Tissue engineering is an approach that promises a limitless supply of replacement tissues and organ to treat a wide variety of diseases. These approaches have worked somewhat with artificial skin but not for cardiovascular tissues. Because adult cardiovascular tissues heal poorly, defects in cardiovascular tissues need to be replaced with synthetic materials or tissues taken from cadavers. For heart valves, cadaveric tissues are in short supply and synthetic devices have long-term complications. In children, neither approach works well and these patients face repeat surgery every few years.
The common approach for heart valve tissue engineering is to harvest the patients own cells, seed them on degradable substrates and hope that the cells somehow regenerate the substrate before it all dissolves away. This has not worked for heart valves. The main limitation appears to be the absence of cells that are capable of generating a mature structurally complex heart valve matrix. Indeed, adult heart valves have limited potential for self repair and the valve cells appear to generate only disorganized scar tissue.
We believe that he field of heart valve tissue engineering can benefit greatly from the availability of heart-valve cells with characteristics similar to those cells that produce valve leaflet tissue during embryonic development. These cells can be harvested in very small numbers from embryos. For medical applications, millions of these cells are needed, however. Growing large numbers of these cells from these few harvested cells is impossible, because during amplification in culture, cells are know to change their characteristic features. After many divisions, these cells no longer possess the desirable features. We believe that human embryonic stem cells can be use for heart valve tissue engineering. Stem cells can be grown in very large numbers and then made to differentiate into the particular cell line with appropriate growth factors. Discovering what these growth factors are is the main objective of this work.
These growth factors will be discovered by culturing human embryonic stem cells inside developing mouse hearts, right next to the developing heart valve tissues. This will be done first inside the whole heart that has been taken from the embryo and grown on a culture dish, and later by isolated portions of the developing valve tissue. If we can induce the stem cells to differentiate into the valve cell type, we can then sample the surrounding fluids for the growth factors that have been secreted by the embryonic tissues. Once identified, these growth factors can be use directly on stem cells grown simply in culture dishes. This can be done in very large numbers.
We expect that such properly differentiated cells will behave in a more constructive way to remodel a matrix into one that is morphologically closer to living valvular tissue.
Californians suffer from cardiac diseases no differently than the rest of the citizens in the US. Valvular heart disease is one such condition and affects over 150,000 individuals in the US per year. It is a slow, debilitating process during which the cardiac valves become stenosed or incompetent. Unless the diseased valve is repaired or replaced, this disease eventually results in cardiopulmonary failure and death. Indeed, many patients are misdiagnosed, fail to receive treatment and become so compromised that surgical correction becomes impossible. The most widely used treatment is the implantation of artificial heart valves – either mechanical or tissue-based devices. Mechanical valves are typically constructed from pyrolytic carbon and tissue valves are fabricated from glutaraldehyde-tanned bovine pericardium or whole porcine aortic valves. These devices, however, are imperfect. Mechanical valves are rigid, require chronic anticoagulant therapy, and can fail suddenly and catastrophically. The chronic anticoagulation is associated with cumulative morbidity and mortality with rates as high as 5% per year, essentially guaranteeing some undesirable event within 20 years. Although animal tissue valves or bioprostheses do not require anticoagulation, they have poor long-term durability and eventually fail through calcification and rupture of the valve cusps. For children, none of the solutions described above are satisfactory. Children grow and the best possible solution for these patients is a living valve that can grow with the patient and support cardiac function for decades into the future.
Tissue engineering technologies offer the promise of an unlimited supply of organs and tissues to treat a wide variety of injuries and diseases. Like many new technologies, however, tissue engineering has promised more than it has delivered. In the cardiovascular field, in particular, tissue engineering has not made great inroads into product lines occupied by conventional synthetic devices. Part of the problem has been that cardiovascular tissues are more complex than initially envisioned and have a capacity for self-repair that is far less effective than tissue engineering principles demand.
Stem cell research, and the harnessing of stem cell technologies, will thus eventually produce devices that are living, rather than inert, and last the life of the patient, rather than a decade or two. California is well-positioned to capitalize on this technology, since two of the three main heart valve manufacturers reside in Santa Anna. Almost all of the new start-up companies making innovative valve technologies are also based in California. Californian is the hub of the world heart valve industry. Advancing this field into stem cell applications is thus best done in California, where the established industry base can translate these technologies into commercial products, and make them available for the treatment of patients in California, and worldwide.