Updates in aortic wall pathology.

Several studies have investigated the pathogenesis of aortic wall abnormalities such as aortic dissection or aneurysm; however, the comprehensive pathological in situ event involved in the development of the disease is not understood well. The vasa vasorum form a network of capillaries or venules around the adventitia and outer media, which play an important role in the aortic wall structure and function. Impairment of their function may induce tissue hypoxia, impede the transfer of cellular nutrients, and cause aortic medial degeneration, which is considered the major predisposing factor to this aortic wall pathology. This review updates our understanding of the pathological changes in the aortic media and vasa vasorum of patients with aortic dissection and aortic aneurysm.

Atherosclerotic Coronary Plaque Is Associated With Adventitial Vasa Vasorum and Local Inflammation in Adjacent Epicardial Adipose Tissue in Fresh Cadavers.

BACKGROUND: The coronary adventitia has recently attracted attention as a source of inflammation because it harbors nutrient blood vessels, termed the vasa vasorum (VV). This study assessed the link between local inflammation in adjacent epicardial adipose tissue (EAT) and coronary arterial atherosclerosis in fresh cadavers.Methods and Results:Lesion characteristics in the left anterior descending coronary artery of 10 fresh cadaveric hearts were evaluated using integrated backscatter intravascular ultrasound (IB-IVUS), and the density of the VV and levels of inflammatory molecules from the adjacent EAT were measured for each of the assessed lesions. The lesions were divided into lipid-rich, lipid-moderate, and lipid-poor groups according to percentage lipid volume assessed by IB-IVUS. Higher expression of inflammatory molecules (i.e., vascular endothelial growth factor A [VEGFA] andVEGFB) was observed in adjacent EAT of lipid-rich (n=11) than in lipid-poor (n=11) lesions (7.99±3.37 vs. 0.45±0.85 arbitrary units [AU], respectively, forVEGFA; 0.27±0.15 vs. 0.11±0.07 AU, respectively, forVEGFB; P<0.05). The density of adventitial VV was greater in lipid-rich than lipid-poor lesions (1.50±0.58% vs. 0.88±0.23%; P<0.05). CONCLUSIONS: Lipid-rich coronary plaques are associated with adventitial VV and local inflammation in adjacent EAT in fresh cadavers. This study suggests that local inflammation of EAT is associated with coronary plaque progression via the VV.

Human pericoronary adipose tissue as storage and possible supply site for oxidized low-density lipoprotein and high-density lipoprotein in coronary artery.

BACKGROUND: Thickening of the pericoronary adipose tissue (PCAT) is a proven risk factor for coronary artery disease, but it is poorly understood whether PCAT stores pro-atherogenic substances with oxidized low-density lipoprotein (oxLDL) and low-density lipoprotein (LDL), and an anti-atherogenic substance with high-density lipoprotein (HDL) and supply them to the coronary intima. METHODS: Using immunohistochemical techniques, the localization of oxLDL, LDL and HDL in PCAT and its adjacent coronary segments was examined in 30 epicardial coronary arteries excised from 11 human autopsy cases. RESULTS: PCAT stored oxLDL and HDL in all, but LDL rarely, in 77 specimens examined, irrespective of the presence or absence of coronary plaques and underlying disease. The percentage (%) incidence of oxLDL, HDL and LDL deposits in intima was, respectively, 28, 10, 35 in 29 normal segments, 80 (p<0.05 vs. normal segments), 12, 75 in 19 white plaques (growth stage), 57, 36, 90 in 15 yellow plaques without necrotic core (NC; mature stage), and 40, 21, 100 (p<0.05 vs. normal segments) in 14 yellow plaques with NC (end-stage of maturation) as classified by angioscopy and histology. In coronary intima, oxLDL deposited in either a dotted or diffuse pattern whereas HDL and LDL showed diffuse patterns. Dotted oxLDL deposits were contained in CD68(+)-macrophages traversing the border of PCAT and adventitia, external and internal elastic laminae. Diffuse oxLDL and HDL deposits colocalized with intimal vasa vasorum. CONCLUSIONS: The results suggested that, as a hitherto unrecognized supplying route, the human PCAT stores oxLDL and HDL and oxLDL is supplied to coronary intima either by CD68(+)-macrophages or vasa vasorum and HDL by vasa vasorum, and that deposition of oxLDL and HDL in the intima increased with plaque growth but the former decreased while the latter increased further with plaque maturation. Molecular therapy targeting PCAT before plaque maturation could be effective in preventing atherosclerosis.

Vasa vasorum in the tunica media and tunica adventitia of the porcine aorta.

Vasa vasorum supply both the tunica adventitia and the tunica media of major arteries with nutrients and oxygen. We estimated the density of von Willebrand factor-positive profiles of vasa vasorum visible in transversal histological sections of 123 tissue samples collected from five anatomical positions in the porcine aortae of growing pigs (n=25). The animals ranged in age from 0 to 230 days. The tunica media of the thoracic aorta had a greater vasa vasorum density, with microvessels penetrating deeper towards the lumen than in the abdominal aorta. The density of vasa vasorum gradually decreased with age in both the media and the adventitia. The relative depth into which the vasa vasorum penetrated and where they branched remained constant during the ageing and growth of the media. The ratio of the tunica media and tunica adventitia thicknesses did not change in the single aortic segments during ageing. The media of older animals received fewer but equally distributed vasa vasorum. A greater density of vasa vasorum in the media was correlated with greater media thickness and a greater elastin fraction (data on elastin taken from another study on the same samples). Immunohistochemical quantification revealed deeper penetration of vasa vasorum towards the adluminal layers of the tunica media that were hitherto reported to be avascular. The complete primary morphometric data, in the form of continuous variables, have been made available as a supplement. Mapping of the vasa vasorum profile density and position has promising illustrative potential for studies on atherosclerotic and inflammatory neovascularization, aortic aneurysms, and drug distribution from arterial stents in experimental porcine models.

Topologic distributions of vasa vasorum and lymphatic vasa vasorum in the aortic adventitia--Implications for the prevalence of aortic diseases.

BACKGROUND AND AIMS: Vasa vasorum (VV) and lymphatic vasa vasorum (LVV) form their own networks in the adventitia. VV supply the aorta with nutrition and oxygen; however, the distribution and role of LVV remains to be determined. The purpose of this study was to investigate differences in the distribution of VV and LVV along the aorta. METHODS: Aortic samples were obtained from 22 autopsy cases without medical history of aortic diseases. Aortic segments were classified as arch (Ar), descending thoracic (De), suprarenal abdominal (S-Ab), and infrarenal abdominal (I-Ab). Adventitial VV and LVV were identified immunohistochemically. RESULTS: VV were most dense in the arch aorta, becoming less dense along the aorta in more distal segments, with the lowest density occurring in the infrarenal abdominal aorta. There was a significant correlation between the numbers of VV and medial thickness in the total aortic segments (r = 0.518, p < 0.01). In contrast, there was no significant correlation between the number of LVV and medial thickness in any aortic segments. However, there was a significant correlation between the number of LVV and intimal thickness in I-Ab (r = 0.425, p < 0.05). CONCLUSIONS: The distributions of adventitial VV and LVV were characteristic along the aortic segments. Differences in the distributions may imply the prevalence of aortic diseases such as dissection, abdominal aortic aneurysm, and atherosclerotic occlusive disease in each aortic segment.

Depiction of the vasa vasorum during carotid endarterectomy by intraoperative videoangiography.

BACKGROUND: The aim of the study was to investigate the depiction of the carotid artery by fluorescein sodium (FS) videoangiography compared with indocyanine green (ICG) videoangiography, focusing on how the vasa vasorum of the carotid artery is depicted. METHODS: Thirty-five patients (19 FS patients, 16 ICG patients, mean age 69.4 ± 5.1 years, mean degree of stenosis 78.7% ± 11.7%) who underwent a carotid endarterectomy (CEA) were enrolled. FS (5-6 mg/kg) or ICG (.2-.3 mg/kg) was injected intravenously as a bolus before the arterectomy during the CEA. The intravascular fluorescence signal was recorded with a digital video camera integrated on a microscope. Magnetic resonance imaging black-blood (BB) T1-weighted imaging (WI) was preoperatively performed using a 1.5-T whole-body imager, and the signal intensity ratio relative to the ipsilateral sternocleidomastoid muscle on BB-T1WI (BB-SIR) was calculated. We also performed an immunohistochemistry study using CD31 and CD68 antibodies for plaque specimens. RESULTS: In the FS videoangiography series, the vasa vasorum of carotid adventitia was depicted first, followed by augmentation of FS of the wall and partially the inner lumen (pattern A) in 6 cases. Augmentation of FS of the wall and inner lumen prior or simultaneous to the depiction of the vasa vasorum of the carotid adventitia (pattern B) were observed in 13 cases. The average BB-SIR value of the pattern B cases was significantly higher than that in the pattern A group (P < .05). Most of the plaques with BB-SIR values higher than 1.25 also belonged to the pattern B group (90.9%). Microvessels stained by CD31 and macrophages stained by CD68 were more frequently observed in the high-BB-SIR plaques. In contrast, the ICG videoangiography uniformly showed pattern B in all 16 cases, because of the fluorolucency of the carotid wall revealed by the ICG. CONCLUSIONS: The early depiction of adventitial vasa vasorum in FS videoangiography was inversely associated with the BB-SIR values of the plaques, along with many microvessels and macrophages that have been reported to have a tendency of intraplaque hemorrhage or symptoms. The present results may support the idea of an intimal origin of the neovascularization in vulnerable carotid plaques, and they demonstrated the potential of intraoperative plaque imaging by FS videoangiography.

Assessment of vasa vasorum in the walls of thrombophlebitic saphenous vein.

AIM: The purpose of this study was to investigate the proliferation of vasa vasorum (VV) in the walls of thrombophlebitic saphenous vein (TSV), and to evaluate the influence of high venous pressure and lack of oxygen on the VV. METHODS: The specimens of the great saphenous vein were collected: 11 primary varicose vein (PVV), 11 TSV and, as a control, eight normal great saphenous vein. Masson staining and immunohistochemistry for CD34 were used to observe the status of VV, and the number of VV were counted under light microscopy. RESULTS: VV of the thrombophlebitic saphenous vein group (TSVG) were clustered together in adventitia, increased linearly in media, and scattered appearance in intima, were increased partially in intima of thrombus rupturing. In TSVG, the number of VV observed in adventitia, media and intima was 16.738±7.685, 4.845±2.537, 2.448±3.030, respectively. In the primary varicose vein group (PVVG), the corresponding values were 14.280±4.440, 2.965±1.125, 0.500±0.548. And the number was 8.911±2.629, 0.150±0.424, 0±0 in the control group (CG). Significantly higher numbers of VV were observed in the TSVG as compared to the PVVG or CG (P<0.05). No significant difference was observed between intima and media (P>0.05). CONCLUSION: Thrombi in varicose veins can induce proliferation of VV, which may be involved in high venous pressure and hypoxia.

Quantitative analysis of carotid plaque vasa vasorum by CEUS and correlation with histology after endarterectomy.

BACKGROUND: Intraplaque neovascularization and vasa vasorum (VV) proliferation contribute in the progression and rupture of atherosclerotic lesions. Contrast enhanced ultrasonography (CEUS) has been reported to attain data regarding intraplaque neovessels and VV. However, whether the detection of microbubbles by CEUS within atherosclerotic plaques truly represents microvessels is a point of concern. We aimed to evaluate stable and unstable carotid artery plaque (CAP) VV pattern by CEUS and its correlation with histology and immunochemistry. PATIENTS AND METHODS: Patients with CAP scheduled for plaque endarterectomy were enrolled. CAP was initially identified by conventional ultrasonography and subsequently CEUS (harmonic ultrasound imaging with simultaneous intravenous contrast agent injection) was performed. The recorded image loops were evaluated by a semi-automated method. Plaque specimens were excised and underwent histological and immunochemical (for CD34, Vascular Endothelial Growth Factor, CD68 and CD3 antibodies) analysis. RESULTS: Fourteen patients (67.6 ± 10.2 years, 10 males) with a 86.9 ± 11.5 % degree of carotid artery stenosis were evaluated. Histology showed that half of the plaques were unstable. Enhancement of plaque brightness on CEUS was significant for both stable and unstable plaque subgroups (p = 0.018 for both). Immunochemistry showed that microvessels, as assessed by CD34 antibody, were more dense in unstable vs. stable plaques (36.6 ± 17.4 vs. 13.0 ± 7.2 respectively, p = 0.002). However, correlation between plaque brigthness enhancement on CEUS and microvessel density was significant only for stable (r = 0.800, p = 0.031) plaques. CONCLUSIONS: The identification of brightness enhacement during CEUS in carotid atherosclerotic plaques may not always reflect the presence of VV.

Dysfunctional vasa vasorum in diabetic peripheral artery obstructive disease with critical lower limb ischaemia.

OBJECTIVES AND DESIGN: To establish whether in diabetic patients with peripheral artery obstructive disease (PAOD) vasa vasorum (vv) neoangiogenesis is altered with increased arterial damage. MATERIALS: Thirty-three patients with PAOD and critical lower limb ischaemia, 22 with type II diabetes. METHODS: Immunohistochemistry for endothelial cell markers (CD34 and von Willebrand Factor); real-time reverse transcription polymerase chain reaction (RT-PCR) to quantify arterial wall expression of vascular endothelial growth factor (VEGF); enzyme-linked immunosorbent assay (ELISA) to assess blood VEGF; flow cytometry to detect circulating endothelial cells (CECs). RESULTS: Patients with PAOD and diabetes have a higher frequency (60% vs. 45%) of advanced atherosclerotic lesions and a significant reduction (p = 0.0003) in CD34(+) capillaries in the arterial media. Adventitial neoangiogenesis was increased equally (CD34(+) and vWF(+)) in all patients. Likewise, all patients have increased CEC and VEGF concentration in the blood as well as in-situ VEGF transcript expression. CONCLUSIONS: Patients with PAOD have remarkable arterial damage despite increased in-situ and circulating expression of the pro-angiogenic VEGF; a dysfunctional vv angiogenesis was seen in diabetics which also showed a higher frequency of parietal damage; it is suggested that in diabetic arterial wall, injury is worsened by vv inability to finalise an effective VEGF-driven arterial wall neoangiogenesis.

Segmental heterogeneity of vasa vasorum neovascularization in human coronary atherosclerosis.

OBJECTIVES: Our aim was to investigate the role of coronary vasa vasorum (VV) neovascularization in the progression and complications of human coronary atherosclerotic plaques. BACKGROUND: Accumulating evidence supports an important role of VV neovascularization in atherogenesis and lesion location determination in coronary artery disease. VV neovascularization can lead to intraplaque hemorrhage, which has been identified as a promoter of plaque progression and complications like plaque rupture. We hypothesized that distinctive patterns of VV neovascularization and associated plaque complications can be found in different stages of human coronary atherosclerosis. METHODS: Hearts from 15 patients (age 52+/-5 years, mean+/-SEM) were obtained at autopsy, perfused with Microfil (Flow Tech, Inc., Carver, Massachusetts), and subsequently scanned with micro-computed tomography (CT). The 2-cm segments (n=50) were histologically classified as either normal (n=12), nonstenotic plaque (<50% stenosis, n=18), calcified (n=10) or noncalcified (n=10) stenotic plaque. Micro-CT images were analyzed for VV density (number/mm2), VV vascular area fraction (mm2/mm2), and VV endothelial surface fraction (mm2/mm3). Histological sections were stained for Mallory's (iron), von Kossa (calcium), and glycophorin-A (erythrocyte fragments) as well as endothelial nitric oxide synthase, vascular endothelial growth factor, and tumor necrosis factor-alpha. RESULTS: VV density was higher in segments with nonstenotic and noncalcified stenotic plaques as compared with normal segments (3.36+/-0.45, 3.72+/-1.03 vs. 1.16+/-0.21, p<0.01). In calcified stenotic plaques, VV spatial density was lowest (0.95+/-0.21, p<0.05 vs. nonstenotic and noncalcified stenotic plaque). The amount of iron and glycophorin A was significantly higher in nonstenotic and stenotic plaques as compared with normal segments, and correlated with VV density (Kendall-Tau correlation coefficient 0.65 and 0.58, respectively, p<0.01). Moreover, relatively high amounts of iron and glycophorin A were found in calcified plaques. Further immunohistochemical characterization of VV revealed positive staining for endothelial nitric oxide synthase and tumor necrosis factor-alpha but not vascular endothelial growth factor. CONCLUSIONS: Our results support a possible role of VV neovascularization, VV rupture, and intraplaque hemorrhage in the progression and complications of human coronary atherosclerosis.

Mild inflammatory activation of mammary arteries in patients with acute coronary syndromes.

Acute coronary syndromes (ACS) are characterized by multiple unstable coronary plaques and elevated circulating levels of inflammatory biomarkers. The endothelium of internal mammary arteries (IMA), which are atherosclerosis resistant, is exposed to proinflammatory stimuli as vessels that develop atherosclerosis. Our study investigated the IMA endothelial expression of inflammatory molecules in patients with ACS or chronic stable angina (CSA). IMA demonstrated normal morphology, intact endothelial lining, and strong immunoreactivity for glucose transporter 1. E-selectin expression was observed more frequently in IMA of ACS patiention than CSA patients (ACS 61% vs. CSA 14%, P = 0.01). High fluorescence for major histocompatibility complex (MHC) was significantly more frequent on the luminal endothelium (ACS 66.7% vs. CSA 17.6%, P = 0.001 for class I; and ACS 66.7% vs. CSA 6.2%, P = 0.0003 for class II-DR) and on the vasa vasorum (ACS 92.9% vs. CSA 33.3% and 7.7%, P = 0.0007 and P < 0.0001 for class I and class II-DR, respectively) of ACS patients than CSA patients. ICAM-1, VCAM-1, Toll-like receptor 4, tissue factor, IL-6, inducible nitric oxide synthase, and TNF-alpha expression were not significantly different in ACS and CSA. Circulating C-reactive protein [ACS 4.8 (2.6-7.3) mg/l vs. CSA 1.8 (0.6-3.5) mg/l, P = 0.01] and IL-6 [ACS 4.0 (2.6-5.5) pg/ml vs. CSA 1.7 (1.4-4.0) pg/ml, P = 0.02] were higher in ACS than CSA, without a correlation with IMA inflammation. The higher E-selectin, MHC class I and MHC class II-DR on the endothelium and vasa vasorum of IMA from ACS patients suggests a mild, endothelial inflammatory activation in ACS, which can be unrelated to the presence of atherosclerotic coronary lesions. These findings indicated IMA as active vessels in coronary syndromes.

Molecular imaging of atherosclerotic plaques using a human antibody against the extra-domain B of fibronectin.

Current imaging modalities of human atherosclerosis, such as angiography, ultrasound, and computed tomography, visualize plaque morphology. However, methods that provide insight into plaque biology using molecular tools are still insufficient. The extra-domain B (ED-B) is inserted into the fibronectin molecule by alternative splicing during angiogenesis and tissue remodeling but is virtually undetectable in normal adult tissues. Angiogenesis and tissue repair are also hallmarks of advanced plaques. For imaging atherosclerotic plaques, the human antibody L19 (specific against ED-B) and a negative control antibody were labeled with radioiodine or infrared fluorophores and injected intravenously into atherosclerotic apolipoprotein E-null (ApoE-/-) or normal wild-type mice. Aortas isolated 4 hours, 24 hours, and 3 days after injection exhibited a selective and stable uptake of L19 when using radiographic or fluorescent imaging. L19 binding was confined to the plaques as assessed by fat staining. Comparisons between fat staining and autoradiographies 24 hours after 125I-labeled L19 revealed a significant correlation (r=0.89; P<0.0001). Minimal antibody uptake was observed in normal vessels from wild-type mice receiving the L19 antibody and in atherosclerotic vessels from ApoE-/- mice receiving the negative control antibody. Immunohistochemical studies revealed increased expression of ED-B not only in murine but also in human plaques, in which it was found predominantly around vasa vasorum and plaque matrix. In summary, we demonstrate selective targeting of atheromas in mice using the human antibody to the ED-B domain of fibronectin. Thus, our findings may set the stage for antibody-based molecular imaging of atherosclerotic plaques in the intact organism.

Hypoxia-induced pulmonary artery adventitial remodeling and neovascularization: contribution of progenitor cells.

Information is rapidly emerging regarding the important role of the arterial vasa vasorum in a variety of systemic vascular diseases. In addition, increasing evidence suggests that progenitor cells of bone marrow (BM) origin may contribute to postnatal neovascularization and/or vascular wall thickening that is characteristic in some forms of systemic vascular disease. Little is known regarding postnatal vasa formation and the role of BM-derived progenitor cells in the setting of pulmonary hypertension (PH). We sought to determine the effects of chronic hypoxia on the density of vasa vasorum in the pulmonary artery and to evaluate if BM-derived progenitor cells contribute to the increased vessel wall mass in a bovine model of hypoxia-induced PH. Quantitative morphometric analyses of lung tissue from normoxic and hypoxic calves revealed that hypoxia results in a dramatic expansion of the pulmonary artery adventitial vasa vasorum. Flow cytometric analysis demonstrated that cells expressing the transmembrane tyrosine kinase receptor for stem cell factor, c-kit, are mobilized from the BM in the circulation in response to hypoxia. Immunohistochemistry revealed an increase in the expression of c-kit+ cells together with vascular endothelial growth factor, fibronectin, and thrombin in the hypoxia-induced remodeled pulmonary artery vessel wall. Circulating mononuclear cells isolated from neonatal calves exposed to hypoxia were found to differentiate into endothelial and smooth muscle cell phenotypes depending on culture conditions. From these observations, we suggest that the vasa vasorum and circulating progenitor cells could be involved in vessel wall thickening in the setting of hypoxia-induced PH.

The presence of collagen III and IV in experimental microsurgical anastomoses of arteries.

PURPOSE: This study investigated the distribution of collagen III and IV in experimental microvascular anastomoses and correlated the findings with the quality of the healing process. MATERIALS AND METHODS: A previously excised and reanastomosed section of the left common carotid artery of 21 female Wistar rats were explanted 4 weeks after surgery. Collagens III and IV were identified in perfusion-fixed specimens by immunohistochemistry. RESULTS: The identification and distribution of collagens reflected the quality of each suturing procedure. A myointimal hyperplasia was present, extending into the vascular lumen, and was closely related to the ends of the vessel segments. The extent of myointimal hyperplasia was dependent on the length and the vitality of the vessel segments. The media was the weakest part of the tissue layers with regard to regenerative potential in the axial direction. The marked cuff of the adventitia around the anastomosis was rich in collagen III-positive vasa vasorum. Collagen IV was distributed in net-like patterns in the media. The distribution pattern in the media was not observed in anastomoses with incomplete approximation and necrotic segment ends. The collagen IV-positive subendothelial basal membrane was incomplete in the aneurysms because of partial necrosis. CONCLUSIONS: The experimental findings support the need for atraumatic suturing in microvascular surgery. Immunohistochemical detection of collagens can be a valuable tool for studying the basic processes of wound healing and the integration of microvascular flaps into their recipient sites.

Endothelial Gi protein expression is markedly low in human coronary microvessels.

Gi protein functionally mediates endothelium-dependent relaxations in large epicardial coronary arteries but not in small coronary arteries, which suggests a different involvement of Gi protein in the endothelium-dependent relaxations between large and small coronary arteries. We previously showed that endothelial Gi protein is present in human epicardial coronary artery. In the present study, we examined the expression of endothelial Gi protein in human coronary microvessels. Immunohistochemical staining with a specific antibody against human Gi protein was performed in intramyocardial coronary microvessels and vasa vasorum from 34 autopsy cases. The immunoreactive levels of the endothelial Gi protein were semiquantitated into four grades (none, 0; slight, +1; moderate, +2; high, +3), and the mean value of the ratings of all endothelial cells was then used as an index of the endothelial Gi protein expression of the vessel. The immunoreactive levels of the endothelial Gi protein were extremely low in intramyocardial coronary microvessels and in vasa vasorum, irrespective of the age of the patients, the presence or absence of coronary risk factors, or the influence of medical treatments. These results may therefore explain in part why endothelium-dependent relaxations in coronary microvessels are not functionally mediated by Gi protein.

Localization of tissue plasminogen activator in the endothelium of a limited number of vessels.

The immunolocalization of tissue plasminogen activator (tPA) was assessed in vessels of various sizes from baboons. Femoral artery and vein, carotid artery, aorta, and sections from basal ganglia and cerebral cortex were stained for tPA and CD31, an endothelial cell-specific surface antigen. In each case, the endothelium of the large vessel stained positively for anti-CD31 but not for tPA. However, vascular structures in the adventitia corresponding to the vasa vasorum were found to be associated with tPA antigen. In situ hybridization of femoral artery with 35S-labeled cRNA probes detected tPA mRNA in the vasa vasorum but not the large vessel endothelium. Analysis of the microvasculature of the basal ganglia and cerebral cortex showed limited immunohistochemical staining for tPA; only 3% of the vessels measuring 4 to 100 mu were positive. Even so, tPA was mostly distributed within a narrow range of vessel size; 90% of the positive vessels were classified as precapillary arterioles and postcapillary venules (7.5 to 30.0 mu), whereas only 3% of the capillaries were positive, despite accounting for 40% of all vessels. Thus, tPA-containing endothelium are distributed mainly in smaller vessels, excluding the capillaries.