Chapter One The History and Scope of Tissue Engineering Joseph Vacanti and Charles A. Vacanti

I. Introduction V. General Scientifi c Issues II. Scientifi c Challenges VI. Social Challenges III. Cells VII. References IV. Materials

I. INTRODUCTION the mid-19th century enabled the rapid evolution of many The dream is as old as humankind. Injury, disease, and surgical techniques. With patients anesthetized, innovative congenital malformation have always been part of the and courageous surgeons could save lives by examining human experience. If only damaged bodies could be and treating internal areas of the body: the thorax, the restored, life could go on for loved ones as though tragedy abdomen, the brain, and the heart. Initially the surgical had not intervened. In recorded history, this possibility fi rst techniques were primarily extirpative, for example, removal was manifested through myth and magic, as in the Greek of tumors, bypass of the bowel in the case of intestinal legend of Prometheus and eternal liver regeneration. Then obstruction, and repair of life-threatening injuries. Main- legend produced miracles, as in the creation of Eve in tenance of life without regard to the crippling effects of Genesis or the miraculous transplantation of a limb by the tissue loss or the psychosocial impact of disfi gurement, saints Cosmos and Damien. With the introduction of the however, was not an acceptable end goal. Techniques that scientifi c method came new understanding of the natural resulted in the restoration of function through structural world. The methodical unraveling of the secrets of biology replacement became integral to the advancement of human was coupled with the scientifi c understanding of disease therapy. and trauma. Artifi cial or prosthetic materials for replacing Now whole fi elds of reconstructive surgery have limbs, teeth, and other tissues resulted in the partial restora- emerged to improve the quality of life by replacing missing tion of lost function. Also, the concept of using one tissue as function through rebuilding body structures. In our current a replacement for another was developed. In the 16th era, modern techniques of transplanting tissue and organs century, Tagliacozzi of Bologna, Italy, reported in his work from one individual into another have been revolutionary Decusorum Chirurgia per Insitionem a description of a and lifesaving. The molecular and cellular events of the nose replacement that he constructed from a forearm fl ap. immune response have been elucidated suffi ciently to sup- With the 19th-century scientifi c understanding of the germ press the response in the clinical setting of transplantation theory of disease and the introduction of sterile technique, and to produce prolonged graft survival and function in modern surgery emerged. The advent of anesthesia by patients. In a sense, transplantation can be viewed as the

Principles of Tissue Engineering, 3rd Edition Copyright © 2007, Elsevier, Inc. ed. by Lanza, Langer, and Vacanti All rights reserved. 4 CHAPTER ONE • THE HISTORY AND SCOPE OF TISSUE ENGINEERING most extreme form of reconstructive surgery, transferring armamentarium of physicians and surgeons. Broadly speak- tissue from one individual into another. ing, the challenges are scientifi c and social. As with any successful undertaking, new problems have emerged. Techniques using implantable foreign body mate- II. SCIENTIFIC CHALLENGES rials have produced dislodgment, infection at the foreign As a fi eld, tissue engineering has been defi ned only body/tissue interface, fracture, and migration over time. since the mid-1980s. As in any new undertaking, its roots are Techniques moving tissue from one position to another fi rmly implanted in what went before. Any discussion of have produced biologic changes because of the abnormal when the fi eld began is inherently fuzzy. Much still needs to interaction of the tissue at its new location. For example, be learned and developed to provide a fi rm scientifi c basis diverting urine into the colon can produce fatal colon for therapeutic application. To date, much of the progress in cancers 20–30 years later. Making esophageal tubes from the this fi eld has been related to the development of model skin can result in skin tumors 30 years later. Using intestine systems, which have suggested a variety of approaches. for urinary tract replacement can result in severe scarring Also, certain principles of cell biology and tissue develop- and obstruction over time. ment have been delineated. The fi eld can draw heavily on Transplantation from one individual into another, the explosion of new knowledge from several interrelated, although very successful, has severe constraints. The major well-established disciplines and in turn may promote the problem is accessing enough tissue and organs for all of the coalescence of relatively new, related fi elds to achieve their patients who need them. Currently, 92,587 people are on potential. The rate of new understanding of complex living transplant waiting lists in the United States, and many will systems has been explosive since the 1970s. Tissue engi- die waiting for available organs. Also, problems with the neering can draw on the knowledge gained in the fi elds of immune system produce chronic rejection and destruction cell and stem cell biology, biochemistry, and molecular over time. Creating an imbalance of immune surveillance biology and apply it to the engineering of new tissues. Like- from immunosuppression can cause new tumor formation. wise, advances in materials science, chemical engineering, The constraints have produced a need for new solutions to and bioengineering allow the rational application of engi- provide needed tissue. neering principles to living systems. Yet another branch of It is within this context that the fi eld of tissue engineering related knowledge is the area of human therapy as applied has emerged. In essence, new and functional living tissue is by surgeons and physicians. In addition, the fi elds of genetic fabricated using living cells, which are usually associated, in engineering, cloning, and stem cell biology may ultimately one way or another, with a matrix or scaffolding to guide develop hand in hand with the fi eld of tissue engineering in tissue development. New sources of cells, including many the treatment of human disease, each discipline depending types of stem cells, have been identifi ed in the past several on developments in the others. years, igniting new interest in the fi eld. In fact, the emergence We are in the midst of a biologic renaissance. Interac- of stem cell biology has led to a new term, regenerative medi- tions of the various scientifi c disciplines can elucidate not cine. Scaffolds can be natural, man-made, or a composite of only the potential direction of each fi eld of study, but also both. Living cells can migrate into the implant after implan- the right questions to address. The scientifi c challenge in tation or can be associated with the matrix in cell culture tissue engineering lies both in understanding cells and their before implantation. Such cells can be isolated as fully dif- mass transfer requirements and the fabrication of materials ferentiated cells of the tissue they are hoped to recreate, or to provide scaffolding and templates. they can be manipulated to produce the desired function when isolated from other tissues or stem cell sources. Con- ceptually, the application of this new discipline to human III. CELLS health care can be thought of as a refi nement of previously If we postulate that living cells are required to fabricate defi ned principles of medicine. The physician has historically new tissue substitutes, much needs to be learned with treated certain disease processes by supporting nutrition, regard to their behavior in two normal circumstances: minimizing hostile factors, and optimizing the environment normal development in morphogenesis and normal wound so that the body can heal itself. In the fi eld of tissue engineer- healing. In both of these circumstances, cells create or recre- ing, the same thing is accomplished on a cellular level. The ate functional structures using preprogrammed informa- harmful tissue is eliminated; the cells necessary for repair tion and signaling. Some approaches to tissue engineering are then introduced in a confi guration optimizing survival rely on guided regeneration of tissue using materials that of the cells in an environment that will permit the body to serve as templates for ingrowth of host cells and tissue. heal itself. Tissue engineering offers an advantage over cell Other approaches rely on cells that have been implanted as transplantation alone in that organized three-dimensional part of an engineered device. As we gain understanding of functional tissue is designed and developed. This chapter normal developmental and wound-healing gene programs summarizes some of the challenges that must be resolved and cell behavior, we can use them to our advantage in the before tissue engineering can become part of the therapeutic rational design of living tissues. V. GENERAL SCIENTIFIC ISSUES • 5

Acquiring cells for creation of body structures is a major developed to be compatible with living systems or with challenge, the solution of which continues to evolve. The living cells in vitro and in vivo. Their interface with the cells ultimate goal in this regard — the large-scale fabrication of and the implant site must be clearly understood so that the structures — may be to create large cell banks composed of interface can be optimized. Their design characteristics are universal cells that would be immunologically transparent major challenges for the fi eld and should be considered at to an individual. These universal cells could be differenti- a molecular chemical level. Systems can be closed, semiper- ated cell types that could be accepted by an individual or meable, or open. Each design should factor into the specifi c could be stem cell reservoirs, which could respond to signals replacement therapy considered. Design of biomaterials to differentiate into differing lineages for specifi c structural can also incorporate the biologic signaling that the materi- applications. Much is already known about stem cells and als may offer. Examples include release of growth and cell lineages in the bone marrow and blood. Studies suggest differentiation factors, design of specifi c receptors and that progenitor cells for many differentiated tissues exist anchorage sites, and three-dimensional site specifi city using within the marrow and blood and may very well be ubiqui- computer-assisted design and manufacture techniques. tous. Our knowledge of the existence and behavior of such New nanotechnologies have been incorporated to design cells in various mesenchymal tissues (muscle, bone, and systems of extreme precision. Combining computational cartilage), endodermally derived tissues (intestine and models with nanofabrication can produce microfl uidic cir- liver), or ectodermally derived tissues (nerves, pancreas, culations to nourish and oxygenate new tissues. and skin) expands on a daily basis. A new area of stem cell biology involving embryonic stem cells holds promise for V. GENERAL SCIENTIFIC ISSUES tissue engineering. The calling to the scientifi c community As new scientifi c knowledge is gained, many conceptual is to understand the principles of stem and progenitor cell issues need to be addressed. Related to mass transfer is the biology and then to apply that understanding to tissue engi- fundamental problem associated with nourishing tissue of neering. The development of immunologically inert univer- large mass as opposed to tissue with a relatively high ratio sal cells may come from advances in genetic manipulation of surface area to mass. Also, functional tissue equivalents as well as stem cell biology. necessitate the creation of composites containing different As intermediate steps, tissue can be harvested as cell types. For example, all tubes in the body are laminated allograft, autograft, or xenograft. The tissues can then be tubes composed of a vascularized mesodermal element, dissociated and placed into cell culture, where proliferation such as smooth muscle, cartilage, or fi brous tissue. The of cells can be initiated. After expansion to the appropriate inner lining of the tube, however, is specifi c to the organ cell number, the cells can then be transferred to templates, system. Urinary tubes have a stratifi ed transitional epithe- where further remodeling can occur. Which of these strate- lium. The trachea has a pseudostratifi ed columnar epithe- gies are practical and possibly applicable in humans remains lium. The esophagus has an epithelium that changes along to be explored. the gradient from mouth to stomach. The intestine has an Large masses of cells for tissue engineering need to be enormous, convoluted surface area of columnar epithelial kept alive, not only in vitro but also in vivo. The design of cells that migrate from a crypt to the tip of the villus. The systems to accomplish this, including in vitro fl ow bioreac- colonic epithelium is, again, different for the purposes of tors and in vivo strategies for maintenance of cell mass, water absorption and storage. presents an enormous challenge, in which signifi cant Even the well-developed manufacture of tissue- advances have been made. The fundamental biophysical engineered skin used only the cellular elements of the constraint of mass transfer of living tissue needs to be dermis for a long period of time. Attention is now focusing understood and dealt with on an individual basis as we on creating new skin consisting of both the dermis and its move toward human application. associated fi broblasts as well as the epithelial layer, consist- ing of keratinocytes. Obviously, this is a signifi cant advance. IV. MATERIALS But for truly “normal” skin to be engineered, all of the cel- There are so many potential applications to tissue lular elements should be contained so that the specialized engineering that the overall scale of the undertaking is appendages can be generated as well. These “simple” com- enormous. The fi eld is ripe for expansion and requires posites will indeed prove to be quite complex and require training of a generation of materials scientists and chemical intricate designs. Thicker structures with relatively high engineers. ratios of surface area to mass, such as liver, kidney, heart, The optimal chemical and physical confi gurations of breast, and the central nervous system, will offer engi- new biomaterials as they interact with living cells to produce neering challenges. tissue-engineered constructs are under study by many Currently, studies for developing and designing materi- research groups. These biomaterials can be permanent or als in three-dimensional space are being developed utiliz- biodegradable. They can be naturally occurring materials, ing both naturally occurring and synthetic molecules. The synthetic materials, or hybrid materials. They need to be applications of computer-assisted design and manufacture 6 CHAPTER ONE • THE HISTORY AND SCOPE OF TISSUE ENGINEERING techniques to the design of these matrices are critically bioreactor in which force vectors can be applied? When is important. Transformation of digital information obtained the optimal timing of this transformation? When does tissue from magnetic resonance scanning or computerized tomog- strength take over the biochemical characteristics as the raphy scanning can then be developed to provide appropri- material degrades? ate templates. Some tissues can be designed as universal tissues that will be suitable for any individual, or they may VI. SOCIAL CHALLENGES be custom-developed tissues specifi c to one patient. An If tissue engineering is to play an important role in important area for future study is the entire fi eld of human therapy, in addition to scientifi c issues, fundamental neural regeneration, neural ingrowth, and neural function issues that are economic, social, and ethical in nature will toward end organ tissues such as skeletal or smooth muscle. arise. Something as simple as a new vocabulary will need to Putting aside the complex architectural structure of these be developed and uniformly applied. A universal problem is tissues, the cells contained in them have a very high meta- funding. Can philanthropic dollars be accessed for the bolic requirement. As such, it is exceedingly diffi cult to purpose of potential new human therapies? Will industry isolate a large number of viable cells. An alternate approach recognize the potential for commercialization and invest may be the use of less mature progenitor cells, or stem cells, heavily? If this occurs, will the focus be changed from that which not only would have a higher rate of survival as a of a purely academic endeavor? What role will governmental result of their lower metabolic demand but also would be agencies play as the fi eld develops? How will the fi eld be more able to survive the insult and hypoxic environment regulated to ensure its safety and effi cacy prior to human of transplantation. As stem cells develop and require application? Is the new tissue to be considered transplanted more oxygen, their differentiation may stimulate the devel- tissue and, therefore, not be subject to regulation, or is it a opment of a vascular complex to nourish them. The pharmaceutical that must be subjected to the closest scru- understanding of and solutions to these problems are tiny by regulatory agencies? If lifesaving, should the track be fundamentally important to the success of any replacement accelerated toward human trials? tissue that needs ongoing neural interaction for mainte- There are legal ramifi cations of this emerging technol- nance and function. ogy as new knowledge is gained. What becomes proprietary It has been shown that some tissues can be driven through patents? Who owns the cells that will be sourced to to completion in vitro in bioreactors. However, optimal provide the living part of the tissue fabrication? incubation times will vary from tissue to tissue. Even so, the In summary, one can see from this brief overview that new tissue will require an intact blood supply at the time of the challenges in the fi eld of tissue engineering remain sig- implantation for successful engraftment and function. nifi cant. All can be encouraged by the progress that has Finally, all of these characteristics need to be under- been made in the past few years, but much discovery lies stood in the fourth dimension, time. If tissues are implanted ahead. Ultimate success will rely on the dedication, creativ- in a growing individual, will the tissues grow at the same ity, and enthusiasm of those who have chosen to work in this rate? Will cells taken from an older individual perform as exciting but still unproved fi eld. Quoting from the epilogue young cells in their new “optimal” environment? How will of the previous edition: “At any given instant in time, human- the biochemical characteristics change over time after ity has never known so much about the physical world and implantation? Can the strength of structural support tissues will never again know so little.” such as bone, cartilage, and ligaments be improved in a

VII. REFERENCES

Langer, R., and Vacanti, J. P. (1993). Tissue engineering. Science 260, Vacanti, C. A. (2006). History of tissue engineering and a glimpse into 920–926. its future. Tissue Eng. 12, 1137–1142. Lanza, R. P., Langer, R., and Vacanti, J. P. (2000). “Principles of Tissue Vacanti, J. P., and Langer, R. (1999). Tissue engineering: the design and Engineering,” 2nd ed., p. 929. fabrication of living replacement devices for surgical reconstruction and transplantation. Lancet 354, SI32–34. Nerem, R. M. (2006). Tissue engineering: the hope, the hype and the future. Tissue Eng. 12, 1143–1150.