone polymers were implanted around the left common carotid artery after balloon injury. In a fourth group, placebo polymers were implanted without balloon injury. Dexamethasone serum and tissue levels after polymer implantation were significantly higher in the 5% dexamethasone group compared with the 0.5% dexamethasone group. There was no neointima formation in any of the arterial segments covered with placebo polymers for 3 wk, but without balloon injury. In the arterial segments covered by the 5 and 0.5% dexamethasone polymers, there was a 76 and 75% reduction in intima/media ratios, respectively, compared with the placebo group (5% dexamethasone, 0.26 +/- 0.04; 0.5% dexamethasone, 0.27 +/- 0.03; placebo, 1.09 +/- 0.16, respectively; P < 0.0001). These results suggest that: (a) silicone polymers wrapped around the common carotid arteries for 3 wk did not, without balloon injury, stimulate neointimal proliferation in the rat model; (b) the activity of the drug-eluting polymer for suppressing intimal proliferation was chiefly, but not exclusively, site specific; and (c) transadventitial local delivery of dexamethasone at two different doses markedly inhibits neointimal proliferation after balloon vascular injury.

Longitudinally oriented smooth muscle cells in rabbit arteries

Intima formation in vessels, spontaneous or experimentally induced, is generally characterized by the presence of longitudinally oriented smooth muscle cells (LSMC). During an experiment of neo-intima induction in carotid arteries in rabbits, by application of a non-constrictive silastic cuff, a study was performed to investigate the presence of LSMC in the systemic and pulmonary circulations, in both elastic and muscular arteries. Three patterns could be distinguished: intimal cushions in muscular arteries, single or small groups of LSMC in the intima in elastic and larger muscular arteries, and intra-medially located layers or columns of LSMC in the aorta, the pulmonary artery, at the bifurcation of the aorta and around orifices of branches. In order to understand this peculiar orientation a biomechanical approach was used: this showed that near the lumen the circumferential stress is 4.5 times higher than the longitudinal. Because the cell surface of the smooth muscle cells exposed to this stress per unit vessel length is much less in the longitudinal than in the circular direction we conclude that the LSMC align in the direction which allows them to cope most effectively with the mechanical stresses.

Triphasic sequence of neointimal formation in the cuffed carotid artery of the rabbit

A nonocclusive silicone cuff placed around the rabbit carotid artery results in a diffuse intimal thickening. The early stages of this phenomenon were studied by light microscopy, immunohistochemistry, and electron microscopy. Neointimal formation appeared to be triphasic. The first phase started 2 hours after cuff placement, with vascular infiltration by polymorphonuclear leukocytes (PMNs). In the second phase, starting within 12 hours, 1.90 +/- 0.36% of the medial smooth muscle cells (SMCs) were replicating, as demonstrated by their immunoreactivity for proliferating cell nuclear antigen (PCNA). The third phase was characterized by the appearance, from day 3 onward, of subendothelial SMCs that were immunoreactive for alpha-SMC actin and vimentin. A few cells showed immunoreactivity for PCNA. During this phase all the PMNs disappeared, but SMC replication in the media was still present, as indicated by the presence of mitoses and the persisting immunoreactivity for PCNA (0.76 +/- 0.22% at day 7). In the third phase the number of subendothelial cells increased (104 +/- 15 SMC nuclei per section at day 7, of which 8.89 +/- 2.26% were PCNA-positive) and was associated with deposition of collagen type IV and fibronectin. At 14 days a complete, circular neointima was present and contained 2.13 +/- 0.28% replicating SMCs. The media showed 0.44 +/- 0.08% cell-cycling SMCs, which was still four times higher than normal. During the first week there was also a significantly higher PCNA activity in the media of sham-operated carotid arteries (no cuff present) than in nonsurgical ones. However, this did not lead to the formation of a neointima. We conclude that in the cuff system SMC replication in the media precedes the neointimal formation. The system can be used to study SMC replication, migration, and neointimal formation with minimal medial SMC damage.

Photodynamic therapy of arteries. A novel approach for treatment of experimental intimal hyperplasia

BACKGROUND

Photodynamic therapy (PDT) uses light activation of otherwise nontoxic dyes for the production of reactive oxygen species that cause cell injury and death.

METHODS AND RESULTS

The inhibition of intimal hyperplasia (IH) by PDT was studied in the balloon injury model of the rat carotid artery. Chloroaluminum-sulfonated phthalocyanine (CASPc) was the drug chosen for PDT because it does not produce skin photosensitivity and has a high absorption peak of light at 675 nm, a wavelength with good tissue penetration. A pilot study indicated that CASPc administration with laser radiant exposure of 100 J/cm2 resulted in a homogeneous, circumferential effect on the whole artery. Male Sprague-Dawley rats received the balloon catheter injury to the left common carotid artery (day 0) and were equally divided into two groups. Nine rats received either CASPc (5 mg/kg i.v., n = 6) or saline (n = 3) at day 2, before IH was present, and nine rats received CASPc or saline in the same manner on day 7, when IH was already present. Twenty minutes after drug injection, the distal left common carotid artery was irradiated under saline with 675-nm laser light at 100 mW/cm2 for 10(3) seconds (100 J/cm2). At this low laser irradiance, there are no thermal effects, but photoactivation of CASPc occurs. The rats were killed at day 14 after balloon injury when IH reaches a maximum. The arteries were harvested after perfusion-fixation for light microscopy, histological and computerized morphometric evaluation, and transmission electron microscopy (TEM) analysis. The cross-sectional areas of the neointima were measured in the PDT-treated arteries and in the laser-only control arteries. There was a significant mean +/- SD decrease of IH in the PDT-irradiated segments of the arteries (0.06 +/- 0.05 mm2) versus the laser-only control ones (0.17 +/- 0.07 mm2) (t test, p less than 0.001), with no statistical difference between the day 2 and day 7 treated rats. Lack of IH was correlated in 90% of cases with histological absence of medial smooth muscle cells or inflammatory cells, but no other structural injury was identified. TEM analysis showed early evidence of PDT-mediated cytotoxic effects at 4 hours and the absence of collagen or elastic tissue structural alterations.

CONCLUSIONS

These data demonstrated that PDT can effectively inhibit the IH response when it is used before or during induction of cellular proliferation in this acute model. Although the long-term implications of PDT in arteries need to be defined, this technique may offer a new method for understanding and treating IH.

Intimal thickening of jugular and femoral veins vs arteries in the rabbit following investment

The authors induced intimal thickening in the jugular and femoral veins and in the common carotid and femoral arteries of rabbits by placement of a polyethylene tube cuff. The comparative effects on the intima were studied by light and electron microscopy. Even in the veins, thickening resulted from the migration of medial smooth muscle cells into the intima with subsequent proliferation. Thickening in the arteries consisted of tightly packed smooth muscle cells and a few elastic fibers, whereas that in the veins was characterized by an abundance of collagen fibers, layers of smooth muscle cells, and a few elastic fibers. Capillaries were often observed in the thickened intima of the veins but not of the arteries.

Light and electron microscopy of the spontaneous intimal thickenings of rabbit aorta. A normal arterial wall growth

A light and electronmicroscopic study in the intima of the smaller and larger curvatures of a healthy rabbit aortic arch was performed. In both curvatures there were areas of intimal thickenings of varying width, and consisting mainly of collagenous and elastic fibers and smooth muscle cells. The structure of the wider thickenings resembled the media layer. There was evidence that such intimal thickenings are media growth in thickness and physiological in nature. It is inferred that the early lipid deposition in the intimal thickenings of the smaller, but not in the larger curvature, is not related to morphological differences, since both curvatures had the same intimal pattern.

Restenosis after coronary angioplasty. Potential biologic determinants and role of intimal hyperplasia

Restenosis after successful PTCA remains a major problem limiting the efficacy of the procedure. The pathophysiologic mechanism of restenosis has been enigmatic so far, but accumulated evidence strongly suggests that intimal hyperplasia is the major mechanism. Based on current understanding of the process of intimal hyperplasia, one unifying concept may be that there are at least two major local biologic determinants influencing this process, lesion characteristics and regional flow dynamics. Lesion characteristics include the plaque structure and the quantity of smooth muscle. These may provide the anatomic substrate that determines the extent of injury and the degree of smooth muscle cell proliferation. The amount of smooth muscle cells in the stenotic lesion activated by injury to undergo proliferation may determine the eventual bulk of the restenotic lesion. In addition, low wall shear stress could promote intimal hyperplasia and cause structural change of vessels to decrease the lumen, whereas high wall shear stress exerts the opposite effects. Intimal hyperplasia after balloon injury is a complex process involving platelets, growth factors, endothelial cells, smooth muscle cells, mechanical injury, wall shear stress, and probably other unknown factors. Platelets not only contribute growth factors such as PDGF but also cause organized thrombus. Different growth factors may be involved in initiating smooth muscle cell proliferation and may come from many different sources, including smooth muscle cells, endothelial cells, and macrophages. Intact confluent endothelial cells may produce heparin sulfates and inhibit intimal proliferation; however, regenerating endothelial cells may have the opposite effect. Thus, the proliferative potential of smooth muscle cells, endothelial recovery, extent of injury, wall shear stress, and other unknown factors may all influence this process. Based on these concepts concerning the biology of restenosis, some research directions concerning potential forms of therapy are proposed.

Identification of macrophages in intimal thickening of rat carotid arteries by cytochemical localization of purine nucleoside phosphorylase

Complete desquamation of the endothelium of the rat carotid artery by balloon catheter stripping resulted within 2 weeks in the formation of a large intimal thickening. After an enzyme cytochemical technique was applied to localize cytosolic purine nucleoside phosphorylase (PNP), light microscopical evaluation indicated that this intimal thickening in normocholesterolemic rats was composed of 5.8% to 11.8% (mean 8.8%) PNP-positive cells. At the electron microscopic level, all these PNP-positive cells were identified as macrophages by the absence of a basement membrane and plasmalemmal vesicles and by the occurrence of specific intracytoplasmic granules. The nearly nonreactive intimal cells were classified as modified smooth muscle cells. Additional evidence of the macrophage nature of the PNP-stained intimal cells was obtained by differential immunogold labeling of these cells with a monoclonal antibody against rat macrophages. Moreover, in hypercholesterolemic rats, only the cells stained for PNP transformed into foam cells (between 8.5% and 11.4% of all nucleated intimal cells; mean 9.6%). This study shows that PNP cytochemistry discriminates macrophages from modified smooth muscle cells in the rat carotid intimal thickening. It further suggests that the intimal thickening in normocholesterolemic rats originates not only from migration and proliferation of smooth muscle cells, but also from a considerable number of leukocyte-derived macrophages. Whether the latter cells are actively involved in the establishment of the intimal thickening as has been suggested in dietary hypercholesterolemia, remains to be verified.

Smooth muscle cell migration induced by inflammatory cell products and its inhibition by a potent calcium antagonist, nilvadipine

The chemotactic activities of inflammatory cell products for rat aortic smooth muscle cells (SMC) were examined in modified Boyden chambers. A checker board analysis revealed that interleukin-1 (IL-1), leukotriene B4 (LTB4), platelet-derived growth factor (PDGF) and inflammatory exudate from zymosan-activated air pouches stimulated chemotaxis of SMC. The chemotaxis, irrespective of the attractants used, was strongly inhibited by nilvadipine, a potent calcium antagonist, and the IC50 values were around 1 x 10(-10) M. Removal of extracellular calcium abolished the chemotactic activities of the attractants. These results suggest that inflammatory cells such as macrophages and polymorphonuclear leukocytes (PMN) have an important role in the migration of SMC into the intima during atherogenesis, and that nilvadipine might be useful for preventing and treating atherosclerosis.

Inflammatory responses in cuff-induced atherosclerosis in rabbits

Cuff-treatment of the rabbit carotid artery produced a diffuse intimal thickening which resembled early lesions of atherosclerosis. A limited amount of focal endothelial damage occurred first (0.5 h), leukocytes infiltrated the subendothelium and extensive endothelial denudation occurred at 24 h. At 3 days, the regenerating endothelium covered the denuded area, and the media was edematous. At 7 days proliferation of intimal cells became visible. Maximum intimal thickening occurred at 3 weeks. Daily injection of dexamethasone (0.01-10 mg/kg i.m.) and ticlopidine (1-100 mg/kg i.m.) dose-dependently attenuated the intimal thickening. Indomethacin had little effect. Inflammatory exudate from zymosan-activated air pouch induced chemotaxis of rat smooth muscle cells (SMC) in vitro. Similar chemotactic activity was observed with leukotriene B4 (LTB4) but not with the other lipoxygenase products tested. The exudate contained reasonable amounts of LTB4, which would account for its chemotactic activity. Dexamethasone inhibited the chemotaxis by the exudate and proliferation of SMC. These results are discussed in relation to the mechanism of atherogenesis. It is concluded that leukocytes play a major role in cuff-induced intimal thickening, and that their products cause endothelial denudation and SMC chemotaxis. Involvement of platelet aggregation in atherogenesis is also suggested.

Antiatherogenic activity of FR34235 (Nilvadipine), a new potent calcium antagonist. Effect on cuff-induced intimal thickening of rabbit carotid artery

The antiatherogenic activity of FR34235 (Nilvadipine), a calcium antagonist, was examined in rabbits with carotid arteries sheathed with polyethylene cuffs, and compared with that of nifedipine, verapamil and diltiazem. The drugs were given intramuscularly in daily doses of 0.01-10 mg/kg for 3 weeks, starting on the day of cuff-placement. FR34235 dose-dependently inhibited the cuff-induced intimal thickening, and was more potent than the other calcium antagonists, whose order of potency was nifedipine, diltiazem and verapamil. In an in vitro experiment on inhibition of migration of rat aortic smooth muscle cells, using zymosan-activated air pouch exudate as a chemoattractant in modified Boyden chambers, FR34235 was also the most potent among the calcium antagonists tested. The IC50 values were 3.3 X 10(-11) M for FR34235, 1.7 X 10(-10) M for nifedipine, 6.0 X 10(-9) M for verapamil and 2.4 X 10(-7) M for diltiazem. Effects of these drugs on proliferation of rat aortic smooth muscle cells and rabbit platelet aggregation were also examined in vitro. At concentrations less than 10(-5) M, none of the drugs inhibited proliferation of the smooth muscle cells, and only verapamil inhibited collagen-induced platelet aggregation (IC50 = 9.0 X 10(-7) M). It is suggested that FR34235 should be useful for preventing and treating atherosclerosis. Inhibition of smooth muscle cell migration is thought to be its mechanism of antiatherogenic activity.

Ultrastructure of human umbilical vessels: a possible role in amniotic fluid formation?

Human umbilical vessels obtained from neonates delivered at term after uneventful pregnancies were examined by light and transmission electron microscopy, with the aim of determining whether or not their structure is compatible with possible fluid exchange between the circulating blood and Wharton's jelly. A comparison of arteries and veins showed that although these vessels have common characteristics, they differ in some elements of their fine structure. The endothelium of both vessels appeared to be highly active metabolically. In the artery, the endothelial cells often protruded into the lumen. This aspect was related to the fine filaments concentrated in the basal part of the cells. This zone, free of organelles, was absent in the venous endothelium, but here pinocytotic vesicles and Weibel-Palade bodies were more abundant. The media included the same elements but was much thicker in the arteries than in the veins. There were two cellular types: typical myocytes and myofibroblasts rich in organelles. Their cytoplasmic processes extended into the interstitial space which was occupied by a material with a loose structure, that is, material containing a well-developed ground substance at the expense of the elastic and collagen fibres. The ultrastructural features of the umbilical vessels suggest an increased endothelial permeability, and it is suggested that transfer across the umbilical vessels may play a role in the formation of amniotic fluid.

Regeneration of the arterial wall in microporous, compliant, biodegradable vascular grafts after implantation into the rat abdominal aorta. Ultrastructural observations

The ultrastructure of a new type of vascular graft, prepared from a mixture of polyurethane (95 weight %) and poly-L-lactic acid (5 weight %), was examined six weeks after implantation into the abdominal aorta of rats. These microporous, compliant, biodegradable, vascular grafts function as temporary scaffolds for the regeneration of the arterial wall. Smooth muscle cells, covering the grafts, regenerated a neo-media underneath an almost completely regenerated endothelial layer (neo-intima). These smooth muscle cells varied in morphology from normal smooth muscle cells to myofibroblasts. They were surrounded by elastic laminae and collagen fibers. Macrophages, epithelioid cells, multinucleated giant cells, fibroblasts and capillaries were present in the disintegrating graft lattices. The epithelioid cells and multinucleated giant cells engulfed polymer particles of the disintegrating grafts. The regeneration of the endothelial and smooth muscle cells is similar to the natural response of arterial tissue upon injury. The presence of macrophages, epithelioid cells, multinucleated giant cells, fibroblasts and capillaries in the graft lattices resembles the natural response of tissue against foreign body implants. Both of these responses result in the formation of a neo-artery that possesses sufficient strength, compliance and thromboresistance to function as a small caliber arterial substitute.

Organisation of experimental thrombosis by blood cells. Evidence of the transformation of mononuclear cells into myofibroblasts and endothelial cells

To clarify whether thrombus organisation was carried out by local cell activity or by elements of the circulating blood we developed an artificial prosthesis, made of an impermeable polyurethane material with an athrombogenic surface but with a central part consisting of a DACRON velour ring which was thrombogenic. We implanted these devices into the aorta of 10 sheep. In these animals, organisation of the central thrombus by local aortic cells could be excluded. After varying periods of time (2-84 days), the device was removed and the organized thrombus investigated by light and electron microscopy. From our investigations the organisation process with the development of mesenchymal cellular elements proceeded in 3 steps: The activation of the mononuclear macrophage system, the appearance of myofibroblastic cells and endothelial formation. The activation of the mononuclear macrophage system is probably induced by chemospecific products of metabolism arising from aging thrombotic material. Apart from mononuclear elements such as monocytes, macrophages, and giant cells we observed fibroblast-like and myofibroblast-like cells. The matrix contained collagen. Endothelium developed on the surface of the organizing thrombus. The final stage was characterized by the formation of a pseudovessel wall, which followed the pattern of the vascular model. Our findings support the hypothesis that a thrombus may be organized by cells derived from the circulating blood.

Relation between arterial intimal thickening and the vasa-vasorum

The histopathological data presented support a new concept for the origin of the cells which cause intimal thickening of arteries. Arterial segments, isolated between ligatures, when examined by intravascular contrast techniques, showed penetration of vasa-vasora and formation of intra-arterial granulation tissue which produced myointimal thickening. Transitional forms between pericytes and myointimal cells were found. In autoradiographic studies on the incorporation of 3H-thymidine, DNA synthesis was first seen in the adventitia, fundamentally in the vasa-vasora pericytes, and in the adjacent media, and later in the intimal thickening. In arterial segments between ligatures typical intimal thickening was produced when intra-arterial granulation tissue was formed and the ligatures were removed thus restoring the circulation. These results were not produced when the arterial segment was sectioned lengthways between ligatures. It is suggested that this intimal thickening originates in cells from the vasa-vasora, in particular from pericytes.

The evolution of experimental endarteritis in the rabbit abdominal aorta. Light and transmission electron microscopy

Experimental aortic intimal thickening has been induced in rabbits by sheathing the vessel with a polyethylene cuff. The alterations have been examined by light and transmission electron microscopy, during 12 months. An irregular intimal thickening develops as soon as the 15th day and includes numerous myofibroblasts with some other cells of monocytic or endothelial type. Microfibrils, elastic aggregates and collagen fibers are found in the intercellular space. Simultaneously, the media undergoes a fragmentation of the elastic laminae and the adventitia shows a capillary angiectasis and a granuloma. After 3 months there is, between the intimal smooth muscle cells, a progressive increase of elastic and collagenous material. In the media, elastic break up becomes more frequent after the 4th month and myocytes appear increasingly atrophic, which facilitates the extension of fibrosis. This is accompanied at times by a thinning of the arterial wall with or without localized disappearance of the media. All these modifications are discussed and compared to what we had previously found in the femoral artery [12].