Building organs piece by piece

doi:doi:10.1038/sj.embor.7400287

Journal home

Aims and scope

Current issue


		editorial
		science & society
		reviews
		scientific reports

Archive:


Authors and referees


	Guide for authors
	Submit Manuscript
	Guide for referees
	Editors and editorial board
	Contact editorial office


Customer Services


	Subscribe
	Order sample copy
	Purchase articles
	Reprints and permissions
	Contact NPG
	Advertising

EMBO reports 5, 11, 1025–1028 (2004)
doi:10.1038/sj.embor.7400287

Building organs piece by piece

Accomplishments and future perspectives in tissue engineering

Alexandra Moreno-Borchart

It might not be long before we see a headline such as: "First tissue-engineered heart valve saves child's life". A team of researchers in Switzerland, Germany and the USA have succeeded in engineering a living heart valve that resembles normal valves in both microstructure and mechanical properties. Simon P. Hoerstrup, a group leader at the University Hospital in Zurich, Switzerland, and his colleagues fabricated these heart valves from bioabsorbable polymers that had been seeded with ovine myofibroblasts and endothelial cells, and then grown for 14 days in a bioreactor (Hoerstrup et al, 2000; Hoerstrup et al, 2002). When implanted into juvenile sheep, they behaved like normal heart valves and lasted for up to five months. But any medical benefit for human patients will take more time. "Although a lot has been done regarding tissue engineering of heart valves over the past ten years, we are still not there," Hoerstrup commented. "Many disciplines are involved and only a longstanding synergic scientific effort will lead to optimized results" (Mol & Hoerstrup, 2004). His comments also apply to tissue engineering in general. Clearly, creating a complex organ, such as a kidney, liver or lung, in vitro presents so many scientific challenges that it is still a thing of the distant future. Nevertheless, tissue engineering has become a multidisciplinary research field, which involves biologists, chemists, materials scientists and clinical researchers, and promising advances have been made over the past two decades, particularly in creating three-dimensional matrices on which to grow new tissues.

...the main problems for engineering more complex tissues are angiogenesis ... and developing three-dimensional matrices on which to grow the new tissue...

The results produced by Hoerstrup and colleagues with artificial heart valves also illustrate the broader social and medical needs for replacement tissues. In 2000, approximately 87,000 heart valves were transplanted in the USA alone, using grafts from deceased organ donors, synthetic valves—which comprised about 55% of the heart valves implanted worldwide—or bioprosthetic valves from animal or human tissue. But each source has its drawbacks. Mechanical heart valves have the advantage of high durability for more than 25 years, but they can significantly increase the risk of thrombosis and infections in recipients. Bioprosthetic and donor valves do not tend to cause thrombosis, but they only last for 10–15 years. Furthermore, patients need to take immunosuppressants for the rest of their lives to prevent immune reactions against foreign tissue. Another major disadvantage is that none of the valve substitutes grow, remodel or repair themselves once they have been transplanted, which is a particular problem in younger patients.

It is also clear that the supply of donated organs cannot meet the demand for several reasons. First, transplantation is not feasible for all organs. Second, donated organs often do not exactly match the immunological properties of the patient, which means that transplant recipients must depend on immunosuppression agents that create a higher risk of infections and malignancies. Moreover, the grafts do not last for long—a transplanted heart, for example, has an average lifetime of only one decade.

Third, there is a drastic shortage of donated organs (Fig 1). Approximately 15% of the candidates for a liver or heart transplant in the USA die while waiting for an organ (Stock & Vacanti, 2001). The American Heart Association (Dallas, TX, USA) states on its web site (www.americanheart.org) that "Every 16 minutes a new name is added to the national organ transplant waiting list. Each year about 16,000 Americans [aged] 55 or younger could benefit from a heart transplant. But only 2,202 heart transplants were done in the USA in 2001."

Figure 1
Shortage of cadaveric organs in Germany. Data obtained from Eurotransplant (http://www.eurotransplant.nl/statistics). Numbers for transplanted organs refer to cadaveric organs only

Engineering new tissues, ideally from the patient's own body cells to prevent rejection by the immune system, seems to be the way forward. This new and emerging technology rests on three pillars: cells, supporting structures (or scaffold) and stimulating biomolecules (Fig 2). It entails the in vitro generation of biological tissue from individual cells with the aid of support structures and growth factors, which differs from in vivo guided tissue regeneration by transplanting matrices that are populated with the patient's cells to support the regenerative abilities of the body. Moreover, although the results of heart-valve engineering seem to offer a glimmer of hope for hundreds of thousands of patients who are waiting for a donor organ, few other organs can actually be grown in vitro. Only skin tissue and cartilage have so far been made available to treat burn victims or patients with knee cartilage defects. The largest advances have been made with skin substitution, of which approximately two-dozen products are now on the market in the USA and Europe (Bock et al, 2003). These consist of a sheet-like matrix of collagen, hyaluronic acid or biodegradable polymers and skin cells. Their manufacture is relatively easy as only a few different cell types are needed and their cultivation is undemanding, which is not the case for other cells, such as hepatocytes. Furthermore, during the in vitro growth phase, it is not necessary to grow blood vessels in the skin graft, something that is a key problem with tissue engineering.

Figure 2
The three pillars of tissue engineering

Indeed, the main problems for engineering more complex tissues are angiogenesis—growing blood vessels to supply the new tissue with blood—and developing three-dimensional matrices on which to grow the new tissue, according to Björn Stark, Professor of Plastic and Hand Surgery at the University Hospital in Freiburg, Germany. As early as the 1980s, surgeon Joseph Vacanti of Harvard Medical School (Boston, MA, USA), a pioneer in the field of tissue engineering, tried to grow tissue layers, an essential prerequisite for constructing entire organs. But the cell layers never grew thicker than a few millimetres. Vacanti finally realized that the cells in the interior of each layer were not getting enough oxygen and nutrients and could not get rid of their metabolic waste. He therefore came up with the idea of using polymer materials with a branching porous structure instead of solid polymers, which was a milestone in tissue engineering (Ferber, 1999). His work triggered much enthusiasm about tissue engineering in the 1990s, mainly in the USA. Since then, the number of publications that include the phrase 'tissue engineering' has grown constantly (Fig 3).

An ideal scaffold should mimic the extracellular matrix and must be biodegradable once the new tissue has formed around it

Figure 3
Number of publications related to tissue engineering over the last decade. PubMed was searched for publications containing the phrase 'tissue engineering'. European numbers refer to papers published in the 15 countries of the EU before the expansion

Engineering efficient structures is not easy and involves the contribution of researchers from other fields, such as materials science and chemistry. A supporting matrix has to sustain and guide the growth of cells, and should provide all of the signals that are needed for cell growth, differentiation and cell interaction. An ideal scaffold should mimic the extracellular matrix and must be biodegradable once the new tissue has formed around it. It must also allow cells to be supplied with nutrients during vascularization—the growth of blood vessels. Furthermore, the composition, structure and, therefore, the mechanical properties of the extracellular matrix can be specific to each organ or tissue. The scaffolds that are currently in use are based on synthetic materials (such as lactides or ceramics), naturally derived materials (such as collagen or polysaccharides) and semi-synthetic polymers. Other research has focused on thermoplastic polymers that can change their conformation if they are subjected to a temperature change (Bock et al, 2003). Another challenge is that the three-dimensional architecture of a tissue is not only determined by the scaffold structure, but also depends on subjecting the cells to physiological stress during cultivation. Growing blood vessels, for example, must be exposed to compression, shear stresses and a pulsated flow of the culture medium to acquire their mechanical properties (Griffith & Naughton, 2002). "Tissue engineering is a novel field marrying cell biology with materials science; the two systems are completely interlinked as you need to have both a good selection of cells and a bioactive support structure which encourages the cells to grow and then gradually disappears. Our work would therefore not be possible without a multidisciplinary team of specialists in everything from cell biology to gene expression and materials science to imaging and clinical applications," according to Julia Polak, the founder and Director of the Tissue Engineering and Regenerative Medicine Centre at Imperial College, London, UK (UKWatch, 2003).

Growing blood vessels ... must be exposed to compression, shear stresses and a pulsated flow of the culture medium to acquire their mechanical properties

The scaffold material should not only be able to deliver nutrients and oxygen, but should also provide developmental stimuli—such as growth, differentiation and angiogenic factors—to encourage the growth of the graft. Delivering these factors in the right concentration to the right place is crucial. Therefore, scaffolds are being constructed that can constantly release growth factors, such as the vascular endothelial growth factor, to induce blood-vessel formation. However, the question remains as to how to achieve physiological concentrations (Griffith & Naughton, 2002). Strictly regulating the activity of signalling molecules, particularly growth factors, is extremely important to avoid uncontrolled cell growth and proliferation.

Another important challenge for biologists is the selection of appropriate cells, and the conditions for their isolation, purification and cultivation. Three sources of cells are available: the patient (autologous cells), another human donor (allogeneic cells) or another species (xenogeneic cells; Griffith & Naughton, 2002; Bock et al, 2003). Deriving cells from the patient by biopsy is advantageous as it avoids immune reactions and has a low risk of infection. However, the availability might be limited, depending on the cells that are needed. Also, for some organs, such as heart valves, biopsies are not feasible. Furthermore, because cells differ between individuals, manufacturing protocols would have to be adapted for every patient. Cells from human or animal donors are available in large numbers, but they carry a higher risk of contamination with pathogens and of immune reactions. Given these constraints, researchers are increasingly concentrating on the potential of stem cells to grow larger tissues and organ parts. But even these have several drawbacks. Human embryonic stem cells, which are taken from spare embryos created by in vitro fertilization, from embryos created in vitro or by somatic cell nuclear transfer (so-called therapeutic cloning), seem to be the most promising in terms of their proliferation and differentiation abilities. However, research on these cells has been hampered by legal constraints and ethical objections in many countries. Furthermore, embryonic stem cells would not solve the problem of rejection by the immune system and therapeutic cloning could take too long—the patients might die while waiting for their stem cells to develop.

The use of the patient's adult stem cells is another, less controversial, option that would also overcome the problem of immune rejection. However, there are doubts about the pluripotency of these cells, and it is not yet clear whether adult stem cells have the same proliferation and differentiation capacities as embryonic stem cells (Vogel, 2004). Furthermore, researchers also lack basic knowledge about the mechanisms of cell–cell and cell–matrix interactions, and how to direct the differentiation of stem cells into the desired cell type.

In light of these problems, scientists are now focusing on generating parts—such as heart valves, cardiac blood vessels and patches of cardiac muscle—to repair damaged tissue in the heart. Alternatively, the direct injection of stem cells into cardiac muscle to improve heart function after heart attacks has been tested (Wollert et al, 2004). The case is similar for liver tissue. Research has been carried out on hepatocyte transplantation using polymeric matrices as an alternative therapy for metabolic liver diseases. Another option is bioartificial liver devices—in other words, extracorporeal bioreactors that house liver cells. The patient's plasma is circulated through hepatocytes that are sandwiched between artificial plates or capillaries. Such devices are used in hospitals to replace liver function until the organ has recovered, which can supersede transplantation in some patients. But a key problem with these devices is the large number of high-quality human hepatocytes required. Furthermore, for some illnesses, in vivo solutions are sometimes more promising; for example, many companies have halted their efforts to construct pancreas grafts for patients who are suffering from diabetes, because it seems to be far more promising to transplant pancreatic islet cells (Bock et al, 2003).

At the moment, in vivo approaches to regenerating tissue in a patient seem far more promising than the ex vivo construction of organs

Despite the difficulties that are involved in creating more complex tissues, some successes have been achieved. In 1999, researchers at the Harvard Medical School reported the construction of urinary bladders from a bladder-shaped polymer matrix that was seeded with urothelial and smooth muscle cells. Three months after they transplanted these artificial bladders into dogs, the polymers had degraded, blood vessels from the animals had colonized the new tissues and the bladders were innervated and functioning (Oberpenning et al, 1999). Less complex is the construction of blood vessels. Scientists at Harvard Medical School and Yale University School of Medicine (New Haven, CT, USA) recently reported the formation of a network of blood vessels in mice. The researchers implanted three-dimensional constructs of a gel matrix that was seeded with vascular endothelial cells and mesenchymal precursor cells into the animals. The resultant vessel networks were stable and functional for one year (Koike et al 2004).

Nevertheless, a refrigerator full of ready-to-transplant organs sounds like science fiction. At the moment, in vivo approaches to regenerating tissue in a patient seem far more promising than the ex vivo construction of organs. "The experiences of the past years have shown that we have to get away from [engineering] complex organs," Stark commented. "Why construct something in vitro if we can do it in vivo?" Nevertheless, the advances that have been made so far yield some hope that tissue engineering could provide solutions for a range of illnesses. But is it also clear that there is still a lot of basic research to be done. Without knowing how organs acquire their structure and function during normal development, and how they are innervated and vascularized, the creation of organs from scratch will remain difficult or even impossible. As Professor C. James Kirkpatrick at the University of Mainz (Germany) stated at a strategic meeting of the German Research Foundation earlier this year, "The mysteries of regenerative therapies can be solved by investigating tissue and organ development in appropriate model organisms, and by exploring human embryology." This interdisciplinary field also requires the concerted action of chemists, materials scientists, cell biologists and physicists. So far, none of the three pillars seems stable enough to support the construction of a tissue, let alone an entire organ.

Top

MORE ARTICLES LIKE THIS

NEWS AND VIEWS