Organ bioengineering and regeneration as the new Holy Grail for organ transplantation. Ann Surg. 2013;258:221-232. [PMID: 23782908 DOI: 10.1097/sla.0b013e31829c79cf]

Towards Safety and Regulation Criteria for Clinical Applications of Decellularized Organ-Derived Matrices

Whole-organ decellularization generates scaffolds containing native extracellular matrix (ECM) components with preserved tissue microarchitecture, providing a promising advancement in tissue engineering and regenerative medicine. Decellularization retains the ECM integrity which is important for supporting cell attachment, growth, differentiation, and biological function. Although there are consensus guidelines to standardize decellularization processes and ECM characterization, no specific criteria or standards regarding matrix sterility and biosafety have been established so far. This regulatory gap in safety, sterilization, and regulation criteria has hampered the clinical translation of decellularized scaffolds. In this review, we identify essential criteria for the safe clinical use of decellularized products from both human and animal sources. These include the decellularization efficacy, levels of chemical residue, preservation of ECM composition and physical characteristics, and criteria for the aseptic processing of decellularization to assure sterility. Furthermore, we explore key considerations for advancing decellularized scaffolds into clinical practice, focusing on regulatory frameworks and safety requirements. Addressing these challenges is crucial for minimizing risks of adverse reactions or infection transmission, thereby accelerating the adoption of tissue-engineered products. This review aims to provide a foundation for establishing robust guidelines, supporting the safe and effective integration of decellularized scaffolds into regenerative medicine applications.

Canonical Wnt signaling directs the generation of functional human PSC-derived atrioventricular canal cardiomyocytes in bioprinted cardiac tissues

The creation of a functional 3D bioprinted human heart remains challenging, largely due to the lack of some crucial cardiac cell types, including the atrioventricular canal (AVC) cardiomyocytes, which are essential to slow down the electrical impulse between the atrium and ventricle. By utilizing single-cell RNA sequencing analysis and a 3D bioprinting technology, we discover that stage-specific activation of canonical Wnt signaling creates functional AVC cardiomyocytes derived from human pluripotent stem cells. These cardiomyocytes display morphological characteristics and express molecular markers of AVC cardiomyocytes, including transcription factors TBX2 and MSX2. When bioprinted in prefabricated cardiac tissues, these cardiomyocytes successfully delay the electrical impulse, demonstrating their capability of functioning as the AVC cardiomyocytes in vitro. Thus, these findings not only identify canonical Wnt signaling as a key regulator of the AVC cardiomyocyte differentiation in vitro, but, more importantly, provide a critical cellular source for the biofabrication of a functional human heart.

Unlocking the Promise of Decellularized Pancreatic Tissue: A Novel Approach to Support Angiogenesis in Engineered Tissue

Advancements in regenerative medicine have highlighted the potential of decellularized extracellular matrix (ECM) as a scaffold for organ bioengineering. Although the potential of ECM in major organ systems is well-recognized, studies focusing on the angiogenic effects of pancreatic ECM are limited. This study investigates the capabilities of pancreatic ECM, particularly its role in promoting angiogenesis. Using a Triton-X-100 solution, porcine pancreas was successfully decellularized, resulting in a significant reduction in DNA content (97.1% removal) while preserving key pancreatic ECM components. A three-dimensional ECM hydrogel was then created from this decellularized tissue and used for cell culture. Biocompatibility tests demonstrated enhanced adhesion and proliferation of mouse embryonic stem cell-derived endothelial cells (mES-ECs) and human umbilical vein endothelial cells (HUVECs) in this hydrogel compared to conventional scaffolds. The angiogenic potential was evaluated through tube formation assays, wherein the cells showed superior tube formation capabilities in ECM hydrogel compared to rat tail collagen. The RT-PCR analysis further confirmed the upregulation of pro-angiogenic genes in HUVECs cultured within the ECM hydrogel. Specifically, HUVECs cultured in the ECM hydrogel exhibited a significant upregulation in the expression of MMP2, VEGF and PAR-1, compared to those cultured in collagen hydrogel or in a monolayer condition. The identification of ECM proteins, specifically PRSS2 and Decorin, further supports the efficacy of pancreatic ECM hydrogel as an angiogenic scaffold. These findings highlight the therapeutic promise of pancreatic ECM hydrogel as a candidate for vascularized tissue engineering application.

Plant-derived nanovesicles: harnessing nature's power for tissue protection and repair

Tissue damage and aging lead to dysfunction, disfigurement, and trauma, posing significant global challenges. Creating a regenerative microenvironment to resist external stimuli and induce stem cell differentiation is essential. Plant-derived nanovesicles (PDNVs) are naturally bioactive lipid bilayer nanovesicles that contain proteins, lipids, ribonucleic acid, and metabolites. They have shown potential in promoting cell growth, migration, and differentiation into various types of tissues. With immunomodulatory, microbiota regulatory, antioxidant, and anti-aging bioactivities, PDNVs are valuable in resisting external stimuli and facilitating tissue repair. The unique structure of PDNVs provides an optimal platform for drug encapsulation, and surface modifications enhance their stability and specificity. Moreover, by employing synergistic administration strategies, PDNVs can maximize their therapeutic potential. This review summarized the progress and prospects of PDNVs as regenerative tools, provided insights into their selection for repair activities based on existing studies, considered the key challenge for clinical application, and anticipated their continued prominent role in the field of biomedicine.

Cardiac regeneration - Past advancements, current challenges, and future directions

Cardiovascular disease is the leading cause of mortality and morbidity worldwide. Despite improvements in the standard of care for patients with heart diseases, including innovation in pharmacotherapy and surgical interventions, none have yet been proven effective to prevent the progression to heart failure. Cardiac transplantation is the last resort for patients with severe heart failure, but donor shortages remain a roadblock. Cardiac regenerative strategies include cell-based therapeutics, gene therapy, direct reprogramming of non-cardiac cells, acellular biologics, and tissue engineering methods to restore damaged hearts. Significant advancements have been made over the past several decades within each of these fields. This review focuses on the advancements of: 1) cell-based cardiac regenerative therapies, 2) the use of noncoding RNA to induce endogenous cell proliferation, and 3) application of bioengineering methods to promote retention and integration of engrafted cells. Different cell sources have been investigated, including adult stem cells derived from bone marrow and adipose cells, cardiosphere-derived cells, skeletal myoblasts, and pluripotent stem cells. In addition to cell-based transplantation approaches, there have been accumulating interest over the past decade in inducing endogenous CM proliferation for heart regeneration, particularly with the use of noncoding RNAs such as miRNAs and lncRNAs. Bioengineering applications have focused on combining cell-transplantation approaches with fabrication of a porous, vascularized scaffold using biomaterials and advanced bio-fabrication techniques that may offer enhanced retention of transplanted cells, with the hope that these cells would better engraft with host tissue to improve cardiac function. This review summarizes the present status and future challenges of cardiac regenerative therapies.

Decellularized vascularized bone grafts: A preliminary in vitro porcine model for bioengineered transplantable bone shafts

Introduction: Durable reconstruction of critical size bone defects is still a surgical challenge despite the availability of numerous autologous and substitute bone options. In this paper, we have investigated the possibility of creating a living bone allograft, using the perfusion/decellularization/recellularization (PDR) technique, which was applied to an original model of vascularized porcine bone graft. Materials and Methods: 11 porcine bone forelimbs, including radius and ulna, were harvested along with their vasculature including the interosseous artery and then decellularized using a sequential detergent perfusion protocol. Cellular clearance, vasculature, extracellular matrix (ECM), and preservation of biomechanical properties were evaluated. The cytocompatibility and in vitro osteoinductive potential of acellular extracellular matrix were studied by static seeding of NIH-3T3 cells and porcine adipose mesenchymal stem cells (pAMSC), respectively. Results: The vascularized bone grafts were successfully decellularized, with an excellent preservation of the 3D morphology and ECM microarchitecture. Measurements of DNA and ECM components revealed complete cellular clearance and preservation of ECM's major proteins. Bone mineral density (BMD) acquisitions revealed a slight, yet non-significant, decrease after decellularization, while biomechanical testing was unmodified. Cone beam computed tomography (CBCT) acquisitions after vascular injection of barium sulphate confirmed the preservation of the vascular network throughout the whole graft. The non-toxicity of the scaffold was proven by the very low amount of residual sodium dodecyl sulfate (SDS) in the ECM and confirmed by the high live/dead ratio of fibroblasts seeded on periosteum and bone ECM-grafts after 3, 7, and 16 days of culture. Moreover, cell proliferation tests showed a significant multiplication of seeded cell populations at the same endpoints. Lastly, the differentiation study using pAMSC confirmed the ECM graft's potential to promote osteogenic differentiation. An osteoid-like deposition occurred when pAMSC were cultured on bone ECM in both proliferative and osteogenic differentiation media. Conclusion: Fully decellularized bone grafts can be obtained by perfusion decellularization, thereby preserving ECM architecture and their vascular network, while promoting cell growth and differentiation. These vascularized decellularized bone shaft allografts thus present a true potential for future in vivo reimplantation. Therefore, they may offer new perspectives for repairing large bone defects and for bone tissue engineering.

Regenerative medicine technologies applied to transplant medicine. An update

Regenerative medicine (RM) is changing how we think and practice transplant medicine. In regenerative medicine, the aim is to develop and employ methods to regenerate, restore or replace damaged/diseased tissues or organs. Regenerative medicine investigates using tools such as novel technologies or techniques, extracellular vesicles, cell-based therapies, and tissue-engineered constructs to design effective patient-specific treatments. This review illustrates current advancements in regenerative medicine that may pertain to transplant medicine. We highlight progress made and various tools designed and employed specifically for each tissue or organ, such as the kidney, heart, liver, lung, vasculature, gastrointestinal tract, and pancreas. By combing both fields of transplant and regenerative medicine, we can harbor a successful collaboration that would be beneficial and efficacious for the repair and design of de novo engineered whole organs for transplantations.

Progress on methods of T lymphocyte development in vitro

T lymphocytes (T cells) play an important role in adoptive cellular immunotherapy (ACT). T cells can be stably derived and easily obtained by various methods of T cell development in vitro, which have more advantages than traditional methods of T cells isolated from autologous or allogeneic tissues. At present, there are mainly three methods for T cell development in vitro: fetal thymus organ culture, recombinant thymus organ culture and two-dimensional culture driven by Notch signal. Fetal thymus organ culture is easy to operate, the isolated thymus can support T cell differentiation and development to maturity in vitro, but the intact thymus has problems of limited maintenance time and difficulty in cell harvesting. In recombinant thymic organ culture, various thymic stromal cells are dispersed and recombined to construct a three-dimensional culture environment, which can support T cell maturation in vitro and in vivo; however, biomaterials and three-dimensional environment may lead to limited culture maintenance time and cell yield. Two-dimensional culture method uses artificial presentation of Notch signaling pathway ligands to drive T cell differentiation and development; the culture architecture is simple and stable, but it can only support T cell development to the early immature stage. This article reviews the research progress of various culture methods of T cell development in vitro, and discusses the existing problems and the future development to facilitate the application of ACT.

Bioengineering approaches to treat the failing heart: from cell biology to 3D printing

Successfully engineering a functional, human, myocardial pump would represent a therapeutic alternative for the millions of patients with end-stage heart disease and provide an alternative to animal-based preclinical models. Although the field of cardiac tissue engineering has made tremendous advances, major challenges remain, which, if properly resolved, might allow the clinical implementation of engineered, functional, complex 3D structures in the future. In this Review, we provide an overview of state-of-the-art studies, challenges that have not yet been overcome and perspectives on cardiac tissue engineering. We begin with the most clinically relevant cell sources used in this field and discuss the use of topological, biophysical and metabolic stimuli to obtain mature phenotypes of cardiomyocytes, particularly in relation to organized cytoskeletal and contractile intracellular structures. We then move from the cellular level to engineering planar cardiac patches and discuss the need for proper vascularization and the main strategies for obtaining it. Finally, we provide an overview of several different approaches for the engineering of volumetric organs and organ parts - from whole-heart decellularization and recellularization to advanced 3D printing technologies.

Genitourinary Tissue Engineering: Reconstruction and Research Models

Tissue engineering is an emerging field of research that initially aimed to produce 3D tissues to bypass the lack of adequate tissues for the repair or replacement of deficient organs. The basis of tissue engineering protocols is to create scaffolds, which can have a synthetic or natural origin, seeded or not with cells. At the same time, more and more studies have indicated the low clinic translation rate of research realised using standard cell culture conditions, i.e., cells on plastic surfaces or using animal models that are too different from humans. New models are needed to mimic the 3D organisation of tissue and the cells themselves and the interaction between cells and the extracellular matrix. In this regard, urology and gynaecology fields are of particular interest. The urethra and vagina can be sites suffering from many pathologies without currently adequate treatment options. Due to the specific organisation of the human urethral/bladder and vaginal epithelium, current research models remain poorly representative. In this review, the anatomy, the current pathologies, and the treatments will be described before focusing on producing tissues and research models using tissue engineering. An emphasis is made on the self-assembly approach, which allows tissue production without the need for biomaterials.

Bioengineering of the digestive tract: approaching the clinic

The field of regenerative medicine is developing technologies that, in the near future, will offer alternative approaches to either cure diseases affecting the gastrointestinal tract or slow their progression by leveraging the intrinsic ability of our tissues and organs to repair after damage. This article will succinctly illustrate the three technologies that are closer to clinical translation-namely, human intestinal organoids, sphincter bioengineering and decellularization, whereby the cellular compartment of a given segment of the digestive tract is removed to obtain a scaffold consisting of the extracellular matrix. The latter will be used as a template for the regeneration of a functional organ, whereby the newly generated cellular compartment will be obtained from the patient's own cells. Although clinical application of this technology is approaching, product development challenges are being tackled to warrant safety and efficacy.

Orthotopic Transplantation of Human Bioartificial Lung Grafts in a Porcine Model: A Feasibility Study

Lung transplantation is the only treatment for end-stage lung disease; however, donor organ shortage and intense immunosuppression limit its broad clinical impact. Bioengineering of lungs with patient-derived cells could overcome these problems. We created bioartificial lungs by seeding human-derived cells onto porcine lung matrices and performed orthotopic transplantation to assess feasibility and in vivo function. Porcine decellularized lung scaffolds were seeded with human airway epithelial cells and human umbilical vein endothelial cells. Following in vitro culture, the bioartificial lungs were orthotopically transplanted into porcine recipients with planned 1-day survival (n = 3). Lungs were assessed with histology and in vivo function. Orthotopic transplantation of cadaveric lungs was performed as control. Engraftment of endothelial and epithelial cells in the grafts were histologically demonstrated. Technically successful orthotopic anastomoses of the vasculatures and airway were achieved in all animals. Perfusion and ventilation of the lung grafts were confirmed intraoperatively. The gas exchange function was evident immediately after transplantation; PO₂ gradient between pulmonary artery and vein were 178 ± 153 mm Hg in the bioartificial lung group and 183 ± 117 mm Hg in the control group. At time of evaluation 24 hours after reperfusion, the pulmonary arteries were found to be occluded with thrombus in all bioartificial lungs. Engineering and orthotopic transplantation of bioartificial lungs with human cells were technically feasible in a porcine model. Early gas exchange function was evident. Further progress in optimizing recellularization and maturation of the grafts will be necessary for sustained perfusability and function.

Comprehensive characterization of the human pancreatic proteome for bioengineering applications

Interactions between the pancreatic extracellular matrix (ECM) and islet cells are known to regulate multiple aspects of islet physiology, including survival, proliferation, and glucose-stimulated insulin secretion. Recognizing the essential role of ECM in islet survival and function, various engineering approaches have been developed that aim to utilize ECM-based materials to recreate a native-like microenvironment. However, a major impediment to the success of these approaches has been the lack of a robust and comprehensive characterization of the human pancreatic proteome. Herein, by combining mass spectrometry (MS) and multiplex ELISA, we have provided an improved workflow for the in-depth profiling of the proteome, including minor constituents that are generally underrepresented. Moreover, we have further validated the effectiveness of our detergent-free decellularization protocol in the removal of cellular proteins and retention of the matrisome. It has also been established that the decellularized ECM and its derivatives can provide more tissue-specific cues than traditionally used biological scaffolds and are therefore more physiologically relevant for the development of hydrogels, bioinks and medium additives, in order to create a pancreatic niche. The data generated in this study would contribute significantly to the efforts of comprehensively defining the ECM atlas and also serve as a standard for the human pancreatic proteome to provide further guidance for design and engineering strategies for improved tissue engineering scaffolds.

Information-Driven Design as a Potential Approach for 3D Printing of Skeletal Muscle Biomimetic Scaffolds

Severe muscle injuries are a real clinical issue that still needs to be successfully addressed. Tissue engineering can represent a potential approach for this aim, but effective healing solutions have not been developed yet. In this regard, novel experimental protocols tailored to a biomimetic approach can thus be defined by properly systematizing the findings acquired so far in the biomaterials and scaffold manufacturing fields. In order to plan a more comprehensive strategy, the extracellular matrix (ECM), with its properties stimulating neomyogenesis and vascularization, should be considered as a valuable biomaterial to be used to fabricate the tissue-specific three-dimensional structure of interest. The skeletal muscle decellularized ECM can be processed and printed, e.g., by means of stereolithography, to prepare bioactive and biomimetic 3D scaffolds, including both biochemical and topographical features specifically oriented to skeletal muscle regenerative applications. This paper aims to focus on the skeletal muscle tissue engineering sector, suggesting a possible approach to develop instructive scaffolds for a guided healing process.

Regenerative medicine, organ bioengineering and transplantation

Background

Organ transplantation is predicted to increase as life expectancy and the incidence of chronic diseases rises. Regenerative medicine-inspired technologies challenge the efficacy of the current allograft transplantation model.

Methods

A literature review was conducted using the PubMed interface of MEDLINE from the National Library of Medicine. Results were examined for relevance to innovations of organ bioengineering to inform analysis of advances in regenerative medicine affecting organ transplantation. Data reports from the Scientific Registry of Transplant Recipient and Organ Procurement Transplantation Network from 2008 to 2019 of kidney, pancreas, liver, heart, lung and intestine transplants performed, and patients currently on waiting lists for respective organs, were reviewed to demonstrate the shortage and need for transplantable organs.

Results

Regenerative medicine technologies aim to repair and regenerate poorly functioning organs. One goal is to achieve an immunosuppression-free state to improve quality of life, reduce complications and toxicities, and eliminate the cost of lifelong antirejection therapy. Innovative strategies include decellularization to fabricate acellular scaffolds that will be used as a template for organ manufacturing, three-dimensional printing and interspecies blastocyst complementation. Induced pluripotent stem cells are an innovation in stem cell technology which mitigate both the ethical concerns associated with embryonic stem cells and the limitation of other progenitor cells, which lack pluripotency. Regenerative medicine technologies hold promise in a wide array of fields and applications, such as promoting regeneration of native cell lines, growth of new tissue or organs, modelling of disease states, and augmenting the viability of existing ex vivo transplanted organs.

Conclusion

The future of organ bioengineering relies on furthering understanding of organogenesis, in vivo regeneration, regenerative immunology and long-term monitoring of implanted bioengineered organs.

Successful muscle regeneration by a homologous microperforated scaffold seeded with autologous mesenchymal stromal cells in a porcine esophageal substitution model

BACKGROUND

Since the esophagus has no redundancy, congenital and acquired esophageal diseases often require esophageal substitution, with complicated surgery and intestinal or gastric transposition. Peri-and-post-operative complications are frequent, with major problems related to the food transit and reflux. During the last years tissue engineering products became an interesting therapeutic alternative for esophageal replacement, since they could mimic the organ structure and potentially help to restore the native functions and physiology. The use of acellular matrices pre-seeded with cells showed promising results for esophageal replacement approaches, but cell homing and adhesion to the scaffold remain an important issue and were investigated.

METHODS

A porcine esophageal substitute constituted of a decellularized scaffold seeded with autologous bone marrow-derived mesenchymal stromal cells (BM-MSCs) was developed. In order to improve cell seeding and distribution throughout the scaffolds, they were micro-perforated by Quantum Molecular Resonance (QMR) technology (Telea Electronic Engineering).

RESULTS

The treatment created a microporous network and cells were able to colonize both outer and inner layers of the scaffolds. Non seeded (NSS) and BM-MSCs seeded scaffolds (SS) were implanted on the thoracic esophagus of 4 and 8 pigs respectively, substituting only the muscle layer in a mucosal sparing technique. After 3 months from surgery, we observed an esophageal substenosis in 2/4 NSS pigs and in 6/8 SS pigs and a non-practicable stricture in 1/4 NSS pigs and 2/8 SS pigs. All the animals exhibited a normal weight increase, except one case in the SS group. Actin and desmin staining of the post-implant scaffolds evidenced the regeneration of a muscular layer from one anastomosis to another in the SS group but not in the NSS one.

CONCLUSIONS

A muscle esophageal substitute starting from a porcine scaffold was developed and it was fully repopulated by BM-MSCs after seeding. The substitute was able to recapitulate in shape and function the original esophageal muscle layer.

Molecular Pathways Underlying Adaptive Repair of the Injured Kidney: Novel Donation After Cardiac Death and Acute Kidney Injury Platforms

OBJECTIVE

To test the hypothesis that gene expression profiling in peripheral blood from patients who have undergone kidney transplantation (KT) will provide mechanistic insights regarding graft repair and regeneration.

BACKGROUND

Renal grafts obtained from living donors (LD) typically function immediately, whereas organs from donation after cardiac death (DCD) or acute kidney injury (AKI) donors may experience delayed function with eventual recovery. Thus, recipients of LD, DCD, and AKI kidneys were studied to provide a more complete understanding of the molecular basis for renal recovery.

METHODS

Peripheral blood was collected from LD and DCD/AKI recipients before transplant and throughout the first 30 days thereafter. Total RNA was isolated and assayed on whole genome microarrays.

RESULTS

Comparison of longitudinal gene expression between LD and AKI/DCD revealed 2 clusters, representing 141 differentially expressed transcripts. A subset of 11 transcripts was found to be differentially expressed in AKI/DCD versus LD. In all recipients, the most robust gene expression changes were observed in the first day after transplantation. After day 1, gene expression profiles differed depending upon the source of the graft. In patients receiving LD grafts, the expression of most genes did not remain markedly elevated beyond the first day post-KT. In the AKI/DCD groups, elevations in gene expression were maintained for at least 5 days post-KT. In all recipients, the pattern of coordinate gene overexpression subsided by 28 to 30 days.

CONCLUSIONS

Gene expression in peripheral blood of AKI/DCD recipients offers a novel platform to understand the potential mechanisms and timing of kidney repair and regeneration after transplantation.

The promise of regenerative medicine in the treatment of urogenital disorders

Polymers and scaffolds are the most significant tools in regenerative medicine. Urogenital disorders are an important group of diseases that greatly affect the patient's life expectancy and quality. Reconstruction of urogenital defects is one of the current challenges in regenerative medicine. Regenerative medicine, as well as tissue engineering, may offer suitable approaches, while the tools needed are appropriate materials and cells. Autologous urothelial cells obtained from biopsy, bone marrow-derived stem cells, adipose stem cells and urine-derived stem cells that expressed mesenchymal cell markers are the cells that mainly used. In addition, two main types of biomaterials mainly exist; synthetic polymers and composite scaffolds that are biodegradable polymers with controllable properties and naturally derived biomaterials such as extracellular matrix components and acellular tissue matrices. In this review, we present and evaluate the most appropriate and suitable scaffolds (naturally derived and synthetic polymers) and cells applied in urogenital reconstruction.

A Hepatic Scaffold from Decellularized Liver Tissue: Food for Thought

Allogeneic liver transplantation is still deemed the gold standard solution for end-stage organ failure; however, donor organ shortages have led to extended waiting lists for organ transplants. In order to overcome the lack of donors, the development of new therapeutic options is mandatory. In the last several years, organ bioengineering has been extensively explored to provide transplantable tissues or whole organs with the final goal of creating a three-dimensional growth microenvironment mimicking the native structure. It has been frequently reported that an extracellular matrix-based scaffold offers a structural support and important biological molecules that could help cellular proliferation during the recellularization process. The aim of the present review is to underline the recent developments in cell-on-scaffold technology for liver bioengineering, taking into account: (1) biological and synthetic scaffolds; (2) animal and human tissue decellularization; (3) scaffold recellularization; (4) 3D bioprinting; and (5) organoid technology. Future possible clinical applications in regenerative medicine for liver tissue engineering and for drug testing were underlined and dissected.

Turning regenerative technologies into treatment to repair myocardial injuries

Regenerative therapies including stem cell treatments hold promise to allow curing patients affected by severe cardiac muscle diseases. However, the clinical efficacy of stem cell therapy remains elusive, so far. The two key roadblocks that still need to be overcome are the poor cell engraftment into the injured myocardium and the limited knowledge of the ideal mixture of bioactive factors to be locally delivered for restoring heart function. Thus, therapeutic strategies for cardiac repair are directed to increase the retention and functional integration of transplanted cells in the damaged myocardium or to enhance the endogenous repair mechanisms through cell‐free therapies. In this context, biomaterial‐based technologies and tissue engineering approaches have the potential to dramatically impact cardiac translational medicine. This review intends to offer some consideration on the cell‐based and cell‐free cardiac therapies, their limitations and the possible future developments.

Regenerative medicine: the red planet for clinicians

Regenerative medicine represents the forefront of health sciences and holds promises for the treatment and, possibly, the cure of a number of challenging conditions. It relies on the use of stem cells, tissue engineering, and gene therapy alone or in different combinations. The goal is to deliver cells, tissues, or organs to repair, regenerate, or replace the damaged ones. Among stem-cell populations, both haematopoietic and mesenchymal stem cells have been employed in the treatment of refractory chronic inflammatory diseases with promising results. However, only mesenchymal stem cells seem advantageous as both systemic and local injections may be performed without the need for immune ablation. Recently, also induced pluripotent stem cells have been exploited for therapeutic purposes given their tremendous potential to be an unlimited source of any tissue-specific cells. Moreover, through the development of technologies that make organ fabrication possible using cells and supporting scaffolding materials, regenerative medicine promises to enable organ-on-demand, whereby patients will receive organs in a timely fashion without the risk of rejection. Finally, gene therapy is emerging as a successful strategy not only in monogenic diseases, but also in multifactorial conditions. Several of these approaches have recently received approval for commercialization, thus opening a new therapeutic era. This is why both General Practitioners and Internists should be aware of these great advancements.

Decellularization of the Porcine Ear Generates a Biocompatible, Nonimmunogenic Extracellular Matrix Platform for Face Subunit Bioengineering

OBJECTIVE

The purpose of this study was to assess whether perfusion-decellularization technology could be applied to facial grafts.

BACKGROUND

Facial allotransplantation remains an experimental procedure. Regenerative medicine techniques allow fabrication of transplantable organs from an individual's own cells, which are seeded into extracellular matrix (ECM) scaffolds from animal or human organs. Therefore, we hypothesized that ECM scaffolds also can be created from facial subunits. We explored the use of the porcine ear as a clinically relevant face subunit model to develop regenerative medicine-related platforms for facial bioengineering.

METHODS

Porcine ear grafts were decellularized and histologic, immunologic, and cell culture studies done to determine whether scaffolds retained their 3D framework and molecular content; were biocompatible in vitro and in vivo, and triggered an anti-MHC immune response from the host.

RESULTS

The cellular compartment of the porcine ear was completely removed except for a few cartilaginous cells, leaving behind an acellular ECM scaffold; this scaffold retained its complex 3D architecture and biochemical components. The framework of the vascular tree was intact at all hierarchical levels and sustained a physiologically relevant blood pressure when implanted in vivo. Scaffolds were biocompatible in vitro and in vivo, and elicited no MHC immune response from the host. Cells from different types remained viable and could even differentiate at the scale of a whole-ear scaffold.

CONCLUSIONS

Acellular scaffolds were produced from the porcine ear, and may be a valuable platform to treat facial deformities using regenerative medicine approaches.

Immune responses towards bioengineered tissues and strategies to control them

PURPOSE OF REVIEW

Research into development of artificial tissues and bioengineered organs to replace physiological functions of injured counterparts has highlighted a previously underestimated challenge for its clinical translatability: the immune response against biomaterials. Herein, we will provide an update and review current knowledge regarding this important barrier to regenerative medicine.

RECENT FINDINGS

Although a clear understanding of the immune reactivity against biomaterials remains elusive, accumulating evidence indicates that innate immune cells, primarily neutrophils and macrophages, play a key role in the initial phases of the immune response. More recently, data have shown that in later phases, T and B cells are also involved. The use of physicochemical modifications of biomaterials and cell-based strategies to modulate the host inflammatory response is being actively investigated for effective biomaterial integration.

SUMMARY

The immune response towards biomaterials and bioengineered organs plays a crucial role in determining their utility as transplantable grafts. Expanding our understanding of these responses is necessary for developing protolerogenic strategies and delivering on the ultimate promise of regenerative medicine.

Bioengineered functional humanized livers: An emerging supportive modality to bridge the gap of organ transplantation for management of end-stage liver diseases

End stage liver diseases (ESLD) represent a major, neglected global public health crisis which requires an urgent action towards finding a proper cure. Orthotropic liver transplantation has been the only definitive treatment modality for ESLD. However, shortage of donor organs, timely unavailability, post-surgery related complications and financial burden on the patients limits the number of patients receiving the transplants. Since last two decades cell-based therapies have revolutionized the field of organ/tissue regeneration. However providing an alternative organ source to address the donor liver shortage still poses potential challenges. The developments made in this direction provide useful futuristic approaches, which could be translated into pre-clinical and clinical settings targeting appropriate applications in specific disease conditions. Earlier studies have demonstrated the applicability of this particular approach to generate functional organ in rodent system by connecting them with portal and hepatic circulatory networks. However, such strategy requires very high level of surgical expertise and also poses the technical and financial questions towards its future applicability. Hence, alternative sites for generating secondary organs are being tested in several types of disease conditions. Among different sites, omentum has been proved to be more appropriate site for implanting several kinds of functional tissue constructs without eliciting much immunological response. Hence, omentum may be considered as better site for transplanting humanized bioengineered ex vivo generated livers, thereby creating a secondary organ at intra-omental site. However, the expertise for generating such bioengineered organs are limited and only very few centres are involved for investigating the potential use of such implants in clinical practice due to gap between the clinical transplant surgeons and basic scientists working on the concept evolution. Herein we discuss the recent advances and challenges to create functional secondary organs through intra-omental transplantation of ex vivo generated bioengineered humanized livers and their further application in the management of ESLD as a supportive bridge for organ transplantation.

Regenerative Medicine and Diabetes: Targeting the Extracellular Matrix Beyond the Stem Cell Approach and Encapsulation Technology

According to the Juvenile Diabetes Research Foundation (JDRF), almost 1. 25 million people in the United States (US) have type 1 diabetes, which makes them dependent on insulin injections. Nationwide, type 2 diabetes rates have nearly doubled in the past 20 years resulting in more than 29 million American adults with diabetes and another 86 million in a pre-diabetic state. The International Diabetes Ferderation (IDF) has estimated that there will be almost 650 million adult diabetic patients worldwide at the end of the next 20 years (excluding patients over the age of 80). At this time, pancreas transplantation is the only available cure for selected patients, but it is offered only to a small percentage of them due to organ shortage and the risks linked to immunosuppressive regimes. Currently, exogenous insulin therapy is still considered to be the gold standard when managing diabetes, though stem cell biology is recognized as one of the most promising strategies for restoring endocrine pancreatic function. However, many issues remain to be solved, and there are currently no recognized treatments for diabetes based on stem cells. In addition to stem cell resesarch, several β-cell substitutive therapies have been explored in the recent era, including the use of acellular extracellular matrix scaffolding as a template for cellular seeding, thus providing an empty template to be repopulated with β-cells. Although this bioengineering approach still has to overcome important hurdles in regards to clinical application (including the origin of insulin producing cells as well as immune-related limitations), it could theoretically provide an inexhaustible source of bio-engineered pancreases.

Report of the Key Opinion Leaders Meeting on Stem Cell-derived Beta Cells

Beta cell replacement has the potential to restore euglycemia in patients with insulin-dependent diabetes. Although great progress has been made in establishing allogeneic islet transplantation from deceased donors as the standard of care for those with the most labile diabetes, it is also clear that the deceased donor organ supply cannot possibly treat all those who could benefit from restoration of a normal beta cell mass, especially if immunosuppression were not required. Against this background, the International Pancreas and Islet Transplant Association in collaboration with the Harvard Stem Cell Institute, the Juvenile Diabetes Research Foundation (JDRF), and the Helmsley Foundation held a 2-day Key Opinion Leaders Meeting in Boston in 2016 to bring together experts in generating and transplanting beta cells derived from stem cells. The following summary highlights current technology, recent significant breakthroughs, unmet needs and roadblocks to stem cell-derived beta cell therapies, with the aim of spurring future preclinical collaborative investigations and progress toward the clinical application of stem cell-derived beta cells.

Avian lungs: A novel scaffold for lung bioengineering

Allogeneic lung transplant is limited both by the shortage of available donor lungs and by the lack of suitable long-term lung assist devices to bridge patients to lung transplantation. Avian lungs have different structure and mechanics resulting in more efficient gas exchange than mammalian lungs. Decellularized avian lungs, recellularized with human lung cells, could therefore provide a powerful novel gas exchange unit for potential use in pulmonary therapeutics. To initially assess this in both small and large avian lung models, chicken (Gallus gallus domesticus) and emu (Dromaius novaehollandiae) lungs were decellularized using modifications of a detergent-based protocol, previously utilized with mammalian lungs. Light and electron microscopy, vascular and airway resistance, quantitation and gel analyses of residual DNA, and immunohistochemical and mass spectrometric analyses of remaining extracellular matrix (ECM) proteins demonstrated maintenance of lung structure, minimal residual DNA, and retention of major ECM proteins in the decellularized scaffolds. Seeding with human bronchial epithelial cells, human pulmonary vascular endothelial cells, human mesenchymal stromal cells, and human lung fibroblasts demonstrated initial cell attachment on decellularized avian lungs and growth over a 7-day period. These initial studies demonstrate that decellularized avian lungs may be a feasible approach for generating functional lung tissue for clinical therapeutics.

Perfusion-decellularization of human ear grafts enables ECM-based scaffolds for auricular vascularized composite tissue engineering

INTRODUCTION

Human ear reconstruction is recognized as the emblematic enterprise in tissue engineering. Up to now, it has failed to reach human applications requiring appropriate tissue complexity along with an accessible vascular tree. We hereby propose a new method to process human auricles in order to provide a poorly immunogenic, complex and vascularized ear graft scaffold.

METHODS

12 human ears with their vascular pedicles were procured. Perfusion-decellularization was applied using a SDS/polar solvent protocol. Cell and antigen removal was examined by histology and DNA was quantified. Preservation of the extracellular matrix (ECM) was assessed by conventional and 3D-histology, proteins and cytokines quantifications. Biocompatibility was assessed by implantation in rats for up to 60 days. Adipose-derived stem cells seeding was conducted on scaffold samples and with human aortic endothelial cells whole graft seeding in a perfusion-bioreactor.

RESULTS

Histology confirmed cell and antigen clearance. DNA reduction was 97.3%. ECM structure and composition were preserved. Implanted scaffolds were tolerated in vivo, with acceptable inflammation, remodeling, and anti-donor antibody formation. Seeding experiments demonstrated cell engraftment and viability.

CONCLUSIONS

Vascularized and complex auricular scaffolds can be obtained from human source to provide a platform for further functional auricular tissue engineered constructs, hence providing an ideal road to the vascularized composite tissue engineering approach.

STATEMENT OF SIGNIFICANCE

The ear is emblematic in the biofabrication of tissues and organs. Current regenerative medicine strategies, with matrix from donor tissues or 3D-printed, didn't reach any application for reconstruction, because critically missing a vascular tree for perfusion and transplantation. We previously described the production of vascularized and cell-compatible scaffolds, from porcine ear grafts. In this study, we ---- applied findings directly to human auricles harvested from postmortem donors, providing a perfusable matrix that retains the ear's original complexity and hosts new viable cells after seeding. This approach unlocks the ability to achieve an auricular tissue engineering approach, associated with possible clinical translation.

Proceedings of the signature series event of the international society for cellular therapy: "Advancements in cellular therapies and regenerative medicine in digestive diseases," London, United Kingdom, May 3, 2017

A summary of the First Signature Series Event, "Advancements in Cellular Therapies and Regenerative Medicine for Digestive Diseases," held on May 3, 2017, in London, United Kingdom, is presented. Twelve speakers from three continents covered major topics in the areas of cellular therapy and regenerative medicine applied to liver and gastrointestinal medicine as well as to diabetes mellitus. Highlights from their presentations, together with an overview of the global impact of digestive diseases and a proposal for a shared online collection and data-monitoring platform tool, are included in this proceedings. Although growing evidence demonstrate the feasibility and safety of exploiting cell-based technologies for the treatment of digestive diseases, regulatory and methodological obstacles will need to be overcome before the successful implementation in the clinic of these novel attractive therapeutic strategies.

Bioengineering a Human Face Graft: The Matrix of Identity

OBJECTIVE

During the last decade, face allotransplantation has been shown to be a revolutionary reconstructive procedure for severe disfigurements. However, offer to patients remains limited due to lifelong immunosuppression. To move forward in the field, a new pathway in tissue engineering is proposed.

BACKGROUND

Our previously reported technique of matrix production of a porcine auricular subunit graft has been translated to a human face model.

METHODS

5 partial and 1 total face grafts were procured from human fresh cadavers. After arterial cannulation, the specimens were perfused using a combined detergent/polar solvent decellularization protocol. Preservation of vascular patency was assessed by imaging, cell and antigen removal by DNA quantification and histology. The main extracellular matrix proteins and associated cytokines were evaluated. Lip scaffolds were cultivated with dermal, muscle progenitor and endothelial cells, either on discs or in a bioreactor.

RESULTS

Decellularization was successful in all facial grafts within 12 days revealing acellular scaffolds with full preservation of innate morphology. Imaging demonstrated a preservation of the entire vascular tree patency. Removal of cells and antigens was confirmed by reduction of DNA and antigen markers negativation. Microscopic evaluation revealed preservation of tissue structures as well as of major proteins. Seeded cells were viable and well distributed within all scaffolds.

CONCLUSIONS

Complex acellular facial scaffolds were obtained, preserving simultaneously a cell-friendly extracellular matrix and a perfusable vascular tree. This step will enable further engineering of postmortem facial grafts, thereby offering new perspectives in composite tissue allotransplantation.

Regenerative immunology: the immunological reaction to biomaterials

Regenerative medicine promises to meet two of the most urgent needs of modern organ transplantation, namely immunosuppression-free transplantation and an inexhaustible source of organs. Ideally, bioengineered organs would be manufactured from a patient's own biomaterials-both cells and the supporting scaffolding materials in which cells would be embedded and allowed to mature to eventually regenerate the organ in question. While some groups are focusing on the feasibility of this approach, few are focusing on the immunogenicity of the scaffolds that are being developed for organ bioengineering purposes. This review will succinctly discuss progress in the understanding of immunological characteristics and behavior of different scaffolds currently under development, with emphasis on the extracellular matrix scaffolds obtained decellularized animal or human organs which seem to provide the ideal template for bioengineering purposes.

Xenogeneic chimera-Generated by blastocyst complementation-As a potential unlimited source of recipient-tailored organs

Blastocyst complementation refers to the injection of cells into a blastocyst. The technology allows for the creation of chimeric animals, which have the potential to be used as an unlimited source of organ donors. Pluripotent stem cells could be generated from a patient in need of a transplantation and injected into a large animal blastocyst (potentially of a pig), leading to the creation of organ(s) allowing immunosuppression-free transplantation. Various chimera combinations have already been generated, but one of the most recent steps leads to the creation of human-pig chimeras, which could be studied at an embryo stage. Although still far from clinical reality, the potential application is almost unlimited. The present review illustrates the historical steps of intra- and interspecific blastocyst complementation in rodents and large animals, specifically looking at its potential for generation of organ grafts. We also speculate on how it could change transplant indications, on its economic impact, and on the linked ethical concerns.

Tissue engineering and surgery: from translational studies to human trials

Tissue engineering was introduced as an innovative and promising field in the mid-1980s. The capacity of cells to migrate and proliferate in growth-inducing medium induced great expectancies on generating custom-shaped bioconstructs for tissue regeneration. Tissue engineering represents a unique multidisciplinary translational forum where the principles of biomaterial engineering, the molecular biology of cells and genes, and the clinical sciences of reconstruction would interact intensively through the combined efforts of scientists, engineers, and clinicians. The anticipated possibilities of cell engineering, matrix development, and growth factor therapies are extensive and would largely expand our clinical reconstructive armamentarium. Application of proangiogenic proteins may stimulate wound repair, restore avascular wound beds, or reverse hypoxia in flaps. Autologous cells procured from biopsies may generate an ‘autologous’ dermal and epidermal laminated cover on extensive burn wounds. Three-dimensional printing may generate ‘custom-made’ preshaped scaffolds – shaped as a nose, an ear, or a mandible – in which these cells can be seeded. The paucity of optimal donor tissues may be solved with off-the-shelf tissues using tissue engineering strategies. However, despite the expectations, the speed of translation of in vitro tissue engineering sciences into clinical reality is very slow due to the intrinsic complexity of human tissues. This review focuses on the transition from translational protocols towards current clinical applications of tissue engineering strategies in surgery.

Unlocking the vital role of host cells in hair follicle reconstruction by semi-permeable capsules

Organ regeneration is becoming a promising choice for many patients; however, many details about the mechanisms underlying organ regeneration remain unknown. As regenerative organs, hair follicles offer a good model to study the mechanisms associated with regenerative medicine. The relevant studies have mainly focused on donor cells, and there are no systematic studies involving the effect of host factors on hair follicle reconstruction. Thus, we intend to explore the effect of host cells on hair follicle reconstruction. Epidermal and dermal cells from red fluorescent protein (RFP) transgenic newborn mice were injected into green fluorescent protein (GFP) transgenic mice. In addition, we wrapped the mixed dermal and epidermal cells from GFP transgenic and RFP transgenic mice by the Cell-in-a-Box kit to form "capsules," so that the cells within would be isolated from host cells. These capsules were cultured in vitro and transplanted in vivo. Fully developed reconstructed hair follicles were observed after the injection of mixed cells. These reconstructed follicles mainly consisted of donor cells, as well as a small number of host cells. The encapsulated cells gradually aggregated into cell spheres in vitro without apparent differentiation towards hair follicles. With respect to the transplanted capsules, concentric circle structures were observed, but no hair follicles or hair shafts formed. When the concentric circle structures were transplanted in vivo, mature hair follicles were observed 30 days later. Host cells were found in the reconstructed hair follicles. Thus, we conclude that host cells participate in the process of hair follicle reconstruction, and they play a vital role in the process, especially for the maturation of reconstructed hair follicles. Furthermore, we established a special hair follicle reconstruction system with the help of capsules: transplant cells were isolated from host, but other factors from host could exchange with cells inside.

Reconstruction of defects of the trachea

The trachea has a complex anatomy to fulfill its tasks. Its unique fibro-cartilaginous structure maintains an open conduit during respiration, and provides vertical elasticity for deglutition, mobility of the neck and speech. Blood vessels pierce the intercartilaginous ligaments to perfuse the ciliated epithelium, which ensures effective mucociliary clearance. Removal of a tracheal segment affected by benign or malignant disease requires airtight restoration of the continuity of the tube. When direct approximation of both tracheal ends is no longer feasible, a reconstruction is needed. This may occur in recurrent short-segment defects in a scarred environment, or in defects comprising more than half the length of the trachea. The resulting gap must be filled with vascularized tissue that restores the mucosal lining and supports the semi-rigid, semi-flexible framework of the trachea. For long-segment or circular defects, restoration of this unique biomechanical profile becomes even more important. Due to the inherent difficulty of creating such a tube, a tracheostomy or palliative stenting are often preferred over permanent reconstruction. To significantly improve and sustain quality of life of these patients, surgeons proposed innovative strategies for complex tracheal repair. In this review, we provide an overview of current clinical applications of tracheal repair using autologous and allogenic tissues. We look at recent advances in the field of tissue engineering, and the areas for improvement of these first human applications. Lastly, we highlight the focus of our research, in an effort to contribute to the development of optimized tracheal reconstructive techniques.

A step towards clinical application of acellular matrix: A clue from macrophage polarization

The outcome of tissue engineered organ transplants depends on the capacity of the biomaterial to promote a pro-healing response once implanted in vivo. Multiple studies, including ours, have demonstrated the possibility of using the extracellular matrix (ECM) of animal organs as platform for tissue engineering and more recently, discarded human organs have also been proposed as scaffold source. In contrast to artificial biomaterials, natural ECM has the advantage of undergoing continuous remodeling which allows adaptation to diverse conditions. It is known that natural matrices present diverse immune properties when compared to artificial biomaterials. However, how these properties compare between diseased and healthy ECM and artificial scaffolds has not yet been defined. To answer this question, we used decellularized renal ECM derived from WT mice and from mice affected by Alport Syndrome at different time-points of disease progression as a model of renal failure with extensive fibrosis. We characterized the morphology and composition of these ECMs and compared their in vitro effects on macrophage activation with that of synthetic scaffolds commonly used in the clinic (collagen type I and poly-L-(lactic) acid, PLLA). We showed that ECM derived from Alport kidneys differed in fibrous protein deposition and cytokine content when compared to ECM derived from WT kidneys. Yet, both WT and Alport renal ECM induced macrophage differentiation mainly towards a reparative (M2) phenotype, while artificial biomaterials towards an inflammatory (M1) phenotype. Anti-inflammatory properties of natural ECMs were lost when homogenized, hence three-dimensional structure of ECM seems crucial for generating an anti-inflammatory response. Together, these data support the notion that natural ECM, even if derived from diseased kidneys promote a M2 protolerogenic macrophage polarization, thus providing novel insights on the applicability of ECM obtained from discarded organs as ideal scaffold for tissue engineering.

New Phase of Growth for Xenogeneic-Based Bioartificial Organs

In this article, we examine the advanced clinical development of bioartificial organs and describe the challenges to implementing such systems into patient care. The case for bioartificial organs is evident: they are meant to reduce patient morbidity and mortality caused by the persistent shortage of organs available for allotransplantation. The widespread introduction and adoption of bioengineered organs, incorporating cells and tissues derived from either human or animal sources, would help address this shortage. Despite the decades of development, the variety of organs studied and bioengineered, and continuous progress in the field, only two bioengineered systems are currently commercially available: Apligraf® and Dermagraft® are both approved by the FDA to treat diabetic foot ulcers, and Apligraf® is approved to treat venous leg ulcers. Currently, no products based on xenotransplantation have been approved by the FDA. Risk factors include immunological barriers and the potential infectivity of porcine endogenous retrovirus (PERV), which is unique to xenotransplantation. Recent breakthroughs in gene editing may, however, mitigate risks related to PERV. Because of its primary role in interrupting progress in xenotransplantation, we present a risk assessment for PERV infection, and conclude that the formerly high risk has been reduced to a moderate level. Advances in gene editing, and more broadly in the field, may make it more likely than ever before that bioartificial organs will alleviate the suffering of patients with organ failure.

The Human Pancreas as a Source of Protolerogenic Extracellular Matrix Scaffold for a New-generation Bioartificial Endocrine Pancreas

OBJECTIVES

Our study aims at producing acellular extracellular matrix scaffolds from the human pancreas (hpaECMs) as a first critical step toward the production of a new-generation, fully human-derived bioartificial endocrine pancreas. In this bioartificial endocrine pancreas, the hardware will be represented by hpaECMs, whereas the software will consist in the cellular compartment generated from patient's own cells.

BACKGROUND

Extracellular matrix (ECM)-based scaffolds obtained through the decellularization of native organs have become the favored platform in the field of complex organ bioengineering. However, the paradigm is now switching from the porcine to the human model.

METHODS

To achieve our goal, human pancreata were decellularized with Triton-based solution and thoroughly characterized. Primary endpoints were complete cell and DNA clearance, preservation of ECM components, growth factors and stiffness, ability to induce angiogenesis, conservation of the framework of the innate vasculature, and immunogenicity. Secondary endpoint was hpaECMs' ability to sustain growth and function of human islet and human primary pancreatic endothelial cells.

RESULTS

Results show that hpaECMs can be successfully and consistently produced from human pancreata and maintain their innate molecular and spatial framework and stiffness, and vital growth factors. Importantly, hpaECMs inhibit human naïve CD4 T-cell expansion in response to polyclonal stimuli by inducing their apoptosis and promoting their conversion into regulatory T cells. hpaECMs are cytocompatible and supportive of representative pancreatic cell types.

DISCUSSION

We, therefore, conclude that hpaECMs has the potential to become an ideal platform for investigations aiming at the manufacturing of a regenerative medicine-inspired bioartificial endocrine pancreas.

Engineering approaches for regeneration of T lymphopoiesis

T cells play a central role in immune-homeostasis; specifically in the induction of antigen-specific adaptive immunity against pathogens and mutated self with immunological memory. The thymus is the unique organ where T cells are generated. In this review, first the complex structures and functions of various thymic microcompartments are briefly discussed to identify critical engineering targets for regeneration of thymic functions in vitro and in vivo. Then the biomimetic regenerative engineering approaches are reviewed in three categories: 1) reconstruction of 3-D thymic architecture, 2) cellular engineering, and 3) biomaterials-based artificial presentation of critical biomolecules. For each engineering approach, remaining challenges and clinical opportunities are also identified and discussed.

Autologous Cells for Kidney Bioengineering

Worldwide, increasing numbers of patients are developing end-stage renal disease, and at present, the only treatment options are dialysis or kidney transplantation. Dialysis is associated with increased morbidity and mortality, poor life quality and high economic costs. Transplantation is by far the better option, but there are insufficient numbers of donor kidneys available. Therefore, there is an urgent need to explore alternative approaches. In this review, we discuss how this problem could potentially be addressed by using autologous cells and appropriate scaffolds to develop 'bioengineered' kidneys for transplantation. In particular, we will highlight recent breakthroughs in pluripotent stem cell biology that have led to the development of autologous renal progenitor cells capable of differentiating to all renal cell types and will discuss how these cells could be combined with appropriate scaffolds to develop a bioengineered kidney.

Tooth Organ Bioengineering: Cell Sources and Innovative Approaches

Various treatment approaches for restoring missing teeth are being utilized nowadays by using artificial dental crowns/bridges or the use of dental implants. All aforementioned restorative modalities are considered to be the conventional way of treating such cases. Although these artificial therapies are commonly used for tooth loss rehabilitation, they are still less conservative, show less biocompatibility and fail to restore the natural biological and physiological function. Adding to that, they are considered to be costly due to the risk of failure and they also require regular maintenance. Regenerative dentistry is currently considered a novel therapeutic concept with high potential for a complete recovery of the natural function and esthetics of teeth. Biological-cell based dental therapies would involve replacement of teeth by using stem cells that will ultimately grow a bioengineered tooth, thereby restoring both the biological and physiological functions of the natural tooth, and are considered to be the ultimate goal in regenerative dentistry. In this review, various stem cell-based therapeutic approaches for tooth organ bioengineering will be discussed.