Progress in tissue engineering and organogenesis in transplantation medicine

Tissue engineering is an attempt to culture living tissues for surgical transplantation. In vitro and in vivo approaches have produced vascular and cardiovascular components, bone, cartilage, gastro-intestinal organs, and liver. Organogenesis is a different approach to create new organs for transplantation from embryonic tissue implanted into the omentum of the recipient. This technique has been employed in creating kidney and pancreas in animals. Tissue engineering and organogenesis are the future of transplantation medicine. The progress in this field is of tremendous importance because it can produce a new generation of morphologically complex tissues and organs. In this review article we have summarized the most relevant experiences in this area, including its perspectives for therapeutical applications.     

Tissue engineering the kidney

The means by which kidney function can be replaced in humans include dialysis and renal allotransplantation. Dialytic therapies are lifesaving, but often poorly tolerated. Transplantation of human kidneys is limited by the availability of donor organs. During the past decades, a number of different approaches have been applied toward tissue engineering the kidney as a means to replace renal function. The goals of one or another of them included the recapitulation of renal filtration, reabsorptive and secretory functions, and replacement of endocrine/metabolic activities. This review will delineate the progress to date recorded for five approaches: (1) integration of new nephrons into the kidney; (2) growing new kidneys in situ; (3) use of stem cells; (4) generation of histocompatible tissues using nuclear transplantation; and (5) bioengineering of an artificial kidney. All five approaches utilize cellular therapy. The first four employ transplantation as well, and the fifth uses dialysis.     

Treatment for end-stage renal disease: an organogenesis/tissue engineering odyssey

The means by which kidney function can be replaced in humans with end-stage renal disease (ESRD) include dialytic therapies and renal allotransplantation. Dialysis, is lifesaving, but often poorly tolerated. Transplantation of human kidneys is limited by the availability of donor organs. During the past decades, several different approaches have been applied towards new means to replace renal function through organogenesis and tissue engineering. These include: (1) incorporation of new nephrons into the kidney; (2) growing new kidneys in situ; (3) use of stem cells; (4) generation of histocompatible tissues using nuclear transplantation; and (5) bioengineering of an artificial kidney. The development of these approaches has depended upon understanding and integrating discoveries made in a diversity of scientific disciplines. The means by which such integration has driven advances in the treatment of ESRD provides a generic roadmap for the successful application of organogenesis and tissue engineering to organ replacement therapy.     

Fusion of approaches to the treatment of organ failure

Because organ transplantation is the preferred treatment for organ failure, the demand for human organs for transplantation is large and growing. From this demand, several fields based on new technologies for the replacement or repair of damaged tissues and organs have emerged. These fields include stem cell biology, cloning, tissue engineering and xenotransplantation. Here we evaluate the potential contribution of these to the devising of alternative approaches to organ replacement. We present our vision for the development of two structurally complex organs – the lung and the kidney – based on a 'fusion' of new and established technologies.     

The bridge between transplantation and regenerative medicine: Beginning a new Banff classification of tissue engineering pathology

The science of regenerative medicine is arguably older than transplantation—the first major textbook was published in 1901—and a major regenerative medicine meeting took place in 1988, three years before the first Banff transplant pathology meeting. However, the subject of regenerative medicine/tissue engineering pathology has never received focused attention. Defining and classifying tissue engineering pathology is long overdue. In the next decades, the field of transplantation will enlarge at least tenfold, through a hybrid of tissue engineering combined with existing approaches to lessening the organ shortage. Gradually, transplantation pathologists will become tissue‐(re‐) engineering pathologists with enhanced skill sets to address concerns involving the use of bioengineered organs. We outline ways of categorizing abnormalities in tissue‐engineered organs through traditional light microscopy or other modalities including biomarkers. We propose creating a new Banff classification of tissue engineering pathology to standardize and assess de novo bioengineered solid organs transplantable success in vivo. We recommend constructing a framework for a classification of tissue engineering pathology now with interdisciplinary consensus discussions to further develop and finalize the classification at future Banff Transplant Pathology meetings, in collaboration with the human cell atlas project. A possible nosology of pathologic abnormalities in tissue‐engineered organs is suggested.     

Stem cells, tissue engineering and organogenesis in transplantation

Tissue engineering is an attempt to generate living tissues for surgical transplantation. In vitro and in vivo approaches have led to the production of vascular and cardiovascular components, bones, cartilages and gastrointestinal tissues. Organogenesis has a different aim, which is to create transplantable organs from embryonic tissue implanted into the recipient's omentum. This approach has been successful in creating kidneys and pancreases in animals. The use of stem cells in organogenesis and in tissue engineering has vastly enlarged the potential for clinical applications. The technique of nuclear transfer offers the possibility of creating cells, which are genetically identical to the host. Tissue engineering and organogenesis represent the future of transplantation in medicine. The progress in this field is of tremendous importance because it can produce a new generation of morphologically complex tissues and organs. In this review, the most relevant experiences in this area are summarized, including its perspectives for therapeutical applications.     

Hepatic tissue engineering for adjunct and temporary liver support: critical technologies.

The severe donor liver shortage, high cost, and complexity of orthotopic liver transplantation have prompted the search for alternative treatment strategies for end-stage liver disease, which would require less donor material, be cheaper, and less invasive. Hepatic tissue engineering encompasses several approaches to develop adjunct internal liver support methods, such as hepatocyte transplantation and implantable hepatocyte-based devices, as well as temporary extracorporeal liver support techniques, such as bioartificial liver assist devices. Many tissue engineered liver support systems have passed the "proof of principle" test in preclinical and clinical studies; however, they have not yet been found sufficiently reliably effective for routine clinical use. In this review we describe, from an engineering perspective, the progress and remaining challenges that must be resolved in order to develop the next generation of implantable and extracorporeal devices for adjunct or temporary liver assist.     

Tissue engineering for the replacement of organ function in the genitourinary system

Acquired and congenital abnormalities may lead to genitourinary organ damage or loss, requiring eventual reconstruction. Tissue engineering follows the principles of cell transplantation, materials science, and engineering toward the development of biological substitutes that would restore and maintain normal function. Tissue engineering may involve matrices alone, wherein the body's natural ability to regenerate is used to orient or direct new tissue growth, or the use of matrices with cells. Both synthetic and natural biodegradable materials have been used, either alone or as cell delivery vehicles. Tissue engineering has been applied experimentally for the reconstitution of several urologic tissues and organs, including bladder, ureter, urethra, kidney, testis, and genitalia. Fetal applications have also been explored. Recently, several tissue engineering technologies have been used clinically including the use of cells as bulking agents for the treatment of vesicoureteral reflux and incontinence and urethral replacement. Recent progress suggests that engineered genitourinary tissues may have clinical applicability in the future.     

Articular cartilage repair using tissue engineering technique--novel approach with minimally invasive procedure

Articular cartilage has very limited potential to spontaneously heal, because it lacks vessels and is isolated from systemic regulation. Although there have been many attempts to treat articular cartilage defects, such as drilling, microfracture techniques, soft tissue grafts or osteochondral grafts, no treatment has managed to repair the defects with long-lasting hyaline cartilage. Recently, a regenerative medicine using a tissue engineering technique for cartilage repair has been given much attention in the orthopedic field. In 1994, Brittberg et al. introduced a new cell technology in which chondrocytes expanded in monolayer culture were transplanted into the cartilage defect of the knee. As a second generation of chondrocyte transplantation, since 1996 we have been performing transplantation of tissue-engineered cartilage made ex vivo for the treatment of osteochondral defects of the joints. This signifies a concept shift from cell transplantation to tissue transplantation made ex vivo using tissue engineering techniques. We have reported good clinical results with this surgical treatment. However, extensive basic research is vital to achieve better clinical results with this tissue engineering technique. This article describes our recent research using a minimally invasive tissue engineering technique to promote cartilage regeneration.     

Tackling the shortage of donor kidneys: how to use the best that we have

Shortage of kidney donor is still a major limitation for renal transplantation programs. This review focuses on the emerging practices, adopted to increase transplant activities, of expanding the criteria for donor and recipient selection without exposing the recipient to the drawbacks of a graft with inadequate nephron mass. Expanding the donor pool inevitably led to consideration for kidney transplantation of organs from older donors or from donors with hypertension, diabetes or other renal diseases. To fit the reduced performance of these suboptimal organs with the renal requirement of the recipient, selection of recipients with reduced metabolic requirements or increase of nephron mass by simultaneous transplantation of two suboptimal kidneys in the same recipient have been pursued. However, a critical aspect of both approaches is to quantify functioning nephron mass provided to the recipient by pre-transplant kidney biopsies. Morphological parameters assessed on kidney biopsies at the time of donor evaluation may serve to quantify the preserved tissue and to discriminate chronic irreversible lesions from acute changes that may account for a transiently impaired renal function in the donor, but that may recover after transplant.     

A Low‐Cost,Small Volume Circuit for Autologous Blood Normothermic Perfusion of Rabbit Organs

We have designed a laboratory extracorporeal normothermic blood perfusion system for whole organs (e.g., kidney) that achieves pulsatile flow, low levels of hemolysis, and a blood priming volume of 60 mL or less. Using this uniquely designed extracorporeal circuit, we have achieved perfusion of two isolated ex vivo constructs. In the first experiment, we successfully perfused a rabbit epigastric flap based on the femoral vessels. In the second experiment, we were able to perfuse the isolated rabbit kidney for 48 h (range for all kidneys was 12–48 h) with excellent urine output, normal arterial blood gasses at 24 h, and normal ex vivo kidney histology at the conclusion of the experiments. These parameters have not been achieved before with any known or previously published laboratory extracorporeal circuits. The study has implications for prolonged organ perfusion prior to transplantation and for tissue engineering of vascularized tissues, such as by the perfusion of decellularized organs.     

Tissue engineering in urology

Congenital abnormalities, cancer, trauma, infection, inflammation, iatrogenic injuries, and other conditions may lead to genitourinary organ damage or loss, requiring eventual reconstruction. Tissue engineering follows the principles of cell transplantation, materials science, and engineering toward the development of biological substitutes that would restore and maintain normal function. Tissue engineering may involve matrices alone, wherein the body’s natural ability to regenerate is used to orient or direct new tissue growth, or the use of matrices with cells. Both synthetic (polyglycolic acid polymer scaffolds alone and with co-polymers of poly-1-lactic acid and poly-DL-lactide-coglycolide) and natural biodegradable materials (processed collagen derived from allogeneic donor bladder submucosa and intestinal submucosa) have been used, either alone or as cell delivery vehicles. Tissue engineering has been applied experimentally for the reconstitution of several urologic tissues and organs, including bladder, ureter, urethra, kidney, testis, and genitalia. Fetal applications have also been explored. Recently, several tissue engineering technologies have been used clinically, including the use of cells as bulking agents for the treatment of vesicoureteral reflux and incontinence, urethral replacement, and bladder reconstruction. Recent progress suggests that engineered urologic tissues may have clinical applicability in the future.     

A note form chair person: International Symposium on transplantation

Orthotopic liver transplantation is now a widely accepted surgical modality for end-stage liver disease, although the shortage of grafts has remained a serious medical and social problem. This has led to the development of new approaches such as split liver transplantation and living related-donor liver transplantation, which have not completely resolved the problem. It is expected that future techniques will rely on better and less toxic agents for preventing rejection, xenotransplantation, and "organ engineering" methods based on tissue engineering techniques. More research is required before xenotransplantation is established clinically due to the current lack of knowledge of physiology, immunology, and infection risks. The mechanism of hyperacute rejection has been the subject of intensive investigation and is gradually being clarified. However, the detailed mechanisms of delayed xengraft rejection and cell-mediated immune response are still poorly understood. Improvement of cell culture conditions in tissue engineering may permit the generation of human organs as substitutes for grafts in the transplantation setting.     

Stem cells in urology

The shortage of donors for organ transplantation has stimulated research on stem cells as a potential resource for cell-based therapy in all human tissues. Stem cells have been used for regenerative medicine applications in many organ systems, including the genitourinary system. The potential applications for stem cell therapy have, however, been restricted by the ethical issues associated with embryonic stem cell research. Instead, scientists have explored other cell sources, including progenitor and stem cells derived from adult tissues and stem cells derived from the amniotic fluid and placenta. In addition, novel techniques for generating stem cells in the laboratory are being developed. These techniques include somatic cell nuclear transfer, in which the nucleus of an adult somatic cell is placed into an oocyte, and reprogramming of adult cells to induce stem-cell-like behavior. Such techniques are now being used in tissue engineering applications, and some of the most successful experiments have been in the field of urology. Techniques to regenerate bladder tissue have reached the clinic, and exciting progress is being made in other areas, such as regeneration of the kidney and urethra. Cell therapy as a treatment for incontinence and infertility might soon become a reality. Physicians should be optimistic that regenerative medicine and tissue engineering will one day provide mainstream treatment options for urologic disorders.     

From deceased to bioengineered graft: New frontiers in liver transplantation

In the worldwide context of graft shortage, several strategies have been explored to increase the number of grafts available for liver transplantation (LT). These include the use of marginal and living donors, split livers, and the improvement of marginal donor grafts (machine perfusion). However, recent advances in the understanding of liver organogenesis, stem cells, and matrix biology provide novel insights in tissue engineering. Today, the newest technologies and discoveries open the door to the development of new methods for organ implementation such as the recellularization of natural scaffolds, liver organoids, bio-printing, and tissue or generation of chimeric organs. These approaches might potentially to generate an unlimited source of grafts (allogenic or chimeric) which will be used in the near future for LT or as a temporary bridge toward LT. This qualitative review focuses on all methods of organ implementation and highlights the newest developments in tissue engineering and regenerative medicine.     

Transmission of a pancreatic adenocarcinoma to a renal transplant recipient

Transmission of donor tumours in solid organ transplant recipients is rare, but has been reported with fatal outcome in some cases depending on the original tumour type and location. We report a case of a pancreatic adenocarcinoma of donor origin presented as lymphangitis carcinomatosa of the lung in a renal transplant recipient 9 months after transplantation, which is likely to have contributed to the death of the patient 15 months after transplantation. The donor tumour was originally diagnosed on adrenal tissue removed from the donor kidney during bench preparation. At the time of the diagnosis this kidney and the liver of the multi-organ donor had been transplanted. The liver patient was urgently retransplanted within 24 h. The renal recipient opted not to have a transplant nephrectomy having been made aware of the risk of tumour transmission. The contralateral kidney was discarded. Management of recipients with potential transmission of initially undiagnosed donor malignancy is difficult. Early transplant nephrectomy may be the safest option.     

Cellular therapies for liver replacement

Insufficient donor organs for orthotopic liver transplantation worldwide have urgently increased the requirement for new therapies for acute and chronic liver disease. Whilst none are yet clinically proven there are at least two different approaches for which there is extensive experimental data, some human anecdotal evidence and some data emerging from Phase 1 clinical trials. Both approaches involve bio-engineering. In vivo tissue engineering involves isolated liver cell transplantation into the liver and/or other ectopic sites and in vitro tissue engineering, using an extracorporeal hepatic support system or bioartificial liver. Some questions are common to both these approaches, such as the best cell source and the therapeutic mass required, and are discussed. Others are specific to each approach. For cell transplantation in vivo the initial engraftment and repopulation will make a critical difference to the outcome, and development of markers for transplanted cells has enabled significant advances in understanding, and therefore manipulating, the process. Moreover, the role of immunosuppression is also important and novel approaches to natural immunosuppression are discussed. For use in a bioartificial liver, the ability for hepatocytes to perform ex vivo at in vivo levels is critical. Three dimensional culture improves cell performance over monolayer cultures. Alginate encapsulated cells offer a suitable 3-D environment for a bioartificial liver since they are both easily manipulatable and cryopreservable. The use of cells derived from stem cells or foetal rather than adult liver cells is also emerging as a potential human cell source which may overcome problems associated with xenogeneic cells.     

Whole-organ re-engineering: a regenerative medicine approach in digestive surgery for organ replacement

Recovery from end-stage organ failure presents a challenge for the medical community, considering the limitations of extracorporeal assist devices and the shortage of donors when organ replacement is needed. There is a need for new methods to promote recovery from organ failure and regenerative medicine is an option that should be considered. Recent progress in the field of tissue engineering has opened avenues for potential clinical applications, including the use of microfluidic devices for diagnostic purposes, and bioreactors or cell/tissue-based therapies for transplantation. Early attempts to engineer tissues produced thin, planar constructs; however, recent approaches using synthetic scaffolds and decellularized tissue have achieved a more complex level of tissue organization in organs such as the urinary bladder and trachea, with some success in clinical trials. In this context, the concept of decellularization technology has been applied to produce whole organ-derived scaffolds by removing cellular content while retaining all the necessary vascular and structural cues of the native organ. In this review, we focus on organ decellularization as a new regenerative medicine approach for whole organs, which may be applied in the field of digestive surgery.     

The role of exocrine tissue in pancreatic islet transplantation

Isolated pancreatic islet transplantation has been proposed as a possible way of treating diabetes, but despite extensive experimental research, successful clinical transplantation remains elusive. A major problem has been the isolation of sufficient viable islet tissue for transplantation, especially from the human pancreas. It is possible to improve the yield of islet tissue by omitting purification steps, and unpurified dispersed pancreas has been successfully transplanted experimentally. However, attempts to apply the same technique clinically have been unsuccessful and have produced unacceptable complications. There is evidence that exocrine contamination may impair the implantation of islet tissue when transplanted to restricted sites, such as the kidney capsule. Yet, complete purification of islet tissue is probably not necessary for safe transplantation with adequate implantation of tissue in sites such as the spleen or liver. Exocrine tissue may be more immunogenic than islet tissue, and complete purification may have advantages for the prevention of rejection.     

The osteochondral dilemma: review of current management and future trends

The management of articular cartilage defects remains challenging and controversial. Hyaline cartilage has limited capacity for self‐repair and post‐injury cartilage is predominantly replaced by fibrocartilage through healing from the subchondral bone. Fibrocartilage lacks the key properties that characterize hyaline cartilage such as capacity for compression, hydrodynamic permeability and smoothness of the articular surface. Many reports relate compromised function associated with repaired cartilage and loss of function of the articular surface. Novel methods have been proposed with the key aim to regenerate hyaline cartilage for repair of osteochondral defects. Over the past decade, with many exciting developments in tissue engineering and regenerative cell‐based technologies, we are now able to consider new combinatorial approaches to overcome the problems associated with osteochondral injuries and damage. In this review, the currently accepted surgical approaches are reviewed and considered; debridement, marrow stimulation, whole tissue transplantation and cellular repair. More recent products, which employ tissue engineering approaches to enhance the traditional methods of repair, are discussed. Future trends must not only focus on recreating the composition of articular cartilage, but more importantly recapitulate the nano‐structure of articular cartilage to improve the functional strength and integration of repair tissue.