Seeding endothelium onto canine arterial prostheses. The effects of graft design

Engineering the multiscale complexity of vascular networks

The survival of vertebrate organisms depends on highly regulated delivery of oxygen and nutrients through vascular networks that pervade nearly all tissues in the body. Dysregulation of these vascular networks is implicated in many common human diseases such as hypertension, coronary artery disease, diabetes and cancer. Therefore, engineers have sought to create vascular networks within engineered tissues for applications such as regenerative therapies, human disease modelling and pharmacological testing. Yet engineering vascular networks has historically remained difficult, owing to both incomplete understanding of vascular structure and technical limitations for vascular fabrication. This Review highlights the materials advances that have enabled transformative progress in vascular engineering by ushering in new tools for both visualizing and building vasculature. New methods such as bioprinting, organoids and microfluidic systems are discussed, which have enabled the fabrication of 3D vascular topologies at a cellular scale with lumen perfusion. These approaches to vascular engineering are categorized into technology-driven and nature-driven approaches. Finally, the remaining knowledge gaps, emerging frontiers and opportunities for this field are highlighted, including the steps required to replicate the multiscale complexity of vascular networks found in nature.

Vascular Tissue Engineering: Challenges and Requirements for an Ideal Large Scale Blood Vessel

Since the emergence of regenerative medicine and tissue engineering more than half a century ago, one obstacle has persisted: the in vitro creation of large-scale vascular tissue (>1 cm³) to meet the clinical needs of viable tissue grafts but also for biological research applications. Considerable advancements in biofabrication have been made since Weinberg and Bell, in 1986, created the first blood vessel from collagen, endothelial cells, smooth muscle cells and fibroblasts. The synergistic combination of advances in fabrication methods, availability of cell source, biomaterials formulation and vascular tissue development, promises new strategies for the creation of autologous blood vessels, recapitulating biological functions, structural functions, but also the mechanical functions of a native blood vessel. In this review, the main technological advancements in bio-fabrication are discussed with a particular highlights on 3D bioprinting technologies. The choice of the main biomaterials and cell sources, the use of dynamic maturation systems such as bioreactors and the associated clinical trials will be detailed. The remaining challenges in this complex engineering field will finally be discussed.

Vascular Tissue Engineering: Progress, Challenges, and Clinical Promise

Although the clinical demand for bioengineered blood vessels continues to rise, current options for vascular conduits remain limited. The synergistic combination of emerging advances in tissue fabrication and stem cell engineering promises new strategies for engineering autologous blood vessels that recapitulate not only the mechanical properties of native vessels but also their biological function. Here we explore recent bioengineering advances in creating functional blood macro and microvessels, particularly featuring stem cells as a seed source. We also highlight progress in integrating engineered vascular tissues with the host after implantation as well as the exciting pre-clinical and clinical applications of this technology.

Local Endovascular Delivery, Gene Therapy, and Cell Transplantation for Peripheral Arterial Disease

Advances in catheter technology, gene identification, and cell biology may provide novel treatment options for patients with peripheral arterial disease (PAD) who are not candidates for standard revascularization procedures. Animal studies and recent results in human beings suggest that transfer of growth factors or regulatory genes and transplantation of progenitor cells may provide novel therapy options by inducing therapeutic angiogenesis or by inhibiting restenosis. This review will discuss the development of a variety of catheters for localized endovascular delivery, as well as the various cellular and genetic strategies that exist to restore blood flow to ischemic tissue and to reduce neointimal hyperplasia.

The anticoagulant effect of PGI2S and tPA in transgenic umbilical vein endothelial cells is linked to up-regulation of PKA and PKC

The selection of vascular grafts for coronary artery bypass surgery is crucial for a positive outcome. This study aimed to establish a novel line of vascular endothelial cells with a potent anticoagulant effect. A lentiviral vector was used to stably transfect human umbilical vein endothelial cells (HUVECs) with PGI2S alone (HUVEC-PGI2S) or both PGI2S and tPA (HUVEC-PGI2S-tPA). Both HUVEC-PGI2S and HUVEC-PGI2S-tPA cells over-expressing PGI2S and tPA were compared to mock-transfected cells. The enzyme-linked immuno sorbent assay (ELISAs) demonstrated that the anticoagulation components, ATIII and PLG, were up-regulated and coagulation factor FVIII was down-regulated in both cell lines. QRT-PCR and western blotting demonstrated the vasodilation and platelet disaggregation proteins PKA, PKC, and PTGIR were up-regulated in both cell lines, but MAPK expression was not altered in either cell line. However, cell viability and colony formation assays and cell cycle analysis demonstrated that both cell lines had a lower rate of cell growth and induced G1 phase arrest. HUVEC-PGI2S and HUVEC-PGI2S-tPA cells have a potent anticoagulant effect and their use in vascular heterografts may decrease the risk of thrombosis.

Prosthetic vascular grafts: Wrong models, wrong questions and no healing

In humans, prosthetic vascular grafts remain largely without an endothelium, even after decades of implantation. While this shortcoming does not affect the clinical performance of large bore prostheses in aortic or iliac position, it contributes significantly to the high failure rate of small- to medium-sized grafts (SMGs). For decades intensive but largely futile research efforts have been under way to address this issue. In spite of the abundance of previous studies, a broad analysis of biological events dominating the incorporation of vascular grafts was hitherto lacking. By focusing on the three main contemporary graft types, expanded polytetrafluoroethylene (ePTFE), Dacron and Polyurethane (PU), accumulated clinical and experimental experience of almost half a century was available. The main outcome of this broad analysis-supported by our own experience in a senescent non-human primate model-was twofold: Firstly, inappropriate animal models, which addressed scientific questions that missed the point of clinical relevance, were largely used. This led to a situation where the vast majority of investigators unintentionally studied transanastomotic rather than transmural or blood-borne endothelialization. Given the fact that in patients transanastomotic endothelialization (TAE) covers only the immediate perianastomotic region of sometimes very long prostheses, TAE is rather irrelevant in the clinical context. Secondly, transmural endothelialization seems to have a time window of opportunity before a build-up of an adverse microenvironment. In selecting animal models that prematurely terminate this build-up through the early presence of an endothelium, the most significant 'impairment factor' for physiological tissue regeneration in vascular grafts remained ignored. By providing insight into mechanisms and experimental designs which obscured the purpose and scope of several decades of vascular graft studies, future research may better address clinical relevance.

Vascular prostheses: performance related to cell-shear responses

BACKGROUND

This work concerned the endothelialization of vascular prostheses and subsequent improvement of functionality with respect to tissue engineering. The aim of the study was to investigate the initial, pre-shear stress cellular behavior with respect to three vascular biomaterials to explain subsequent cellular responses to physiological shear stresses.

MATERIALS AND METHODS

Expanded polytetrafluoroethylene (ePTFE), polyethyleneterephthalate (polyester; Dacron; PET), and electrostatically spun polyurethane (PU) (all pre-impregnated with collagen I/III) were cell-seeded with L929 immortalized murine fibroblasts or human umbilical vein endothelial cells (HUVECs). Cytoskeletal involvement, cell height profiles, and immunohistochemistry were examined after 7 d static culture.

RESULTS

All three vascular biomaterials demonstrated different structures. Cell behavior varied both between the materials and the two cell types: cytoskeletal involvement was greater for the HUVECs and the more fibrous surfaces; height profiles were greater for the L929 and PET, and lowest on PU. Immunohistochemistry of HUVEC samples also showed differences: PU revealed the greatest expression of intercellular adhesion molecule-1 and E-selectin (PET and ePTFE the lowest, respectively); ePTFE produced the greatest for vascular cell adhesion molecule-1 (PET the lowest).

CONCLUSIONS

Material substrate influenced the cellular response. Cells demonstrating firm adhesion increased their cytoskeletal processes and expression of cell-substratum and inter-cellular adhesion markers, which may explain their ability to adapt more readily to shear stress. The fibrous PU structure appeared to be most suited to further shear stress exposure. This study demonstrated the potential of the underlying vascular material to affect the long-term cellular functionality of the prosthesis.

Development of small-diameter vascular grafts

INTRODUCTION

Cardiovascular disease, including coronary artery and peripheral vascular pathologies, is the leading cause of mortality in the United States and Western countries. There is a pressing need to develop small-diameter vascular vessels for bypass surgery and other vascular reconstructive procedures. Tissue engineering offers the prospect of being able to meet the demand for replacement of diseased vessels. Significant advances have been made in recent studies and provide confidence that success is attainable. For instance, a completely cellular approach culturing cells into tissue sheets and wrapping these layers was able to form a layered cellular vascular graft with impressive strength.

METHODS/RESULTS

In our experiments, decellularization and heparin immobilization grafts from porcine tissues implanted in a canine model could be repopulated from the host cells, indicating the grafts' potential to develop into living tissues that can adapt and respond to changes in the body.

CONCLUSIONS

This review summarizes the current status of vascular grafts used clinically, updates the most recent developments on vascular tissue engineering, and discusses the challenges for the future.

Transmural communication at a subcellular level may play a critical role in the fallout based-endothelialization of dacron vascular prostheses in canine

A microporous and permeable wall is important for the healing of vascular prostheses, however, the significance of its permeability to soluble substances at subcellular level has not been demonstrated. Polyester arterial prostheses were prepared in such a way that each of them contained three segments, of which at least one segment was impervious and another segment was permeable to water but impermeable to cells. Twenty graft segments were implanted in 7 dogs as a thoraco-abdominal bypass for 2 months. The prostheses were then harvested, photographed, and treated for histological and morphological studies. The low porosity graft capped by two thrombogenic segments was fully endothelialized, proving the fallout mechanism. The striking contrast with its impermeable counterpart demonstrated that a wall permeable to small substances of subcellular level was critical for the endothelial healing. A wide range of water permeabilities did not reveal advantages of high water permeable segments over low water permeable ones. Endothelial ingrowth from anastomoses was also jeopardized in the absence of wall permeability. In conclusion, transmural communication at a subcellular level may have played a critical role in the fallout based-endothelialization of arterial prostheses in canine. This highlights the potential function of perigraft cytokines and growth factors in endothelial healing.

Local endovascular delivery, gene therapy, and cell transplantation for peripheral arterial disease

Advances in catheter technology, gene identification, and cell biology may provide novel treatment options for patients with peripheral arterial disease (PAD) who are not candidates for standard revascularization procedures. Animal studies and recent results in human beings suggest that transfer of growth factors or regulatory genes and transplantation of progenitor cells may provide novel therapy options by inducing therapeutic angiogenesis or by inhibiting restenosis. This review will discuss the development of a variety of catheters for localized endovascular delivery, as well as the various cellular and genetic strategies that exist to restore blood flow to ischemic tissue and to reduce neointimal hyperplasia.

Basic Fibroblast Growth Factor Coating and Endothelial Cell Seeding of a Decellularized Heparin-coated Vascular Graft

The objective of this study was to determine the effect of basic fibroblast growth factor (bFGF) coating on endothelial cell seeding and proliferation on a decellularized heparin coated vascular graft and to determine the retention of seeded cells on the graft under flow conditions. Disks of heparin coated decellularized grafts were incubated for 24 h as controls or with bFGF. Human microvascular endothelial cells (HMECs) or canine peripheral blood endothelial progenitor cells (CEPC) were seeded onto the disks and incubated for 96 h or 48 h, respectively. HMECs were also seeded onto the luminal surfaces of two heparin-coated decellularized grafts for 3 h. One graft was placed in a perfusion culture system and cultured for an additional 6 h with flow and pressure. After culturing, there were 4.7 +/- 1.4 cells/mm(2) HMECs on control grafts and 11.4 +/- 1.4 cells/mm(2) in bFGF treated grafts (P < 0.05). Likewise, with CEPCs, there were 14.8 +/- 4.8 cells/mm(2) in control grafts and 33.3 +/- 7.3 cells/mm(2) in bFGF treated grafts. After only 3 h of cell attachment, 60% of HMECs were retained in the intact graft exposed flow relative to the static control graft, which is an acceptable level. These data demonstrate that bFGF coating on the heparin bound decellularized grafts significantly increases both HMEC and dog EPC proliferation and that seeded cells are stable under perfusion conditions.

Tissue engineering therapy for cardiovascular disease

The present treatments for the loss or failure of cardiovascular function include organ transplantation, surgical reconstruction, mechanical or synthetic devices, or the administration of metabolic products. Although routinely used, these treatments are not without constraints and complications. The emerging and interdisciplinary field of tissue engineering has evolved to provide solutions to tissue creation and repair. Tissue engineering applies the principles of engineering, material science, and biology toward the development of biological substitutes that restore, maintain, or improve tissue function. Progress has been made in engineering the various components of the cardiovascular system, including blood vessels, heart valves, and cardiac muscle. Many pivotal studies have been performed in recent years that may support the move toward the widespread application of tissue-engineered therapy for cardiovascular diseases. The studies discussed include endothelial cell seeding of vascular grafts, tissue-engineered vascular conduits, generation of heart valve leaflets, cardiomyoplasty, genetic manipulation, and in vitro conditions for optimizing tissue-engineered cardiovascular constructs.

Acutely sodded expanded polytetrafluoroethylene grafts produce only prostacyclin: a qualitative difference from saphenous veins

Select subsets of patients require prosthetic graft material for revascularization. Although arterial prosthetic grafts of large caliber perform acceptably, grafts of <6 mm exhibit a high attrition rate. Microvessel endothelial sodding, a method resulting in the lining of prosthetic grafts with autologous endothelium, improves graft patency; however, aggressive antiplatelet therapy is still required, because terminating an antiplatelet regimen accelerates graft attrition. The present investigation was designed to address the acute production of vasoactive substances in microvessel endothelial cell sodded expanded polytetrafluoroethylene (ePTFE) grafts in an attempt to delineate a possible mechanism behind the continued requirement for antiplatelet therapy. Equal lengths of acutely sodded ePTFE grafts (canine falciform ligament source) and saphenous veins (SV) (canine source) were evaluated by superfusion bioassay. Basal secretion from ePTFE grafts relaxed the biodetector ring 1 +/- 3%, whereas SV relaxed the ring 10 +/- 3% (p < 0.05, ePTFE vs. SV). Relaxation with acetylcholine stimulation was 49 +/- 7% in grafts and 50 +/- 10% in veins (p = NS). Calcium ionophore stimulation produced relaxation of 37 +/- 9% from ePTFE grafts and 100 +/- 23% from SV (p < 0.05). Indomethacin added to perfusate reduced relaxations from sodded ePTFE grafts to 20.2 +/- 9.2% with acetylcholine stimulation and 12.5 +/- 4.3% with calcium ionophore (p < 0.05 vs. control); addition of N(G)-monomethyl-L-arginine (L-NMMA) had no effect on the release of vasoactive substances from ePTFE grafts. In contrast, relaxations of effluent from SV stimulated by acetylcholine and calcium ionophore were significantly attenuated with indomethacin and L-NMMA (p < 0.05 vs. control). Scanning electron microscopy demonstrated confluent endothelium in SV and a nonconfluent endothelial cell layer in grafts. Acutely sodded ePTFE grafts produce vasoactive substances that quantitatively and qualitatively differ from those produced by canine SV. The ePTFE grafts produce mainly prostanoids, whereas SV produce both nitric oxide and prostanoids. The endothelial cell isolation procedure and absence of immediate graft luminal confluence may contribute to the observed differences.

Endothelial cell delivery for cardiovascular therapy

Obstructive atherosclerotic vascular disease stands as one of the greatest public health threats in the world. While a number of therapies have been developed to combat vascular disease, endothelial cell delivery has emerged as a distinct therapeutic modality. In this article, we will review the anatomy of the normal blood vessel and the biology of the intact endothelium, focusing upon its centrality in vascular biology and control over the components of the vascular response to injury so as to understand better the motivation for a cell-based form of therapy. Our discussion of cell delivery for cardiovascular therapy will be divided into surgical and interventional approaches. We will briefly recount the development of artificial grafts for surgical vascular bypass before turning our attention towards endothelial cell seeded vascular grafts, in which endothelial cells effectively provide local delivery of endogenous endothelial secretory products to maintain prosthetic integrity after surgical implantation. New techniques in tissue and genetic engineering of vascular grafts and whole blood vessels will be presented. Methods for percutaneous interventions will be examined as well. We will evaluate results of endoluminal endothelial cell seeding for treatment of restenosis and gene therapy approaches to enhance endogenous re-endothelialization. Finally, we will examine some innovations in endothelial cell delivery that may lead to the development of endothelial cell implants as a novel therapy for controlling proliferative vascular arteriopathy.

Release of PDGF-BB and bFGF by human endothelial cells seeded on expanded polytetrafluoroethylene vascular grafts

BACKGROUND

The majority of endothelial cell (EC) seeded graft failures are due to anastomotic neointimal fibrous hyperplasia. We investigated the PDGF-BB and bFGF release in vitro by umbilical vein EC seeded on precoated expanded polytetrafluoroethylene (ePTFE) prostheses.

MATERIALS

EC harvested from human umbilical veins were seeded into ePTFE (30 microns internodal distance, 1 cm2 in diameter) disks. ePTFE disks uncoated or precoated with collagen type I, fibronectin, and Matrigel were used, and EC seeded into plastic wells coated as ePTFE disks or uncoated plastic wells served as controls. Scanning electron microscopy study assessed EC coverage. The presence of bFGF and PDGF-BB in serum-free conditioned media from EC seeded into ePTFE grafts and EC seeded into wells was determined by the inhibition antibody-binding assay 72 h after seeding.

RESULTS

EC coverage was similar in uncoated and coated ePTFE grafts. The release of PDGF-BB and bFGF by EC seeded into ePTFE grafts was significantly higher than that observed in EC seeded into plastic wells. The release of PDGF-BB and bFGF was independent from the various substrates used in the experiments in EC seeded into either ePTFE grafts or plastic wells.

CONCLUSIONS

Our findings pointed out that in seeded ePTFE grafts, anastomotic smooth muscle cell proliferation and intimal thickening could take place underneath an intact endothelium because seeded EC may release several growth factors.

Endothelial cells on Dacron vascular prostheses: adherence, growth, and susceptibility to neutrophils

Human umbilical vein endothelial cells (HUVEC) on knitted and woven Dacron prostheses were compared with HUVEC on smooth surfaces (tissue culture polystyrene, PET film, and Natrix) with regard to adherence, growth, and susceptibility to injury by neutrophils (PMN). These are properties of importance for successful seeding or coating of prostheses. For prosthetic material of given macroscopic dimensions, more endothelial cells (EC) adhered than to smooth surfaces. However, the prostheses had a greater effective surface area as determined by the number of EC at confluency. When this parameter was taken into account, fewer EC were found adherent to prosthetic material per unit effective surface area than for the smooth surface substrates. Growth on prostheses was clearly inferior to that on smooth surfaces, and EC on prostheses were more susceptible to attack by activated PMN than on smooth surfaces. These differences may reflect the topographic differences in cells attached to fibers where they assume more distorted shapes by stretching to span fibers.

Culture of human adult endothelial cells on endarterectomy surfaces

OBJECTIVES

Endothelial cell seeding of prosthetic grafts has not been as successful as initially hoped and the application of seeding technology to alternative reconstructive procedures such as endarterectomy and angioplasty has been increasingly considered. The success of such seeding depends on the ability of the seeded cells to attach to, and form a monolayer on the endarterectomised vessel wall which was the aim of this study.

METHODS

Using a seeding chamber model, heterologous human adult endothelial cells were seeded onto fresh human endarterectomy specimens and cultured. Studies of endothelial call adherence to endarterectomy specimens were performed using 111-Indium oxine labelled cells using methodology analogous to graft seeding.

RESULTS

Mean endothelial cell adherence of 70% (S.D. 10%) after 1 h incubation was achieved and the successful development of a monolayer of human adult venous endothelial cells on endarterectomised arteries was demonstrated in vitro.

CONCLUSIONS

These results indicate that closed endarterectomy appears to offer a surface with cell attachment that is superior to prosthetic grafts. Where femoral endarterectomy is appropriate, endothelial seeding potentially offers a method of reducing thrombogenicity and intimal hyperplasia, improving patency and avoiding a prosthetic graft whilst preserving collateral circulation and autologous vein.

Comparison of tissue factor and prostacyclin production by human umbilical vein endothelial cells on Dacron vascular prostheses and Dacron smooth films

The functional capacity of human umbilical vein endothelial cells (HUVEC) grown on Dacron (polyethylene terephthalate; PET) vascular prosthetic material was compared with the function of cells on smooth surfaced PET, tissue culture polystyrene (TCPS), and Natrix-coated TCPS. Prosthetic materials include two knitted fabrics (Bionit I and II) and two woven preparations (DeBakey Soft Woven and Extra Low Porosity). Two entities produced by HUVEC that influence blood coagulation were assessed: the procoagulant tissue factor (TF) and the anticoagulant prostacyclin (PGI2). Although TF activity was stimulated on all substrates by endotoxin (LPS), there was no difference among prostheses and no difference among smooth surface materials, but TF was reduced in cells on the prosthetic materials relative to those on smooth surface substrates. The reduced TF production by HUVEC on prosthetic material could be reversed by returning them to TCPS. In contrast, PGI2 production on prostheses was comparable to that on smooth surfaces for both stimulated and unstimulated cells. Stimulation with histamine (1 microM) gave a 2.4-fold increase in PGI2 whereas mellitin (10 micrograms/ml) increased production 12.5-fold. The differential response of HUVEC with regard to these two coagulation factors, one of which is secreted and the other membrane bound, may reflect the distorted shape of cells on fibers of the prosthesis.

The endothelium: a key to the future

The vascular endothelium is a complex modulator of a variety of biological systems and may well be the key to definitive success in the treatment of cardiovascular disorders. Surgically-induced endothelial injury may occur preoperatively during cardiac catheterization and intraoperatively from mechanical manipulation, ischemia, hypothermia, and exposure to cardioplegic solutions. The normal endothelium is antithrombogenic and yet promotes platelet aggregation and coagulation if injured. Vasospasm, occlusive intimal hyperplasia, and accelerated arteriosclerosis can also all occur as a result of endothelial injury. Furthermore, endothelial injury is harmful even in the absence of disruption of its monolayer integrity. Thus, preservation of the endothelium should be an additional objective for all cardiovascular surgeons. Synthetic vascular grafts, cardiac valves, and artificial ventricles do not spontaneously endothelialize and thus usually require some form of anticoagulation to maintain patency. Hence, endothelialization of prosthetic implants became an attractive concept. A number of different methods of obtaining an endothelial lining of prosthetic material has since been developed; these include facilitated endothelial cell migration, and endothelial cell seeding by using either venous or microvascular endothelial cells. Manipulating the endothelium might well provide the next major advancement for therapeutic and preventive measures for cardiovascular disease.

Endothelial cell seeding: a review

The concept of endothelial cell seeding, designed to provide vascular grafts with a nonthrombogenic lining, has progressed from crude animal experiments during the past two decades to detailed in vitro functional studies using human cells. Although favorable results have been obtained in animal studies this has yet to be translated to humans, where current application of these techniques has been limited to a very few clinical trials. The history, current status and future directions are reviewed herein.

Optimal seeding conditions for human endothelial cells

An in vitro model of endothelial cell seeding has been developed to individually evaluate the steps required for seeding arterial prostheses. Human saphenous vein endothelial cells are radiolabeled with tritiated thymidine and seeded onto 4 mm polytetrafluoroethylene grafts. Grafts are then placed into a perfusion circuit for determination of cellular retention. Using this model, the following variables were studied: (1) graft coating (fibronectin versus serum versus plasma); (2) time of incubation of cells with graft (0, 20, 90 minutes); (3) density of the initial seeding solution (4 x 10(3)-6 x 10(5) cells/cm2). The data suggest that incubation of a graft with plasma provides an adhesive surface that is as effective as fibronectin for enhancing cell retention. With this particular model, seeding densities between 1 and 2 x 10(5) cells/cm2 produce a confluent monolayer with optimal utilization of cells. A shorter 20 minute incubation period resulted in the retention of only half of the seeded cells, while postperfusion attachment increased significantly with a 90 minute incubation period. Data derived from this system can be used to construct a protocol that may be useful for clinical in vivo seeding trials.

Endothelial cell seeding

Endothelial cell seeding is a technique that has developed over the past 15 years in response to the need for a high performance synthetic vascular graft. This review details our present knowledge of seeding and examines the various problems that have hampered its introduction into clinical practice.

Human microvessel endothelial cell isolation and vascular graft sodding in the operating room

We have evaluated multiple factors inherent to an operating room-compatible endothelial cell procurement and sodding procedure. Microvessel endothelial cell isolations have been performed on fat tissue obtained from over 140 patients with a 100% success rate. Liposuction-derived fat was optimal with respect to cell yield, and isolation time. The devices and equipment used were acceptable to the operating room and the complete cell procurement procedure was successful even in the hands of personnel with minimal training. Fat digestion was achieved using crude clostridial collagenase, with an average cell yield of 1 x 10(6) microvessel endothelial cells/gm of fat. Evaluation of this procedure with canine fat using an operating room acceptable procedure resulted in a 100% procurement success rate requiring 1.5 hours (+/- .5 hrs) for completion of the fat isolation, and cell isolation procedure. Microvessel EC could subsequently be used in graft seeding or sodding techniques to establish endothelial cell monolayers on vascular grafts. Our results indicate that one person with minimal cell isolation background can reproducibly isolate large quantities of sterile autologous endothelial cells in the operating room for immediate use in endothelial cell seeding/sodding procedures.

Endothelialization of polymer surfaces

The role that substrate surface properties play in influencing the extent of endothelialization of polymer surfaces has been investigated. For a wide range of polymer surfaces, the degree of endothelialization for both porcine and bovine endothelial cells is directly related to polymer surface tension: increased endothelialization occurring with increasing substrate surface tension. As a result of adsorption of the proteins in the culture media, the surface properties of the polymers are altered considerably. The protein-coated polymers were characterized by means of liquid-liquid contact angle measurements under non-denaturing conditions. A striking correlation is observed between the degree of endothelialization and the measured dextran contact angle. The degree of endothelial cell spreading is not related to polymer surface tension. Cell morphology and extracellular matrix production, however, are influenced by substrate surface properties.

Improved endothelial cell seeding with cultured cells and fibronectin-coated grafts

A possible approach to the low seeding efficiency of endothelial cells into prosthetic grafts is to increase the number of cells to be seeded in cell culture and improve seeding efficiency by graft precoating with fibronectin. The effect of cell culture on cell adhesion is unknown, however, and fibronectin also binds fibrin, which may increase the thrombogenicity of the graft luminal surface. To investigate these questions, freshly harvested canine jugular vein endothelial cells from six animals and similar cells harvested from six primary and eight secondary cell cultures were labeled with 111Indium and seeded into 5 cm, 4 mm PTFE grafts coated with fibronectin, using similar uncoated PTFE grafts as controls. Platelet accumulation and distribution on six similar coated and uncoated grafts placed in canine carotid, external jugular arterial venous shunts for 2 hr were also determined using autogenous 111Indium-labeled platelets. Significant differences between group means were determined using the paired Student's t test. Results reveal that seeding efficiency is significantly better in all groups of coated grafts compared to uncoated grafts (P less than 0.01). Cells derived from cell culture also had significantly higher seeding efficiencies than freshly harvested cells when seeded into coated grafts (P less than 0.05) and tended to have higher seeding efficiencies than harvested cells when seeded into uncoated grafts (P = 0.53). Fibronectin coating increased mean platelet accumulation on the entire graft luminal surface, but not to a statistically significant degree (P greater than 0.1). Whether this increased seeding efficiency will improve graft endothelialization remains to be investigated.

History of (micro) vascular surgery and the development of small-caliber blood vessel prostheses (with some notes on patency rates and re-endothelialization)

The historical development of vascular surgery is reviewed from ancient times (Ruphus of Ephesus, Aëtius of Amida) to recent developments (sutured anastomosis by Carrel). Attempts to anastomose blood vessels by means of nonsuturing technique, using a ring or short tube of diverse materials called prostheses, were undertaken at the start of this century and continued until shortly after World War II. With the advent of modern polymeric materials, prostheses of different types, sizes, structures, and fabrics have been used to substitute for blood vessels, both experimentally and clinically. Recently, blood vessel prostheses with small (1-1.5 mm) internal diameters became available and have been implanted experimentally. Patency rates, biophysical and structural properties, the re-endothelialization and the neointima formation of several types of microvascular prostheses are briefly reviewed.