The effect of fibronectin coating on endothelial cell kinetics in polytetrafluoroethylene grafts

Advanced Glycation End Products Upregulate CD40 in Human Retinal Endothelial and Müller Cells: Relevance to Diabetic Retinopathy

CD40 induces pro-inflammatory responses in endothelial and Müller cells and is required for the development of diabetic retinopathy (DR). CD40 is upregulated in these cells in patients with DR. CD40 upregulation is a central feature of CD40-driven inflammatory disorders. What drives CD40 upregulation in the diabetic retina remains unknown. We examined the role of advanced glycation end products (AGEs) in CD40 upregulation in endothelial cells and Müller cells. Human endothelial cells and Müller cells were incubated with unmodified or methylglyoxal (MGO)-modified fibronectin. CD40 expression was assessed by flow cytometry. The expression of ICAM-1 and CCL2 was examined by flow cytometry or ELISA after stimulation with CD154 (CD40 ligand). The expression of carboxymethyl lysine (CML), fibronectin, and laminin as well as CD40 in endothelial and Müller cells from patients with DR was examined by confocal microscopy. Fibronectin modified by MGO upregulated CD40 in endothelial and Müller cells. CD40 upregulation was functionally relevant. MGO-modified fibronectin enhanced CD154-driven upregulation of ICAM-1 and CCL2 in endothelial and Müller cells. Increased CD40 expression in endothelial and Müller cells from patients with DR was associated with increased CML expression in fibronectin and laminin. These findings identify AGEs as inducers of CD40 upregulation in endothelial and Müller cells and enhancers of CD40-dependent pro-inflammatory responses. CD40 upregulation in these cells is associated with higher CML expression in fibronectin and laminin in patients with DR. This study revealed that CD40 and AGEs, two important drivers of DR, are interconnected.

Current Progress in Vascular Engineering and Its Clinical Applications

Coronary heart disease (CHD) is caused by narrowing or blockage of coronary arteries due to atherosclerosis. Coronary artery bypass grafting (CABG) is widely used for the treatment of severe CHD cases. Although autologous vessels are a preferred choice, healthy autologous vessels are not always available; hence there is a demand for tissue engineered vascular grafts (TEVGs) to be used as alternatives. However, producing clinical grade implantable TEVGs that could healthily survive in the host with long-term patency is still a great challenge. There are additional difficulties in producing small diameter (<6 mm) vascular conduits. As a result, there have not been TEVGs that are commercially available. Properties of vascular scaffolds such as tensile strength, thrombogenicity and immunogenicity are key factors that determine the biocompatibility of TEVGs. The source of vascular cells employed to produce TEVGs is a limiting factor for large-scale productions. Advanced technologies including the combined use of natural and biodegradable synthetic materials for scaffolds in conjunction with the use of mesenchyme stem cells or induced pluripotent stem cells (iPSCs) provide promising solutions for vascular tissue engineering. The aim of this review is to provide an update on various aspects in this field and the current status of TEVG clinical applications.

Human Vascular Endothelial Cells on Expanded Ptfe Precoated with An Engineered Protein Adhesion Factor

To elucidate the role of the molecular structure of adhesive proteins in an endothelialization of synthetic vascular prosthesis in vitro, a recombinant fibronectin-like engineered adhesion factor (FP) constructed from the specific Arg-Gly-Asp cell adhesion repeats was assayed as adhesive substrate to culture human saphenous vein endothelial cells on ePTFE. ePTFE samples (1 cm2) inserted into cell culture chambers were coated by incubation with increasing amounts of FP (up to 100 μg/cm2) prior to cell seeding. At 24 hours after low density cell seeding and 20 μg/ml/cm2 FP concentration, the number of adhered cells reached a plateau and the adhered cells did not proliferate up to 6 days of culture. At 24 hours after high density seeding (105 cells/cm2), the number of adhered cells was significantly higher on ePTFE with preadsorbed FP than on fibronectin coated PTFE. About 55% of the initially adhered cells survived up to 7 days on FP, whereas cell debris and free nuclei were predominant on fibronectin coated PTFE. In the investigated model the engineered RGD polymer potentialized a short-term adhesion of vascular endothelial cells to PTFE, nevertheless it did not ensure proliferation and long-term survival of these cells.

Tissue engineering; strategies, tissues, and biomaterials

Current tissue regenerative strategies rely mainly on tissue repair by transplantation of the synthetic/natural implants. However, limitations of the existing strategies have increased the demand for tissue engineering approaches. Appropriate cell source, effective cell modification, and proper supportive matrices are three bases of tissue engineering. Selection of appropriate methods for cell stimulation, scaffold synthesis, and tissue transplantation play a definitive role in successful tissue engineering. Although the variety of the players are available, but proper combination and functional synergism determine the practical efficacy. Hence, in this review, a comprehensive view of tissue engineering and its different aspects are investigated.

Quantitative Analysis of Unidirectional 2-D Tissue Formation of Endothelial Cells

We developed a reliable and quantitative method for measuring the dynamic process of unidirectional two-dimensional (2-D) tissue formation of endothelial cells (ECs) in vitro. The culturing of bovine ECs in an assembled culture chamber provided a square monolayered cell sheet with a linear margin when disassembled at the confluency. The cell sheet maintained in culture showed a unidirectional endothelialization in vitro. The cell population-distance histogram, which was determined from the daily observation of tissue, allowed us to determine quantitatively the dynamic process of unidirectional endothelialization in vitro. The endothelialized distance and the endothelializing zone on a glass slide were found to be nearly 500 μm/day and 750 μm in width, respectively. Thus, the method developed here provided information of the 2-D tissue formation process. This model would be useful as an in vitro model which simulates the anastomotic endothelialization of an artificial vascular graft.

Electrostatic Endothelial Cell Seeding Technique for Small-Diameter (<6 MM) Vascular Prostheses: Feasibility Testing

Multiple studies have indicated the importance of surface charge in the adhesion of multiple cardiovascular cell lines including platelets and endothelial cells on the substrate materials (1,4,7-10,12-15). It is the purpose of this article to report a feasibility study conducted using an electrostatic endothelial cell seeding technique. The feasibility study was conducted using human umbilical vein endothelial cells (HUVEC), a static pool apparatus, a voltage source, and a parallel plate capacitor. The HUVEC concentration and seeding times were constant at 560,000 HUVEC/ml and 30 min, respectively. Scanning electron microscopy examination of the endothelial cell adhesion indicated that an induced temporary positive surface charge on e-PTFE graft material enhances the number and the maturation (flattening) of HUVECs adhered. The results indicated that the total number of endothelial cells adhered (70.9 mm2) was increased from 9198 ± 1194 HUVECs on the control (no induced surface charge) e-PTFE to 22,482 ± 4814 HUVECs (2.4 × control) on the maximum induced positive surface charge. The total number of cells in the flattened phase of adhesion increased from 837 ± 275 to 6785 ± 1012 HUVECs (8.1 ×) under identical conditions. Thus, the results of the feasibility study support the premise that electrostatic interaction is an important factor in both the endothelial cell adhesion and spreading processes and suggest that the electrostatic seeding technique may lead to an increased patency of small diameter (<6 mm) vascular prostheses.

Living scaffolds: surgical repair using scaffolds seeded with human adipose-derived stem cells

BACKGROUND

Decellularized porcine small intestinal submucosa (SIS) is a biological scaffold used surgically for tissue repair. Here, we demonstrate a model of SIS as a scaffold for human adipose-derived stem cells (ASCs) in vitro and apply it in vivo in a rat ventral hernia repair model.

STUDY DESIGN

ASCs adherence was examined by confocal microscopy and proliferation rate was measured by growth curves. Multipotency of ASCs seeded onto SIS was tested using adipogenic, chondrogenic, and osteogenic induction media. For in vivo testing, midline abdominal musculofascial and peritoneal defects were created in Sprague-Dawley rats. Samples were evaluated for tensile strength, histopathology and immunohistochemistry.

RESULTS

All test groups showed cell adherence and proliferation on SIS. Fibronectin-treated scaffolds retained more cells than those treated with vehicle alone (p < 0.05). Fresh stromal vascular fraction (SVF) pellets containing ASCs were injected onto the SIS scaffold and showed similar results to cultured ASCs. Maintenance of multipotency on SIS was confirmed by lineage-specific markers and dyes. Histopathology revealed neovascularization and cell influx to ASC-seeded SIS samples following animal implantation. ASC-seeded SIS appeared to offer a stronger repair than plain SIS, but these results were not statistically significant. Immunohistochemistry showed continued presence of cells of human origin in ASC-seeded repairs at 1 month postoperation.

CONCLUSION

Pretreatment of the scaffold with fibronectin offers a method to increase cell adhesion and delivery. ASCs maintain their immunophenotype and ability to differentiate while on SIS. Seeding freshly isolated SVF onto the scaffold demonstrated that minimally manipulated cells may be useful for perioperative surgical applications within the OR suite. We have shown that this model for a "living mesh" can be successfully used in abdominal wall reconstruction.

Endothelialization of electrospun polycaprolactone (PCL) small caliber vascular grafts spun from different polymer blends

Small caliber vascular grafts represent a challenge to material scientists. In contrast to large caliber grafts, prostheses with diameter <6 mm, lead to increased hemodynamic disturbances and thrombogenic complications. Thus, endothelialization of small caliber grafts should create a compatible interface for hemodynamic processes. The purpose of our study was to compare different compositions of electrospun scaffolds with conventional ePTFE grafts with an inner diameter of 4 mm as well as different pre-coatings to create an optimized physiological interface for endothelialization. Polycaprolactone, polylactide, and polyethylenglycol (PCL/PLA and PCL/PLA/PEG) electrospun grafts and ePTFE grafts were pre-coated with blood, gelatine or fibronectin and seeded with endothelial cells from the human term placenta. Best results were obtained with fibronectin-coated PCL/PLA/PEG grafts. Here, the number of attached viable cells was 78-81% higher than on fibronectin pre-treated ePTFE grafts. Cells attached to PCL/PLA/PEG grafts appeared in physiological cobblestone morphology. Viability analysis showed a high cell viability of more than 98%. Fibronectin-coated PCL/PLA/PEG grafts may be a promising improvement to conventionally used ePTFE grafts.

Cell-biomaterial mechanical interaction in the framework of tissue engineering: insights, computational modeling and perspectives

Tissue engineering is an emerging field of research which combines the use of cell-seeded biomaterials both in vitro and/or in vivo with the aim of promoting new tissue formation or regeneration. In this context, how cells colonize and interact with the biomaterial is critical in order to get a functional tissue engineering product. Cell-biomaterial interaction is referred to here as the phenomenon involved in adherent cells attachment to the biomaterial surface, and their related cell functions such as growth, differentiation, migration or apoptosis. This process is inherently complex in nature involving many physico-chemical events which take place at different scales ranging from molecular to cell body (organelle) levels. Moreover, it has been demonstrated that the mechanical environment at the cell-biomaterial location may play an important role in the subsequent cell function, which remains to be elucidated. In this paper, the state-of-the-art research in the physics and mechanics of cell-biomaterial interaction is reviewed with an emphasis on focal adhesions. The paper is focused on the different models developed at different scales available to simulate certain features of cell-biomaterial interaction. A proper understanding of cell-biomaterial interaction, as well as the development of predictive models in this sense, may add some light in tissue engineering and regenerative medicine fields.

Seeding of endothelial cells on the luminal surface of a sheet model of cold-stored (at 4°C) sheep carotid arteries

Cold-stored arteries are biomaterials that potentially represent an abundant "off-the-shelf" source of vascular grafts for use in vascular surgery. One of the keys to reestablishing the antithrombogenic endothelial cell (EC) lining of cold-stored arterial grafts is to maximize the number of ECs that attach following seeding. In this study, the cold-stored sheep carotid artery is used as a substrate to determine the conditions that maximize EC adherence following seeding. The effect of serum concentration, duration of seeding incubation, seeding density, and period of cold storage on attachment of ECs following seeding of 4-week cold-stored sheep carotid arteries (n = 5 arteries), 8-week cold-stored sheep carotid arteries (n = 5 arteries), and 12-week cold-stored sheep carotid arteries (n = 5 arteries) was examined. Three experiments (serum concentration, time of incubation, and seeding density) were conducted to determine the conditions that maximized the number of cultured sheep carotid artery ECs that attached to cold-stored sheep carotid artery following seeding. A flat sheet model was used. Serum concentration (0%, 10%, 20%, and 30%) in the seeding suspension did not have a significant effect on overall EC adherence on 4-, 8-, and 12-week cold-stored arteries. Time of seeding incubation (30, 60, and 90 min) did not have a significant effect on overall EC adherence on 4-, 8-, and 12-week cold-stored arteries. Seeding density (500,000 cells/ml, 1,000,000 cells/ml, and 2,000,000 cells/ml) had a significant effect (p = 0.036) on overall EC adherence on 4-, 8-, and 12-week cold-stored arteries. The period of cold storage (4, 8, and 12 weeks) of the artery had a significant effect (p = 0.002, p < 0.0001, p < 0.0001 in serum, time, and seeding density experiments, respectively) on overall EC adherence following seeding. Pairwise comparisons of EC adherence revealed the following. In the serum experiment, EC adherence on 4-week cold-stored arteries was significantly greater than on 8-week cold-stored arteries (p = 0.003) and 12-week cold-stored arteries (p = 0.002). This effect was due largely to the significant difference between EC adherence on 4-week and 8-week cold-stored arteries (p = 0.0002) and between EC adherence on 4-week and 12-week cold-stored arteries (p = 0.0091) at 20% serum. In the time experiment, EC adherence on 4-week cold-stored arteries was significantly greater than on 12-week cold-stored arteries (p < 0.0001). In the seeding density experiment, EC adherence on 4-week cold-stored arteries was significantly greater than on 8-week cold-stored arteries (p < 0.0001) and 12-week cold-stored arteries (p < 0.0001). In the same experiment, EC adherence following seeding at a density of 1,000,000 cells/ml and 2,000,000 cells/ml was significantly greater (p = 0.03 and p = 0.02, respectively) than EC adherence following seeding at a density of 500,000 cells/ml. Thus, it was determined that 4-week cold-stored arteries were superior to 8- and 12-week cold-stored arteries in terms of the number of ECs that adhered. It was also determined that a seeding density of 1,000,000 or 2,000,000 cells/ml was superior to a seeding density of 500,000 cells/ml in terms of producing maximal EC attachment. The ideal conditions, from those examined, for maximizing EC attachment to cold-stored arteries were 4 weeks of cold storage, a serum concentration of 20%, a seeding density of 2,000,000 cells/ml, and a duration of incubation of 30-90 min.

The influence of polyelectrolyte charges of polyurethane membrane surface on the growth of human endothelial cells

A novel technique to introduce free amino groups onto polyester scaffolds via aminolyzing the ester groups with diamine has been developed recently. The introduction of the free amino groups on these polyester surfaces provides us the possibility to modify polymer surface in a simpler manner, e.g. layer-by-layer assembly of charged species. By this technique, many negatively and positively charged biopolymers were deposited alternatively on polyurethane surface. The deposition process was monitored by fluorescence spectroscopy and advancing contact angle measurements. The result of human endothelial cells cultured in vitro showed that cells on negatively charged surface could not spread and flatten well due to the electrostatic repulsion. The lower attachment ratio induced the lower proliferation ratio. However, after the surface charge was inversed by collagen, both attachment and proliferation ratios increased to different extent. Observed under SEM, cells also presented a flat and spreading morphology.

Endothelial cell seeding of a 4-mm I.D. polyurethane vascular graft

We evaluated the extent (luminal coverage) of the endothelial cell (EC) lining/neointimal development and the thromboresistance of electrostatically EC seeded small diameter ChronoFlex-polyurethane vascular grafts. The evaluation consisted of harvesting autologous, canine jugular vein ECs, electrostatically seeding the polyurethane grafts (4-mm I.D., length = 6 cm) with the harvested ECs, implanting the grafts in a canine femoral artery model for four to six weeks, and excising the grafts for histological and scanning electron microscopy evaluations. Results of the histological evaluation (mid-graft region only) indicated that electrostatic EC seeding led to neointimal development and to minimal to no thrombus formation within the EC seeded grafts. The unseeded control grafts resulted in no neointimal development and substantial thrombus formation on the graft luminal surfaces. Scanning electron microscopy examination demonstrated a mature, confluent endothelium with a "cobblestone" appearance on the EC seeded graft luminal surface after six weeks. We conclude that electrostatic EC seeding enhanced the development of a neointima and reduced the incidence of thrombosis in polyurethane grafts implanted in a canine femoral artery model.

Vascular graft endothelialization: comparative analysis of canine and human endothelial cell migration on natural biomaterials

Canines are typically used as the standard preclinical model to gauge the success of vascular graft materials. However, canines spontaneously re-endothelialize vascular grafts, whereas humans do not, even after years. This raises questions of why there are differences in vascular healing between humans and other species and whether the canine is the appropriate preclinical model. In the present study we evaluated human and canine endothelial cell (EC) migration on the novel cross-linked collagen biomaterial PhotoFix(TM) pericardium. We compared in vitro migration of these cells on PhotoFix alone and on PhotoFix adsorbed with various growth factors (aFGF and bFGF) and adhesion proteins (fibronectin, collagen IV, vitronectin, and laminin). We also compared human and canine ECs in terms of their morphologies and prostacyclin production. We found that human umbilical vein ECs (HUVECs) and canine ECs (CECs) migrated well on PhotoFix, suggesting that this biomaterial may be a good vascular graft candidate. Both cell types responded similarly to different growth factors and adhesive proteins, but HUVEC migration was consistently higher than that for CECs. This suggested that human in vivo graft re-endothelialization is likely not hindered by poor endothelial migration but is hindered by other cellular or graft properties.

Adherence of human saphenous vein endothelial cell monolayers to tissue-engineered biomatrix vascular conduits

This study investigates the adherence and retention under in vitro flow conditions of endothelium grown on the luminal surface of 4-mm-internal-diameter, biomatrix vascular conduits. The biomatrix vascular conduits are produced in living animals and consist of a naturally formed extracellular matrix wall incorporating a polyester mesh. We propose that the microarchitecture of the luminal surface may be conducive to endothelial cell (EC) seeding and to the formation of a firmly adherent endothelium without prior treatment of the surface with cell adhesives. ECs were isolated from segments of human saphenous vein and grown to confluence in a culture. Cultured cells were characterized by morphology and immunocytochemistry with anti-CD31, Von Willebrand factor, smooth muscle actin, cytokeratin, and the lectin Ulex Europaeus agglutinin. After culture, ECs were seeded (1 x 10(6) cell/mL) by rotation onto the luminal surface of 20-cm-long, biomatrix vascular conduits (n = 3). The seeded conduits were incubated for 72 h, and at set time points postseeding (1, 24, 48, and 72 h), the morphology, percentage luminal surface cover, and cell density (cell/cm(2)) were determined from en face preparations and histological cross sections. After 72 h in culture, the seeded conduits were subjected to a nonpulsatile flow for 1 h with culture media. A flow rate of 480 mL/min generated physiological-range shear stresses of 12 dyn/cm(2) on the endothelialized surface. After the flow, the conduits were fixed for histology, and the EC morphology and percentage luminal surface cover were determined. Quantification of the extent of luminal surface endothelialization, preflow and postflow, and cell densities at confluence was performed with digital imaging light microscopy and image analysis software. An analysis of the results demonstrated that confluent EC monolayers may be established on the luminal surface of biomatrix vascular conduits within 48 h. The formed endothelium was firmly adherent and was retained under physiological-range flow.

Determination of the prime electrostatic endothelial cell transplantation procedure for e-PTFE vascular prostheses

The purpose of this study was to evaluate the extent of cellular adhesion (density and morphological maturation), cellular membrane damage, and cellular viability after an electrostatic transplantation of human umbilical vein endothelial cells (HUVECs) onto 6-cm segments of 4-mm I.D. e-PTFE (GORE-TEX) vascular prostheses using a prototype electrostatic endothelial cell transplantation device (EECTD). The electrostatic transplantation parameters evaluated were the apparatus-applied voltage and transplantation time. By our definition, the combination of applied voltage and transplantation time that met the a priori criteria of: 1) maximum transplanted cellular viability, 2) maximum transplantation density, 3) maximum morphological maturation (degree of cellular flattening), and 4) minimal cellular membrane damage would be the prime transplantation procedure. The results of the experimentation indicated that the prime conditions for HUVEC transplantation were obtained when +1.0 V was applied for a transplantation time of 16 min. These conditions achieved an average viable graft surface coverage of 97.4+/-1.6% with an average transplantation density of 73,540+/-8.514 HUVECs/cm2. Furthermore, the transplanted HUVECs were morphologically mature (flattened) with minimal apparent cellular membrane damage (lysis or pitting). The overall clinical significance of this study is that viable endothelial cell transplantation to synthetic vascular grafts can be accomplished at high cellular densities and morphological maturation in 16 min using the EECTD. With the promising in vitro transplantation results, the next logical investigations will include additional in vitro evaluations (cellular retention upon shear stress exposure and biochemical assays) followed by in vivo evaluations to examine thromboresistance and influence on intimal/anastomotic hyperplasia.

Blood compatibility of surfaces with immobilized albumin-heparin conjugate and effect of endothelial cell seeding on platelet adhesion

Endothelial cell (EC) seeding significantly improves the blood compatibility of artificial surfaces. Although a coating consisting of albumin and heparin (alb-hep) is a suitable substrate for seeded ECs, binding of ECs to the substrate further improves when small amounts of fibronectin are present in the alb-hep coating. Alb-hep conjugate was immobilized on carbon dioxide gas plasma-treated polystyrene (PS-CO(2)), thereby significantly increasing the recalcification time of blood plasma exposed to this surface. Furthermore, surface-immobilized alb-hep conjugate inhibited exogenous thrombin. Heparin activity was reduced by adding fibronectin on top of a monolayer of alb-hep conjugate, but not by simultaneous coating of fibronectin and alb-hep conjugate. Coating of PS-CO(2) with alb-hep conjugate significantly decreased contact activation (FXII activation). The number of platelets deposited from blood plasma on PS-CO(2) coated with alb-hep conjugate was twice as high as on PS-CO(2) coated with albumin. Addition of fibronectin to alb-hep conjugate-coated PS-CO(2) had no significant effect on the number of adhered platelets. Seeding of the substrates with ECs significantly reduced the number of adhered platelets under stationary conditions. Platelets deposited onto endothelialized surfaces were primarily found on endothelial cell edges, and sparingly on areas between ECs. In conclusion, alb-hep conjugate-coated surfaces display anticoagulant activity. ECs adhering to and proliferating on this coating significantly decrease the number of platelets which adhere to the surface. Therefore, alb-hep conjugate-coated surfaces form a suitable substrate for seeding of ECs in low density. Although application of fibronectin on top of the coating decreases the anticoagulant activity to some extent, it might be useful in view of the improved adherence of ECs to the coating.

Improving endothelial cell adhesion to vascular graft surfaces: clinical need and strategies

Synthetic vascular grafts do not spontaneously endothelialize in humans and require some form of anticoagulation to maintain patency. Preseeding synthetic graft materials such as expanded polytetrafluoroethylene (ePTFE) and polyethylene terephthalate (PET) with endothelial cells (EC) has been examined in various in vitro and in vivo models. Although various studies provide encouraging results, clinical trials for EC seeding on synthetic grafts have not been equally successful. This paper provides a brief review of the various reports on EC seeding in animal and clinical studies. We discuss the inefficiencies associated with the EC seeding process and examine plasma protein treatment of the graft surfaces as a viable option for improving EC attachment, retention and spreading. As an alternative to existing therapies we present data on a heterogeneous ligand treatment of fibronectin (Fn) and avidin-biotin for enhanced human umbilical vein endothelial cell (HUVEC) adhesion to ePTFE graft surfaces. Control consisted of HUVECs seeded on Fn treated ePTFE graft surfaces. Functionality of HUVECs was assessed by measuring prostacyclin production of cells on both homogeneous and heterogeneous ligand treated surfaces. Laminar flow studies with a variable width flow chamber and scanning electron microscopy were used to measure initial cell retention and observe initial cell spreading on ePTFE surfaces, respectively. HUVEC retention on heterogeneous ligand treated graft surface was significantly (p < 0.001) higher compared to homogeneous ligand treated surfaces for shear stress in the range of 10-30 dyn cm(-2). HUVEC showed more cellular spreading on the heterogeneous ligand treated surface after seeding for 1-2 h. In vivo experimentation was performed in immune deficient (nude) rats by replacing a section of both the femoral arteries with 8 mnm long, 1 mm internal diameter denucleated ePTFE grafts treated with homogeneous and heterogeneous ligands respectively. Both grafts were seeded with similar cell density for 15 min prior to implantation. EC attachment and retention was measured by staining EC with hematoxylin and counting the cells before and after flow using light microscopy. The results indicate that a heterogeneous ligand treatment of graft surfaces using avidin-biotin and Fn-integrin attachment mechanisms increase cell seeding efficiency, initial cell retention and cellular spreading.

Fibronectin and avidin-biotin as a heterogeneous ligand system for enhanced endothelial cell adhesion

A preadsorbed layer of "heterogeneous" integrin-dependent and -independent protein was used to enhance initial integrin-mediated endothelial cell attachment and spreading. Glass substrates were treated with fibronectin (Fn) and avidin coupled through adsorbed biotinylated bovine serum albumin (b-BSA). The slides then were seeded with biotinylated BAEC. Control "homogeneous" surfaces were slides adsorbed with either Fn or avidin coupled to b-BSA. The cells were incubated for 0.5 h in serum-containing media and exposed to a range of shear stresses in a laminar flow variable-height flow chamber. The critical shear stress to detach 50% of the seeded cells on the heterogeneous ligand surface was significantly greater than for either of the control homogeneous ligand systems (p < 0.001). Cellular spreading during the initial period of 0-2 h also was higher (p < 0.05) on the heterogeneous ligand-treated surface than on the surface of either of the homogeneous controls. The close contact area of the cell membrane with the substrate 1 h after seeding in serum-containing media was measured using TIRFM. Cells attached onto the heterogeneous ligand-treated surfaces had a significantly (p < 0.01) higher area of close contact with the substrate, which is consistent with a greater degree of attachment and spreading. The results indicate that the combination of integrin-dependent and -independent adhesion systems using heterogeneous ligands further enhances initial endothelial cell attachment and spreading.

The effect of flow shear stress on endothelialization of impervious Dacron grafts from circulating cells in the arterial and venous systems of the same dog

The purpose of this report was to study effects of shear force and hemodynamic conditions that influence fallout healing in the arterial and venous systems of the same dog. Knitted Dacron grafts made impervious by a 1.5 mm thick coat of silicone rubber bonded to the external surface were implanted for 4 weeks during the same surgery in the descending thoracic aorta (DTA), abdominal aorta (AA) and inferior vena cava (IVC) of each of five dogs. Flow rates were measured during surgery and shear stresses calculated with the Hagen-Poiseuille formula. Full-wall thickness longitudinal tissue sections were embedded in resin and stained with hematoxylin and eosin for light microscopy, and in paraffin for immunocytochemistry studies with Factor VIII/von Willebrand factor, smooth muscle alpha-actin, collagen IV, laminin, and proliferating cell nuclear antigen. Scanning electron microscopy and transmission electron microscopy studies were also performed. AgNO3 was used to determine percentage of endothelial-like cell coverage on the flow surface. All grafts were patent, without hematoma or seroma. Endothelial-like cell coverage was highest in the IVC grafts and lowest in the DTA. Shear stress and flow velocity were significantly lower in IVC grafts than DTA and AA. Proliferating cell nuclear antigen indicated extensive cellular proliferation in the intima and in the interstices of the inner portion of the graft wall. The degree of fallout healing in knitted Dacron grafts made impervious by an external coat of silicone rubber varies inversely with the sheer force of blood flow in these grafts.

Small-diameter vascular graft prostheses: current status

In contrast to large-diameter vascular grafts (i.e. larger than 5 mm) which remain excellent for more than 10 years after implantation, small-diameter vascular grafts of both Dacron and Teflon occlude rapidly upon implantation. In this overview article, the strategies used to improve the patency of these small-diameter grafts, the current status in clinical trials, and further perspectives in the field of artificial vascular graft development are reviewed. It is concluded that, in view of recent developments in tissue engineering approaches, the future of small-diameter vascular prostheses looks promising.

Using avidin-mediated binding to enhance initial endothelial cell attachment and spreading

Binding between the protein avidin and the vitamin biotin was used as an extrinsic, high affinity receptor-ligand system to augment the intrinsic integrin-dependent cellular adhesion mechanism. Glass substrates were coupled with avidin receptors through an adsorbed film of biotinylated bovine serum albumin (b-BSA). The avidin-treated slides then were seeded with biotinylated bovine aortic endothelial cells (BAEC). A 3:1 ratio of BSA:b-BSA provided the best results in terms of specific cellular attachment, growth, and spreading. Control surfaces consisted of bare glass or glass with adsorbed BSA. Attachment of unmodified BAEC to glass decreased in the presence of anti-beta 1 integrin antibody. Adhesion of biotinylated BAEC to avidin-treated slides was not affected by anti-beta 1 integrin antibody, consistent with integrin-independent avidin-mediated adhesion. The initial rate of cell spreading was greatest for avidin-biotin-mediated adhesion (80.0 +/- 25.6 microns2/h), followed by integrin-dependent cellular adhesion on plain glass (35.7 +/- 7.7 microns2/h) and, finally, by adhesion on BSA-coated protein surfaces (10.2 +/- 0.3 microns2/h). Biotinylated and unmodified BAEC, cultured for 1 h in serum-containing media, were subjected to laminar flow in a variable-height flow chamber that provided a range of shear stresses from 0.2 to 75 dynes/cm2. The critical shear stress required to detach 50% of the cells in serum-containing media increased from 4.6 +/- 0.8 dynes/cm2 for integrin-dependent adhesion to 12.6 +/- 1.2 dynes/cm2 for avidin-biotin-mediated adhesion. Avidin-mediated attachment for biotinylated BAEC increased initial cellular spreading rates and strength of attachment (i.e., at 1 h) by a factor of two and three, respectively. These results support the hypothesis that integrin-mediated cell attachment and spreading can be enhanced using high affinity integrin-independent binding.

In vitro evaluation of electrostatic endothelial cell transplantation onto 4 mm interior diameter expanded polytetrafluoroethylene grafts

PURPOSE

To perform an in vitro evaluation of electrostatic endothelial cell transplantation of human umbilical vein endothelial cells (HUVEC) onto segments of 4 mm internal diameter expanded polytetrafluoroethylene (ePTFE) vascular prostheses.

METHODS

This evaluation consisted of exposing vascular graft segments that had been subjected to either electrostatic or gravitation transplantation with HUVEC to a physiologic shear stress (15 dynes/cm2) under steady flow conditions within a flow loop system. Biochemical assays were performed on freshly transplanted grafts by means of radioimmunoassay for prostacyclin and thromboxane A2.

RESULTS

There was a 30% loss of HUVEC after 30 minutes of shear stress exposure from the grafts subjected to gravitational transplantation with no additional significant (alpha = 0.05) loss after 120 minutes. Grafts subjected to electrostatic transplantation had no significant (alpha = 0.05) loss of HUVEC during exposure to physiologic shear stress. Furthermore, after 120 minutes of shear-stress exposure, the grafts subjected to electrostatic transplantation (78,420 +/- 6274 HUVEC/cm2) retained 2.3 times more HUVEC than the counterparts subjected to gravitational transplantation (34,427 +/- 4637 HUVEC/cm2). The biochemical assay results indicated no significant (alpha = 0.05) production of prostacyclin or thromboxane A2 regardless of the method of cell transplantation.

CONCLUSIONS

(1) The electrostatic transplantation technique was superior to the gravitational transplantation technique in terms of cellular retention when the ePTFE grafts were exposed to physiologic shear stress. (2) Production of prostacyclin and thromboxane A2 did not differ between transplanted HUVEC subjected to gravitational or electrostatic procedures.

Retroviral mediated gene transduction alters integrin expression on vascular endothelial cells

Genetically recombinant endothelial cells (rEC) may improve the patency of small diameter vascular grafts by preventing thrombosis or limiting neointimal hyperplasia. Previous work has shown that rEC have reduced adhesion to vascular bypass grafts in vivo. Poor adhesion may be due to altered adhesion (integrin) receptors. This study evaluated the expression of the alpha 5 beta 1 (fibronectin), alpha 2 beta 1 (collagen IV), and alpha v beta 3 (vitronectin) integrin subunits on rEC. Human umbilical vein EC or canine jugular vein EC were transduced with neoR, neoR and human tPA or hygromycin resistance genes using retroviral vectors. Naive EC and EC exposed to empty viral particles (mEC) were controls. Naive EC, mEC, and all rEC's were evaluated for alpha and beta subunits for each integrin receptor studied using immunoblotting. Blotting for alpha 2, alpha 5, and alpha v exhibited expression of the alpha integrin subunits in all cells. The beta 1 and beta 3 subunits were present in mEC and nEC but were absent or truncated in all rEC. The decreased adhesion of rEC's to synthetic vascular grafts may be accounted for by their altered beta 1 and beta 3 integrin subunit expression. The beta subunit is critical for organization of the cytoskeleton and cellular signal transduction. Diminished beta subunit expression in rEC is neither vector specific nor related to retroviral exposure alone. Alteration of beta integrin expression may be to associated with the over-expression of phosphotransferase genes such as neoR or hygromycin B used as selectable markers in gene transfer protocols.

Endothelial cell seeding kinetics under chronic flow in prosthetic grafts

Improved patency of endothelial cell seeded grafts relies on good initial adherence and cell retention when the circulation is restored. In this study human adult endothelial cells (HAECs) were used to evaluate the suitability of commercially available prostheses for seeding. Acutely seeded indium-111 oxine labeled HAECs were used to measure cell adherence to plain and fibronectin (FN)-coated expanded polytetrafluoroethylene (ePTFE), gelatin-impregnated Dacron (Gelseal), and collagen-impregnated Dacron (Hemashield) grafts. Cell loss from FN-coated prostheses, when exposed to a simulated human arterial blood flow of 200 ml/min in an artificial pulsatile circulation, was quantified from the loss of gamma activity from the graft over 24 hours, pressure in the circulation being reduced to 15 mm Hg to reduce fluid loss. Initial HAEC adherence (mean [SD]) to plain grafts was 3(1)%, 47(9)%, and 53(9)% for ePTFE, Gelseal, and Hemashield, respectively. This improved significantly with FN coating (78[6]%, 60[8]%, and 76[4]%). Cell retention after 24 hours of flow to FN-coated grafts was 16(10)%, 25(5)%, and 65(4)% and was confirmed qualitatively by scanning electron microscopy and environmental scanning electron microscopy. FN significantly improved initial cell adherence with Dacron grafts showing the better adherence. Cell retention after 24 hours of flow was better with FN-coated Dacron than with ePTFE but was best with Hemashield grafts.

The influence of micro-topography on cellular response and the implications for silicone implants

Tissue attachment to substratum surfaces is of central importance to the in vivo performance of prosthetic implant materials. It is not yet understood why connective tissue does not attach to the surface of silicone or any other polymeric material. Recently the authors have conclusively demonstrated that micro-range surface roughness modifies cellular responses in cell culture and modifies biocompatibility and tissue attachment in vivo significantly. In order to better understand the basic interactions between living cells or tissues on one hand and man-made substratum surfaces on the other hand, the germane literature is reviewed here. Cells adhere to substratum surfaces mainly through focal adhesions which are a complex of intracellular transmembrane and extracellular proteins. Adhesion is facilitated and modified by proteins adsorbed to the substratum surface. Protein adsorption in turn is modified by the underlying substratum surface properties including surface chemistry, charge, and free energy. When silicone and other polymeric implants having well-defined surface topographic features including pores, pillars, or grooves were implanted, the tissue response to these implants was strongly influenced by the dimensions of these features as well as by other geometric details. Highest biocompatibility along with tissue attachment was seen when topographic features had dimensions of 1-3 microns and a uniform distribution. Cell culture studies revealed that topographic features affect cellular alignment, direction of proliferation, cellular attachment, growth rate, metabolism, and cytoskeletal arrangement. Since discontinuities or curvatures associated with topographic features may represent local changes in surface free energy, it is hypothesized that these discontinuities trigger changes in protein adsorption, protein configuration, and cellular response.

Endothelial cell-lined skeletal muscle ventricles in circulation

Skeletal muscle ventricles were constructed from the latissimus dorsi in six dogs by wrapping the muscle around a polypropylene mandrel. Jugular vein endothelial cells were harvested enzymatically and grown in tissue culture. After 3 weeks of vascular delay and 4 weeks of electrical conditioning, five skeletal muscle ventricles were seeded with 5 to 8 x 10(6) autologous endothelial cells by percutaneous injection of a cellular suspension into the lumen of the skeletal muscle ventricle; one skeletal muscle ventricle was injected with culture medium alone as an unseeded control. The autologous endothelial cells were all prelabeled with a lipid-bound cellular marker, PKH-26. After an additional 4 weeks of electrical conditioning, the mandrels were removed and the skeletal muscle ventricles were connected to the descending thoracic aorta and activated to contract during cardiac diastole at a 1:2 ratio with the heart. After 3 hours of continuous pumping, mean diastolic pressure was increased by 35% (58 +/- 7 versus 78 +/- 6 mm Hg, p < 0.05). At this time, the skeletal muscle ventricles were excised for histologic examination. Sections stained with hematoxylin and eosin revealed a continuous cellular layer lining the skeletal muscle ventricle; no cells were present on the lumen of the control skeletal muscle ventricle. All seeded skeletal muscle ventricles exhibited fluorescence as a result of the PKH-26 cellular marker. Immunofluorescent staining with antibodies to von Willebrand factor and ultrastructural analysis with an electron microscope confirmed the endothelial character of these cells lining the lumen of the skeletal muscle ventricles. The ability to create endothelial cell-lined muscular pumping chambers holds important implications for the resolution of thrombotic events in cardiac assist devices as well as toward the clinical application of skeletal muscle ventricles.

Fluid shear induced endothelial cell detachment from glass--influence of adhesion time and shear stress

In this study, human umbilical vein and human saphenous vein endothelial cells were seeded on glass and exposed to fluid shear in a parallel-plate flow chamber. Cell retention, morphology and migration were studied as a function of shear stress and of adhesion time prior to exposure to shear. Three-hour and 24-h adhesion times gave rise to comparable cell retention values after 2 h of flow for both cell types. Cell retention decreased from 85 to 20% as shear stress increased from 88 to 264 dynes cm-2 (8.8 to 26 Pa). Mean spreading areas decreased after the onset of flow, but subsequently stabilized to plateau values, which were smaller at higher shear stresses. Shape factors increased faster to higher values as cells were exposed to higher shear stresses, without any obvious preference in orientation of the cells with respect to the direction of flow. Migration was unidirectional with flow and linear with time. Migration was faster for cells which had adhered for 24 h than for cells which had adhered for 3 h and was accompanied by the presence of fibrillar structures left behind on the surface upstream of migrating cells. It is concluded that after 3 h adhesion to glass, cells have adhered with an adhesion strength that does not substantially increase during the next 21 h. However, during this time changes in cell-substratum interactions seem to occur judging by the differences in, e.g., migration rates.

Effect of pulsatile shear stress on endothelial attachment to native vascular surfaces

An in vitro model of vascular damage was used to investigate the ability of seeded endothelial cells to resist shear stresses generated in a perfusion circuit. At perfusion rates of 100 ml/min the maximum shear stress reached 16.5 dyn/cm2. At this level the rate of cell detachment from the damaged vascular surface was 88 per cent per h for the first 20 min of flow but gradually decreased to 5 per cent per h after 90 min. These findings suggest that endothelial cells may be retained on damaged vascular surfaces in conditions that approximate to arterial flow.

A new and simple technique to achieve a confluent and flow resistant endothelium on vascular ePTFE-grafts using human serum

Confluent endothelialisation of vascular grafts in vitro before implantation has been suggested as a way to improve patency. With the aim of creating a confluent endothelium or expanded polytetrafluoroethylene (ePTFE) vascular grafts, using a minimum of allo- or xenogenic material, we have investigated the use of human serum for endothelial cell (EC) growth and adhesion to the grafts. Loss of fibronectin from the serum was measured before and after coating. After serum- or collagen I-precoating, the grafts were seeded with cultured adult human endothelial cells from the great saphenous vein and then kept under rotational culture conditions for 2 days. To evaluate the endothelial resistance to detachment the grafts were mounted in parallel to a heart-lung machine, delivering a pulsatile flow of human blood for 1 hour. Evaluation was performed using scanning electron microscopy (SEM). By simply pressing serum through the graft wall, fibronectin corresponding to 19 micrograms/cm2 graft surface was consumed. It was possible to achieve a confluent endothelium on both serum- and collagen I-precoated grafts (n = 8). After being subjected to blood flow, SEM revealed a complete endothelial lining of the grafts (n = 12) except for two serum-precoated grafts which showed < or = 10% and between 10 and 20% denuded areas, and one collagen-coated graft that showed < or = 10% of denuded area. This method provides a way to achieve a confluent and flow resistant endothelial lining on ePTFE vascular grafts.

Mesothelial cell adherence to vascular prostheses and their subsequent growth in vitro

Cell seeding may decrease the thrombogenicity of implanted vascular grafts, but its application is hampered by the limited availability of autologous endothelial cells. Human peritoneal mesothelial cells have blood flow supporting qualities and are readily available. This study investigated the adherence of mesothelial cells to vascular prostheses and their subsequent growth in vitro. Circular pieces of various vascular prosthetic materials were seeded with 51Chromium-labeled mesothelial and endothelial cells and left for either 5, 15, 30, 60, and 120 minutes. The unattached cells were removed and the degree of cell attachment was measured. The number of mesothelial cells to Dacron increased during the first 60 min up to 35.2% of the seeded inoculum whereafter a plateau was reached. Scanning electron microscopy showed spread mesothelial cells adherent to the Dacron fibers. A significant increase in adherence was observed after preincubation of Dacron with 10 micrograms/mL fibronectin, but no improvement was found after preincubation with human serum albumin or gelatin. Mesothelial cells adhered better to Gel-coated than to Gel-sealed or plain Dacron. The adherence of mesothelial cells to ePTFE (Teflon) was significantly poorer. No significant differences in adherence were found between mesothelial and endothelial cells. Mesothelial cell growth on Dacron resulted in a modest increase in the number of viable cells during 27 days, which implies biocompatibility of Dacron and mesothelial cells in vitro.

Cell growth on immobilized cell growth factor. 9. Covalent immobilization of insulin, transferrin, and collagen to enhance growth of bovine endothelial cells

Bovine endothelial cells were cultivated on the surface of polyurethane membranes or tubes immobilized with insulin, transferrin, and/or collagen, and growth was measured. For the purpose of immobilization, amino groups were introduced on the membranes or tubes by glow-discharge treatment in the presence of ammonia gas. The proteins were coupled to the amino groups on the surfaces by using dimethyl suberimidate (DMS) or water-soluble carbodiimide (WSC). Growth of bovine endothelial cells was higher on immobilized insulin or transferrin using DMS than that using WSC. The cell growth was higher on immobilized insulin or transferrin than on free insulin or transferrin, respectively. Though immobilized collagen did not affect the cell growth, coimmobilization with insulin or transferrin brought about greater acceleration of cell growth. In addition, the immobilized collagen was indispensible to maintain the endothelial cells on the material surface for a long time. The endothelial cells grown in the polyurethane tubes, on which insulin and collagen are coimmobilized, kept a normal cobblestone-like appearance and maintained the ability to secrete prostacyclin for 9 months.

Mesothelial seeding of knitted Dacron

Bilateral superficial femoral artery replacement using knitted Dacron was performed in 38 dogs. One side was seeded with omental mesothelium and the other acted as an unseeded control. 111In-labelled platelet accumulation on grafts was measured at 5 days and 2 months and the thrombogenicity index of seeded and unseeded grafts calculated. Patency was monitored for 2 months, at which time grafts were removed and luminal thrombus, ultrastructural cell cover and prostacyclin release were measured. Cell seeding did not influence the mean(s.e.m.) thrombogenicity index of 0.95(0.25) and 0.88(0.24) at 5 days in control and seeded grafts respectively; nor was there any difference between the groups at 2 months. Occlusion occurred in six control and four seeded grafts. Seeding did not significantly improve the percentage thrombus-free area or luminal cell cover. Neither did it enhance mean(s.e.m.) luminal 6-keto-prostaglandin F1 alpha release of 2.58(0.80) pg cm-2 in controls and 2.63(0.78) pg cm-2 in seeded grafts. Further studies demonstrated that only a mean(s.e.m.) of 4.4(1.9) per cent of the seeded inoculum was present on grafts 48 h after implantation, providing too few cells to achieve confluent cover. Mesothelial cell seeding might be useful in promoting a healed graft surface but critical levels of seeding density must be achieved before the technique can be properly evaluated.

A cobblestone cell isolated from the human omentum: the mesothelial cell; isolation, identification, and growth characteristics

Normal human mesothelial cells (NHMC) were isolated from pieces of human omentum. The cell yield was approximately one million cells per square centimeter omentum. The mesothelial cells were identified by their positive staining with monoclonal antibodies against cytokeratins 6 and 18. Transmission electronmicroscopy of cultured NHMC revealed many microvilli on the apical surface and many mitochondria and pinocytotic vesicles in the cytoplasm, indicating active transmembrane transport. Growth of NHMC was directly related to the concentration of human serum or of fetal bovine serum in the growth medium. Addition of epidermal growth factor with or without hydrocortisone resulted in a significant increase of NHMC growth; when endothelial cell growth factor, insulin, or hydrocortisone were added no such increase was observed. Seeding NHMC at densities less than 3000/cm2 did not result in monolayer formation. The mesothelial cells were serially passed in growth medium M199 with added 10% fetal bovine serum up to 7 passages. However, after Passage 4 the cells changed into giant cells with an irregular pattern, and a lack of intracellular cytokeratin expression was observed for most of the cells.

Serum proteins provide a matrix for cultured endothelial cells on expanded polytetrafluoroethylene vascular grafts

Endothelialization of prosthetic vascular grafts has been shown to increase patency. We studied the adhesion of cultured endothelial cells of adult human saphenous vein to serum protein coating on expanded polytetrafluoroethylene vascular grafts. The adhesion was compared with that on native or collagen type I-coated grafts. Scanning electron microscopy was used to analyze the precoating and the appearance of the added cultured cells. Presence of radioactively labeled saphenous vein endothelial cells was compared between precoated and uncoated grafts before and after exposure to pulsatile plasma flow. The adherence of saphenous vein endothelial cells was markedly increased on grafts coated with serum proteins or collagen. On both graft types the cells formed confluent areas, which were also present after the plasma flow. The technique was designed to promote endothelialization, using autologous serum as the main source for both growth stimulation during culture and adherence of endothelial cells to grafts. The method may be clinically useful for precoating and endothelialization of vascular grafts.

An artificial blood vessel with an endothelial-cell monolayer

An artificial blood vessel with an endothelial-cell monolayer was used as an arterial substitute in rats. Endothelial cells were isolated from the aorta of a Wistar rat by the digestion method. The cell identification was established by the cobblestone appearance of a confluent cell monolayer, by an expression of factor VIII-related antigen, and by the presence of Weibel-Palade bodies. The luminal surface of the thin-walled polytetrafluoroethylene (PTFE) graft (4 mm in diameter and 10 mm in length) was coated with an endothelial-cell monolayer for 7 days in vitro. An interpositional graft was placed using the endothelial cell-coated PTFE prosthesis on the right common carotid artery in seven rats. A total of 10 rats received an interpositional graft with the noncoated PTFE prosthesis as a control. The patency rate at 1 month after implantation was significantly higher in the coated group than in the control group. The vascular prosthesis with an endothelial-cell monolayer is a promising technique to inhibit the development of thrombosis.

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.

Fibronectin binding to gelatin-impregnated Dacron (Gelseal) prostheses

Endothelial cell adherence to uncoated gelatin-impregnated Dacron (Gelseal) is poor but can be significantly improved by precoating with a suitable basement membrane such as fibronectin. To assess the suitability of fibronectin-coated Gelseal for endothelial cell seeding, fibronectin binding to Gelseal and its dissociation kinetics were investigated. Fibronectin binding was quantified by radiolabeling human fibronectin with iodine 125, concentrations of 10, 25, 50, 150, and 250 micrograms/ml being used to coat Gelseal at 30, 60, and 90 min of incubation. The amount of fibronectin bound was directly proportional to the concentration used and increased with time of incubation (p < 0.05). However, the percentage attachment decreased with increasing concentration (p < 0.001). The number of molecules bound per centimeter squared of graft was calculated. In the first 30 min, 75% of bound fibronectin was lost after exposure to a flow rate of 200 ml/min in a pulsatile artificial circulation; thereafter, the fibronectin-Gelseal bond was stable for up to 2 h.

Seeding of human microvascular endothelial cells onto polytetrafluoroethylene graft material

Microvascular endothelial cells from human omental samples were isolated and grown. Adhesion to polytetrafluoroethylene vascular graft material was then studied using scanning electron microscopy and adhesion of cells labelled with indium-111. Grafts coated with fibronectin and type I collagen were found to promote the best adhesion of cells at times up to 90 min. A coating of blood clot matrix was less effective but still resulted in a threefold increase in cell adhesion compared with controls.

Effects of two methods of endothelial cell seeding on cell retention during blood flow

The potential benefits of endothelial cell seeding depend not only on effective cell attachment, but also on the ability of the cells to resist the stresses of blood flow. We have investigated the effect of different blood flow rates on immediately seeded grafts (SHORT) and also on preformed confluent monolayers formed by overnight incubation (LONG). Cells were labelled with indium and then exposed to either 100 or 200 ml/min of pulsatile blood flow. Cell retention was measured up to 120 min. At both flow rates the LONG seeding procedure gave significantly better cell retention than the SHORT method (P less than 0.01 at all times). There was no difference in cell retention between the two flow rates in the LONG group, but more cells detached at the higher flow rate in the SHORT group (P less than 0.01). We conclude that the new seeding method, using preformed confluent monolayers, significantly improves cell retention during blood flow.