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Tissue engineering and regenerative medicine: from first principles to state of the art
Journal of Pediatric Surgery (2010) 45, 291–294
www.elsevier.com/locate/jpedsurg
Journal of Pediatric Surgery Lecture
Tissue engineering and regenerative medicine: from first principles to state of the art Joseph Vacanti ⁎ Massachusetts General Hospital, Warren 1151, Boston, Massachusetts 02114, USA Received 10 October 2009; accepted 27 October 2009
Key words: Tissue engineering; Regenerative medicine; Stem cells; Organ shortage
Abstract This lecture updates pediatric surgeons on the state of the science of tissue engineering and regenerative medicine. © 2010 Published by Elsevier Inc.
The purpose of this study is to update pediatric surgeons on the state of the science of tissue engineering and regenerative medicine. We can take some pride as a surgical specialty in our contributions to the conceptualization of this field as well as contributions to the science and engineering involved in the field. Also, in the year 2009, we can point to several clinical trials involving children and the surgical specialists in children's care.
1. History This field emerged from the need for reconstruction in children and adults in whom tissue has been destroyed by disease, trauma, and congenital anomalies. The term Tissue Engineering arose in the late 1980s after early work in the modern era suggested approaches to creating living tissue substitutes for human therapy [1,2]. Reconstruction in humans is likely to predate the development of the written word, but with the advent of
Presented at the 56th Annual Meeting of the British Association of Paediatric Surgeons, Graz, Austria, June 18-20, 2009. ⁎ Tel.: +1 617 724 1725; fax: +1 617 726 2167. E-mail address: [email protected]. 0022-3468/$ – see front matter © 2010 Published by Elsevier Inc. doi:10.1016/j.jpedsurg.2009.10.063
surgical anesthesia and the understanding of the causes and prevention of infection in the 19th century, rapid progress in surgery emerged. At first, life-saving techniques were used for problems such as the drainage of infection, the control of hemorrhage, or the removal of tumors. Quickly, clinicians realized that a form of reconstruction to return the patient to as normal a life as possible was integral to success in lifesaving maneuvers. Tissue substitutes were created using nonliving materials and also moving living structures into different locations for different functions. The ultimate manifestation of this was the emergence of organ transplantation in the mid-1950s, at first, the transplant of a kidney from an identical twin brother [3]. Although enormous progress followed such strategies, they also had serious limitations. Our work began in the mid-1980s when we were faced with the insoluble problem of organ shortages for liver transplants. We postulated that living organs might be designed and built based on the principles of biologic science and technologic advances in the engineering disciplines. We presented our first work experimentally generating new liver, intestinal, and pancreatic tissue at the spring meeting of the American Pediatric Surgical Association in May 1987 in Hilton Head Island, South Carolina [4]. That first work was then published in this journal in 1988 [5]. Needless to say, it was met with varying reactions ranging from skepticism to
292 enthusiasm. Within a decade, much work from many laboratories across the globe had demonstrated the fundamental principles to successful tissue creation and had shown the practical application of these principles in many systems eventually applied to human therapy. Also, the discovery of stem cell sources other than the blood forming elements had been made and the observation of the existence of embryonic stem cells and their potential useful application for human therapy was likewise demonstrated [6,7]. With the rapid emergence of stem cell science, the term “Regenerative Medicine” combined the elements of tissue engineering and stem cell science to encompass the broad range of scientific disciplines that were represented and necessary to further the field. However, a central point from the beginning is the necessity for an adequate blood supply, and this reality needs to be incorporated into any tissue engineering strategy. Although this is obvious to every surgeon involved in wound repair and healing, it is not so obvious to stem cell biologists.
2. Biologic and engineering principles To build a living system to replace lost structures in humans, an investigator needs to understand the fundamental principles upon which living systems are created. The central component of any living system is the single cell. Nature by
J. Vacanti trial and error within the first 1 billion years of the earth's creation had generated the single living cell. Each individual cell to reproduce and function has simple metabolic requirements. It needs to get food in, it needs to get gas in, and it needs to dump waste and remove waste gas. Two billion years after the creation of individual cells, nature evolved multicellular systems. These had evolutionary advantage and thrived over the past 1.5 billion years. As observers of nature, then, we needed to understand which designs evolved and were successful for multicellular systems to deliver food and gas into each cell and to rid it of waste. Because most of the structures that we are interested in as surgeons are large and complex multicellular systems, this design feature is central to any solution in the field of tissue engineering. Fig. 1 illustrates that the problem here is demonstrated by comparing a small spheroid of cells and then a larger spheroid. The fundamental biophysical constraint from going from small to large is the fact that exchange of nutrients and gas and waste occurs at the surface, whereas the volume or mass of cells needing the exchange is much greater. In fact, the surface only increases as r2, whereas the mass increases as r3. Therefore, any strategy to be successful in this field of tissue engineering and regenerative medicine must in one way or another try to match surface area to volume for effective mass transfer of nutrients and gas. The design solution was worked out at least 1.5 billion years ago by nature. She solved the problem by the generation of branching fractal mathematic algorithms
Fig. 1 Spheroids of cells need the surface for exchange of nutrients, gas, and waste. However, without a circulation, the spheroid outstrips its ability for exchange because the mass or volume increases as the cube of the radius, whereas the surface only increases as the square of the radius. Therefore, every design in tissue engineering must include strategies to improve this mismatch.
Tissue engineering and regenerative medicine to break up the surface and provide exchange with massive increases in surface area matching volume or mass (Fig. 2). This branching pattern is what was mimicked in our first experiments in the mid-1980s. We reasoned that we could generate large living structures combining large numbers of cells on degradable scaffolding materials that were based on fiber arrays 12 to 14 μm in diameter. By combining these 2 elements in cell culture or bioreactors, the cells were well nourished and oxygenated and, over time, began to aggregate by laying down extracellular matrix and forming tissues even before implantation. Once implanted in an animal model, this “construct” induced an inflammatory response and an angiogenesis response. Over time the inflammation subsided, the cells proliferated and reorganized, and a permanent vascular circulation was integrated into the new living tissue [8]. Variations of this technique evolved over the next 12 years, and many tissues were constructed and reported. In fact, more than 30 tissues of the body were demonstrated experimentally using variations of these tissue engineering techniques [9]. By the late 1990s, the advantages as well as the limitations of this technique were demonstrated. Unfortunately, the creation of a large organ such as a liver, kidney, or lung was hampered by the thickness of the living tissue that was needed. Therefore, work was begun in 1998 to overcome the problem by the creation of a complete microfluidic vascular circulation as part of the living engineered construct [10]. The important components of this study included an engineering understanding of the network topology of vascular circulations for all of the various organs and tissues and understanding of rheology of blood flow through them as well as the physiology of flow. Once understood, engineering technologies could then be used to design and build such microfluidic circulations to accompany the needed tissue cell population. The past 10 years has seen a remarkable advancement of this approach and has shown its potential utility in systems ultimately destined to become organ replacement structures for liver,
293 Table 1 Structures in human use in clinical trails created using tissue engineering principles Human application of tissue engineering—February 2009 • • • • • • •
Skin Cartilage Bone Blood vessels Corneas Urinary structures Left mainstem bronchus
kidney, and lung [11]. Alternate approaches decellularizing living organs and maintaining the vascular scaffolding of a tissue have been reported. In fact, the demonstration of using this matrix with the circulation intact and flowing nutrient medium through the circulation while repopulating the tissue with cells has been a remarkable experimental achievement [12]. A stunning graphic example is the report of the creation of a beating structure from the cells of a rodent heart in a bioreactor being nourished through the native circulation of a heart extracellular matrix.
3. Present and future Much work remains, but tissues approved either for human use or in clinical trials are listed in Table 1. The commercialization of these technologies to allow widespread human application has recent been reported by Lysaght et al [13]. These early achievements point to a very optimistic future not only for the field but also for its potential in helping many patients. The field continues to be populated with creative, engaged, and committed young people who have the ability to work in multidisciplinary teams for the common purpose of improved patient care. The addition of stem cell technologies into the equation has brought even more hope to this potential solution to many problems vexing clinicians in patient care. We as pediatric surgeons can be proud of the role we have played in the development of this field and should be encouraged to continue to make productive contributions in the future.
References
Fig. 2 Seaweed on a Cape Cod beach demonstrating nature's matching of surface area to volume by branching structures corresponding to the rules of fractal mathematics [2].
[1] Langer R, Vacanti JP. Tissue engineering. Science 1993;260:920-6. [2] Vacanti JP. Tissue engineering and regenerative medicine. Proc Am Philos Soc 2007;151:395-402. [3] Murray JE, Merrill JP, Harrison JH. Renal homotransplantation in identical twins. Surgical Forum 1955;6:432-6. [4] Vacanti JP, Morse MA, Domb A, et al. Chimeric neomorphogenesis of organs: a proposal for organ replacement using controlled cellular implantation on bioabsorbable artificial matrices. Presented at the American Pediatric Surgical Association, Hilton Head Island, SC, May 6-9; 1987.
294 [5] Vacanti JP, Morse MA, Saltzman WM, et al. Selective cell transplantation using bioabsorbable artificial polymers as matrices. J Pediatr Surg 1988;23:3-9. [6] Caplan AI. Mesenchymal stem cells. J Ortho Res 1991;9:641-50. [7] Thomson JA, Itskovitz-Eldor J, Shapiro SS, et al. Embryonic stem cell lines derived from human blastocysts. Science 1998;282: 1145-7. [8] Vacanti JP. Beyond transplantation. Third Annual Samuel Jason Mixter Lecture. Arch Surg 1988;123:545-9. [9] Vacanti JP, Langer R. Tissue engineering: the design and fabrication of living replacement devices for surgical reconstruction and transplantation. Lancet 1999;354(Suppl 1):32-4.
J. Vacanti [10] Kaihara S, Borenstein J, Koka R, et al. Silicon micromachining to tissue engineer branched vascular channels for liver fabrication. Tissue Eng Part A 2000;6:105-17. [11] Hoganson D, Pryor II HI, Vacanti JP. Tissue engineering and organ structure: a vascularized approach to liver and lung. Pediatr Res 2008;63:520-6. [12] Ott HC, Matthiesen TS, Goh SK, et al. Perfusion-decellularized matrix: using nature's platform to engineer a bioartificial heart. Nature (Medicine) 2008;14:213-21. [13] Lysaght MJ, Jaklenec A, Deweerd E. Great expectations: private sector activity in tissue engineering, regenerative medicine, and stem cell therapeutics. Tissue Eng Part A 2008;14:305-15.
www.elsevier.com/locate/jpedsurg
Journal of Pediatric Surgery Lecture
Tissue engineering and regenerative medicine: from first principles to state of the art Joseph Vacanti ⁎ Massachusetts General Hospital, Warren 1151, Boston, Massachusetts 02114, USA Received 10 October 2009; accepted 27 October 2009
Key words: Tissue engineering; Regenerative medicine; Stem cells; Organ shortage
Abstract This lecture updates pediatric surgeons on the state of the science of tissue engineering and regenerative medicine. © 2010 Published by Elsevier Inc.
The purpose of this study is to update pediatric surgeons on the state of the science of tissue engineering and regenerative medicine. We can take some pride as a surgical specialty in our contributions to the conceptualization of this field as well as contributions to the science and engineering involved in the field. Also, in the year 2009, we can point to several clinical trials involving children and the surgical specialists in children's care.
1. History This field emerged from the need for reconstruction in children and adults in whom tissue has been destroyed by disease, trauma, and congenital anomalies. The term Tissue Engineering arose in the late 1980s after early work in the modern era suggested approaches to creating living tissue substitutes for human therapy [1,2]. Reconstruction in humans is likely to predate the development of the written word, but with the advent of
Presented at the 56th Annual Meeting of the British Association of Paediatric Surgeons, Graz, Austria, June 18-20, 2009. ⁎ Tel.: +1 617 724 1725; fax: +1 617 726 2167. E-mail address: [email protected]. 0022-3468/$ – see front matter © 2010 Published by Elsevier Inc. doi:10.1016/j.jpedsurg.2009.10.063
surgical anesthesia and the understanding of the causes and prevention of infection in the 19th century, rapid progress in surgery emerged. At first, life-saving techniques were used for problems such as the drainage of infection, the control of hemorrhage, or the removal of tumors. Quickly, clinicians realized that a form of reconstruction to return the patient to as normal a life as possible was integral to success in lifesaving maneuvers. Tissue substitutes were created using nonliving materials and also moving living structures into different locations for different functions. The ultimate manifestation of this was the emergence of organ transplantation in the mid-1950s, at first, the transplant of a kidney from an identical twin brother [3]. Although enormous progress followed such strategies, they also had serious limitations. Our work began in the mid-1980s when we were faced with the insoluble problem of organ shortages for liver transplants. We postulated that living organs might be designed and built based on the principles of biologic science and technologic advances in the engineering disciplines. We presented our first work experimentally generating new liver, intestinal, and pancreatic tissue at the spring meeting of the American Pediatric Surgical Association in May 1987 in Hilton Head Island, South Carolina [4]. That first work was then published in this journal in 1988 [5]. Needless to say, it was met with varying reactions ranging from skepticism to
292 enthusiasm. Within a decade, much work from many laboratories across the globe had demonstrated the fundamental principles to successful tissue creation and had shown the practical application of these principles in many systems eventually applied to human therapy. Also, the discovery of stem cell sources other than the blood forming elements had been made and the observation of the existence of embryonic stem cells and their potential useful application for human therapy was likewise demonstrated [6,7]. With the rapid emergence of stem cell science, the term “Regenerative Medicine” combined the elements of tissue engineering and stem cell science to encompass the broad range of scientific disciplines that were represented and necessary to further the field. However, a central point from the beginning is the necessity for an adequate blood supply, and this reality needs to be incorporated into any tissue engineering strategy. Although this is obvious to every surgeon involved in wound repair and healing, it is not so obvious to stem cell biologists.
2. Biologic and engineering principles To build a living system to replace lost structures in humans, an investigator needs to understand the fundamental principles upon which living systems are created. The central component of any living system is the single cell. Nature by
J. Vacanti trial and error within the first 1 billion years of the earth's creation had generated the single living cell. Each individual cell to reproduce and function has simple metabolic requirements. It needs to get food in, it needs to get gas in, and it needs to dump waste and remove waste gas. Two billion years after the creation of individual cells, nature evolved multicellular systems. These had evolutionary advantage and thrived over the past 1.5 billion years. As observers of nature, then, we needed to understand which designs evolved and were successful for multicellular systems to deliver food and gas into each cell and to rid it of waste. Because most of the structures that we are interested in as surgeons are large and complex multicellular systems, this design feature is central to any solution in the field of tissue engineering. Fig. 1 illustrates that the problem here is demonstrated by comparing a small spheroid of cells and then a larger spheroid. The fundamental biophysical constraint from going from small to large is the fact that exchange of nutrients and gas and waste occurs at the surface, whereas the volume or mass of cells needing the exchange is much greater. In fact, the surface only increases as r2, whereas the mass increases as r3. Therefore, any strategy to be successful in this field of tissue engineering and regenerative medicine must in one way or another try to match surface area to volume for effective mass transfer of nutrients and gas. The design solution was worked out at least 1.5 billion years ago by nature. She solved the problem by the generation of branching fractal mathematic algorithms
Fig. 1 Spheroids of cells need the surface for exchange of nutrients, gas, and waste. However, without a circulation, the spheroid outstrips its ability for exchange because the mass or volume increases as the cube of the radius, whereas the surface only increases as the square of the radius. Therefore, every design in tissue engineering must include strategies to improve this mismatch.
Tissue engineering and regenerative medicine to break up the surface and provide exchange with massive increases in surface area matching volume or mass (Fig. 2). This branching pattern is what was mimicked in our first experiments in the mid-1980s. We reasoned that we could generate large living structures combining large numbers of cells on degradable scaffolding materials that were based on fiber arrays 12 to 14 μm in diameter. By combining these 2 elements in cell culture or bioreactors, the cells were well nourished and oxygenated and, over time, began to aggregate by laying down extracellular matrix and forming tissues even before implantation. Once implanted in an animal model, this “construct” induced an inflammatory response and an angiogenesis response. Over time the inflammation subsided, the cells proliferated and reorganized, and a permanent vascular circulation was integrated into the new living tissue [8]. Variations of this technique evolved over the next 12 years, and many tissues were constructed and reported. In fact, more than 30 tissues of the body were demonstrated experimentally using variations of these tissue engineering techniques [9]. By the late 1990s, the advantages as well as the limitations of this technique were demonstrated. Unfortunately, the creation of a large organ such as a liver, kidney, or lung was hampered by the thickness of the living tissue that was needed. Therefore, work was begun in 1998 to overcome the problem by the creation of a complete microfluidic vascular circulation as part of the living engineered construct [10]. The important components of this study included an engineering understanding of the network topology of vascular circulations for all of the various organs and tissues and understanding of rheology of blood flow through them as well as the physiology of flow. Once understood, engineering technologies could then be used to design and build such microfluidic circulations to accompany the needed tissue cell population. The past 10 years has seen a remarkable advancement of this approach and has shown its potential utility in systems ultimately destined to become organ replacement structures for liver,
293 Table 1 Structures in human use in clinical trails created using tissue engineering principles Human application of tissue engineering—February 2009 • • • • • • •
Skin Cartilage Bone Blood vessels Corneas Urinary structures Left mainstem bronchus
kidney, and lung [11]. Alternate approaches decellularizing living organs and maintaining the vascular scaffolding of a tissue have been reported. In fact, the demonstration of using this matrix with the circulation intact and flowing nutrient medium through the circulation while repopulating the tissue with cells has been a remarkable experimental achievement [12]. A stunning graphic example is the report of the creation of a beating structure from the cells of a rodent heart in a bioreactor being nourished through the native circulation of a heart extracellular matrix.
3. Present and future Much work remains, but tissues approved either for human use or in clinical trials are listed in Table 1. The commercialization of these technologies to allow widespread human application has recent been reported by Lysaght et al [13]. These early achievements point to a very optimistic future not only for the field but also for its potential in helping many patients. The field continues to be populated with creative, engaged, and committed young people who have the ability to work in multidisciplinary teams for the common purpose of improved patient care. The addition of stem cell technologies into the equation has brought even more hope to this potential solution to many problems vexing clinicians in patient care. We as pediatric surgeons can be proud of the role we have played in the development of this field and should be encouraged to continue to make productive contributions in the future.
References
Fig. 2 Seaweed on a Cape Cod beach demonstrating nature's matching of surface area to volume by branching structures corresponding to the rules of fractal mathematics [2].
[1] Langer R, Vacanti JP. Tissue engineering. Science 1993;260:920-6. [2] Vacanti JP. Tissue engineering and regenerative medicine. Proc Am Philos Soc 2007;151:395-402. [3] Murray JE, Merrill JP, Harrison JH. Renal homotransplantation in identical twins. Surgical Forum 1955;6:432-6. [4] Vacanti JP, Morse MA, Domb A, et al. Chimeric neomorphogenesis of organs: a proposal for organ replacement using controlled cellular implantation on bioabsorbable artificial matrices. Presented at the American Pediatric Surgical Association, Hilton Head Island, SC, May 6-9; 1987.
294 [5] Vacanti JP, Morse MA, Saltzman WM, et al. Selective cell transplantation using bioabsorbable artificial polymers as matrices. J Pediatr Surg 1988;23:3-9. [6] Caplan AI. Mesenchymal stem cells. J Ortho Res 1991;9:641-50. [7] Thomson JA, Itskovitz-Eldor J, Shapiro SS, et al. Embryonic stem cell lines derived from human blastocysts. Science 1998;282: 1145-7. [8] Vacanti JP. Beyond transplantation. Third Annual Samuel Jason Mixter Lecture. Arch Surg 1988;123:545-9. [9] Vacanti JP, Langer R. Tissue engineering: the design and fabrication of living replacement devices for surgical reconstruction and transplantation. Lancet 1999;354(Suppl 1):32-4.
J. Vacanti [10] Kaihara S, Borenstein J, Koka R, et al. Silicon micromachining to tissue engineer branched vascular channels for liver fabrication. Tissue Eng Part A 2000;6:105-17. [11] Hoganson D, Pryor II HI, Vacanti JP. Tissue engineering and organ structure: a vascularized approach to liver and lung. Pediatr Res 2008;63:520-6. [12] Ott HC, Matthiesen TS, Goh SK, et al. Perfusion-decellularized matrix: using nature's platform to engineer a bioartificial heart. Nature (Medicine) 2008;14:213-21. [13] Lysaght MJ, Jaklenec A, Deweerd E. Great expectations: private sector activity in tissue engineering, regenerative medicine, and stem cell therapeutics. Tissue Eng Part A 2008;14:305-15.