First human transplantation of a bioengineered airway tissue

Regenerative Engineering: Current Applications and Future Perspectives

Many pathologies, congenital defects, and traumatic injuries are untreatable by conventional pharmacologic or surgical interventions. Regenerative engineering represents an ever-growing interdisciplinary field aimed at creating biological replacements for injured tissues and dysfunctional organs. The need for bioengineered replacement parts is ubiquitous among all surgical disciplines. However, to date, clinical translation has been limited to thin, small, and/or acellular structures. Development of thicker tissues continues to be limited by vascularization and other impediments. Nevertheless, currently available materials, methods, and technologies serve as robust platforms for more complex tissue fabrication in the future. This review article highlights the current methodologies, clinical achievements, tenacious barriers, and future perspectives of regenerative engineering.

Differentiation Behaviour of Adipose-Derived Stromal Cells (ASCs) Seeded on Polyurethane-Fibrin Scaffolds In Vitro and In Vivo

Adipose-derived stromal cells (ASCs) are a promising cell source for tissue engineering and regenerative medicine approaches for cartilage replacement. For chondrogenic differentiation, human (h)ASCs were seeded on three-dimensional polyurethane (PU) fibrin composites and induced with a chondrogenic differentiation medium containing TGF-ß3, BMP-6, and IGF-1 in various combinations. In addition, in vitro predifferentiated cell-seeded constructs were implanted into auricular cartilage defects of New Zealand White Rabbits for 4 and 12 weeks. Histological, immunohistochemical, and RT-PCR analyses were performed on the constructs maintained in vitro to determine extracellular matrix (ECM) deposition and expression of specific cartilage markers. Chondrogenic differentiated constructs showed a uniform distribution of cells and ECM proteins. RT-PCR showed increased gene expression of collagen II, collagen X, and aggrecan and nearly stable expression of SOX-9 and collagen I. Rabbit (r)ASC-seeded PU-fibrin composites implanted in ear cartilage defects of New Zealand White Rabbits showed deposition of ECM with structures resembling cartilage lacunae by Alcian blue staining. However, extracellular calcium deposition became detectable over the course of 12 weeks. RT-PCR showed evidence of endochondral ossification during the time course with the expression of specific marker genes (collagen X and RUNX-2). In conclusion, hASCs show chondrogenic differentiation capacity in vitro with the expression of specific marker genes and deposition of cartilage-specific ECM proteins. After implantation of predifferentiated rASC-seeded PU-fibrin scaffolds into a cartilage defect, the constructs undergo the route of endochondral ossification.

Strategies for treating oesophageal diseases with stem cells

There is a wide range of oesophageal diseases, the most general of which are inflammation, injury and tumours, and treatment methods are constantly being developed and updated. With an increasingly comprehensive understanding of stem cells and their characteristics of multilineage differentiation, self-renewal and homing as well as the combination of stem cells with regenerative medicine, tissue engineering and gene therapy, stem cells are playing an important role in the treatment of a variety of diseases. Mesenchymal stem cells have many advantages and are most commonly applied; however, most of these applications have been in experimental studies, with few related clinical trials for comparison. Therefore, the methods, positive significance and limitations of stem cells in the treatment of oesophageal diseases remain incompletely understood. Thus, the purpose of this paper is to review the current literature and summarize the efficacy of stem cells in the treatment of oesophageal diseases, including oesophageal ulceration, acute radiation-induced oesophageal injury, corrosive oesophageal injury, oesophageal stricture formation after endoscopic submucosal dissection and oesophageal reconstruction, as well as gene therapy for oesophageal cancer.

A Survival Model of In Vivo Partial Liver Lobe Decellularization Towards In Vivo Liver Engineering

In vivo liver decellularization has become a promising strategy to study in vivo liver engineering. However, long-term survival after in vivo liver decellularization has not yet been achieved due to anatomical and technical challenges. This study aimed at establishing a survival model of in vivo partial liver lobe perfusion-decellularization in rats. We compared three decellularization protocols (1% Triton X100 followed by 1% sodium dodecyl sulfate [SDS], 1% SDS vs. 1% Triton X100, n = 6/group). Using the optimal one as judged by macroscopy, histology and DNA content, we characterized the structural integrity and matrix proteins by using histology, scanning electron microscopy, computed tomography scanning, and immunohistochemistry (IHC). We prevented contamination of the abdominal cavity with the corrosive detergents by using polyvinylidene chloride (PVDC) film + dry gauze in comparison to PVDC film + dry gauze + aspiration tube (n = 6/group). Physiological reperfusion was assessed by histology. Survival rate was determined after a 7-day observation period. Only perfusion with 1% SDS resulted in an acellular scaffold (fully translucent without histologically detectable tissue remnants, DNA concentration is <2% of that in native lobe) with remarkable structural and ultrastructural integrity as well as preservation of main matrix proteins (IHC positive for collagen IV, laminin, and elastin). Contamination of abdominal organs with the potentially toxic SDS solution was achieved by placing a suction tube in addition to the PVDC film + dry gauze and allowed a 7-day survival of all animals without severe postoperative complications. On reperfusion, the liver turned red within seconds without any leakage from the surface of the liver. About 12 h after reperfusion, not only blood cells but also some clots were visible in the portal vein, sinusoidal matrix network, and central vein, suggesting physiological perfusion. In conclusion, our results of this study show the first available data on generation of a survival model of in vivo parenchymal organ decellularization, creating a critical step toward in vivo organ engineering. Impact Statement Recently, in vivo liver decellularization has been considered a promising approach to study in vivo liver repopulation of a scaffold compared with ex vivo liver repopulation. However, long-term survival of in vivo liver decellularization has not yet been achieved. Here, despite anatomical and technical challenges, we successfully created a survival model of in vivo selected liver lobe decellularization in rats, providing a major step toward in vivo organ engineering.

Biocompatibility and cellular compatibility of decellularized tracheal matrix derived from rabbits

Objective:

To study the different concentrations of Triton X-100 and nuclease needed to remove cells from the tracheal matrix of rabbits and analyse their biocompatibility and cellular compatibility.

Methods:

Fifty tracheas were harvested from donor New Zealand rabbits. Thirty tracheas were randomly divided into five groups (n = 6 each). The tracheas in group A were untreated and served as a control group, and those in groups B, C, D and E were treated with different concentrations of Triton X-100 (1%, 2%, 3% and 4%), respectively. The tracheas of the five groups were assessed by histological observation, scanning electron microscopy and mechanical evaluation. The remaining 20 donor tracheas, which were divided into a control group and an optimally decellularized group, were used for xenogeneic transplantation and cell seeding.

Results:

Many epithelial cells and cartilage cells were observed in the tracheas of group A. There were fewer cartilage cells in the tracheas of groups C, D and E than in the tracheas of groups A and B under histological observation. In scanning electron microscopy, there were many ciliated epithelial cells in the tracheas of group A; in groups B and C, the ciliated epithelial cells disappeared, but the basement membrane was intact. The basement membranes were broken in the tracheas of groups D and E. Implanted decellularized tracheas showed good biocompatibility. Bone marrow mesenchymal stem cells grown in the decellularized tracheal matrix grew well.

Conclusion:

Decellularized tracheal matrix obtained from rabbits by 2% Triton X-100 may be suitable for the construction of tissue-engineered trachea because of its favourable morphological and biomechanical properties as well as its biocompatibility and cellular compatibly.

[Decellularization of organs and tissues]

The use of allogenic materials in reconstructive surgery is of great scientific interest due to high availability of donor tissues. The positive aspects of allogenous tissue transplantation are complicated by the histological incompatibility of donor tissue and recipient organism. This incompatibility results hypersensitivity reaction towards the allogenous transplant followed by rejection of allogenic tissue and even death in some cases. Cellular biological incompatibility may be managed by decellularization of allogenous organs and tissues prior to transplantation. The improvement of decellularization techniques will facilitate application of allogenous tissues in complex reconstructive procedures and significantly increase the capabilities of reconstructive surgery.

Decellularization of Trachea With Combined Techniques for Tissue-Engineered Trachea Transplantation

Objectives

The purpose of this study is to shorten the decellularization time of trachea by using combination of physical, chemical, and enzymatic techniques.

Methods

Approximately 3.5-cm-long tracheal segments from 42 New Zealand rabbits (3.5±0.5 kg) were separated into seven groups according to decellularization protocols. After decellularization, cellular regions, matrix and strength and endurance of the scaffold were followed up.

Results

DNA content in all groups was measured under 50 ng/mg and there was no significant difference for the glycosaminoglycan content between group 3 (lyophilization+deoxycholic acid+de-oxyribonuclease method) and control group (P=0.46). None of the decellularized groups was different than the normal trachea in tensile stress values (P>0.05). Glucose consumption and lactic acid levels measured from supernatants of all decellularized groups were close to group with cells only (76 mg/dL and 53 mg/L).

Conclusion

Using combination methods may reduce exposure to chemicals, prevent the excessive influence of the matrix, and shorten the decellularization time.

Bioengineering Approaches for Bladder Regeneration

Current clinical strategies for bladder reconstruction or substitution are associated to serious problems. Therefore, new alternative approaches are becoming more and more necessary. The purpose of this work is to review the state of the art of the current bioengineering advances and obstacles reported in bladder regeneration. Tissue bladder engineering requires an ideal engineered bladder scaffold composed of a biocompatible material suitable to sustain the mechanical forces necessary for bladder filling and emptying. In addition, an engineered bladder needs to reconstruct a compliant muscular wall and a highly specialized urothelium, well-orchestrated under control of autonomic and sensory innervations. Bioreactors play a very important role allowing cell growth and specialization into a tissue-engineered vascular construct within a physiological environment. Bioprinting technology is rapidly progressing, achieving the generation of custom-made structural supports using an increasing number of different polymers as ink with a high capacity of reproducibility. Although many promising results have been achieved, few of them have been tested with clinical success. This lack of satisfactory applications is a good reason to discourage researchers in this field and explains, somehow, the limited high-impact scientific production in this area during the last decade, emphasizing that still much more progress is required before bioengineered bladders become a commonplace in the clinical setting.

Study of Mechanical Properties of Engineered Cartilage in an in Vivo Culture for Design of a Biodegradable Scaffold

Introduction

An engineered trachea with an absorbable scaffold should be used to augment the repair of a stenotic tracheal section in infants and children because this type of engineered airway structure can grow as the child grows. Our strategy for relief of tracheal stenosis is tracheoplasty by engineered cartilage implantation in accordance with the concept of costal cartilage grafting to enlarge the lumen. This study investigated the mechanical properties of regenerative cartilage with a biodegradable scaffold, Neoveil®, to aid in design of a composite scaffold that maintained semi-rigid properties until cartilage could be generated.

Materials and methods

New Zealand White rabbit (n=3) chondrocytes were isolated from auricular cartilage with collagenase type 2 digestion. Then 10×106/cm3 chondrocytes in atelocollagen solution were seeded onto polyglycolic acid (PGA) mesh. A total of 36 constructs, 12 from each rabbit, were implanted into athymic mice (3 constructs/mouse). Constructs were retrieved after 8 weeks and evaluated by measurements of mechanical and biochemical properties as well as histological examination. Thirty-six PGA mesh sheets of the same size but without cells were implanted in control mice.

Results

After 6 weeks of implantation, staining of sections with Safranin O revealed cartilage accumulation. Glycosaminoglycan was gradually produced from chondrocytes in the engineered constructs, correlating with the duration of implantation. Mechanical parameters had the same values as those for rabbit tracheal cartilage 8 weeks after implantation.

Conclusions

Biodegradable Neoveil® had good biocompatibility and was able to support extracellular matrix formation in engineered cartilage in an animal model.

Evaluation of a Miniaturized Biologically Vascularized Scaffold in vitro and in vivo

In tissue engineering, the generation and functional maintenance of dense voluminous tissues is mainly restricted due to insufficient nutrient supply. Larger three-dimensional constructs, which exceed the nutrient diffusion limit become necrotic and/or apoptotic in long-term culture if not provided with an appropriate vascularization. Here, we established protocols for the generation of a pre-vascularized biological scaffold with intact arterio-venous capillary loops from rat intestine, which is decellularized under preservation of the feeding and draining vascular tree. Vessel integrity was proven by marker expression, media/blood reflow and endothelial LDL uptake. In vitro maintenance persisted up to 7 weeks in a bioreactor system allowing a stepwise reconstruction of fully vascularized human tissues and successful in vivo implantation for up to 4 weeks, although with time-dependent decrease of cell viability. The vascularization of the construct lead to a 1.5× increase in cellular drug release compared to a conventional static culture in vitro. For the first time, we performed proof-of-concept studies demonstrating that 3D tissues can be maintained within a miniaturized vascularized scaffold in vitro and successfully implanted after re-anastomosis to the intrinsic blood circulation in vivo. We hypothesize that this technology could serve as a powerful platform technology in tissue engineering and regenerative medicine.

Autologous Cell Seeding in Tracheal Tissue Engineering

Purpose of Review

There is no consensus on the best technology to be employed for tracheal replacement. One particularly promising approach is based upon tissue engineering and involves applying autologous cells to transplantable scaffolds. Here, we present the reported pre-clinical and clinical data exploring the various options for achieving such seeding.

Recent Findings

Various cell combinations, delivery strategies, and outcome measures are described. Mesenchymal stem cells (MSCs) are the most widely employed cell type in tracheal bioengineering. Airway epithelial cell luminal seeding is also widely employed, alone or in combination with other cell types. Combinations have thus far shown the greatest promise. Chondrocytes may improve mechanical outcomes in pre-clinical models, but have not been clinically tested. Rapid or pre-vascularization of scaffolds is an important consideration. Overall, there are few published objective measures of post-seeding cell viability, survival, or overall efficacy.

Summary

There is no clear consensus on the optimal cell-scaffold combination and mechanisms for seeding. Systematic in vivo work is required to assess differences between tracheal grafts seeded with combinations of clinically deliverable cell types using objective outcome measures, including those for functionality and host immune response.

Electronic supplementary material

The online version of this article (10.1007/s40778-017-0108-2) contains supplementary material, which is available to authorized users.

Airway reconstruction using decellularized tracheal allografts in a porcine model

PURPOSE

Tracheal cartilage reconstruction is an essential approach for the treatment of tracheal congenital abnormalities or injury. Here, we evaluated the use of allogeneic decellularized tracheas as novel support scaffolds.

METHODS

Six weaned pigs (4-week-old domestic males) were transplanted with allogeneic tracheal graft patches (three decellularized and three fresh tracheal scaffolds) onto artificial defects (approximately 15 × 15 mm). After 11 weeks, the tracheas were evaluated by bronchoscopy and histological studies.

RESULTS

No pigs displayed airway symptoms during the observation period. Tracheal lumen restored by fresh graft patches showed more advanced narrowing than that treated with decellularized grafts by bronchoscopy. Histologically, fresh grafts induced typical cellular rejection; this was decreased with decellularized grafts. In addition, immunohistochemistry demonstrated regenerating foci of recipient cartilage along the adjacent surface of decellularized tracheal grafts.

CONCLUSION

Decellularized allogeneic tracheal scaffolds could be effective materials for restoring impaired trachea.

Current Solutions for Long-Segment Tracheal Reconstruction

This article is a continuation of previous reviews about the appropriate method for long-segment tracheal reconstruction. We attempted to cover the most recent, successful and promising results of the different solutions for reconstruction that are rather innovative and suitable for imminent clinical application. Latest efforts to minimize the limitations associated with each method have been covered as well. In summary, autologous and allogenic tissue reconstruction of the trachea have been successful methods for reconstruction experimentally and clinically. Autologous tissues were best utilized clinically to enhance revascularization, whether as a definitive airway or as an adjunct to allografts or tissue-engineered trachea (TET). Allogenic tissue transplantation is, currently, the most suitable for clinical application, especially after elimination of the need for immunosuppressive therapy with unlimited supply of tissues. Similar results have been reported in many studies that used TET. However, clinical application of this method was limited to use as a salvage treatment in a few studies with promising results. These results still need to be solidified by further clinical and long-term follow-up reports. Combining different methods of reconstruction was often required to establish a physiological rather than an anatomical trachea and have shown superior outcomes.

An Investigation of Topics and Trends of Tracheal Replacement Studies Using Co-Occurrence Analysis

This study evaluated tracheal regeneration studies using scientometric and co-occurrence analysis to identify the most important topics and assess their trends over time. To provide the adequate search options, PubMed, Scopus, and Web of Science (WOS) were used to cover various categories such as keywords, countries, organizations, and authors. Search results were obtained by employing Bibexcel. Co-occurrence analysis was applied to evaluate the publications. Finally, scientific maps, author's network, and country contributions were depicted using VOSviewer and NetDraw. Furthermore, the first 25 countries and 130 of the most productive authors were determined. Regarding the trend analysis, 10 co-occurrence terms out of highly frequent words were examined at 5-year intervals. Our findings indicated that the field of trachea regeneration has tested different approaches over the time. In total, 65 countries have contributed to scientific progress both in experimental and clinical fields. Special keywords such as tissue engineering and different types of stem cells have been increasingly used since 1995. Studies have addressed topics such as angiogenesis, decellularization methods, extracellular matrix, and mechanical properties since 2011. These findings will offer evidence-based information about the current status and trends of tracheal replacement research topics over time, as well as countries' contributions.

The effective bioengineering method of implantation decellularized renal extracellular matrix scaffolds

End stage renal disease (ESRD) is a progressive loss of kidney function with a high rate of morbidity and mortality. Transplantable organs are hard to come by and hold a high risk of recipient immune rejection. We intended to establish a more effective and faster method to decellularize and recellularize the kidney scaffold for transplant and regeneration. We successfully produced renal scaffolds by decellularizing rat kidneys with 0.5% sodium dodecyl sulfate (SDS), while still preserving the extracellular matrix (ECM) 3D architecture, an intact vascular tree and biochemical components. We recellularized the kidney scaffolds with mouse embryonic stem (ES) cells that then populated and proliferated within the glomerular, vascular, and tubular structures. After in vivo implantation, these recellularized scaffolds were easily reperfused, tolerated blood pressure and produced urine with no blood leakage. Our methods can successfully decellularize and recellularize rat kidneys to produce functional renal ECM scaffolds. These scaffolds maintain their basic components, retain intact vasculature and show promise for kidney regeneration.

In-vivo organ engineering: Perfusion of hepatocytes in a single liver lobe scaffold of living rats

Organ decellularization is emerging as a promising regenerative medicine approach as it is able to provide an acellular, three-dimensional biological scaffold material that can be seeded with living cells for organ reengineering. However this application is currently limited to donor-derived decellularized organs for reengineering in vitro and no study has been conducted for re-engineering the decellularized organ in vivo. We developed a novel technique of a single liver lobe decellularization in vivo in live animals. Using a surgical method to generate a by-pass circulation through the portal vein and infra-hepatic vena cava with a perfusion chamber system, we decellularized the single liver lobe and recellularized it with allogenic primary hepatocytes. Our results showed that the decellularization process in vivo can preserve the vascular structural network and functional characteristics of the native liver lobe. It allows for efficient recellularization of the decellularized liver lobe matrix with allogenic primary hepatocytes. Upon the re-establishment of blood circulation, the recellularized liver lobe is able to gain the function and the allogenic hepatocytes are able to secret albumin. Our findings provide a proof of principle for the in vivo reengineering of liver.

Mesenchymal Stem Cell Fate: Applying Biomaterials for Control of Stem Cell Behavior

The materials pipeline for biomaterials and tissue engineering applications is under continuous development. Specifically, there is great interest in the use of designed materials in the stem cell arena as materials can be used to manipulate the cells providing control of behavior. This is important as the ability to "engineer" complexity and subsequent in vitro growth of tissues and organs is a key objective for tissue engineers. This review will describe the nature of the materials strategies, both static and dynamic, and their influence specifically on mesenchymal stem cell fate.

Engineered Vascularized Muscle Flap

One of the main factors limiting the thickness of a tissue construct and its consequential viability and applicability in vivo, is the control of oxygen supply to the cell microenvironment, as passive diffusion is limited to a very thin layer. Although various materials have been described to restore the integrity of full-thickness defects of the abdominal wall, no material has yet proved to be optimal, due to low graft vascularization, tissue rejection, infection, or inadequate mechanical properties. This protocol describes a means of engineering a fully vascularized flap, with a thickness relevant for muscle tissue reconstruction. Cell-embedded poly L-lactic acid/poly lactic-co-glycolic acid constructs are implanted around the mouse femoral artery and vein and maintained in vivo for a period of one or two weeks. The vascularized graft is then transferred as a flap towards a full thickness defect made in the abdomen. This technique replaces the need for autologous tissue sacrifications and may enable the use of in vitro engineered vascularized flaps in many surgical applications.

Re-epithelialization: a key element in tracheal tissue engineering

Trachea-tissue engineering is a thriving new field in regenerative medicine that is reaching maturity and yielding numerous promising results. In view of the crucial role that the epithelium plays in the trachea, re-epithelialization of tracheal substitutes has gradually emerged as the focus of studies in tissue-engineered trachea. Recent progress in our understanding of stem cell biology, growth factor interactions and transplantation immunobiology offer the prospect of optimization of a tissue-engineered tracheal epithelium. In addition, advances in cell culture technology and successful applications of clinical transplantation are opening up new avenues for the construction of a tissue-engineered tracheal epithelium. Therefore, this review summarizes current advances, unresolved obstacles and future directions in the reconstruction of a tissue-engineered tracheal epithelium.

Stem cells from amniotic fluid--Potential for regenerative medicine

Regenerative medicine has recently been established as an emerging field focussing on repair, replacement or regeneration of cells, tissues and whole organs. The significant recent advances in the field have intensified the search for novel sources of stem cells with potential for therapy. Recently, researchers have identified the amniotic fluid as an untapped source of stem cells that are multipotent, possess immunomodulatory properties and do not have the ethical and legal limitations of embryonic stem cells. Stem cells from the amniotic fluid have been shown to differentiate into cell lineages representing all three embryonic germ layers without generating tumours, which make them an ideal candidate for tissue engineering applications. In addition, their ability to engraft in injured organs and modulate immune and repair responses of host tissues suggest that transplantation of such cells may be useful for the treatment of various degenerative and inflammatory diseases affecting major tissues/organs. This review summarises the evidence on amniotic fluid cells over the past 15 years and explores the potential therapeutic applications of amniotic fluid stem cells and amniotic fluid mesenchymal stem cells.

Bioengineering for Organ Transplantation: Progress and Challenges

Organ transplantation can offer a curative option for patients with end stage organ failure. Unfortunately the treatment is severely limited by the availability of donor organs. Organ bioengineering could provide a solution to the worldwide critical organ shortage. The majority of protocols to date have employed the use of decellularization-recellularization technology of naturally occurring tissues and organs with promising results in heart, lung, liver, pancreas, intestine and kidney engineering. Successful decellularization has provided researchers with suitable scaffolds to attempt cell reseeding. Future work will need to focus on the optimization of organ specific recellularization techniques before organ bioengineering can become clinically translatable. This review will examine the current progress in organ bioengineering and highlight future challenges in the field.

Respiratory Tissue Engineering: Current Status and Opportunities for the Future

Currently, lung disease and major airway trauma constitute a major global healthcare burden with limited treatment options. Airway diseases such as chronic obstructive pulmonary disease and cystic fibrosis have been identified as the fifth highest cause of mortality worldwide and are estimated to rise to fourth place by 2030. Alternate approaches and therapeutic modalities are urgently needed to improve clinical outcomes for chronic lung disease. This can be achieved through tissue engineering of the respiratory tract. Interest is growing in the use of airway tissue-engineered constructs as both a research tool, to further our understanding of airway pathology, validate new drugs, and pave the way for novel drug therapies, and also as regenerative medical devices or as an alternative to transplant tissue. This review provides a concise summary of the field of respiratory tissue engineering to date. An initial overview of airway anatomy and physiology is given, followed by a description of the stem cell populations and signaling processes involved in parenchymal healing and tissue repair. We then focus on the different biomaterials and tissue-engineered systems employed in upper and lower respiratory tract engineering and give a final perspective of the opportunities and challenges facing the field of respiratory tissue engineering.

Preparation and characterization of a decellularized cartilage scaffold for ear cartilage reconstruction

Scaffolds are widely used to reconstruct cartilage. Yet, the fabrication of a scaffold with a highly organized microenvironment that closely resembles native cartilage remains a major challenge. Scaffolds derived from acellular extracellular matrices are able to provide such a microenvironment. Currently, no report specifically on decellularization of full thickness ear cartilage has been published. In this study, decellularized ear cartilage scaffolds were prepared and extensively characterized. Cartilage decellularization was optimized to remove cells and cell remnants from elastic cartilage. Following removal of nuclear material, the obtained scaffolds retained their native collagen and elastin contents as well as their architecture and shape. High magnification scanning electron microscopy showed no obvious difference in matrix density after decellularization. However, glycosaminoglycan content was significantly reduced, resulting in a loss of viscoelastic properties. Additionally, in contact with the scaffolds, human bone-marrow-derived mesenchymal stem cells remained viable and are able to differentiate toward the chondrogenic lineage when cultured in vitro. These results, including the ability to decellularize whole human ears, highlight the clinical potential of decellularization as an improved cartilage reconstruction strategy.

Current progress in xenotransplantation and organ bioengineering

Organ transplantation represents a unique method of treatment to cure people with end-stage organ failure. Since the first successful organ transplant in 1954, the field of transplantation has made great strides forward. However, despite the ability to transform and save lives, transplant surgery is still faced with a fundamental problem the number of people requiring organ transplants is simply higher than the number of organs available. To put this in stark perspective, because of this critical organ shortage 18 people every day in the United States alone die on a transplant waiting list (U.S. Department of Health & Human Services, http://organdonor.gov/about/data.html). To address this problem, attempts have been made to increase the organ supply through xenotransplantation and more recently, bioengineering. Here we trace the development of both fields, discuss their current status and highlight limitations going forward. Ultimately, lessons learned in each field may prove widely applicable and lead to the successful development of xenografts, bioengineered constructs, and bioengineered xeno-organs, thereby increasing the supply of organs for transplantation.

Host-integration of a tissue-engineered airway patch: two-year follow-up in a single patient

Different bioengineering techniques have been applied repeatedly for the reconstruction of extensive airway defects in the last few years. While short-term surgical success is evident, there is a lack of long-term results in patients. Here, we report the case of a young male who received a 5×2 cm bioartificial airway patch for tracheoesophageal reconstruction focusing on clinical defect healing and histomorphological tissue reorganization 2.5 years after surgery. We generated bioartificial airway tissue using a cell-free biological vascularized scaffold that was re-endothelialized and reseeded with the recipient's autologous primary cells and we implanted it into the recipient's left main bronchus. To investigate host-integration 2.5 years after the implantation, we obtained biopsies of the implant and adjacent tracheal tissue and processed these for histological and immunohistochemical analyses. The early postoperative course was uneventful and the transplanted airway tissue was integrated into the host. 2.5 years after transplantation, a bronchoscopy confirmed the scar-free reconstruction of the former airway defect. Histological work-up documented respiratory airway mucosa lining the bronchial reconstruction, making it indistinguishable from native airway mucosa. After transplantation, our bioartificial airway tissue provided perfect airway healing, with no histological evidence of tissue dedifferentiation.

Advances in tracheal reconstruction

SUMMARY

A recent revival of global interest for reconstruction of long-segment tracheal defects, which represents one of the most interesting and complex problems in head and neck and thoracic reconstructive surgery, has been witnessed. The trachea functions as a conduit for air, and its subunits including the epithelial layer, hyaline cartilage, and segmental blood supply make it particularly challenging to reconstruct. A myriad of attempts at replacing the trachea have been described. These along with the anatomy, indications, and approaches including microsurgical tracheal reconstruction will be reviewed. Novel techniques such as tissue-engineering approaches will also be discussed. Multiple attempts at replacing the trachea with synthetic scaffolds have been met with failure. The main lesson learned from such failures is that the trachea must not be treated as a "simple tube." Understanding the anatomy, developmental biology, physiology, and diseases affecting the trachea are required for solving this problem.

A microfluidic device to apply shear stresses to polarizing ciliated airway epithelium using air flow

Organization of airway epithelium determines ciliary beat direction and coordination for proper mucociliary clearance. Fluidic shear stresses have the potential to influence ciliary organization. Here, an in vitro fluidic flow system was developed for inducing long-term airflow shear stresses on airway epithelium with a view to influencing epithelial organization. Our system consists of a fluidic device for cell culture, integrated into a humidified airflow circuit. The fluidic device has a modular design and is made from a combination of polystyrene and adhesive components incorporated into a 6-well filter membrane insert. We demonstrate the system operates within physiologically relevant shear and pressure ranges and estimate the shear stress exerted on the epithelial cell layer as a result of air flow using a computational model. For both the bronchial epithelial cell line BEAS2B and primary human tracheal airway epithelial cells, we demonstrate that cells remain viable within the device when exposed to airflow for 24 h and that normal differentiation and cilia formation occurs. Furthermore, we demonstrate the utility of our device for exploring the impact of exposing cells to airflow: our tool enables quantification of cytoskeletal organization, and is compatible with in situ bead assays to assess the orientation of cilia beating.

Organ bioengineering for the newborn

Regenerative medicine has recently been established as an emerging interdisciplinary field focused on the repair; replacement or regeneration of cells, tissues and organs. It involves various disciplines, which are focused on different aspects of the regeneration process such as cell biology, gene therapy, bioengineering, material science and pharmacology. In this article, we will outline progress on tissue engineering of specific tissues and organs relevant to paediatric surgery.

Decellularized tracheal extracellular matrix supports epithelial migration, differentiation, and function

Tracheal loss is a source of significant morbidity for affected patients with no acceptable solution. Interest in engineering tracheal transplants has created a demand for small animal models of orthotopic tracheal transplantation. Here, we examine the use of a decellularized graft in a murine model of tracheal replacement. Fresh or decellularized tracheas harvested from age-matched female donor C57BL/6 mice were transplanted into syngeneic recipients. Tracheas were decellularized using repeated washes of water, 3% Triton X-100, and 3 M NaCl under cyclic pressure changes, followed by disinfection with 0.1% peracetic acid/4% ethanol, and terminal sterilization by gamma irradiation. Tracheas were explanted for immunolabeling at 1, 4, and 8 weeks following surgery. Video microscopy and computed tomography were performed to assess function and structure. Decellularized grafts supported complete reepithelialization by 8 weeks and motile cilia were observed. Cartilaginous portions of the trachea were maintained in mice receiving fresh transplants, but repopulation of the cartilage was not seen in mice receiving decellularized transplants. We observed superior postsurgical survival, weight gain, and ciliary function in mice receiving fresh transplants compared with those receiving decellularized transplants. The murine orthotopic tracheal transplant provides an appropriate model to assess the repopulation and functional regeneration of decellularized tracheal grafts.

Advances in tracheal reconstruction

Summary:

A recent revival of global interest for reconstruction of long-segment tracheal defects, which represents one of the most interesting and complex problems in head and neck and thoracic reconstructive surgery, has been witnessed. The trachea functions as a conduit for air, and its subunits including the epithelial layer, hyaline cartilage, and segmental blood supply make it particularly challenging to reconstruct. A myriad of attempts at replacing the trachea have been described. These along with the anatomy, indications, and approaches including microsurgical tracheal reconstruction will be reviewed. Novel techniques such as tissue-engineering approaches will also be discussed. Multiple attempts at replacing the trachea with synthetic scaffolds have been met with failure. The main lesson learned from such failures is that the trachea must not be treated as a “simple tube.” Understanding the anatomy, developmental biology, physiology, and diseases affecting the trachea are required for solving this problem.

Repair and regeneration of the respiratory system: complexity, plasticity, and mechanisms of lung stem cell function

Respiratory disease is the third leading cause of death in the industrialized world. Consequently, the trachea, lungs, and cardiopulmonary vasculature have been the focus of extensive investigations. Recent studies have provided new information about the mechanisms driving lung development and differentiation. However, there is still much to learn about the ability of the adult respiratory system to undergo repair and to replace cells lost in response to injury and disease. This Review highlights the multiple stem/progenitor populations in different regions of the adult lung, the plasticity of their behavior in injury models, and molecular pathways that support homeostasis and repair.

Assessment of the mechanics of a tissue-engineered rat trachea in an image-processing environment

OBJECTIVES

Despite the recent success regarding the transplantation of tissue-engineered airways, the mechanical properties of these grafts are not well understood. Mechanical assessment of a tissue-engineered airway graft before implantation may be used in the future as a predictor of function. The aim of this preliminary work was to develop a noninvasive image-processing environment for the assessment of airway mechanics.

METHOD

Decellularized, recellularized and normal tracheas (groups DECEL, RECEL, and CONTROL, respectively) immersed in Krebs-Henseleit solution were ventilated by a small-animal ventilator connected to a Fleisch pneumotachograph and two pressure transducers (differential and gauge). A camera connected to a stereomicroscope captured images of the pulsation of the trachea before instillation of saline solution and after instillation of Krebs-Henseleit solution, followed by instillation with Krebs-Henseleit with methacholine 0.1 M (protocols A, K and KMCh, respectively). The data were post-processed with computer software and statistical comparisons between groups and protocols were performed.

RESULTS

There were statistically significant variations in the image measurements of the medial region of the trachea between the groups (two-way analysis of variance [ANOVA], p<0.01) and of the proximal region between the groups and protocols (two-way ANOVA, p<0.01).

CONCLUSIONS

The technique developed in this study is an innovative method for performing a mechanical assessment of engineered tracheal grafts that will enable evaluation of the viscoelastic properties of neo-tracheas prior to transplantation.

Bioengineering kidneys for transplantation

One in 10 Americans suffers from chronic kidney disease, and close to 90,000 people die each year from causes related to kidney failure. Patients with end-stage renal disease are faced with two options: hemodialysis or transplantation. Unfortunately, the transplantation option is limited because of the shortage of donor organs and the need for immunosuppression. Bioengineered kidney grafts theoretically present a novel solution to both problems. Herein, we discuss the history of bioengineering organs, the current status of bioengineered kidneys, considerations for the future of the field, and challenges to clinical translation. We hope that by integrating principles of tissue engineering, and stem cell and developmental biology, bioengineered kidney grafts will advance the field of regenerative medicine while meeting a critical clinical need.

Clinical potentials of human pluripotent stem cells in lung diseases

Lung possesses very limited regenerative capacity. Failure to maintain homeostasis of lung epithelial cell populations has been implicated in the development of many life-threatening pulmonary diseases leading to substantial morbidity and mortality worldwide, and currently there is no known cure for these end-stage pulmonary diseases. Embryonic stem cells (ESCs) and somatic cell-derived induced pluripotent stem cells (iPSCs) possess unlimited self-renewal capacity and great potential to differentiate to various cell types of three embryonic germ layers (ectodermal, mesodermal, and endodermal). Therapeutic use of human ESC/iPSC-derived lung progenitor cells for regeneration of injured or diseased lungs will have an enormous clinical impact. This article provides an overview of recent advances in research on pluripotent stem cells in lung tissue regeneration and discusses technical challenges that must be overcome for their clinical applications in the future.

An engineered 3D human airway mucosa model based on an SIS scaffold

To investigate interrelations of human obligate airway pathogens, such as Bordetella pertussis, and their hosts test systems with high in vitro/in vivo correlation are of urgent need. Using a tissue engineering approach, we generated a 3D test system of the airway mucosa with human tracheobronchial epithelial cells (hTEC) and fibroblasts seeded on a clinically implemented biological scaffold. To investigate if hTEC display tumour-specific characteristics we analysed Raman spectra of hTEC and the adenocarcinoma cell line Calu-3. To establish optimal conditions for infection studies, we treated human native airway mucosa segments with B. pertussis. Samples were processed for morphologic analysis. Whereas our test system consisting of differentiated epithelial cells and migrating fibroblasts shows high in vitro/in vivo correlation, hTEC seeded on the scaffold as monocultures did not resemble the in vivo situation. Differences in Raman spectra of hTEC and Calu-3 were identified in distinct wave number ranges between 720 and 1662 cm(-1) indicating that hTEC do not display tumour-specific characteristics. Infection of native tissue with B. pertussis led to cytoplasmic vacuoles, damaged mitochondria and destroyed epithelial cells. Our test system is suitable for infection studies with human obligate airway pathogens by mimicking the physiological microenvironment of the human airway mucosa.

Transdifferentiation of autologous bone marrow cells on a collagen-poly(ε-caprolactone) scaffold for tissue engineering in complete lack of native urothelium

Urological reconstructive surgery is sometimes hampered by a lack of tissue. In some cases, autologous urothelial cells (UCs) are not available for cell expansion and ordinary tissue engineering. In these cases, we wanted to explore whether autologous mesenchymal stem cells (MSCs) from bone marrow could be used to create urological transplants. MSCs from human bone marrow were cultured in vitro with medium conditioned by normal human UCs or by indirect co-culturing in culture well inserts. Changes in gene expression, protein expression and cell morphology were studied after two weeks using western blot, RT-PCR and immune staining. Cells cultured in standard epithelial growth medium served as controls. Bone marrow MSCs changed their phenotype with respect to growth characteristics and cell morphology, as well as gene and protein expression, to a UC lineage in both culture methods, but not in controls. Urothelial differentiation was also accomplished in human bone marrow MSCs seeded on a three-dimensional poly(ε-caprolactone) (PCL)-collagen construct. Human MSCs could easily be harvested by bone marrow aspiration and expanded and differentiated into urothelium. Differentiation could take place on a three-dimensional hybrid PCL-reinforced collagen-based scaffold for creation of a tissue-engineered autologous transplant for urological reconstructive surgery.