An integrated approach to the design and engineering of hybrid arterial prostheses

Concise Review: Patency of Small-Diameter Tissue-Engineered Vascular Grafts: A Meta-Analysis of Preclinical Trials

Several patient groups undergoing small‐diameter (<6 mm) vessel bypass surgery have limited autologous vessels for use as grafts. Tissue‐engineered vascular grafts (TEVG) have been suggested as an alternative, but the ideal TEVG remains to be generated, and a systematic overview and meta‐analysis of clinically relevant studies is lacking. We systematically searched PubMed and Embase databases for (pre)clinical trials and identified three clinical and 68 preclinical trials ([>rabbit]; 873 TEVGs) meeting the inclusion criteria. Preclinical trials represented low to medium risk of bias, and binary logistic regression revealed that patency was significantly affected by recellularization, TEVG length, TEVG diameter, surface modification, and preconditioning. In contrast, scaffold types were less important. The patency was 63.5%, 89%, and 100% for TEVGs with a median diameter of 3 mm, 4 mm, and 5 mm, respectively. In the group of recellularized TEVGs, patency was not improved by using smooth muscle cells in addition to endothelial cells nor affected by the endothelial origin, but seems to benefit from a long‐term (46–240 hours) recellularization time. Finally, data showed that median TEVG length (5 cm) and median follow‐up (56 days) used in preclinical settings are relatively inadequate for direct clinical translation. In conclusion, our data imply that future studies should consider a TEVG design that at least includes endothelial recellularization and bioreactor preconditioning, and we suggest that more standard guidelines for testing and reporting TEVGs in large animals should be considered to enable interstudy comparisons and favor a robust and reproducible outcome as well as clinical translation.

Muscular Tissue Engineering: Capillary-Incorporated Hybrid Muscular Tissues in Vivo Tissue Culture

Requirements for a functional hybrid muscular tissue are 1) a high density of multinucleated cells, 2) a high degree of cellular orientation, and 3) the presence of a capillary network in the hybrid tissue. Rod-shaped hybrid muscular tissues composed of C2C12 cells (skeletal muscle myoblast cell line) and type I collagen, which were prepared using the centrifugal cell-packing method reported in our previous article, were implanted into nude mice. The grafts, comprised three hybrid tissues (each dimension, diameter, approximately 0.3 mm, length, approximately 1 mm, respectively), were inserted into the subcutaneous spaces on the backs of nude mice. All nude mice that survived the implantation were sacrificed at 1, 2, and 4 wk after the implantation. The grafts were easily distinguishable from the subcutaneous tissues of host mice with implantation time. The grafts increased in size with time after implantation, and capillary networks were formed in the vicinities and on the surfaces of the grafts. One week after implantation, many capillaries formed in the vicinities of the grafts. In the central portion of the graft, few capillaries and necrotic cells were observed. Mononucleated myoblasts were densely distributed and a low number of multinucleated myotubes were scattered. Two weeks after implantation, the formation of a capillary network was induced, resulting in the surfaces of the grafts being covered by capillaries. Numerous elongated multinucleated myotubes and mononucleated myoblasts were densely distributed and numerous capillaries were observed throughout the grafts. Four weeks after implantation a dense capillary network was formed in the vicinities and on the surfaces of the grafts. In the peripheral portion of the graft, multinucleated myotubes in the vicinities of the rich capillaries were observed. Thus, hybrid muscular tissues in vitro preconstructed was remodeled in vivo, which resulted in facilitating the incorporation of capillary networks into the tissues. © 1998 Elsevier Science Inc.

Hybrid Muscular Tissues: Preparation of Skeletal Muscle Cell-Incorporated Collagen Gels

We prepared three different types of hybrid muscular tissues in which C2C12 cells (skeletal muscle myoblast cell line) were incorporated in type I collagen gels and then differentiated to myotubes upon culture: a disctype, a polyester mesh-reinforced sheet-type, and a tubular type. A cold mixed solution of the cells and type I collagen was poured into three different types of molds and was kept at 37°C in an incubator to form C2C12 cell-incorporated gels. A polyester mesh was incorporated into a gel to form the sheet-type tissue. The tubular hybrid tissue was prepared by pouring a mixed solution into the interstitial space of a tubular mold consisting of an outer sheath and a mandrel and subsequently culturing after removal of the outer sheath. Hybrid tissues were incubated in a growth medium (20% fetal bovine serum medium) for the first 4 days and then in a differentiation medium (2% horse serum medium) to induce formation of myotubes. Transparent fragile gels shrank with time to form opaque gels, irrespective of type, resulting in the formation of quite dense hybrid tissues. On day 14 of incubation, myoblasts fused and differentiated to form multinucleated myotubes. For a tubular type hybrid tissue, both cells and collagen fiber bundles became circumferentially oriented with incubation time. Periodic mechanical stress loading to a mesh-reinforced hybrid tissue accelerated the cellular orientation along the axis of the stretch. The potential applications for use as living tissue substitutes in damaged and diseased skeletal and cardiac muscle and as vascular grafts are discussed.

Phenotypic Reversion of Smooth Muscle Cells in Hybrid Vascular Prostheses

Our purpose was to evaluate whether or not and when phenotypic modulation of smooth muscle cells (SMCs) in hybrid vascular prostheses preincorporated with SMCs occurs upon implantation. Two types of hybrid vascular grafts incorporated with vascular cells derived from canine jugular veins were prepared: grafts containing a collagen gel layer covered with an endothelial monolayer at the luminal surface (Model I graft) and those containing an endothelial monolayer and SMC multilayer (Model II graft). They were bilaterally implanted into carotid arteries of the same dogs from which the cells had been harvested for 2 wk (n = 3) and 12 wk (n = 3). The time-dependent changes in populations of three SMC phenotypes (synthetic, intermediate, and contractile) in the neoarterial layers were quantified by morphometric evaluation using a transmission electron microscope in hybrid vascular grafts. Before implantation, all the SMCs were of the synthetic phenotype. In Model II grafts at 2 wk, synthetic and intermediate SMCs were dominant especially in the luminal layer. On the other hand, neoarterial layers at 12 wk were dominated by contractile SMCs, which were evenly distributed throughout the entire neoarterial tissues. A markedly delayed phenotypic reversion was noted for the Model I grafts at 12 wk. In the hybrid grafts, during about 3 mo of implantation, neoarterial SMCs transformed from the synthetic to the contractile phenotypes, which was promoted by SMC incorporation.

Tissue Engineered Skeletal Muscle: Preparation of Highly Dense, Highly Oriented Hybrid Muscular Tissues

We prepared highly dense, highly oriented hybrid muscular tissues that are composed of C2C12 cells (skeletal muscle myoblast cell line) and type I collagen. A cold mixture of C2C12 cells suspended in DMEM and type I collagen solution was poured into capillary tube molds of two different sizes (inner diameters; 0.90 and 0.53 mm, respectively). One end of each mold was sealed. Upon centrifugation (1000 rpm, 5 min) and subsequent thermal gelation, a rod-shaped gel was obtained. It was cultured in an agarose gel-coated dish for 7 days (first for 3 days in a growth medium and then for 4 days in a differentiation medium), during which time it shrank to become a highly dense tissue. Small-diameter rod-shaped, highly dense cellular assemblages with multinucleated myotubes were formed and only few necrotic cells at the core of the tissue were observed. On the other hand, a ring-shaped tissue prepared using a specially devised agarose gel mold was subjected to cyclic stretching at 60 rpm, resulting in the formation of a highly dense, highly oriented hybrid muscular tissue in which both densely accumulated cells and collagen fiber bundles tended to be aligned in the direction of stretching. The hybrid muscular tissues that were prepared using via sequential procedures of a centrifugal cell packing method and a mechanical stress-loading method became closer to native muscular tissues in terms of cell density and orientation.

Self-Organized, Tubular Hybrid Vascular Tissue Composed of Vascular Cells and Collagen for Low-Pressure-Loaded Venous System

A tubular, hierarchically structured hybrid vascular tissue composed of vascular cells and collagen was prepared. First, a cold mixed solution of bovine aortic smooth muscle cells (SMCs) and Type I collagen was poured into a tubular glass mold composed of a mandrel and a sheath (example of dimensions: inner diameter, 1.5 mm; outer diameter, 7 mm; length, 7 cm). Upon incubation at 37°C, an SMC-incorporated collagenous gel was formed. After the sheath was removed, the resulting fragile tissue, when cultured in medium, thinned in a time-dependent manner to form an opaque, dense tissue. Higher SMC seeding density and lower initial collagen concentration induced more rapid and prominent shrinkage of the tissue. Morphologic investigation showed that over time, bipolarly elongated SMCs and collagen fiber bundles became positioned around the mandrel. Both components became circumferentially oriented. When the mandrel was removed, a tubular hybrid medial tissue was formed. A hybrid vascular tissue with a hierarchical structure was constructed by seeding endothelial cells onto the inner surface of the hybrid medial tissue. Prepared tissues tolerated luminal pressures as great as 100 mmHg and mechanical stress applied during an anastomotic procedure. This method allowed us to prepare a tubular hybrid medial tissue of predetermined size (inner diameter, wail thickness, and length) by selecting appropriate mold design, initial collagen concentration, and SMC seeding density. Such hybrid vascular tissues may provide physiological functions when implanted into the venous system.

Tissue-engineered acellular small diameter long-bypass grafts with neointima-inducing activity

Researchers have attempted to develop efficient antithrombogenic surfaces, and yet small-caliber artificial vascular grafts are still unavailable. Here, we demonstrate the excellent patency of tissue-engineered small-caliber long-bypass grafts measuring 20-30 cm in length and having a 2-mm inner diameter. The inner surface of an acellular ostrich carotid artery was modified with a novel heterobifunctional peptide composed of a collagen-binding region and the integrin α4β1 ligand, REDV. Six grafts were transplanted in the femoral-femoral artery crossover bypass method. Animals were observed for 20 days and received no anticoagulant medication. No thrombogenesis was observed on the luminal surface and five cases were patent. In contrast, all unmodified grafts became occluded, and severe thrombosis was observed. The vascular grafts reported here are the first successful demonstrations of short-term patency at clinically applicable sizes.

Tissue engineered vascular grafts--preclinical aspects

Tissue engineering enables the development of fully biological vascular substitutes that restore, maintain and improve tissue function in a manner identical to natural host tissue. However the development of the appropriate preclinical evaluation techniques for the generation of fully functional tissue-engineered vascular graft (TEVG) is required to establish their safety for use in clinical trials and to test clinical effectiveness. This review gives an insight on the various preclinical studies performed in the area of tissue engineered vascular grafts highlighting the different strategies used with respect to cells and scaffolds, typical animal models used and the major in vivo evaluation studies that have been carried out. The review emphasizes the combined effort of engineers, biologists and clinicians which can take this clinical research to new heights of regenerative therapy.

Arterial shear stress augments the differentiation of endothelial progenitor cells adhered to VEGF-bound surfaces

Our ongoing studies show that vascular endothelial cell growth factor (VEGF)-bound surfaces selectively capture endothelial progenitor cells (EPCs) in vitro and in vivo, and that surface-bound VEGF stimulates intracellular signal transduction pathways over prolonged culture periods, resulting in inductive differentiation of EPCs. In this article, we investigated whether simulated arterial shear stress augments the differentiation of EPCs adhered to a VEGF-bound surface. Human peripheral blood-derived mononuclear cells adhered to a VEGF-bound surface were exposed to 1 day of shear stress (15 dynes/cm(2), corresponding to shear load in arteries). Shear stress suppressed the expression of mRNAs encoding CD34 and CD133, which are markers for EPCs, and augmented the expression of mRNAs encoding CD31 and von Willebrand factor (vWF) as well as vWF protein, which are markers for endothelial cells (ECs). Shear stress enhanced expression of ephrinB2 mRNA, a marker for arterial ECs, but did not significantly change expression of EphB4 mRNA, a marker for venous ECs. Focused protein array analysis showed that mechanotransduction by shear stress activated the p38 and MAPK pathways in EPCs. Thus, arterial shear stress, in concert with surface-bound VEGF, augments the differentiation of EPCs. These results strongly support previous observation of rapid differentiation of EPCs captured on VEGF-bound stents in a porcine model.

Quantification and regulation of cell migration

Cell migration is essential for many physiological and pathological processes that include embryonic development, the immune response, wound healing, angiogenesis, and cancer metastasis. It is also important for emerging tissue engineering applications such as tissue reconstitution and the colonization of biomedical implants. By summarizing results from recent experimental and theoretical studies, this review outlines the role played by growth factors or substrate-adhesion molecules in modulating cell motility and shows that cell motility can be an important factor in determining the rates of tissue formation. The application of cell motility assays and the use of theoretical models for analyzing cell migration and proliferation are also discussed.

Luminal surface design of electrospun small-diameter graft aiming at in situ capture of endothelial progenitor cell

If endothelial progenitor cells (EPCs), which circulate in blood flow, are captured on the luminal surface of an implanted artificial graft and sooner or later proliferate to form a fully endothelialized surface, such a small-diameter artificial graft must exhibit a high patency rate. This study aimed at designing a luminal surface of elastomeric electrospun mesh graft, which is capable of selective capture of EPCs under arterial flow and has a high antithrombogenic potential until full endothelization is achieved. The designed luminal surface layer is composed of a photopolymerized gelatin gel layer that enables the release of impregnated heparin and selective adhesion of circulating EPCs via complexation between surface-fixed vascular endothelial growth factor (VEGF) and cellular VEGF receptor. Human mononuclear cells seeded and cultured on such a gel layer expressed endothelial cell surface markers. Confocal laser scanning microscopy observation revealed that VEGF is highly surface-enriched, and heparin is homogeneously distributed in the gel layer. A continuously slow release of heparin was observed. Thus, a prototype luminal surface was fabricated on electrospun segmented polyurethane tubes for in vivo study.

Cell Sorting Technique Based on Thermoresponsive Differential Cell Adhesiveness

Cell sorting of specific target cells from a mixture of different cell types is a prerequisite for development of functional engineered tissues based on stem-cell and tissue engineering. This paper presents a new method of cell sorting that uses a mixture of thermoresponsive cell-adhesive and non-cell-adhesive substances. The former substance is poly(N-isopropylacrylamide)-grafted gelatin (PNIPAM-gelatin) and the latter is PNIPAM. Graded cell adhesion, produced by mixed coating of these thermoresponsive substances at an appropriate mixing ratio, clearly differentiated the adhesive potentials of two bovine vascular cell types (endothelial cell and smooth muscle cell). The sequential procedures of detachment at room temperature and subsequent replating at 37 degrees C on dishes coated with a mixed coating with the same composition as that employed previously yielded remarkably pure target cells, as determined using confocal laser scanning fluorescence microscopy. This method, leading to harvesting of target cells, is characteristic of simple manipulation with no cell damage. Such advantages are expected to facilitate stem-cell and tissue engineering.

The effect of gradually graded shear stress on the morphological integrity of a huvec-seeded compliant small-diameter vascular graft

The premature endothelialization of tissue-engineered grafts had often induced cellular detachment at an early period of implantation in arterial circulation, resulting in occlusion at an early period of implantation. This study was aimed to determine whether gradually increased shear stress applied ex vivo improves cell retention and tissue morphological integrity including cell shape and alignment, actin fiber alignment and expression of vascular endothelial (VE) cadherin. Tissue-engineered grafts used for this study were human umbilical vein endothelial cell (HUVEC)-seeded compliant small-diameter grafts made of poly(L-lactide-co-epsilon-caprolactone) fiber meshes fabricated by electrospinning. The shear stresses applied to grafts, generated using a custom-designed mock circulatory apparatus, were 3.2, 8.7 and 19.6 dyn/cm(2). The grafts completely monolayered prior to shear stress exposure exhibited a polygonal cobblestone morphology with randomly distributed actin fibers and VE cadherin at the continuous peripheral region of adjacent cells. The 24-h-loading of high shear stresses (8.7 and 19.6 dyn/cm(2)) equivalent to those of the arterial circulatory system resulted in severe cellular damage resulting in the complete loss of cells. However, a gradually increased graded exposure from a low (3.2 dyn/cm(2)) to a high shear stress (19.6 dyn/cm(2)) resulted in a markedly reduced cell detachment, a highly elongated cell shape, and orientation or alignment of both cells and actin fibers, which were parallel to the direction of flow. Although VE-cadherin expression was not detected yet, a higher degree of tissue integrity was achieved, which may greatly improve the performance particularly at an early period of implantation.

Tissue-engineered valves with commissural alignment

We present a novel approach to producing bioartificial valves using the tissue-equivalent method of entrapping cells within a biopolymer gel and using a mold design that presents appropriate mechanical constraints to the cell-induced gel compaction to yield both the fibril alignment and the geometry of a native valve. Bileaflet valves were fabricated from bovine collagen and neonatal human dermal fibroblasts as proof of principle. The resultant valves possessed both commissure-tocommissure alignment of collagen fibers in the leaflets and circumferential alignment in the root. While this alignment was manifested in planar biaxial tensile mechanical properties, histology of the leaflets revealed an aligned collagen matrix but lacking other extracellular matrix (ECM) components present in the native valve. The apparent lack of ECM production by the fibroblasts after contracting and aligning the collagen fibrils is consistent with peak loads during biaxial testing being only approximately 10% of native leaflet values and a 0:1 coupling index that was only approximately 50% of native leaflet values despite exhibiting comparable values for the anisotropy index.

Cell transplantation for heart failure, and tissue engineering of cardiovascular structures

The emergence of selective cell transplantation and tissue engineering has opened up new alternatives for the treatment of heart failure. These involve the use of stem and progenitor cells, as well as new biodegradable polymer scaffolds. The last two decades have seen a dramatic increase in our knowledge of how to fabricate a wide array of tissues, including cardiovascular structures. This article reviews current trends in selective cell transplantation and tissue engineering, and summarises recent achievements and remaining questions in constructing functional cardiac tissue.

Tissue engineering of blood vessels

BACKGROUND

Tissue engineering techniques have been employed successfully in the management of wounds, burns and cartilage repair. Current prosthetic alternatives to autologous vascular bypass grafts remain poor in terms of patency and infection risk. Growing biological blood vessels has been proposed as an alternative.

METHODS

This review is based on a literature search using Medline, PubMed, ISIS and CAS of original articles and reviews, and unpublished material and abstracts.

RESULTS AND CONCLUSIONS

Complete incorporation into host tissues and the maintenance of a viable and self-renewing endothelial layer are the fundamental goals to be achieved when developing a tissue-engineered blood vessel. Sourcing of cells and modulating their interaction with extracellular matrix and supporting scaffold have been the focus of intense research. Although the use of tissue-engineered blood vessels in humans is so far limited, advances in our knowledge of stem cell precursors and the development of new biomaterials should enable this technology to reach routine clinical practice within a decade.

In vivo tissue-engineered small-caliber arterial graft prosthesis consisting of autologous tissue (biotube)

In this study, vascular-like tubular tissues called biotubes, consisting of autologous tissues, were prepared using in vivo tissue engineering. Their mechanical properties were evaluated for application as a small-caliber artificial vascular prosthesis. The biotubes were prepared by embedding six kinds of polymeric rods [poly(ethylene) (PE), poly(fluoroacetate) (PFA), poly(methyl methacrylate) (PMMA), segmented poly(urethane) (PU), poly(vinyl chloride) (PVC), and silicone (Si)] as a mold in six subcutaneous pouches in the dorsal skin of New Zealand White rabbits. For rods apart from PFA, biotubes were constructed after 1 month of implantation by encapsulation around the polymeric implants. The wall thickness of the biotubes ranged from about 50 to 200 microm depending on the implant material and were in the order PFA < PVC < PMMA < PU < PE. As for PE, PMMA, and PVC, the thickness increased after 3 months of implantation and ranged from 1.5-to 2-fold. None of the biotubes were ruptured when a hydrostatic pressure was gradually applied to their lumen up to 200 mmHg. The relationship between the intraluminal pressure and the external diameter, which was highly reproducible, showed a "J"-shaped curve similar to the native artery. The tissue mostly consisted of collagen-rich extracellular matrices and fibroblasts. Generally, the tissue was relatively firm and inelastic for Si and soft for PMMA. For PMMA, PE, and PVC the stiffness parameter (beta value; one of the indexes for compliance) of the biotubes obtained was similar to those of the human coronary, femoral, and carotid arteries, respectively. Biotubes, which possess the ability for wide adjustments in their matrices, mechanics, shape, and luminal surface design, can be applied for use as small-caliber blood vessels and are an ideal implant because they avoid immunological rejection.

Cardiovascular tissue engineering: state of the art

In patients requiring coronary or peripheral vascular bypass procedures, autogenous arterial or vein grafts remain as the conduit of choice even in the case of redo patients. It is in this class of redo patients that often natural tissue of suitable quality becomes unavailable; so that prosthetic material is then used. Prosthetic grafts are liable to fail due to graft occlusion caused by surface thrombogenicity and lack of elasticity. To prevent this, seeding of the graft lumen with endothelial cells has been undertaken and recent clinical studies have evidenced patency rates approaching reasonable vein grafts. Recent advances have also looked at developing a completely artificial biological graft engineered from the patient's cells with surface and viscoelastic properties similar to autogenous vessels. This review encompasses both endothelialisation of grafts and the construction of biological cardiovascular conduits.

Use of a fibrin preparation in the engineering of a vascular graft model

OBJECTIVE

Morphological and functional characterization of cocultured endothelial cells (EC) and myofibroblasts (MFB) seeded on a matrix composed of a fibrin preparation mimicking the microenvironment of a vascular wall.

METHODS

MFB and EC were isolated from human saphenous veins and expanded separately in vitro. MFB were seeded on a composite matrix consisting of a fibrin preparation (with or without transforming growth factor-beta2) and a polyglactin-mesh to form a 3-dimensional structure, which was consecutively reseeded with EC. Seeded matrices were incubated in a bioreactor. Characterization was done including fluorescence staining, live-/dead-assay and immunohistochemistry.

RESULTS

High density cocultures in hierarchical structure mimicking the formation of a vascular wall were obtained with nearly complete coverage of the surface with EC. Distribution of preseeded MFB in a 519+/-27 microm thick layer (day 14) was achieved. Cell viability was shown in fluorescence staining for at least 19 days. In deeper layers, no viable cells could be detected within the fibrin preparation. EC covered the surface, had uniform morphology, and their preserved viability was shown for at least 5 days. No EC-ingrowth was found into the fibrin preparation. Neoformation of the matrix proteins laminin and collagen IV was observed.

CONCLUSION

A structured coculture of MFB and EC was obtained mimicking the formation of a vascular wall with preserved viability utilizing a fibrin preparation. Nutrition problems seem to limit the maximal extent of MFB in the matrix.

Novel 70-kDa chondroitin sulfate/dermatan sulfate hybrid chains with a unique heterogeneous sulfation pattern from shark skin, which exhibit neuritogenic activity and binding activities for growth factors and neurotrophic factors

Chondroitin sulfate (CS) and dermatan sulfate (DS) hybrid chains of proteoglycans are critical in growth factor binding, neuritogenesis, and brain development. Here we isolated CS/DS hybrid chains from shark skin aiming to develop therapeutic agents. Digestion with various chondroitinases showed that both GlcUA- and IdoUA-containing disaccharides are scattered along the polysaccharide chains with an unusually large average molecular mass of 70 kDa. The CS/DS chains were separated into major (80%) and minor (20%) fractions by anion-exchange chromatography. Both fractions had relatively low degrees of sulfation (sulfate/disaccharide molar ratio=1.17 versus 0.87), showing a unique feature compared with the marine CS and DS isolated to date, most of which are oversulfated. They were highly heterogeneous and characterized by multiple disaccharides including GlcUA-GalNAc, GlcUA-GalNAc(6S), GlcUA-GalNAc(4S), IdoUA-GalNAc(4S), GlcUA-GalNAc(4S,6S), IdoUA-GalNAc(4S,6S), GlcUA(2S)-GalNAc(6S), and/or IdoUA(2S)-GalNAc(6S), IdoUA(2S)-GalNAc(4S) and novel GlcUA(2S)-GalNAc(4S), where 2S, 4S, and 6S represent 2-O-, 4-O- and 6-O-sulfate, respectively. The CS/DS chains bound two neurotrophic factors and various growth factors expressed in the brain with high affinity as evaluated for the major fraction by kinetic analysis using a surface plasmon resonance detector, and also promoted the outgrowth of neurites of both an axonic and a dendritic nature. The neuritogenic activity was abolished completely by digestion with chondroitinase ABC, AC-I, or B, suggesting the importance of both GlcUA- and IdoUA-containing moieties. It also showed anti-heparin cofactor II activity comparable to that exhibited by DS from porcine skin. Thus, by virtue of its unique structure and biological activities, DS will find a potential use in therapeutics.

Engineering of bypass conduits to improve patency

For patients with severe coronary artery and distal peripheral vascular disease not amenable to angioplasty and lacking sufficient autologous vessels there is a pressing need for improvements to current surgical bypass options. It has been decades since any real progress in bypass material has reached mainstream surgical practice. This review looks at possible remedies to this situation. Options considered are methods to reduce prosthetic graft thrombogenicity, including endothelial cell seeding and developments of new prosthetic materials. The promise of tissue-engineered blood vessels is examined with a specific look at how peptides can improve cell adhesion to scaffolds.

The use of animal models in developing the discipline of cardiovascular tissue engineering: a review

Cardiovascular disease remains one of the major causes of death and disability in the Western world. Tissue engineering offers the prospect of being able to meet the demand for replacement of heart valves, vessels for coronary and lower limb bypass surgery and the generation of cardiac tissue for addition to the diseased heart. In order to test prospective tissue-engineered devices, these constructs must first be proven in animal models before receiving CE marking or FDA approval for a clinical trial. The choice of animal depends on the nature of the tissue-engineered construct being tested. Factors that need to be considered include technical requirements of implanting the construct, availability of the animal, cost and ethical considerations. In this paper, we review the history of animal studies in cardiovascular tissue engineering and the uses of animal tissue as sources for tissue engineering.

Recent progress of vascular graft engineering in Japan

The development of a small-diameter vascular graft has long been awaited. This review covers research activities, achievements and progress on vascular engineering in Japan, which was conducted over the last decade. The article includes recently developed experimental scaffolds, biologically active artificial extracellular matrices (ECMs) or non-fouling synthetic coatings, cell sourcing including the autologous vascular cell type, endothelial progenitor cells and genetically-engineered, temporary endothelial-like cells. The discussions were presented from biomechanical, biomaterial, cellular and tissue aspects. Once the mechano-biological and biologically active extracellular milieus are established in a designed vascular graft, the functional, structural and mechanical tissue morphogenesis and adaptation of implanted vascular grafts may proceed with implantation duration, and the spatio-temporal tissue modulations at cytokine, cellular, ECM levels under physiological stress proceed to regenerate vascular tissue architecture. The ultimate solution to a small-diameter vascular graft should be realized by optimal combinations of these factors.

Purification and characterization of dermatan sulfate from the skin of the eel, Anguilla japonica

Glycosaminoglycans were isolated from the eel skin (Anguilla japonica) by actinase and endonuclease digestions, followed by a beta-elimination reaction and DEAE-Sephacel chromatography. Dermatan sulfate was the major glycosaminoglycan in the eel skin with 88% of the total uronic acid. The content of the IdoA2Salpha1-->4GalNAc4S sequence in eel skin, which shows anticoagulant activity through binding to heparin cofactor II, was two times higher than that of dermatan sulfate from porcine skin. The anti-IIa activity of eel skin dermatan sulfate was determined to be 2.4 units/mg, whereas dermatan sulfate from porcine skin shows 23.2 units/mg. The average molecular weight of dermatan sulfate was determined by gel chromatography on a TSKgel G3000SWXL column as 14 kDa. Based on 1H NMR spectroscopy, the presence of 3-sulfated and/or 2,3-sulfated IdoA residues was suggested. The reason why highly sulfated dermatan sulfate does not show anticoagulant activity is discussed. In addition to dermatan sulfate, the eel skin contained a small amount of keratan sulfate, which was identified by keratanase treatment.

Improving the clinical patency of prosthetic vascular and coronary bypass grafts: the role of seeding and tissue engineering

In patients requiring coronary or peripheral vascular bypass procedures, autogenous vein is currently the conduit of choice. If this is unavailable, then a prosthetic material is used. Prosthetic graft is liable to fail due to occlusion of the graft. To prevent graft occlusion, seeding of the graft lumen with endothelial cells is undertaken. Recent advances have also looked at developing a completely artificial biological graft engineered from the patient's cells with properties similar to autogenous vessels. This review encompasses the developments in the two principal technologies used in developing hybrid coronary and peripheral vascular bypass grafts, that is, seeding and tissue engineering.

Tissue engineering: a 21st century solution to surgical reconstruction

Tissue engineering has emerged as a rapidly expanding approach to address the organ shortage problem. It is an "interdisciplinary field that applies the principles and methods of engineering and the life sciences toward the development of biological substitutes that can restore, maintain, or improve tissue function." Much progress has been made in the tissue engineering of structures relevant to cardiothoracic surgery, including heart valves, blood vessels, myocardium, esophagus, and trachea.

Surface microarchitectural design in biomedical applications: in vivo analysis of tissue ingrowth in excimer laser-directed micropored scaffold for cardiovascular tissue engineering

A micropatterned microporous segmented polyurethane film (20 x 12 mm in size, 30 micrometer thick) with four regions was prepared by excimer laser microprocessing to provide an in vivo model of transmural tissue ingrowth in an open cell-structured scaffold specially designed for cardiovascular tissue engineering. Three microporous regions had the same circular micropores (30 micrometer diameter) but different pore density arrangements (percentage of total pore area against unit area was 0.3%, 1.1%, and 4.5%), and the other region remained nonporous. The covered stent, prepared by wrapping the regionally different density-microporous film on an expandable metallic stent (approximately 3.1 mm in diameter), was delivered to the luminal surface of canine common carotid arteries and placed after expansion of the stent to a diameter of approximately 8 mm using a balloon catheter. At 4 weeks of implantation, all the covered stents (n = 10) were patent. The luminal surfaces of the covered stents were almost confluently endothelialized both in nonporous and microporous regions. Histological examination showed that the neointimal wall was formed by tissue ingrowth from host through micropores (transmural) and anastomotic sites. Thrombus formation occurred frequently in the lowest density porous region and nonporous region. With an increase in pore density, the thickness of the neointimal wall decreased. This study demonstrated how the micropore density of implanted devices influences tissue ingrowth in arterial implantation.

Progenitor endothelial cells on vascular grafts: an ultrastructural study

The morphology of progenitor endothelial cells on vascular graft surfaces is addressed in this report. Such cells were seen to attach to intima-expressed CD34 and Flk-1 antigen and showed positive 5-bromo-2-deoxyuridine (BrdU) uptake. We examined CD34 and Flk-1 antigen-expressing endothelial progenitor cells three-dimensionally using confocal laser scanning microscopy (CLSM). Under detailed CLSM observation, through an ameboid-form cell, these progenitor endothelial cells changed from a globular to a flattened form. We also investigated these morphological changes using scanning electron microscopy. From these results, progenitor endothelial cells were observed not only near the advancing edge of endothelium, but also around the developing intimal site. Their form also changed from globular to flattened as observed in the CLSM results. These morphological changes were seen more frequently near the advancing edge and around the developing intimal site. They attached directly to vascular prosthesis fibers and likewise covered the graft luminal surface. Progenitor endothelial cells in any form had a common surface structure. We conclude from our results that progenitor endothelial cells can attach to graft fibers directly without clotting and directly cover the graft luminal surfaces.

Preparation and characterization of hydrophobic polymeric films that are thromboresistant via nitric oxide release

The preparation of hydrophobic polymer films (polyurethane and poly(vinyl chloride)) containing nitric oxide (NO)-releasing diazeniumdiolate functions is reported as a basis for improving the thromboresistivity of such polymeric materials for biomedical applications. Several different approaches for preparing NO-releasing polymer films are presented, including: (1) dispersion of diazeniumdiolate molecules within the polymer matrix; (2) covalent attachment of the diazeniumdiolate to the polymer backbone; and (3) ion-pairing of a diazeniumdiolated heparin species to form an organic soluble complex that can be blended into the polymer. Each approach is characterized in terms of NO release rates and in vitro biocompatibility. Results presented indicate that the polymer films prepared by each approach release NO for variable periods of time (10-72 h), although they differ in the mechanism, location and amount of NO released. In vitro platelet adhesion studies demonstrate that the localized NO release may prove to be an effective strategy for improving blood compatibility of polymer materials for a wide range of medical devices.

Fabrication of compliant hybrid grafts supported with elastomeric meshes

We devised tubular hybrid medial tissues with mechanical properties similar to those of native arteries, which were composed of bovine smooth muscle cells (SMCs) and type I collagen with minimal reinforcement with knitted fabric meshes made of synthetic elastomers. Three hybrid medial tissue models that incorporated segmented polyester (mesh A) or polyurethane-nylon (mesh B) meshes were designed: the inner, sandwich, and wrapping models. Hybrid medial tissues were prepared by pouring a cold mixed solution of SMCs and collagen into a tubular glass mold consisting of an inner mandrel and an outer sheath and subsequent thermal gelation, followed by further culture for 7 days. For the inner model, the mandrel was wrapped with a mesh. For the sandwich model, a cylindrically shaped mesh was incorporated into a space between the mandrel and the sheath. The wrapping model was prepared by wrapping a 7-day-incubated nonmesh gel with a mesh. The inner diameter was 3 mm, irrespective of the model, and the length was 2.5-4.0 cm, depending on the model. The intraluminal pressure-external diameter relationship showed that nonmesh and inner models had a very low burst strength below 50 mmHg, while the sandwich model ruptured at around 110-120 mmHg; no rupturing below 240 mmHg was observed for the wrapping model, regardless of the type of mesh used. Compliance values of wrapping and sandwich models were close to those of native arteries. Pressure-dependent distensibility characteristics similar to native arteries were observed for a mesh A wrapping model, whereas a mesh B wrapping model expanded almost linearly as intraluminal pressure increased, which appeared to be due to elasticity of the incorporated mesh. Thus, design criteria for hybrid vascular grafts with appropriate biomechanical matching with host arteries were established. Such hybrid grafts may be mechanically adapted in an arterial system.

The role of vascular smooth muscle cell integrins in the compaction and mechanical strengthening of a tissue-engineered blood vessel

Vascular smooth muscle cells (VSMC) influence vessel structure and function during normal development, and in disease states. VSMC interactions with extracellular matrix, via cell surface integrins, play an important role in these processes. A greater understanding of the molecular basis of these interactions is also critical to advances in the field of cardiovascular tissue engineering. This study examined the role of VSMC integrins in the spontaneous compaction and eventual strengthening of a rudimentary tissue-engineered blood vessel (TEBV) consisting of a fibrillar type I collagen network populated by human aortic smooth muscle cells. Using integrin subunit-specific antibodies, we demonstrated that anti-beta1 (Mab13 and P4C10) and anti-alpha2 (P1E6) antibodies that inhibit aortic smooth muscle cell (AoSMC) adhesion to collagen, also significantly inhibit TEBV compaction during the 24-hour period following TEBV construction. However, no difference in the tensile stress of antibody-treated and control TEBVs was observed at this time point. In contrast, 72 hours after construction, the inhibitory effect of anti-integrin antibodies on compaction had been overcome but tensile stress was decreased in TEBVs treated with anti-alpha2/anti-beta1 antibodies when compared to controls. These data provide evidence linking VSMC integrins, specifically the alpha2beta1 integrin, with the initial compaction, as well as, the postcompaction strengthening of the TEBV.

A new generation of polyurethane vascular prostheses: rara avis or ignis fatuus?

Three polyurethane (PU) vascular grafts with novel designs were investigated and compared in terms of the microporous structure, reinforcement technology, polymer chemistry, microphase separation, and mechanical properties. The Corvita graft, composed of a poly(carbonate urethane) polymer, displayed a helically wound filament structure with communicating inter-fiber spaces. The reinforced model contained an external PET mesh impregnated with a protein sealant, and displayed good microphase separation, the highest Young's modulus in the longitudinal direction, and the second highest in the radial direction. The Thoratec graft was made of a polyetherurethaneurea with an average micropore size of 15 microns. Silicone was observed on both surfaces of the graft. The Thoratec device displayed a low degree of hydrogen-bonding among the urethane groups and had no well-organized hard-segment domains. Its mechanical strength was superior to that of the Pulse-Tec graft. A solid PU layer underneath the luminal surface precluded any communication between the luminal and adventitial sides. The Pulse-Tec prosthesis was composed of polyetherurethane, with an average micropore size of 28 microns. It offered the highest radial compliance, a high degree of hydrogen-bonding, a narrow molecular weight distribution, and a certain degree of microphase separation. Its tensile strength and hysteresis loss were inferior to those of the other two grafts.

Chronic in vitro shear stress stimulates endothelial cell retention on prosthetic vascular grafts and reduces subsequent in vivo neointimal thickness

OBJECTIVE

The absence of endothelial cells at the luminal surface of a prosthetic vascular graft potentiates thrombosis and neointimal hyperplasia, which are common causes of graft failure in humans. This study tested the hypothesis that pretreatment with chronic in vitro shear stress enhances subsequent endothelial cell retention on vascular grafts implanted in vivo.

METHODS

Cultured endothelial cells derived from Fischer 344 rat aorta were seeded onto the luminal surface of 1.5-mm internal diameter polyurethane vascular grafts. The seeded grafts were treated for 3 days with 1 dyne/cm2 shear stress and then for an additional 3 days with 1 or 25 dyne/cm2 shear stress in vitro. The grafts then were implanted as aortic interposition grafts into syngeneic rats in vivo. Grafts that were similarly seeded with endothelial cells but not treated with shear stress and grafts that were not seeded with endothelial cells served as controls. The surgical hemostasis time was monitored. Endothelial cell identity, density, and graft patency rate were evaluated 24 hours after implantation. Endothelial cell identity in vivo was confirmed with cells transduced in vitro with beta-galactosidase complementary DNA in a replication-deficient adenoviral vector. Histologic, scanning electron microscopic, and immunohistochemical analyses were performed 1 week and 3 months after implantation to establish cell identity and to measure neointimal thickness.

RESULTS

The pretreatment with 25 dyne/cm2 but not with 0 or 1 dyne/cm2 shear stress resulted in the retention of fully confluent endothelial cell monolayers on the grafts 24 hours after implantation in vivo. Retention of seeded endothelial cells was confirmed by the observation that beta-galactosidase transduced cells were retained as a monolayer 24 hours after implantation in vivo. In the grafts with adherent endothelial cells that were pretreated with shear stress, immediate graft thrombosis was inhibited and surgical hemostasis time was significantly prolonged. Confluent intimal endothelial cell monolayers also were present 1 week and 3 months after implantation. However, 1 week after implantation, macrophage infiltration was observed beneath the luminal cell monolayer. Three months after the implantation in vivo, subendothelial neointimal cells that contained alpha-smooth muscle actin were present. The thickness of this neointima averaged 41 +/- 12 micrometer and 60 +/- 23 micrometer in endothelial cell-seeded grafts that were pretreated with 25 dyne/cm2 shear stress and 1 dyne/cm2 shear stress, respectively, and 158 +/- 46 micrometer in grafts that were not seeded with endothelial cells.

CONCLUSION

The effect of chronic shear stress on the enhancement of endothelial cell retention in vitro can be exploited to fully endothelialize synthetic vascular grafts, which reduces immediate in vivo graft thrombosis and subsequent neointimal thickness.

A rat model for the evaluation of small-caliber vascular grafts

This article describes the development of a new experimental model using rats for the evaluation of small-caliber vascular grafts. By modifying heterotopic heart transplantation, two 1.5- to 2.0-cm long vascular prostheses were interposed between a syngeneic donor heart and the recipient abdominal vessels in the form of vascular bridges. Once blood flow through the vascular grafts was reestablished, the donor heart resumed normal beating. The status of the vascular grafts could be easily monitored by palpation. Occlusion of the grafts stopped donor heart beating without affecting survival of the animals. Once the surgical method was mastered, the postoperative mortality was approximately 10%, and the total procedure took less than 2 hours. Although microvascular surgical technique and equipment are required, this model has several advantages, including easy detection of thrombotic occlusion of the grafts, the use of small animals of defined genetic background, the absence of effect of graft occlusion on the recipient's life, and possible repeated operation on the same animal.

Enhanced vascularization in a microporous polyurethane graft impregnated with basic fibroblast growth factor and heparin

Rapid and controlled neoarterial regeneration via perianastomotic as well as transmural tissue ingrowth is critical to patency of implanted small-caliber artificial vascular grafts. Microporous polyurethane (PU) grafts (inner diameter, 1.5 mm; wall thickness, 100 microns; length, 20 mm; pore size, 100 microns), fabricated using an excimer laser ablation technique, were coated with a mixed solution of photoreactive gelatin, basic fibroblast factor (bFGF), and heparin, and were photocured by ultraviolet irradiation. Control grafts were treated with only photoreactive gelatin. An in vitro study showed that coimmobilization of bFGF and heparin (bFGF/heparin) in a crosslinked gelatin gel significantly enhances proliferation of endothelial cells. The bFGF/heparin-impregnated grafts (n = 6) and nonimpregnated (control) grafts (n = 9) were implanted in aortas of rats for 4 weeks. All the implanted grafts were patent, but there was a marked difference in the extent of neoarterial regeneration between the two groups. Irrespective of group, endothelialization proceeded from anastomotic sites and little occurrence of transmural capillary ingrowth was observed. The extent of endothelialization was much greater for bFGF/heparin-immobilized grafts than that for controls. In subendothelial tissues for the impregnated group, a significantly profound peripheral transmural tissue ingrowth including recruits of smooth muscle cells and fibroblasts from adjacent native tissue was observed near anastomotic sites; subendothelial tissue regeneration was noticed at the midportions of the grafts. However, only a fibrin layer was formed on control grafts. Thus, coimmobilization of bFGF and heparin significantly accelerated neoarterial regeneration via both perianastomotic and transmural tissue ingrowth. The former was more extensive than the latter.

Hybrid muscular tissues: preparation of skeletal muscle cell-incorporated collagen gels

We prepared three different types of hybrid muscular tissues in which C2C12 cells (skeletal muscle myoblast cell line) were incorporated in type I collagen gels and then differentiated to myotubes upon culture: a disc-type, a polyester mesh-reinforced sheet-type, and a tubular type. A cold mixed solution of the cells and type I collagen was poured into three different types of molds and was kept at 37 degrees C in an incubator to form C2C12 cell-incorporated gels. A polyester mesh was incorporated into a gel to form the sheet-type tissue. The tubular hybrid tissue was prepared by pouring a mixed solution into the interstitial space of a tubular mold consisting of an outer sheath and a mandrel and subsequently culturing after removal of the outer sheath. Hybrid tissues were incubated in a growth medium (20% fetal bovine serum medium) for the first 4 days and then in a differentiation medium (2% horse serum medium) to induce formation of myotubes. Transparent fragile gels shrank with time to form opaque gels, irrespective of type, resulting in the formation of quite dense hybrid tissues. On day 14 of incubation, myoblasts fused and differentiated to form multinucleated myotubes. For a tubular type hybrid tissue, both cells and collagen fiber bundles became circumferentially oriented with incubation time. Periodic mechanical stress loading to a mesh-reinforced hybrid tissue accelerated the cellular orientation along the axis of the stretch. The potential applications for use as living tissue substitutes in damaged and diseased skeletal and cardiac muscle and as vascular grafts are discussed.

Vascular endothelial cell responses to different electrically charged poly(vinylidene fluoride) supports under static and oscillating flow conditions

We investigated the effect of electrically charged surface copolymers on endothelialization of four types of poly(vinylidene fluoride) (PVDF) copolymer surface films with different electrical characteristics. PVDF films without a surface charge, with a remanent surface (5 and 7 microC) and with piezoelectric characteristics were studied through the secretion by an endothelial cell (EC) line culture, under static and oscillating flow conditions of prostacyclin (PGI2) and thromboxane (TXA2), two metabolites which have directly opposing actions on platelet function. The surface electrical properties of PVDF are suitable for promoting cell adhesion. Secretion of thrombomodulatory mediators varied, depending on the surface electrical charge and on the molecular structure of the PVDF substrate. Under static conditions the ECs respond to the substrates by a similar increase of PGI2. Under oscillating flow conditions, the ratio of PGI2 to TXA2 is higher with the piezoelectric PVDF film. The piezoelectricity generated from shear stress along the entire length of the fibres may be appropriate in vivo to keep the [PGI2]/[TXA2] ratio at a level which could counteract the build-up of surface deposits which could be at the origin of thrombosis.

Effect of forms of collagen linked to polyurethane on endothelial cell growth

Collagen has been widely coated or grafted onto polymer surfaces to improve the biocompatibility of materials. To better support the growth of endothelial cells on polyurethane (PU), collagen was grafted to the carboxyl group enriched PU through 1,2-bis(2,3-epoxypropoxy)ethane linking. Our results demonstrated that collagen in various conditions may result in different forms being grafted to the PU substrate, which subsequently affected the growth of endothelial cells. Collagen predialyzed against physiological phosphate buffered saline (PBS) could be reconstituted into native type fibrils with a bigger diameter at 37 degrees C than could collagen neutralized by titration with NaOH. At low temperature, titrated collagen formed floss-like fibrils packed in a ball with cobblestone-like morphology. The amount of collagen grafted was related to the condition of the collagen used, which in consequence affected the diameter of the collagen fibril formed and the growth of endothelial cells. In conclusion, reconstituted collagen fibrils formed from collagen in PBS at 37 degrees C grafted in the highest amounts to an epoxy-PU substrate and that optimally supported the growth of endothelial cells. Such prepared materials may be potentially good vascular bioprosthetic materials and may provide a wide range of biological applications.

Surface microarchitectural design in biomedical applications: in vitro transmural endothelialization on microporous segmented polyurethane films fabricated using an excimer laser

We describe the preparation of segmented polyurethane (SPU) films with round micropores and present a quantitative assay method of endothelial cell (EC) migration through micropores of and growth on microprocessed SPU films as an in vitro model of transmural endothelialization in open-cell-structured small-diameter vascular grafts. The micropored films, pores of which ranged from 9 to 100 microns in diameter, were microfabricated using an excimer laser. Time-dependent processes of EC ingrowth through micropores of SPU films with different pore sizes, which have a confluent monolayer sheet on one face and are cell free on the other, and subsequent endothelialization were quantitatively studied. The circular cellular sheet centered at the micropores expanded as incubation proceeded. Markedly retarded migration was found for the smallest pore size (9 microns in diameter). The larger the pore, the higher was the endothelialization rate. The endothelialization characteristics were studied on multiply microspored films of different pore sizes and densities, each of which was prepared so as to provide a fixed total pore area per unit area (0.01 mm2 per mm2). The highest endothelialization rates in an early incubation period were found on films with microspores between 18 and 50 microns in diameter.

A hybrid vascular model biomimicking the hierarchic structure of arterial wall: neointimal stability and neoarterial regeneration process under arterial circulation

A layered-structure hybrid vascular graft, mimicking the hierarchic structure of the intima and media of a natural artery, is expected to exhibit antithrombogenicity and to accelerate neoarterial tissue formation. Two models of hybrid vascular grafts were prepared on knitted Dacron fabric grafts (inner diameter 4 mm, length 6 cm). Model I grafts consisted of an endothelial cell monolayer formed on collagenous matrix, and model II grafts consisted of an endothelial cell monolayer that formed on hybrid collagenous medial tissue in which smooth muscle cells were incorporated. Both models (n = 17 for each model) were implanted bilaterally in carotid arteries and left in place for up to 26 weeks. Although all of the implanted grafts were patent, the two models significantly differed in the degree of maturity of the regenerated neoarterial wall especially at earlier implantation periods. At 2 weeks, model II grafts showed a much higher degree of neointimal integrity than model I grafts: a smooth and organized neointimal layer was formed, which was free from leukocyte adhesion. On further increase of implantation time, the formation of neomedia (subendothelial smooth muscle cell layers) and circumferential orientation of both smooth muscle cells and collagenous extracellular matrix were much more advanced in model II grafts than in model I grafts. At 26 weeks after implantation, layered elastic laminae regenerated along circumferentially oriented smooth muscle cells in neomedia were observed only in model II grafts. Irrespective of model, little excessive smooth muscle cell proliferation occurred. A hierarchically structured hybrid graft eventually provided a more integrated neointimal layer and accelerated neoarterial tissue formation much more than model I grafts. The significance of the incorporation of smooth muscle cells into hybrid grafts is discussed.

Dermatan sulfate as a potential therapeutic agent

1. Dermatan sulfate is a linear, sulfated polysaccharide and is a glycosaminoglycan component of several important proteoglycans. This minireview discusses the biosynthesis, structure and biological function of dermatan sulfate proteoglycans. 2. Dermatan sulfate and its derivatives are being investigated as a new class of anticoagulant and antithrombotic agents. 3. The preparation, chemistry and structure-activity relationship of dermatan sulfate is described. 4. Dermatan sulfate, low molecular weight dermatan sulfate and glycosaminoglycan mixtures containing dermatan sulfate have been used clinically. 5. The future prospects of these agents and other new, potentially useful dermatan sulfate based therapeutics are discussed.

Mechanical stress-induced orientation and ultrastructural change of smooth muscle cells cultured in three-dimensional collagen lattices

The effect of tensile stress on the orientation and phenotype of arterial smooth muscle cells (SMCs) cultured in three-dimensional (3D) type I collagen gels was morphologically investigated. Ring-shaped hybrid tissues were prepared by thermal gelation of a cold mixed solution of type I collagen and SMCs derived from bovine aorta. The tissues were subjected to three different modes of tensile stress. They were floated (isotonic control), stretched isometrically (static stress) and periodically stretched and recoiled by 5% above and below the resting tissue length at 60 RPM frequency (dynamic stress). After incubation for up to four wk, the tissues were investigated under a light microscope (LM) and a transmission electron microscope (TEM). Hematoxylin and eosin-stained LM samples revealed that, irrespective of static or dynamic stress loading, SMCs in stress-loaded tissues exhibited elongated bipolar spindle shape and were regularly oriented parallel to the direction of the strain, whereas those in isotonic control tissues were polygonal or spherical and had no preferential orientation. In Azan-stained samples, collagen fiber bundles in isotonic control tissues were somewhat retracted around the polygonal SMCs to form a random network. On the other hand, those in statically and dynamically stressed tissues were accumulated and prominently oriented parallel to the stretch direction. Ultrastructural investigation using a TEM showed that SMCs in control and statically stressed tissues were almost totally filled with synthetic organelles such as rough endoplasmic reticulums, free ribosomes, Golgi complexes and mitochondria, indicating that the cells remained in the synthetic phenotype. On the other hand, SMCs in dynamically stressed tissues had increased fractions of contractile apparatus, such as myofilaments, dense bodies and extracellular filamentous materials equivalent to basement membranes, that progressed with incubation time. These results indicate that periodic stretch, in concert with 3-D extracellular collagen matrices, play a significant role in the phenotypic modulation of SMCs from the synthetic to the contractile state, as well as cellular and biomolecular orientation.