血管紧张素转换酶抑制剂抑制血管平滑肌细胞增殖的机制

目的: 探讨血管紧张素转换酶抑制剂(ACEI)抑制动脉平滑肌细胞增殖及向内膜迁徙的机理。方法: 球囊导管损伤Wistar大鼠颈总动脉, 实验组于术前2 d开始给与ACEI(temocapril-HCl 10 mg·kg^-1·d^-1), 术后2 d、3 d、5 d分批处死。用抗人PDGF-A、-B及其受体, 抗人MMP-1、MMP-9等抗体以ABC法行免疫染色。动脉组织行放射自显影乳剂原位酶谱分析。电镜下观察细胞质内小器官及细胞周围弹性、胶原纤维的密度。结果: 给ACEI后, PDGF及其受体、MMP-1、-9蛋白阳性细胞率及明胶酶活性显著降低, 并抑制了中膜平滑肌细胞的表型转换。结论: ACEI可能通过继发地抑制PDGFs、MMPs蛋白表达, 阻碍细胞表型转换, 从而阻止中膜平滑肌细胞增殖及向内膜迁徙。     

TMLR对缺血心肌乳酸代谢及线粒体的影响

目的:探讨激光心肌血运重建术(TMLR)对急性心肌缺血(AMI)的作用与机制。方法: 将18条犬随机等分为假手术组、AMI组和TMLR组,采用连续型Nd:YAG激光行TMLR术。测动脉和冠状窦血乳酸含量(A.Lat 和CS.Lat),心肌乳酸代谢速率(MLR)和心肌乳酸吸收分数(MLE);超微电镜结合生物体视学原理定量观察心肌细胞线粒体形态和数量。结果: 结扎左前降支冠状动脉(LAD)后60 min,AMI组和TMLR组CS.Lat分别为(7.63±4.27)mmol/L和(5.78±3.98)mmol/L,P<0.05;MLR分别为(0.03±0.01)mmol·100 g心肌^-1·min^-1和(0.06±0.02)mmol·100 g心肌^-1·min^-1,P<0.05;MLE分别为(12.04±3.04)% 和(21.84±8.49)%,P<0.05。LAD结扎后4 h,AMI组和TMLR组线粒体体密度分别为(27.51±7.93)% 和(31.26±3.85)%,P>0.05;面密度分别为(1.25±0.18)μm^-1和(1.64±0.28)μm^-1,P<0.01;数密度分别为(0.10±0.03)μm^-3和(0.18±0.05)μm^-3,P<0.01;平均体积分别为(5.27±2.85)μm³和(2.80±0.54)μm³,P<0.05;平均外径分别为(2.06±0.36)μm和(1.78±0.12)μm,P<0.05。结论: TMLR可纠正急性心肌缺血犬的心肌乳酸代谢障碍和减轻心肌细胞损伤。     

血管紧张素转换酶抑制剂对损伤后动脉弹性蛋白酶的影响

目的：研究球囊导管损伤后早期血管紧张素转换酶抑制剂对动脉中膜弹性蛋白酶的影响。方法：12周雄性Wistar大鼠颈动脉和主动脉用球囊导管损伤,分成实验组和对照组,分别于术前2 d投与血管紧张素转换酶抑制剂（temocapril-HCl, 10 mg·kg^-1·d^-1）和溶剂,术后2、3、5和10 d处死。用原位杂交、免疫组织化学和酶活性测定研究弹性蛋白酶。结果：实验组动脉损伤10 d时的内膜面积与对照组相比明显被抑制（P<0.01）,实验组2、3和5 d弹性蛋白酶mRNA阳性细胞率、弹性蛋白酶阳性细胞率及其活性明显低于对照组（P<0.01, P<0.05）。结论：ACEI（temocapril-HCl）明显抑制损伤后动脉中膜平滑肌弹性蛋白酶表达和活性,提示弹性蛋白酶的表达与血管紧张素转换酶的作用有关。     

强力霉素影响基质金属蛋白酶活性和血管重构的实验研究

目的：观察强力霉素对损伤动脉组织中基质金属蛋白酶（MMP）活性的抑制作用，并探讨强力霉素对血管平滑肌细胞增殖、动脉内膜增生、管腔重构的影响。方法:球囊导管扩张动脉的方法建立大鼠颈总动脉损伤模型。治疗组用强力霉素30 mg·kg^-1·d^-1干预。明胶酶谱法测定损伤动脉组织中MMPs的活性。用HE染色、VVG染色、免疫组化标记α-actin和增殖细胞核抗原的方法观察损伤动脉内膜厚度、管腔重构及平滑肌细胞增殖的情况。结果:①强力霉素治疗组MMP-9活性在术后24 h、3 d分别比对照组低26.3％、34.5％（P<0.01）；MMP-2活性在术后7 d比对照组低40.0％（P<0.01）。②强力霉素治疗使术后7 d内膜平滑肌细胞增殖率（43.23％±1.06％）显著低于对照组（62.76％±1.02％）（P<0.01）；使术后14 d、28 d新生内膜厚度比对照组分别少32.0％、38.8％（P<0.01），而管腔面积比对照组多58.0％、90.4％（P<0.01） 。结论：强力霉素可以显著降低血管损伤后MMPs活性，抑制内膜平滑肌细胞的增殖、新生内膜增生以及管腔重构，提示它可能具有防治PTCA术后再狭窄的作用。     

剪切力改变后动脉内皮细胞通透性的变化

目的:研究剪切力改变后内皮通透性及其超微结构。方法:饲以普通或高脂肪食的兔腹主动脉狭窄60.7%,高速摄像微粒子示踪技术血流分析,依万斯蓝和苏丹Ⅳ动脉染色,辣根过氧化物酶法电镜观察。结果:距狭窄近侧1mm的动脉前、后壁剪切力分别为74dyn/cm²和317dyn/cm²,为高剪切力区;狭窄远侧3mm前、后壁分别为-18dyn/cm²和-71dyn/cm²,为返流、涡流、湍流和停滞的低剪切力区。狭窄近、远段内皮通透性均增高,在蓝染或苏丹Ⅳ阳性区,狭窄远段内皮细胞连接部完全开放型的百分率(88.5%或88.1%)明显大于近段(22.7%或30%)(P＜0.01),远段标记HRP小胞的密度(2.57±1.14)×10¹²/m²或(2.72±1.81)×10¹²/m²明显高于近段(1.24±1.06)×10¹²/m²或(1.90±1.47)×10¹²/m² (P＜0.01)。结论:低剪切力更易导致内皮细胞对大分子物质通透性增高和动脉硬化形成。     

全反式维甲酸对大鼠腹主动脉球囊损伤后内膜增生和血管重塑的影响

目的：研究全反式维甲酸（ATRA）对球囊损伤大鼠腹主动脉内膜增生和血管重塑的影响。方法：复制球囊损伤的大鼠腹主动脉剥脱模型，12只Wistar大鼠随机分为①对照组：6只腹腔内注射intralipid（1 mL/d）；②ATRA组：6只腹腔内注射溶解于intralipid的ATRA（4 mg·kg^-1·d^-1）。球囊损伤14 d后，损伤区域的血管段用10%甲醛固定，然后染色，进行组织学观察。结果：ATRA组新生内膜面积（IA）明显小于对照组，管腔面积（LA）和外弹力膜包绕面积（EEL）则大于对照组。结论：ATRA具有抑制损伤血管的内膜增生和促进有益的血管重塑的作用。     

蛋白酶体抑制剂MG132对脑室室管膜下区神经干细胞增殖潜能的影响

目的: 观察侧脑室注射蛋白酶体抑制剂MG132对脑室室管膜下区(SVZ)神经干细胞(NSCs)增殖能力的影响,了解蛋白酶体对NSCs增殖潜能的调控作用,从而进一步探讨蛋白酶体在帕金森病(PD)等老年性疾病神经元损伤中的可能作用机制。方法: 选90日龄健康BALB/c小鼠,左侧侧脑室注射10 μg MG132,于注射后3 d、7 d、14 d提取SVZ总蛋白,荧光酶标仪检测蛋白酶体活性。同时腹腔注射5-溴-2'-脱氧尿嘧啶核苷(BrdU),BrdU免疫荧光染色标记NSCs,动态观察蛋白酶体活性改变对NSCs增殖能力的影响。另设溶剂对照组左侧侧脑室注射等剂量DMSO,进行上述实验。结果: 侧脑室注射MG132 3 d、7 d后,SVZ蛋白酶体活性较DMSO对照组显著降低(P<0.05),同时SVZ BrdU⁺ NSCs也显著减少至21±4和22±3,与DMSO对照组相比差异显著(P<0.05)。随着MG132注射时间延长,14 d后SVZ蛋白酶体活性恢复至正常水平(P＞0.05),BrdU⁺ NSCs数量MG132组(82±4)与DMSO组(67±6)相比无显著差别(P＞0.05)。结论: 应用蛋白酶体可逆性抑制剂MG132短时期内能显著抑制SVZ蛋白酶体活性,并且能够降低NSCs增殖能力,提示蛋白酶体活性与NSCs增殖潜能密切相关。     

普罗布可抗兔动脉成形术后再狭窄与调节血管重塑的关系

目的:研究普罗布可抗动脉成形术后再狭窄与血管重塑的关系。方法:用3.5F球囊导管构建兔动脉粥样硬化动脉成形术后再狭窄模型, 动脉成形术后2周, 观察普罗布可抗动脉成形术后再狭窄作用;组织形态学观察及计算机图像分析, 了解普罗布可对病理性血管重塑的影响;血脂含量测定。结果:普罗布可抗再狭窄作用显著, 能明显增加血管内外径及管腔面积, 减少新生内膜的形成;能调节血管重塑, 增加动脉成形术后内弹力层包围的面积[IEL, ( 3.50±0.20) mm² υs (1.59±0.23) mm², P<0.01]、外弹力层包围的面积[EEL, (4.61±0.29) mm²υs (2.56±0.28) mm², P<0.01];能调节血脂水平, 降低血清总胆固醇及甘油三酯含量。结论:普罗布可通过调节动脉成形术后血管病理性重塑达到抗再狭窄作用。     

增强型体外反搏对冠心病患者血浆内皮素和一氧化氮的影响

目的:探讨增强型体外反搏对冠心病患者血浆内皮素(ET)和一氧化氮(NO)的影响。方法:把62例确诊冠心病的患者随机分为体外反搏治疗组(29例)和药物治疗组(32例), 反搏组在常规药物治疗的基础上接受增强型体外反搏仪治疗36d(1h/d), 药物组接受常规药物治疗相同天数;分别于治疗前后应用放射免疫法测定患者的血浆ET含量, 应用硝酸盐还原酶法测定患者血浆NO^-₂/NO^-₃含量, 以间接反映NO的浓度;并测定30例健康人的ET和NO^-₂/NO^-₃值作为对照。结果:治疗前反搏组和药物组的ET水平(116.4±44.9)ng/L, (111.9±44.4)ng/L明显高于正常人(65.8±15.6)ng/L(P＜0.01)。治疗后反搏组ET水平(78.9±30.2)ng/L明显低于药物组(148.0±39.5)ng/L(P＜0.01)。NO^-₂/NO^-₃水平, 治疗前反搏组(64.4±14.8)μmol/L和药物组(67.0±24.0)μmol/L, 稍低于正常人(70.1±13.9)μmol/L, 但P＞0.05;治疗后反搏组(89.6±30.3)μmol/L高于正常人(P＜0.01), 药物组NO^-₂/NO^-₃(83.4±23.0)μmol/L与正常人比较无明显差异(P＞0.05)。体现血管收缩和舒张平衡关系的ET/(NO^-₂/NO^-₃)比值, 治疗前反搏组(1.9±0.8)和对照组(1.8±0.9)均高于正常人(1.0±0.3)(P＜0.01), 治疗后反搏组该值(0.9±0.4)下降(P＜0.01), 并接近正常人水平(P＞0.05), 而药物组(1.8±0.7)与治疗前比较无明显差异(P＞0.05)。结论:增强型体外反搏可改善冠心病患者的内皮功能。     

L-精氨酸对大鼠血管损伤后内膜增生及相关细胞周期调控基因表达的影响

目的:研究L-精氨酸(L-Arg)对大鼠血管损伤后内膜增生及相关细胞周期调控基因表达的影响。方法:大鼠分为假手术组,球囊损伤组(又分为术后48h、7d和14d亚组)及球囊损伤+L-Arg组。取各组实验动脉段测新生内膜面积,并采用免疫组化及计算机图像分析法测细胞周期依赖性激酶2(CDK2)、细胞周期蛋白E(cyclinE)和增殖细胞核抗原(PCNA)的表达水平。结果:球囊损伤后14d组的血浆NO水平低于假手术组(P＜0.01),CDK2、cyclinE及PCNA均于球囊损伤术后48h在中膜表达,第7d、14d在内膜表达,中膜几无表达,随着内膜增厚表达水平上升。与球囊损伤后14d组相比,球囊损伤+L-Arg组的血浆NO水平增高(P＜0.01),新生内膜面积少59.1%(P＜0.01),CDK2、cyclinE及PCNA的阳性表达指数分别低36.1%,46.3%和76.2%(P均＜0.01)。结论:L-Arg可有效抑制血管损伤后内膜增生,其机制可能与逆转CDK2、cyclinE及PCNA的过表达从而抑制血管平滑肌细胞增殖有关。     

Intimal smooth muscle cell proliferation after balloon catheter injury. The role of basic fibroblast growth factor.

The localization and synthesis of basic fibroblast growth factor (bFGF) in the rat carotid artery were investigated at times of chronic smooth muscle cell proliferation. Immunocytochemical staining showed the presence of bFGF in the uninjured arterial wall, and after balloon injury, this cellular staining was decreased. Western and northern blot analyses likewise showed that the amount of bFGF protein and mRNA decreased after injury. A neutralizing antibody to bFGF was administered 4 and 5 days after injury and was found to have no effect on intimal smooth muscle cell proliferation. These data suggest that an increase in the expression of bFGF is not necessary for chronic smooth muscle cell proliferation observed after balloon catheter injury and that bFGF is not the major mitogen responsible for intimal smooth muscle cell proliferation.     

Kinetics of cellular proliferation after arterial injury. II. Inhibition of smooth muscle growth by heparin

Heparin inhibits intimal thickening after arterial injury. Whether this effect is due to inhibition of medial smooth muscle cell (SMC) migration, SMC proliferation in the intima, or synthesis and deposition of connective tissue has not been evident. In this study we have investigated these possibilities in a rat carotid balloon injury model. Heparin (0.3 mg/kg/hour) was administered intravenously by means of osmotic pumps to experimental animals, and controls received lactated Ringer's solution. Smooth muscle proliferation (thymidine index), intimal smooth muscle accumulation, and endothelial regeneration were measured at intervals between 0 and 28 days. Total smooth muscle growth as determined biochemically at 14 days was markedly inhibited by heparin if the pumps were placed 24 hours before or at the time of injury and less so if inserted 48 or 96 hours after injury. SMC thymidine indices were maximal in the media at 4 days and in the intima at 7 days for injured arteries of both heparin-treated and control rats; at each time point SMC proliferation and intimal thickening were less in heparin-treated rats. The volume of connective tissue in the intima was the same in both groups at 28 days. Medial SMC migration into the intima was diminished by heparin treatment, but endothelial regeneration was not affected. These results support the hypothesis that heparin is a specific inhibitor of SMC migration and proliferation and is most effective if started before SMC enter S-phase.     

Doxycycline modulates smooth muscle cell growth, migration, and matrix remodeling after arterial injury

The tetracyclines function as antibiotics by inhibiting bacterial protein synthesis, but recent work has shown that they are pluripotent drugs that affect many mammalian cell functions including proliferation, migration, apoptosis, and matrix remodeling. Because all of these processes have been implicated in arterial intimal lesion development, the objective of these studies was to examine the effect of doxycycline treatment using a well-characterized model of neointimal thickening, balloon catheter denudation of the rat carotid artery. Rats were treated with 30-mg/kg/day doxycycline. Doxycycline reduced the activity of matrix metalloproteinase (MMP)-2 and MMP-9 in the arterial wall, and inhibited smooth muscle cell migration from media to intima by 77% at 4 days after balloon injury. Replication of smooth muscle cells in the intima at 7 days was reduced from 28.3 plus minus 2.5% in controls to 17.0 +/- 2.8% in doxycycline-treated rats. The synthesis of elastin and collagen was not affected, but accumulation of elastin was blocked in the doxycycline-treated rats. By contrast, collagen accumulation was not affected, which led to the formation of a more collagen-rich intima. At 28 days after injury, the intimal:medial ratio was significantly reduced from 1.67 +/- 0.09 in control rats to 1.36 +/- 0.06 in the doxycycline-treated rats. This study shows that doxycycline is an effective inhibitor of cell proliferation, migration, and MMP activity in vivo. Further study in more complicated models of atherosclerosis and restenosis is warranted.     

Eliminating Arterial Pulsatile Strain by External Banding Induces Medial but Not Neointimal Atrophy and Apoptosis in the Rabbit

We have examined the role of vessel pulsation and wall tension on remodeling and intimal proliferation in the rabbit infrarenal abdominal aorta. A rigid perivascular polyethylene cuff was used to reduce vessel systolic diameter by 25%, producing a region of reduced circumferential strain. At 6 weeks postoperatively, reduced circumferential strain caused medial atrophy, with 45% reduction of medial area and 30% loss of medial smooth muscle cells. Apoptotic cell death was indicated by DNA fragmentation, propidium iodide staining, and cell morphology. Cuffing the aorta after balloon denudation produced medial atrophy but did not inhibit neointimal growth. At 1 week postoperatively, intimal thickness was slightly decreased in regions with reduced strain; however, intimal thickening in regions of reduced strain was not different from control segments at 3 weeks postoperatively (intimal area was 0.37 ± 0.05 mm² with reduced strain and 0.50 ± 0.08 for controls, mean ± SEM). We conclude that circumferential strain is a major factor controlling medial structure and cell number, whereas growth of the neointima after injury is not significantly affected by either reduced strain or extensive medial cell death. Vessel cuffing represents a new model of blood vessel remodeling in vivo that involves extensive smooth muscle cell apoptosis.     

Wound healing in the media of the normolipemic rabbit carotid artery injured by air drying or by balloon catheter de-endothelialization.

The response to air-dry injury to the carotid artery of the normolipemic rabbit was compared with the response to de-endothelialization with a balloon catheter. Air drying induced an inflammatory response that resembled arteritis rather than atherosclerosis. There was medial damage, neutrophil but not macrophage infiltration, and fibrin formation, limited smooth muscle proliferation, which regressed after 3 months, and no lipid deposition. Within 1 week the smooth muscle cells were mainly of the secretory phenotype, and a neointima had formed. At 4 weeks the neointimal proliferation continued, but most cells showed a contractile phenotype. By 3 months, the lesion consisted of fibromuscular thickening with few small smooth muscle cells. Balloon injury induced minimal medial damage and continuing intimal proliferation with no evidence of regression by 3 months. It is concluded that air drying the carotid artery induces smooth muscle damage as well as endothelial cell loss, and this stimulates a wound-healing mechanism that is different from the response to selective intimal injury.     

Phenotypic features of smooth muscle cells during the evolution of experimental carotid artery intimal thickening. Biochemical and morphologic studies

Balloon catheter denudation of rat carotid artery that results in significant medial damage is followed by marked intimal smooth muscle cell (SMC) proliferation associated with limited endothelial regrowth. In this report we demonstrate that: (a) SMC of the carotid media, preceding their intimal proliferation, develop a cytoskeletal profile and morphology consistent with a de-differentiated SMC phenotype; and (b) both medial and intimal SMC subsequently revert to a cytoskeletal profile and morphology reflecting incomplete but significant re-differentiation toward normal SMC phenotype. Specifically, early after balloon injury, SMC of the media and those that have migrated into the intima contain decreased amounts of actin, desmin, and tropomyosin and increased amounts of vimentin; moreover, beta-actin becomes the dominant actin isoform, whereas alpha-actin decreases as compared with that found in normal medial SMC. Late after balloon injury, actin is still less abundant, however, desmin, tropomyosin, and vimentin return toward normal values and both medial and intimal SMC again show a predominance of alpha-actin, although the endothelium does not regenerate over the central surface of intimal thickening in this model. The SMC surface to volume ratio significantly decreases early after balloon injury, whereas it is not significantly different late after balloon injury as compared with that of SMC of the normal carotid media. We demonstrate, furthermore that: (c) adjacent luminal SMC are interconnected by gap junctions and develop focal tight junctions, a feature not reported previously to occur in smooth muscle; these cells however do not form any well defined membrane specialization with the leading edge of endothelium, supporting the view that presence of modified SMC on the luminal surface of chronically denuded vessels is not responsible for the cessation of endothelial regrowth.     

Smooth muscle cells of the coronary arterial tunica media express tumor necrosis factor-alpha and proliferate during acute rejection of rabbit cardiac allografts.

Graft coronary arteriosclerosis (GCA) frequently limits the long-term success of cardiac transplantation. The pathogenic mechanisms of and stimuli that provoke GCA remain uncertain. Whatever the initiating factors, deranged control of smooth muscle cells (SMC) proliferation likely contributes to the intimal hyperplasia that produces obstructive lesions. To identify mediators that may contribute to ongoing modulation of SMC functions during acute rejection and to explore the mechanisms of the pathogenesis of graft coronary arteriosclerosis, we studied the kinetics of proliferation and the expression of tumor necrosis factor-alpha (TNF-alpha), a proinflammatory and SMC growth-promoting cytokine, in coronary arterial SMCs in rabbit hearts transplanted heterotopically without immunosuppression. Hearts were harvested at 2 (n = 5), 5 (n = 5), and 8.2 +/- 0.4 (mean +/- SD, n = 5) days after transplantation, just before graft failure as judged clinically. SMC proliferation was assessed by continuous bromodeoxyuridine labeling (BrdU 10 mg/kg/d. s.q.). Whole heart cross sections were stained immunohistochemically with monoclonal antibodies that recognize TNF-alpha, BrdU, and SMCs (muscle alpha-actin). Major epicardial coronary arteries (five to nine profiles in each animal) were evaluated. Histological rejection grades by the International Society for Heart and Lung Transplantation scale at 2, 5, and 10 days were 1.6 +/- 0.9, 2.8 +/- 1.1, and 4.0 +/- 0.0, respectively. Medial SMCs in normal hearts and 2 days after transplant expressed little or no TNF-alpha and displayed negligible BrdU incorporation. At 5 days after transplantation, some medial SMCs stained for TNF-alpha and had a low BrdU labeling index (0.5 +/- 0.8%). At 8.2 days after transplant, almost all medial SMCs expressed TNF-alpha intensely and had a high labeling index (29.8 +/- 8.0%). These results demonstrate that acute rejection activates medial SMCs in coronary arteries to express TNF-alpha and that SMC-derived TNF-alpha may contribute to medial SMC proliferation in coronary arteries during acute rejection. This finding of early medial SMC replication suggests a novel and heretofore unsuspected mechanism of intimal expansion consequent to the allogeneic state. These results furnish additional insight into the possible mechanisms that link acute rejection with graft coronary arteriosclerosis. Furthermore, the close association of TNF-alpha expression with SMC replication provides not only a novel marker of SMC activation but also a potential new therapeutic target for the prevention of graft coronary disease.     

Sequence of cellular responses in rabbit aortas following one and two injuries with a balloon catheter

In order to further elucidate the pathogenesis of intimal proliferation and increased thrombogenesis following repeated arterial injuries we studied the sequence of the cellular changes following two injuries of rabbit aortas with a balloon catheter. Following the first injury, the de-endothelialized surface was covered by a platelet monolayer. Polymorphonuclear leucocytes adhered to the inner surface of this monolayer and did not appear to penetrate the vessel wall. By 4 to 7 days, areas of neointima had formed. Within seconds after the reinjury at 7 days after the de-endothelialization small platelet aggregates formed on injured neointimal smooth muscle cells. Within I min platelet thrombi and fibrin strands formed. At 30 min most of the platelet thrombi had become fibrin-rich. Polymorphonuclear leucocytes had accumulated and many had begun to penetrate into the neointimal tissue. The number and extent of penetration of leucocytes into the inner parts of the arterial wall increased with time. Four days after the injury the neointimal cushions were restored and thickened. Both following the first and second injury the formation of neointimal cushions was accompanied by a change in the polarity of the inner layers of medial smooth muscle cells, some of which appeared to have migrated into the neointima.     

Sequence of cellular responses in rabbit aortas following one and two injuries with a balloon catheter.

In order to further elucidate the pathogenesis of intimal proliferation and increased thrombogenesis following repeated arterial injuries we studied the sequence of the cellular changes following two injuries of rabbit aortas with a balloon catheter. Following the first injury, the de-endothelialized surface was covered by a platelet monolayer. Polymorphonuclear leucocytes adhered to the inner surface of this monolayer and did not appear to penetrate the vessel wall. By 4 to 7 days, areas of neointima had formed. Within seconds after the reinjury at 7 days after the de-endothelialization small platelet aggregates formed on injured neointimal smooth muscle cells. Within I min platelet thrombi and fibrin strands formed. At 30 min most of the platelet thrombi had become fibrin-rich. Polymorphonuclear leucocytes had accumulated and many had begun to penetrate into the neointimal tissue. The number and extent of penetration of leucocytes into the inner parts of the arterial wall increased with time. Four days after the injury the neointimal cushions were restored and thickened. Both following the first and second injury the formation of neointimal cushions was accompanied by a change in the polarity of the inner layers of medial smooth muscle cells, some of which appeared to have migrated into the neointima.