Tissue engineering of vascular grafts

Abstract

The challenge of tissue engineering blood vessels with the mechanical properties of native vessels, and with the anti-thrombotic properties required is immense. Recent advances, however, indicate that the goal of providing a tissue-engineered vascular graft that will remain patent in vivo for substantial periods of time, is achievable. For instance, collagen gels have been used to fabricate a tissue in vitro that is representative of a native vessel: an acellular collagen tubular structure, when implanted as a vascular graft, was able to function, and to become populated with host cells. A completely cellular approach culturing cells into tissue sheets and wrapping these around a mandel was able to form a layered tubular structure with impressive strength. Culture of cells onto a biodegradable scaffold within a dynamic bioreactor, generated a tissue-engineered vascular graft with substantial stiffness and, when lined with endothelial cells, was able to remain patent for up to 4 weeks in vivo. In our experiments, use of a non-degradable polyurethane scaffold and culture with smooth muscle cells generated a construct with mechanical properties similar to native vessels. This composite tissue engineered vascular graft with an endothelial layer formed using fluid shear stress to align the endothelial cells, was able to remain patent with an neointima for up to 4 weeks. These results show that tissue engineering of vascular grafts has true potential for application in the clinical situation.

Introduction

Blood vessels function to carry blood from and to the heart, and to and from the tissues and organs. They form a branched system of arteries and veins that vary in size, mechanical properties, biochemical and cellular content, and ultrastructural organization, depending on their location and specific function (for example, volume of blood to be carried, flow rate, vasoactivity). The largest arteries such as the aorta function to transport blood originating from the heart. The smaller diameter muscular arteries deliver the blood from the large arteries to the tissues and organs. These then branch into smaller arterioles and capillaries, which function to distribute blood within the tissues and organs. Blood is returned to the heart through venules, which combine to form veins.

The large and medium sized arteries have distinct structural features, primarily the intima, media and adventitia, although these are less obvious in the smaller arterioles and do not exist in the capillaries. The intima forms the layer closest to the blood flow and consists of a lining of endothelial cells attached to a connective tissue bed of basement membrane and matrix molecules. This is adjacent to the internal elastic lamina, which is a band of elastic tissue, found most prominently in the larger arteries, and which separates the intima from the media. The media contains a dense population of smooth muscle cells, organized concentrically, with bands or fibers of elastic tissue. The external elastic lamina separates the media from the adventitia, which contains a collagenous extracellular matrix containing fibroblasts, blood vessels and nerves, and functions to add rigidity and form to the blood vessel. The veins are thin-walled vessels, lack the distinct molecular and tissue organization of arteries, and deform more easily.

The extracellular matrix surrounding the vascular cells is complex and combines to provide the biomechanical properties of the tissue (Wight, 1996). The molecular network consists primarily of collagen (primarily types I and III), elastin in the form of fibers, proteoglycans (including versican, decorin and biglycan, lumican and perlican), hyaluronan, glycoproteins (for example, laminin, fibronectin, thrombospondin and tenascin). The mechanical properties critical to blood vessel function include the tensile stiffness, elasticity, compressibility and viscoelasticity. The collagens provide the tensile stiffness, the elastin the elastic properties, the proteoglycans contribute to the compressibility, and combined with the collagen and elastin they are responsible for the viscoelastic properties. This complex mixture of molecules and their organization provide the blood vessels with their properties that allow them to function throughout life.

The pathology that affects the small and medium-sized blood vessels is the primary cause of death in the USA, and is one of the major causes of death in Western society (Tu et al., 1997). Atherosclerosis is the major disease of blood vessels, and affects the larger and medium sized arteries that contain an intima (Benditt and Schwartz, 1988). The atherosclerotic lesion consists of a raised focal plaque within the intima consisting of a lipid core, surrounded by extracellular matrix and smooth muscle cells and covered by a fibrous cap. As it increases in size through intimal hyperplasia it restricts blood flow and eventually blocks vessels. In cardiac and peripheral bypass surgery, these are usually replaced by autologous veins, or sometimes with autologous arteries. However, many patients do not have appropriate blood vessels for use as replacements, either because of diseased blood vessels or because the blood vessels were used in a previous surgery. In these cases, the patients are restricted to modest treatment modalities, with the results often leading to myocardial infarction or limb amputation. Unfortunately, although synthetic vascular grafts such as ePTFE and Dacron have been used successfully in treating the pathology of large arteries (>6 mm internal diameter), these have generally not proved successful in replacing the smaller diameter (6 mm internal diameter and below) vessels. There is therefore, a massive clinical need for an alternative supply of vessels to replace diseased arteries. Tissue engineering offers the potential of providing vessels that can be used to replace diseased and damaged native blood vessels.

Tissue engineering has already had success in generating materials for repair of chronic wounds (Naughton et al., 1997, Mansbridge et al., 1998) and burns (Noordenbos et al., 1999) and at the experimental level, in repair of cartilage defects (Schreiber et al., 1999). The challenges faced by the approach of tissue engineering for replacing blood vessels are substantial. They include providing a conduit that will have sufficient strength not to burst with changes in blood pressure, a vessel wall that is elastic and can withstand cyclic loading, matching compliance of the graft with the adjacent host vessel, and a lining of the lumen that is antithrombotic. Many tissue engineering approaches can rely on remodeling of the tissue in vivo to approach functionality with time, however the tissue engineered vascular graft must function immediately on implantation. It is considered that tissue engineering offers the opportunity to use cell and tissue growth, biomaterial selection and scaffold fabrication, and cell type and phenotypic regulation, to manufacture a blood vessel that can function long term in vivo. The studies below provide a review of the progress to date, and indicate that the development of a functional tissue engineered vascular graft is a feasible approach to treating a major clinical pathology.