IN THE LAST 10 TO 15 YEARS, tremendous progress in cell biology and cell culture have led to the birth of the field of tissue engineering. One of the primary objectives in this field is to use cultured human cells to recreate functional tissues and organs in order to provide "replacement parts" that can be grafted into humans. Historically, tissue-engineered organs have relied on synthetic materials to provide a scaffolding effect and mechanical strength. This has been especially true for tissue-engineered vascular grafts where mechanical strength and suturing characteristics are critical. However, despite vast improvements in the field of biomaterials, a completely biocompatible material is still not available for the production of vascular grafts.
The small-diameter synthetic vascular grafts that are presently implanted (inside diameter 5 mm) have shown poor patency rates, particularly in below-the-knee locations where low blood flow and high resistance increase the risks of thrombus formation (1–3). Initial strategies to increase patency focused on graft pacification, using various protein coatings to minimize blood/biomaterial interactions (4–6). More recently, several investigators have seeded the lumen of synthetic vascular grafts with various cell types, mostly endothelial cells, to create a living hemocompatible lining (7–10). Although some of these reports have shown promising results, synthetic grafts may still induce a low-level foreign body reaction/chronic inflammation, which could be a key event in late graft failure (11). Furthermore, vascular grafts made from synthetic materials can lead to bacterial colonization and subsequent graft infection, which is associated with mortality and amputation rates (12, 13).
We hypothesize that a tissue-engineered blood vessel (TEBV)3 composed exclusively of biological ma~terials and autologous vascular cells would have several theoretical advantages. First, a living graft implies a responsive and self-renewing tissue with an inherent healing potential. Second, its biological matrix can be remodeled by the body according to the needs of the environment. Third, the absence of synthetic material will preclude foreign body reaction, allow complete graft integration, limit graft infection, and may result in increased overall patency rates (14–16). In 1986, Weinberg and Bell (17) were the first to produce a completely biological TEBV from animal collagen gels and cultured bovine endothelial cells (ECs), smooth muscle cells (SMC), and fibroblasts. Unfortunately, this model did not display the requisite mechanical strength and, even reinforced with Dacron meshes, failed to show a burst strength that would allow its in vivo grafting. Using similar techniques, our group (18) has developed and reported a completely biological TEBV from cultured human vascular cells and human collagens and encountered the same mechanical limitations. Recently, Hirai and Matsuda (19) constructed a similar canine model that withstood venous grafting when supported by a Dacron mesh, but ruptured when grafted alone.
In this report, we present a radically different approach to TEBV production based exclusively on the use of cultured human cells, without synthetic or exogenous biological material. This is the first biological model to demonstrate sufficient mechanical strength to warrant its in vivo grafting. Our innovative approach takes advantage of the abundant endogenous synthesis of extracellular matrix (ECM) by mesenchymal cells when cultured in the presence of ascorbic acid. This TEBV displayed histological organization, ECM composition, cell differentiation markers, and cellular functions observed in normal human blood vessels. Furthermore, its grafting potential was established by a short-term implantation in a canine model.