Tissue Engineering Heart Valves

The State of the Art in Tissue Engineered Heart Valves

Introduction

As advances in science and medicine have occurred, the idea of "tissue engineering", which focuses on fabricating living replacement body tissue and organs by cultivating cells has evolved. In the last decade the field of tissue engineering has grown dramatically and its use to combat disease and injury has the potential to revolutionise methods of health care treatment and improve the quality of life for millions of people.

One such disease that many people suffer from is valvular heart disease which affects the regulation of blood flow through the heart. It is estimated that 300,000 valve replacement procedures are performed annually around the world .

Conventional treatment for this type of disease is undertaken using mechanical devices and prostheses. However, they come with significant disadvantages such as noise, rejection and transmission of infection. Biological valves such as hetrographs or homographs are also available for valve repair and replacement. However these also have draw backs mainly relating to progressive tissue deterioration. Due to this tissue deterioration, biological tissue valves need replacing every 10-15 years causing further discomfort to the patient.

As a result of the problems associated with mechanical and biological heart valves, tissue engineered heart valves have now been produced. These valves combine most of the characteristics of the healthy native heart valves and have the potential to revolutionize the treatment of valvular heart disease.

Tissue engineered heart valves

In order to successfully produce heart valves by tissue engineering, it is essential that three matters are considered. Firstly, the scaffold: which is a three dimensional support and serves as the initial guidance structure to organise cells in space for tissue development. Secondly, the sources of the cells that will be used to grow living tissue and finally, the growing conditions for the cells on the scaffold.

The Scaffold

Cells lack the ability to grow into a three dimensional structure. Instead, they randomly migrate to form a two dimensional monolayer of cells. This poses an issue when heart valves need to take a three dimensional structure to replace their native tissue. The three dimensional structure is therefore produced by seeding the cells onto a porous matrix known as a scaffold which the cells grow upon. The scaffold must have certain characteristics:-

Ð'* It should possess interconnecting pores of appropriate scale to favour tissue integration and flow of blood through the tissue.

Ð'* Be made from a material with is biodegradable and biocompatible.

Ð'* Have appropriate surface chemistry to ensure that the cells attach to the structure.

Ð'* It must be easy to handle and fabricate into a variety of shapes and sizes.

There are many different materials that can be used to fabricate these scaffolds. However synthetic polymers such as polyglycolic acid (PGA) and polylactic acid PLA, are the most commonly used materials for most aspects of tissue engineering scaffolds. They are particularly favoured as their degradation products are glycolic acid and lactic acid which are naturally present in the human body and are removed by natural metabolic pathways. Consequentially these types of scaffolds will not cause immunogenic reactions or undesirable side effects.

Even though these synthetic polymers are biodegradable, they are not truly autologous. As a result new studies have been undertaken, which involve developing autologous scaffolds using fibrin, which is readily available from the patients own blood. This process is more favorable as it further reduces the risk of rejection and the transmission of disease. Also, the degradation rate of fibrin can be controlled by the use of proteins incorporated into the scaffold structure enabling targeted promotion of specific tissue growth. The structural properties of fibrin also make it a good scaffold as it can easily be molded into complex valve structures using established techniques. However fibrin does suffer from insufficient mechanical strength and consequentially is unlikely to withstand the stresses exerted on heart valves. Bioreactors have been used to improve this mechanical strength, but no information on the conditions required to perform this strengthening has yet been documented.

Cell Source

Once the scaffold has been fabricated it must be sterilized by ethylenoxide before cells can be seeded onto it. The scaffold is often seeded with myofibroblasts which are a form of fibroblast cell that has differentiated partially towards a smooth muscle phenotype. Once these cells have been seeded onto the scaffold, they are covered with an endothelial cell layer. These cells are obtained from the peripheral segment of vein or artery. Usually the endothelial cells are obtained by labelling the cells with low density lipoproteine and subsequent fluorescence activated cell sorting. The myofibrolasts layer is obtained from the remaining artery or vein (once the endothelial cells are removed) by cutting it into small pieces. The disadvantage associated with using vascular cells for tissue engineering of heart valves is that it is a very invasive procedure which sacrifices an intact vascular structure.

Problems can arise with using these vascular cells if the patient suffers from systemic atherosclerotic vascular disease as the cells of their vascular system will not be suitable for the procedure. As a result cells need to be sourced from a different part of the body. An attractive alternative is the use of bone marrow derived cells. These are becoming a popular choice for tissue engineering heart valves as they can be easily obtained by a simple puncture of the iliac crest. This procedure can be carried out under local anaesthesia and is thus less invasive and more comfortable to the patient. The other attractive factor in using bone marrow is that it is a strong source of red blood cells, platelets, monocytes, granulocytes and lymphocytes.

Recently human umbilical cord cells have been used as a cell source for tissue engineering in creating pulmonary artery conduits. These cells are a readily available cell source for tissue engineering and have great potential for future use in tissue engineering heart valves as they do not entail sacrificing intact vascular donor structures. The results from