What do the stem cells, aging and tissue engineering have in common? Stem cells are the main functionaries of all organs and tissues it means they are responsible for renovation. Aging is evolutionary gained process responsible for elimination of older and not highly adapted organisms from population; introduction of new, more advanced genotypes into the population. Tissue engineering- 10 years old scientific field where stem cells are used to engineer living tissue and organs, which could be transplanted/introduced into aged organisms to make them feel better and rejuvenate. What is exactly made in this promising area for this time? Does this help us to prolong our living and change the way of living driving us to theoretical immortality? We hope everyone has its own opinion on these questions and here we display some theory just to let people easier understand the progress of science in this area.

Tissue Engineering: Definitions and Concepts

Tissue engineering is a developing field that combines principles from cell biology and engineering in order to create biologic replacement structures that restore, maintain, or improve tissue function. Fig 1 shows the essential concepts of tissue engineering a cardiac valve. Cells from peripheral blood or bone marrow are harvested from the patient and cultured in the laboratory. Selected subpopulations of these cells are then delivered onto a supporting scaffold that has been fashioned into the shape of a valve; the scaffold is biodegradable so that it can be removed and replaced over time. The cell-scaffold construct is cultured to expand the number of cells and further developed in the laboratory before implantation into the same patient.

Fig. 1. Basic steps in the tissue engineering of a cardiac valve. Donor blood (arm) or bone marrow (sternum) cells are harvested, cultured in the laboratory, and then seeded onto the scaffold. Following additional culturing and development of the cell-scaffold construct in the laboratory, it is implanted on place of the defective or missing valve in the donor patient.

Tissue-engineered cardiovascular structures, including valves, blood vessels, and vascular patches, may potentially offer several advantages over current replacement options. First, because a tissue-engineered structure contains living cells, it has the potential to grow and remodel over time. Therefore, an implanted valve or vessel might function well for decades or even a lifetime, reducing or eliminating the need for repeat operations. Second, because the cells would be derived from the patient who receives the implant, there should be no rejection by the patient's immune system. Third, if the cells in contact with the blood behave similarly to native endothelial cells, the likelihood of thrombus formation and infection should be low. Anti-coagulation therapy could thus be avoided.

Cell Source for Tissue-engineered Valves and Vessels

An understanding of the cells comprising normal heart valve leaflets and blood vessels has guided the selection of cell types for tissue-engineered constructs. The central region of both heart valve leaflets and blood vessels contains cells that can synthesize, deposit, organize, and maintain the basic structural components of the tissue (primarily extracellular matrix proteins such as collagen and elastin) over a lifetime. The surface of a valve or vessel in contact with the blood is composed of a layer of endothelial cells. These cells have multiple functions, including serving as a selective barrier, preventing thrombus formation, and producing chemical signals. Therefore, it has seemed logical to choose a cell type for the central region of a tissue-engineered heart valve or blood vessel that has the capacity to generate and maintain extracellular matrix proteins and endothelial cells to cover the surface in contact with the blood stream.

Most published reports on tissue-engineered heart valves or blood vessels describe the use of cells obtained from an animal's carotid artery. Cells populating the central region (interstitium) of the carotid artery, called smooth muscle cells or myofibroblasts, have the ability to grow and divide on a heart valve scaffold and synthesize extracellular matrix proteins to maintain tissue structure. Carotid artery endothelial cells can be cultured in the laboratory and then added to a scaffold containing smooth muscle cells to create an endothelial lining. However, a segment of carotid artery must be removed to obtain these cells, leading to loss of artery function, a major disadvantage in humans. Therefore, less damaging alternative cell sources must be developed for human applications.

Heart Valve and Blood Vessel Development

The developmental biology of fetal heart valves and blood vessels has provided insights into the mechanisms of cell function in tissue-engineered structures. Although the exact mechanisms of development are not entirely understood, it is clear that a series of well-orchestrated steps must occur for heart valves and blood vessels to form correctly.

Formation of heart valve leaflets. During early fetal development, primitive blood vessel precursor cells called angioblasts emerge from tissue known as mesoderm and coalesce into a network of blood vessels. A fraction of these angioblasts mature into endothelial cells that line blood vessels; the remainder appears to become hematopoietic cells that populate the bloodstream. As fetal development continues, a segment of the blood vessel network becomes an endothelialized heart tube within the pericardial cavity. During the formation of the mature four-chamber heart from the heart tube, the walls thicken and swellings within the right and left ventricular outflow tracts appear that later develop into the trileaflet aortic and pulmonary valves. Endothelial cells on the surface of these swellings appear to give rise to leaflet interstitial (also called mesenchymal) cells through a process of cell differentiation regulated by local growth factors. This process has been termed endothelial-to-mesenchymal cell transdifferentiation (EMT). Endothelial cells isolated from the surface of mature heart valve leaflets retain the ability to undergo EMT because they can differentiate in vitro (i.e., in an artificial environment) into cells that look and behave like valvar interstitial cells. If EMT occurs normally in valves, it could provide a mechanism for ongoing replacement of aging interstitial cells from endothelial cells.

Formation of blood vessels. The original blood vessel network is formed during fetal development in a process known as vasculogenesis. Vasculogenesis is defined as the in situ (i.e., at the site of origin) differentiation of angioblasts into endothelial cells that aggregate into a capillary network. New blood vessels also are formed during and after fetal development through a distinct process known as angiogenesis. Postnatal angiogenesis may occur by recruitment of endothelial cells termed endothelial progenitor cells (EPCs) that appear to originate in bone marrow and are released to circulate in the blood. EPCs may be "left over" in the bone marrow from the time of fetal development. If angioblasts give rise to valve and vessel endothelial cells, and if these endothelial cells in turn become interstitial cells by EMT, then it is conceivable that a single cell type could be used to construct a complete heart valve or blood vessel by tissue engineering (Fig 2).

Fig. 2. Potential mechanism for endothelial progenitor cells (EPCs) to give rise to endothelial and interstitial valve cells by endothelial-to-mesenchymal transdifferentiation (EMT). Top: EPCs from bone marrow (dark cells with light nucleus) circulate in the blood where they can attach to valve leaflet endothelium and subsequently undergo EMT to maintain and adequate population of interstitial cells. Bottom: Bone marrow-derived EPCs are seeded onto a valve scaffold where they initially become endothelial cells (round light cells with dark nucleus): they later trandifferentiate into interstitial cells (larger, elongated cells) by EMT.

New potential cell sources for cardiovascular tissue engineering. Progenitor cells such as EPCs and even more primitive cells, commonly termed stem cells, have recently been used in cardiovascular tissue engineering. Stem cells can be thought of as the "original" cells in the fetus. During fetal development, stem cells migrate throughout the body and generate mature tissues and organs. Stem cells persist in adults and appear to replace damaged or dead cells. Stem and progenitor cells in bone marrow and umbilical cord and peripheral blood all theoretically could be accessible, single cell types with ideal properties as donor cells for cardiovascular tissue engineering.

Bone marrow. Although the existence of mesenchymal stem cells (MSCs) was first proposed over 100 years ago and later described in 1966, the technique for isolating a relatively pure population of MSC from bone marrow published only recently. MSC were originally shown to have the potential to differentiate into fat, bone, cartilage, and muscle. A number of groups have used MSC from bone marrow to tissue engineer heart valves. Investigators recently described the use of MSC from bone marrow to create heart valves that functioned well in bioreactors in the laboratory. MSC cultured in vitro can attach to a valve scaffold, multiply, and deposit extracellular matrix proteins to form interstitial tissue. Because of these encouraging preliminary results, valve constructs engineered from bone marrow-derived MSC are being implanted as a substitute pulmonary valve in sheep. Since the constructs lack endothelial cells at implantation, it will be intriguing to evaluate the tissue after explantation to determine if endothelial cells populate the constructs.

Peripheral blood and umbilical cord. Isolation of EPCs from the peripheral blood of sheep showed that these cells could form the endothelial lining of tissue-engineered, small-diameter blood vessels. These vessels not only remained open following implantation as artery segments with no evidence of thrombosis for up to 130 days, but also were capable of both vasodilation and vasoconstriction in response to vasoactive agents in a pattern similar to normal arteries. The diagnosis of congenital heart disease in utero is now common. If cells could be harvested from the fetus, which is already technically feasible, then a heart valve or conduit artery could be prepared for implantation soon after delivery. This vision has been the stimulus for efforts to isolate progenitor cells from the umbilical cord. Isolation of EPCs from human umbilical cord blood demonstrated that these cells could grow in culture in response to vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), spontaneously form capillary-like tubes (early blood vessels), and could be cryopreserved and "banked" for future use. Cells from human umbilical cords, cultured in a pulse duplicator bioreactor on a conduit artery scaffold, showed that the constructs had cellular, extracellular matrix, and biomechanical properties similar to native tissue.

Cardiomyocytes. Research in an animal model has recently shown that bone marrow-derived MSCs injected into the heart muscle following a myocardial infarction can develop into cardiac muscle cells (cardiomyocytes) that augment contractile function, as measured by improved ejection fraction and ventricular wall motion. Although the mechanism is still unknown, these cell transplant studies suggest that when placed in direct contact with native cardiomyocytes, MSCs can change into functional cardiac muscle cells. This phenomenon has been termed site-specific differentiation. Therefore, MSCs from bone marrow have the potential to be used in the future for the treatment of patients with decreased ventricular function after a myocardial infarction.

Creating replacement structures and organs using tissue engineering offers an exciting alternative to existing technologies. The possibility of creating valves and vessels from fetal cells for implantation into infants is particularly appealing. Tissue-engineered skin, bone, and cartilage have been developed for use in patients, and heart valves and blood vessels have functioned well in several animal models. Many researchers in cardiovascular tissue engineering are now exploring the optimal cell sources that would maximize versatility and minimize trauma to the donor at cell harvesting.

Research in humans and animals suggests that multipotent progenitor or stem cells exist in the bone marrow and peripheral blood of adults as well as in umbilical cord blood. During 2001 several groups published or presented results from both in vitro and in vivo experiments with MSCs from bone marrow and EPCs from peripheral and umbilical cord blood. These cells potentially can be stimulated to differentiate into all of the cell types found in normal heart valve leaflets. Additional experimentation with these cells should increase our knowledge of the normal early development of valves and vessels, as well as allow us to construct new tissue-engineered structures for in vivo testing. However, even if these new cell types produce excellent tissue constructs in animal models, it is likely to be several years before the initial implantation of tissue-engineered human structures occurs.