Tissue-engineered tracheal transplantation

The Development of Lung Tissue Engineering: From Biomaterials to Multicellular Systems

The challenge of the treatment of end-stage lung disease poses an urgent clinical demand for lung tissue engineering. Over the past few years, various lung tissue-engineered constructs are developed for lung tissue regeneration and respiratory pathology study. In this review, an overview of recent achievements in the field of lung tissue engineering is proposed. The introduction of lung structure and lung injury are stated briefly at first. After that, the lung tissue-engineered constructs are categorized into three types: acellular, monocellular, and multicellular systems. The different bioengineered constructs included in each system that can be applied to the reconstruction of the trachea, airway epithelium, alveoli, and even whole lung are described in detail, followed by the highlight of relevant representative research. Finally, the challenges and future directions of biomaterials, manufacturing technologies, and cells involved in lung tissue engineering are discussed. Overall, this review can provide referable ideas for the realization of functional lung regeneration and permanent lung substitution.

Bioengineering and Clinical Translation of Human Lung and its Components

Clinical lung transplantation has rapidly established itself as the gold standard of treatment for end-stage lung diseases in a restricted group of patients since the first successful lung transplant occurred. Although significant progress has been made in lung transplantation, there are still numerous obstacles on the path to clinical success. The development of bioartificial lung grafts using patient-derived cells may serve as an alternative treatment modality; however, challenges include developing appropriate scaffold materials, advanced culture strategies for lung-specific multiple cell populations, and fully matured constructs to ensure increased transplant lifetime following implantation. This review highlights the development of tissue-engineered tracheal and lung equivalents over the past two decades, key problems in lung transplantation in a clinical environment, the advancements made in scaffolds, bioprinting technologies, bioreactors, organoids, and organ-on-a-chip technologies. The review aims to fill the lacuna in existing literature toward a holistic bioartificial lung tissue, including trachea, capillaries, airways, bifurcating bronchioles, lung disease models, and their clinical translation. Herein, the efforts are on bridging the application of lung tissue engineering methods in a clinical environment as it is thought that tissue engineering holds enormous promise for overcoming the challenges associated with the clinical translation of bioengineered human lung and its components.

Directly construct microvascularization of tissue engineering trachea in orthotopic transplantation

Tissue engineering technology provides effective alternative treatments for tracheal reconstruction. The formation of a functional microvascular network is essential to support cell metabolism and ensure the long-term survival of grafts. However, given the lack of an identifiable vascular pedicle of the trachea that could be anastomosed to the blood vessels directly in the recipient's neck, successful tracheal transplantation faces significant challenges in rebuilding the adequate blood supply of the graft. Herein, we describe a one-step method to construct microvascularization of tissue-engineered trachea in orthotopic transplantation. Forty rabbit tracheae were decellularized using a vacuum-assisted decellularization (VAD) method. Histological appearance and immunohistochemical (IHC) analysis demonstrated efficient removal of cellular components and nuclear material from natural tissue, which was also confirmed by 4'-6-diamidino-2-phenylindole(DAPI) staining and DNA quantitative analysis, thus significantly reducing the antigenicity. Scanning electron microscopy (SEM), immunofluorescence (IF) analysis, GAG and collagen quantitative analysis showed that the hierarchical structures, composition and integrity of the extracellular matrix (ECM) were protected. IF analysis also demonstrated that basic fibroblast growth factor (b-FGF) was preserved during the decellularization process, and also exerted biocompatibility and proangiogenic properties by the chick chorioallantoic membrane(CAM) assay. Xenotransplantation assays indicated that the VAD tracheal matrix would no longer induced inflammatory reactions implanted in the body for 4 weeks after treated by VAD more than 16 h. Furthermore, we seeded the matrix with bone marrow-derived endothelial cells (BMECs) in vitro and performed in vivo tracheal patch repair assays to prove the biocompatibility and neovascularization of VAD-treated tracheal matrix, and the formation of a vascular network around the patch promoted the crawling of surrounding ciliated epithelial cells to the surface of the graft. We conclude that this natural VAD tracheal matrix is non-immunogenic and no inflammatory reactions in vivo transplantation. Seeding with BMECs on the grafts and then performing orthotopic transplantation can effectively promote the microvascularization and accelerate the native epithelium cells crawling to the lumen of the tracheal graft.

Vascularization strategies for tissue engineering for tracheal reconstruction

Tissue engineering technology provides effective alternative treatments for tracheal reconstruction. The formation of a functional microvascular network is essential to support cell metabolism and ensure the long-term survival of grafts. Although several tracheal replacement therapy strategies have been developed in the past, the critical significance of the formation of microvascular networks in 3D scaffolds has not attracted sufficient attention. Here, we review key technologies and related factors of microvascular network construction in tissue-engineered trachea and explore optimized preparation processes of vascularized functional tissues for clinical applications.

Refunctionalization of Decellularized Organ Scaffold of Pancreas by Recellularization: Whole Organ Regeneration into Functional Pancreas

BACKGROUND

Tissue engineering centers on creating a niche similar to the natural one, with a purpose of developing an organ construct. A natural scaffold can replace none while creating a scaffold unique to each tissue in composition, architecture and cues that regulate the character of cells.

METHODS

Whole pancreas from mouse was decellularized using detergent and enzymes, followed by recellularizing with MSC from human placenta. This construct was transplanted in streptozotocin induced diabetic mice. Histopathology of both decellularized and recellularized transplanted pancreas and qPCR analysis were performed to assess its recovery.

RESULTS

Decellularization removes the cells leaving behind extracellular matrix rich natural scaffold. After reseeding with mesenchymal stem cells, these cells differentiate into pancreas specific cells. Upon transplantation in streptozotocin induced diabetic mice, this organ was capable of restoring its histomorphology and functioning. Restoration of endocrine (islets), the exocrine region (acinar) and vascular network was seen in transplanted pancreas. The process of functional recovery of endocrine system took about 20 days when the mice start showing blood glucose reduction, though none achieved gluconormalization.

CONCLUSION

Natural decellularized scaffolds of soft organs can be refunctionalized using recipient's mesenchymal stem cells to restore structure and function; and counter immune problems arising during transplantation.

Photocrosslinked natural hydrogel composed of hyaluronic acid and gelatin enhances cartilage regeneration of decellularized trachea matrix

Repair of long segmental trachea defects is always a great challenge in the clinic. The key to solving this problem is to develop an ideal trachea substitute with biological function. Using of a decellularized trachea matrix based on laser micropore technique (LDTM) demonstrated the possibility of preparing ideal trachea substitutes with tubular shape and satisfactory cartilage regeneration for tissue-engineered trachea regeneration. However, as a result of the very low cell adhesion of LDTM, an overly high concentration of seeding cell is required, which greatly restricts its clinical translation. To address this issue, the current study proposed a novel strategy using a photocrosslinked natural hydrogel (PNH) carrier to enhance cell retention efficiency and improve tracheal cartilage regeneration. Our results demonstrated that PNH underwent a rapid liquid-solid phase conversion under ultraviolet light. Moreover, the photo-generated aldehyde groups in PNH could rapidly react with inherent amino groups on LDTM surfaces to form imine bonds, which efficiently immobilized the cell-PNH composite to the surfaces of LDTM and/or maintained the composite in the LDTM micropores. Therefore, PNH significantly enhanced cell-seeding efficiency and achieved both stable cell retention and homogenous cell distribution throughout the LDTM. Moreover, PNH exhibited excellent biocompatibility and low cytotoxicity, and provided a natural three-dimensional biomimetic microenvironment to efficiently promote chondrocyte survival and proliferation, extracellular matrix production, and cartilage regeneration. Most importantly, at a relatively low cell-seeding concentration, homogeneous tubular cartilage was successfully regenerated with an accurate tracheal shape, sufficient mechanical strength, good elasticity, typical lacuna structure, and cartilage-specific extracellular matrix deposition. Our findings establish a versatile and efficient cell-seeding strategy for regeneration of various tissue and provide a satisfactory trachea substitute for repair and functional reconstruction of long segmental tracheal defects.

Applied Bioengineering in Tissue Reconstruction, Replacement, and Regeneration

IMPACT STATEMENT

The use of autologous tissue in the reconstruction of tissue defects has been the gold standard. However, current standards still face many limitations and complications. Improving patient outcomes and quality of life by addressing these barriers remain imperative. This article provides historical perspective, covers the major limitations of current standards of care, and reviews recent advances and future prospects in applied bioengineering in the context of tissue reconstruction, replacement, and regeneration.

Advances in regenerative therapy: A review of the literature and future directions

There is enormous global anticipation for stem cell-based therapies that are safe and effective. Numerous pre-clinical studies present encouraging results on the therapeutic potential of different cell types including tissue derived stem cells. Emerging evidences in different fields of research suggest several cell types are safe, whereas their therapeutic application and effectiveness remain challenged. Multiple factors that influence treatment outcomes are proposed including immunocompatibility and potency, owing to variations in tissue origin, ex-vivo methodologies for preparation and handling of the cells. This communication gives an overview of literature data on the different types of cells that are potentially promising for regenerative therapy. As a case in point, the recent trends in research and development of the mesenchymal stem cells (MSCs) for cell therapy are considered in detail. MSCs can be isolated from a variety of tissues and organs in the human body including bone marrow, adipose, synovium, and perinatal tissues. However, MSC products from the different tissue sources exhibit unique or varied levels of regenerative abilities. The review finally focuses on adipose tissue-derived MSCs (ASCs), with the unique properties such as easier accessibility and abundance, excellent proliferation and differentiation capacities, low immunogenicity, immunomodulatory and many other trophic properties. The suitability and application of the ASCs, and strategies to improve the innate regenerative capacities of stem cells in general are highlighted among others.

Biomedical grafts for tracheal tissue repairing and regeneration "Tracheal tissue engineering: an overview"

Airway system is a vital part of the living being body. Trachea is the upper respiratory portion that connects nostril and lungs and has multiple functions such as breathing and entrapment of dust/pathogen particles. Tracheal reconstruction by artificial prosthesis, stents, and grafts are performed clinically for the repairing of damaged tissue. Although these (above-mentioned) methods repair the damaged parts, they have limited applicability like small area wounds and lack of functional tissue regeneration. Tissue engineering helps to overcome the above-mentioned problems by modifying the traditional used stents and grafts, not only repair but also regenerate the damaged area to functional tissue. Bioengineered tracheal replacements are biocompatible, nontoxic, porous, and having 3D biomimetic ultrastructure with good mechanical strength, which results in faster and better tissue regeneration. Till date, the bioengineered tracheal replacements studies have been going on preclinical and clinical levels. Besides that, still many researchers are working at advance level to make extracellular matrix-based acellular, 3D printed, cell-seeded grafts including living cells to overcome the demand of tissue or organ and making the ready to use tracheal reconstructs for clinical application. Thus, in this review, we summarized the tracheal tissue engineering aspects and their outcomes.

Tissue-engineered trachea regeneration using decellularized trachea matrix treated with laser micropore technique

Tissue-engineered trachea provides a promising approach for reconstruction of long segmental tracheal defects. However, a lack of ideal biodegradable scaffolds greatly restricts its clinical translation. Decellularized trachea matrix (DTM) is considered a proper scaffold for trachea cartilage regeneration owing to natural tubular structure, cartilage matrix components, and biodegradability. However, cell residual and low porosity of DTM easily result in immunogenicity and incomplete cartilage regeneration. To address these problems, a laser micropore technique (LMT) was applied in the current study to modify trachea sample porosity to facilitate decellular treatment and cell ingrowth. Decellularization processing demonstrated that cells in LMT treated samples were more easily removed compared with untreated native trachea. Furthermore, after optimizing the protocols of LMT and decellular treatments, the LMT-treated DTM (LDTM) could retain their original tubular shape with only mild extracellular matrix damage. After seeding with chondrocytes and culture in vitro for 8 weeks, the cell-LDTM constructs formed tubular cartilage with relatively homogenous cell distribution in both micropores and bilateral surfaces. In vivo results further confirmed that the constructs could form mature tubular cartilage with increased DNA and cartilage matrix contents, as well as enhanced mechanical strength, compared with native trachea. Collectively, these results indicate that LDTM is an ideal scaffold for tubular cartilage regeneration and, thus, provides a promising strategy for functional reconstruction of trachea cartilage.

STATEMENT OF SIGNIFICANCE

Lacking ideal biodegradable scaffolds greatly restricts development of tissue-engineered trachea. Decellularized trachea matrix (DTM) is considered a proper scaffold for trachea cartilage regeneration. However, cell residual and low porosity of DTM easily result in immunogenicity and incomplete cartilage regeneration. By laser micropore technique (LMT), the current study efficiently enhanced the porosity and decellularized efficacy of DTM. The LMT-treated DTM basically retained the original tubular shape with mild matrix damage. After chondrocyte seeding followed by in vitro culture and in vivo implantation, the constructs formed mature tubular cartilage with matrix content and mechanical strength similar to native trachea. The current study provides an ideal scaffold and a promising strategy for cartilage regeneration and functional reconstruction of trachea.

Tissue-Engineered Solutions in Plastic and Reconstructive Surgery: Principles and Practice

Recent advances in microsurgery, imaging, and transplantation have led to significant refinements in autologous reconstructive options; however, the morbidity of donor sites remains. This would be eliminated by successful clinical translation of tissue-engineered solutions into surgical practice. Plastic surgeons are uniquely placed to be intrinsically involved in the research and development of laboratory engineered tissues and their subsequent use. In this article, we present an overview of the field of tissue engineering, with the practicing plastic surgeon in mind. The Medical Research Council states that regenerative medicine and tissue engineering “holds the promise of revolutionizing patient care in the twenty-first century.” The UK government highlighted regenerative medicine as one of the key eight great technologies in their industrial strategy worthy of significant investment. The long-term aim of successful biomanufacture to repair composite defects depends on interdisciplinary collaboration between cell biologists, material scientists, engineers, and associated medical specialties; however currently, there is a current lack of coordination in the field as a whole. Barriers to translation are deep rooted at the basic science level, manifested by a lack of consensus on the ideal cell source, scaffold, molecular cues, and environment and manufacturing strategy. There is also insufficient understanding of the long-term safety and durability of tissue-engineered constructs. This review aims to highlight that individualized approaches to the field are not adequate, and research collaboratives will be essential to bring together differing areas of expertise to expedite future clinical translation. The use of tissue engineering in reconstructive surgery would result in a paradigm shift but it is important to maintain realistic expectations. It is generally accepted that it takes 20–30 years from the start of basic science research to clinical utility, demonstrated by contemporary treatments such as bone marrow transplantation. Although great advances have been made in the tissue engineering field, we highlight the barriers that need to be overcome before we see the routine use of tissue-engineered solutions.

Biocompatibility of Experimental Polymeric Tracheal Matrices

Biocompatibility of a new tracheal matrix is studied. The new matrix is based on polymeric ultra-fiber material colonized by mesenchymal multipotent stromal cells. The experiments demonstrate cytoconductivity of the synthetic matrices and no signs of their degradation within 2 months after their implantation to recipient mice. These data suggest further studies of the synthetic tracheal matrices on large laboratory animals.

Pharmacological Correction of Alcohol Motivation Depends on the Phenotype of the Response to Emotional Stress

The specific features of alcohol behavior were studied in MR and MNRA rats that exhibit an opposite reaction to emotional stress. We evaluated the effect of a dipeptide anxiolytic GB-115 (N-phenyl-hexanoyl-glycyl-L-tryptophan amide, neuropeptide cholecystokinin-4 analogue with antagonistic activity) on alcohol motivation in rats, which was formed over 12 months. High-emotionality MR rats were more sensitive to the anxiolytic effect of ethanol in the conflict situation test than low-emotionality MNRA rats. MNRA rats consumed a greater amount of ethanol under a free-choice condition with 15% ethanol solution and water (as in comparison with MR rats). However, the behavior of MR rats was transformed due to a significant increase in alcohol motivation from the 5th month of long-term free access to ethanol. An anxiolytic GB-115 (0.025 mg/kg intraperitoneally for 14 days) with selective activity in high-emotionality rats was shown to reduce significantly the average daily consumption and alcohol-deprivation effect in MR rats, but did not modulate ethanol addiction in MNRA rats.

Recent advancements in regenerative dentistry: A review

Although human mouth benefits from remarkable mechanical properties, it is very susceptible to traumatic damages, exposure to microbial attacks, and congenital maladies. Since the human dentition plays a crucial role in mastication, phonation and esthetics, finding promising and more efficient strategies to reestablish its functionality in the event of disruption has been important. Dating back to antiquity, conventional dentistry has been offering evacuation, restoration, and replacement of the diseased dental tissue. However, due to the limited ability and short lifespan of traditional restorative solutions, scientists have taken advantage of current advancements in medicine to create better solutions for the oral health field and have coined it "regenerative dentistry." This new field takes advantage of the recent innovations in stem cell research, cellular and molecular biology, tissue engineering, and materials science etc. In this review, the recently known resources and approaches used for regeneration of dental and oral tissues were evaluated using the databases of Scopus and Web of Science. Scientists have used a wide range of biomaterials and scaffolds (artificial and natural), genes (with viral and non-viral vectors), stem cells (isolated from deciduous teeth, dental pulp, periodontal ligament, adipose tissue, salivary glands, and dental follicle) and growth factors (used for stimulating cell differentiation) in order to apply tissue engineering approaches to dentistry. Although they have been successful in preclinical and clinical partial regeneration of dental tissues, whole-tooth engineering still seems to be far-fetched, unless certain shortcomings are addressed.

In vitro characterization of design and compressive properties of 3D-biofabricated/decellularized hybrid grafts for tracheal tissue engineering

Infection or damage to the trachea, a thin walled and cartilage reinforced conduit that connects the pharynx and larynx to the lungs, leads to serious respiratory medical conditions which can often prove fatal. Current clinical strategies for complex tracheal reconstruction are of limited availability and efficacy, but tissue engineering and regenerative medicine approaches may provide viable alternatives. In this study, we have developed a new "hybrid graft" approach that utilizes decellularized tracheal tissue along with a resorbable polymer scaffold, and holds promise for potential clinical applications. First, we evaluated the effect of our decellularization process on the compression properties of porcine tracheal segments, and noted approximately 63% decrease in resistance to compression following decellularization. Next we developed four C-shape scaffold designs by varying the base geometry and thickness, and fabricated polycaprolactone scaffolds using a combination of 3D-Bioplotting and thermally-assisted forming. All scaffolds designs were evaluated in vitro under three different environmental testing conditions to determine the design that offered the best resistance to compression. These were further studied to determine the effect of gamma radiation sterilization and cyclic compression loading. Finally, hybrid grafts were developed by securing these optimal design scaffolds to decellularized tracheal segments and evaluated in vitro under physiological testing conditions. Results show that the resistance to compression offered by the hybrid grafts created using gamma radiation sterilized scaffolds was comparable to that of fresh tracheal segments. Given that current clinical attempts at tracheal transplantation using decellularized tissue have been fraught with luminal collapse and complications, our data support the possibility that future embodiments using a hybrid graft approach may reduce the need for intraluminal stenting in tracheal transplant recipients.

[The methods for the treatment and prevention of cicatrix stenoses of trachea]

The objective of the present study was to analyze the current literature concerning mechanisms underlying the development of tracheal stenosis, new methods for the treatment and prevention of this condition. The main cause behind the formation of cicatrical stenosis of trachea is believed to be long-term artificial lung ventilation whereas the principal factors responsible for the injury to the tracheal wall include the impact of the cuff and the free end of the endotracheal tube, reflux of duodenal and gastric contents, concomitant infection, and the involvement of the autoimmune component. These pathogenic factors produce morphological changes in all layers of the tracheal wall with the formation of the granulation tissue the appearance of which serves as a forerunner of irreversible changes leading to tracheal stenosis. The biomedical technologies including auto- and allo-transplantation, tissue engineering, gene and cell-based therapy are considered to be the most promising methods for the treatment and prevention of this condition likely to improve the outcome of the management of cicatrical tracheal stenosis.

Tissue-Engineering Approaches to Restore Kidney Function

Kidney transplantation for the treatment of chronic kidney disease has established outcome and quality of life. However, its implementation is severely limited by a chronic shortage of donor organs; consequently, most candidates remain on dialysis and on the waiting list while accruing further morbidity and mortality. Furthermore, those patients that do receive kidney transplants are committed to a life-long regimen of immunosuppressive drugs that also carry significant adverse risk profiles. The disciplines of tissue engineering and regenerative medicine have the potential to produce alternative therapies which circumvent the obstacles posed by organ shortage and immunorejection. This review paper describes some of the most promising tissue-engineering solutions currently under investigation for the treatment of acute and chronic kidney diseases. The various stem cell therapies, whole embryo transplantation, and bioengineering with ECM scaffolds are outlined and summarized.

Patient-specific carbon nanocomposite tracheal prosthesis

PURPOSE

Surgical removal of the trachea is the current gold standard for treating severe airway carcinoma and stenosis. Resection of 6 cm or more of the trachea requires a replacement graft due to anastomotic tension. The high failure rates of current grafts are attributed to a mismatching of mechanical properties and slow epithelium formation on the inner lumen surface. There is also a current lack of tracheal prostheses that are closely tailored to the patient's anatomy.

METHODS

We propose the development of a patient-specific, artificial trachea made of carbon nanotubes and poly-di-methyl-siloxane (CNT-PDMS) composite material. Computational simulations and finite element analysis were used to study the stress behavior of the designed implant in a patient-specific, tracheal model.

RESULTS

Finite element studies indicated that the patient-specific carbon nanocomposite prosthesis produced stress distributions that are closer to that of the natural trachea. In vitro studies conducted on the proposed material have demonstrated its biocompatibility and suitability for sustaining tracheal epithelial cell proliferation and differentiation. In vivo studies done in porcine models showed no adverse side effects or breathing difficulties, with complete regeneration of the epithelium in the prosthesis lumen within 2 weeks.

CONCLUSIONS

This paper highlights the potential of a patient-specific CNT-PDMS graft as a viable airway replacement in severe tracheal carcinoma.

Tissue engineering

Ultimately much work remains to be done in the companion fields of biomaterials and stem cells. Nonetheless, the monumental progress in TE that has been reported in the studies summarized here demonstrates that regenerative approaches to problems in general surgery need to be explored in more depth. Furthermore, the surgical disciplines of reconstruction and transplantation need to recognize their research counterparts in TE, given its potential to actualize freedom from immunosuppression, one of the most elusive goals in modern surgery. The engineering and proliferation of autologous cells, tissues, and organs ex vivo before surgical operation can significantly reduce the obstacles current practitioners are intimately familiar with: donor site morbidity and immunologic rejection. Therefore, in addition to the truly exciting research and development prospects and implications for the commercial sector, patients with end-stage diseases and debilitating injury stand to gain the most from clinically adapted TE therapies.

Biomechanical changes from long-term freezer storage and cellular reduction of tracheal scaffoldings

OBJECTIVES/HYPOTHESIS

To determine structural biomechanical changes in tracheal scaffolds resulting from cellular reduction and storage at -80(o) C.

STUDY DESIGN

Laboratory-based study.

METHODS

Forty-four rabbit tracheal segments were separated into four treatment groups: untreated (group A, control), cellular-reduced (group B), storage at -80(o) C followed by cellular reduction (group C), and cellular-reduced followed by storage at -80(o) C (group D). Tracheal segments were subjected to uniaxial tension (n = 21) or compression (n = 23) using a universal testing machine to determine sutured tensile yield load and radial compressive strengths at 50% lumen occlusion. Mean differences among groups for tension and compression were compared by analysis of variance with post-hoc Tukey-Kramer test.

RESULTS

The untreated trachea (group A) demonstrated mean yield strength of 5.93 (± 1.65) N and compressive strength of 2.10 (± 0.51) N. Following treatment/storage, the tensile yield strength was not impaired (group B = 6.79 [± 1.58] N, C = 6.21 [± 1.40] N, D = 6.26 [± 1.18]; P > 0.10 each). Following cellular reduction, there was a significant reduction in compressive strength (group B = 0.44 N [± 0.13], P < 0.0001), but no further reduction due to storage (group C = 0.39 N [± 0.10]; P = 0.97 compared to group B).

CONCLUSION

The data suggest cellular reduction leads to loss of compressive strength. Freezing at -80°C (either before, or subsequent to cellular reduction) may be a viable storage method for tracheal grafts.

Decellularization and Recellularization Technologies in Tissue Engineering

Decellularization is the process by which cells are discharged from tissues/organs, but all of the essential cues for cell preservation and homeostasis are retained in a three-dimensional structure of the organ and its extracellular matrix components. During tissue decellularization, maintenance of the native ultrastructure and composition of the extracellular matrix (ECM) is extremely acceptable. For recellularization, the scaffold/matrix is seeded with cells, the final goal being to form a practical organ. In this review, we focus on the biological properties of the ECM that remains when a variety of decellularization methods are used, comparing recellularization technologies, including bioreactor expansion for perfusion-based bioartificial organs, and we discuss cell sources. In the future, decellularization–recellularization procedures may solve the problem of organ assembly on demand.

Tissue engineering and regeneration of lymphatic structures

Tissue engineering is the process by which biological structures are recreated using a combination of molecular signals, cellular components and scaffolds. Although the perceived potential of this approach to reconstruct damaged or missing tissues is seemingly limitless, application of these ideas in vivo has been more difficult than expected. However, despite these obstacles, important advancements have been reported for a number of organ systems, including recent reports on the lymphatic system. These advancements are important since the lymphatic system plays a central role in immune responses, regulation of inflammation, lipid absorption and interstitial fluid homeostasis. Insights obtained over the past two decades have advanced our understanding of the molecular and cellular mechanisms that govern lymphatic development and function. Utilizing this knowledge has led to important advancements in lymphatic tissue engineering, which is the topic of this review.

Liver support strategies: cutting-edge technologies

The treatment of end-stage liver disease and acute liver failure remains a clinically relevant issue. Although orthotopic liver transplantation is a well-established procedure, whole-organ transplantation is invasive and increasingly limited by the unavailability of suitable donor organs. Artificial and bioartificial liver support systems have been developed to provide an alternative to whole organ transplantation, but despite three decades of scientific efforts, the results are still not convincing with respect to clinical outcome. In this Review, conceptual limitations of clinically available liver support therapy systems are discussed. Furthermore, alternative concepts, such as hepatocyte transplantation, and cutting-edge developments in the field of liver support strategies, including the repopulation of decellularized organs and the biofabrication of entirely new organs by printing techniques or induced organogenesis are analysed with respect to clinical relevance. Whereas hepatocyte transplantation shows promising clinical results, at least for the temporary treatment of inborn metabolic diseases, so far data regarding implantation of engineered hepatic tissue have only emerged from preclinical experiments. However, the evolving techniques presented here raise hope for bioengineered liver support therapies in the future.

Extracellular Matrix Scaffold Technology for Bioartificial Pancreas Engineering: State of the Art and Future Challenges

Emergent technologies in regenerative medicine may soon overcome the limitations of conventional diabetes therapies. Collaborative efforts across the subfields of stem cell technology, islet encapsulation, and biomaterial carriers seek to produce a bioengineered pancreas capable of restoring endocrine function in patients with insulin-dependent diabetes. These technologies rely on a robust understanding of the extracellular matrix (ECM), the supportive 3-dimensional network of proteins necessary for cellular attachment, proliferation, and differentiation. Although these functions can be partially approximated by biosynthetic carriers, novel decellularization protocols have allowed researchers to discover the advantages afforded by the native pancreatic ECM. The native ECM has proven to be an optimal platform for recellularization and whole-organ pancreas bioengineering, an exciting new field with the potential to resolve the dire shortage of transplantable organs. This review seeks to contextualize recent findings, discuss current research goals, and identify future challenges of regenerative medicine as it applies to diabetes management.

Tracheal reconstruction in a canine model

OBJECTIVE

Tracheal reconstruction using a stem cell-based engineered trachea has recently shown promise. Our goal is to achieve a single-stage stem cell-based tracheal replacement.

STUDY DESIGN

Prospective feasibility study.

SETTING

Wake Forest Institute of Regenerative Medicine.

SUBJECTS AND METHODS

Five healthy male beagles were implanted with a 2.5-cm segment of decellularized trachea. A sixth animal, planned for the control arm, died of anesthetic complications prior to tracheal implantation. The remaining 5 beagles were divided into 2 study arms: 4 had adipose-derived stem cells coating the lumen of the donor trachea, and a control animal had the trachea implanted cell free. The donor tracheas were obtained from previously sacrificed size-matched canines and decellularized. The adipose tissue was harvested from a recipient animal and the trachea prepared, seeded, and then implanted, all in one operation. Adipose stem cells were labeled fluorescently.

RESULTS

Five of 6 planned surgical procedures were completed successfully. All required sacrifice for airway distress at approximately 1 week postoperatively. All tracheal grafts were found to be malacic and compromised.

CONCLUSION

In a canine model using a decellularized tracheal scaffold and adipose stem cells, the postoperative inflammatory response and evidence of rejection was minimal. However, all scaffolds exhibited breakdown, compromising the animals' airways, necessitating euthanasia earlier than planned. For future study, a similar animal model using a single-stage approach with a more robust scaffold may allow for greater survival and stem cell differentiation.

Human flexor tendon tissue engineering: in vivo effects of stem cell reseeding

BACKGROUND

Tissue-engineered human flexor tendons may be an option to aid in reconstruction of complex upper extremity injuries with significant tendon loss. The authors hypothesize that human adipose-derived stem cells remain viable following reseeding on human tendon scaffolds in vivo and aid in graft integration.

METHODS

Decellularized human flexor tendons harvested from fresh-frozen cadavers and reseeded with green fluorescent protein-labeled pooled human adipose-derived stem cells were examined with bioluminescent imaging and immunohistochemistry. Reseeded repaired tendons were compared biomechanically with unseeded controls following implantation in athymic rats at 2 and 4 weeks. The ratio of collagen I to collagen III at the repair site was examined using Sirius red staining. To confirm cell migration, reseeded and unseeded tendons were placed either in contact or with a 1-mm gap for 12 days. Green fluorescent protein signal was then detected.

RESULTS

Following reseeding, viable cells were visualized at 12 days in vitro and 4 weeks in vivo. Biomechanical testing revealed no significant difference in ultimate load to failure and 2-mm gap force. Histologic evaluation showed host cell invasion and proliferation of the repair sites. No increase in collagen III was noted in reseeded constructs. Cell migration was confirmed from reseeded constructs to unseeded tendon scaffolds with tendon contact.

CONCLUSIONS

Human adipose-derived stem cells reseeded onto decellularized allograft scaffolds are viable over 4 weeks in vivo. The movement of host cells into the scaffold and movement of adipose-derived stem cells along and into the scaffold suggests biointegration of the allograft.

Organ bioengineering and regeneration as the new Holy Grail for organ transplantation

Organ transplantation is a victim of its own success. In view of the excellent results achieved to date, the demand for organs is escalating whereas the supply has reached a plateau. Consequently, waiting times and mortality on the waiting list are increasing dramatically. Recent achievements in organ bioengineering and regeneration have provided proof of principle that the application of organ bioengineering and regeneration technologies to manufacture organs for transplant purposes may offer the quickest route to clinical application. As investigators are focusing their interest on the utilization and manipulation of autologous cells, ideally the end product will be the equivalent of an autograft such that the recipient will not require any antirejection medication. Achievement of an immunosuppression-free state has been pursued but has proven to be a difficult odyssey since the early days of the transplant era, yet an immediate, stable, durable, and reproducible immunosuppression-free state remains an unfulfilled quest. As organ bioengineering and regeneration has shown the potential to meet both the needs for a new source of organs that may eclipse the increasing organ demand and an immunosuppression-free state, advances in this field could become the new Holy Grail for transplant sciences.

Spatial patterning of endothelial cells and vascular network formation using ultrasound standing wave fields

The spatial organization of cells is essential for proper tissue assembly and organ function. Thus, successful engineering of complex tissues and organs requires methods to control cell organization in three dimensions. In particular, technologies that facilitate endothelial cell alignment and vascular network formation in three-dimensional tissue constructs would provide a means to supply essential oxygen and nutrients to newly forming tissue. Acoustic radiation forces associated with ultrasound standing wave fields can rapidly and non-invasively organize cells into distinct multicellular planar bands within three-dimensional collagen gels. Results presented herein demonstrate that the spatial pattern of endothelial cells within three-dimensional collagen gels can be controlled by design of acoustic parameters of the sound field. Different ultrasound standing wave field exposure parameters were used to organize endothelial cells into either loosely aggregated or densely packed planar bands. The rate of vessel formation and the morphology of the resulting endothelial cell networks were affected by the initial density of the ultrasound-induced planar bands of cells. Ultrasound standing wave fields provide a rapid, non-invasive approach to pattern cells in three-dimensions and direct vascular network formation and morphology within engineered tissue constructs.

Will regenerative medicine replace transplantation?

Recent groundbreaking advances in organ bioengineering and regeneration have provided evidence that regenerative medicine holds promise to dramatically improve the approach to organ transplantation. The two fields, however, share a common heritage. Alexis Carrel can be considered the father of both regenerative medicine and organ transplantation, and it is now clear that his legacy is equally applicable for the present and future generations of transplant and regenerative medicine investigators. In this review, we will briefly illustrate the interplay that should be established between these two complementary disciplines of health sciences. Although regenerative medicine has shown to the transplant field its potential, transplantation is destined to align with regenerative medicine and foster further progress probably more than either discipline alone. Organ bioengineering and regeneration technologies hold the promise to meet at the same time the two most urgent needs in organ transplantation, namely, the identification of a new, potentially inexhaustible source of organs and immunosuppression-free transplantation of tissues and organs.

Production and implantation of renal extracellular matrix scaffolds from porcine kidneys as a platform for renal bioengineering investigations

BACKGROUND

It is important to identify new sources of transplantable organs because of the critical shortage of donor organs. Tissue engineering holds the potential to address this issue through the implementation of decellularization-recellularization technology.

OBJECTIVE

To produce and examine acellular renal extracellular matrix (ECM) scaffolds as a platform for kidney bioengineering.

METHODS

Porcine kidneys were decellularized with distilled water and sodium dodecyl sulfate-based solution. After rinsing with buffer solution to remove the sodium dodecyl sulfate, the so-obtained renal ECM scaffolds were processed for vascular imaging, histology, and cell seeding to investigate the vascular patency, degree of decellularization, and scaffold biocompatibility in vitro. Four whole renal scaffolds were implanted in pigs to assess whether these constructs would sustain normal blood pressure and to determine their biocompatibility in vivo. Pigs were sacrificed after 2 weeks and the explanted scaffolds were processed for histology.

RESULTS

Renal ECM scaffolds were successfully produced from porcine kidneys. Scaffolds retained their essential ECM architecture and an intact vascular tree and allowed cell growth. On implantation, unseeded scaffolds were easily reperfused, sustained blood pressure, and were tolerated throughout the study period. No blood extravasation occurred. Pathology of explanted scaffolds showed maintenance of renal ultrastructure. Presence of inflammatory cells in the pericapsular region and complete thrombosis of the vascular tree were evident.

CONCLUSIONS

Our investigations show that pig kidneys can be successfully decellularized to produce renal ECM scaffolds. These scaffolds maintain their basic components, are biocompatible, and show intact, though thrombosed, vasculature.

Regeneration and bioengineering of the gastrointestinal tract: current status and future perspectives

The present review aims to illustrate the strategies that are being implemented in regenerative medicine to treat diseases that affect the digestive tract. Possible avenues are twofold: organ bioengineering, where cells are seeded on biological or synthetic scaffolding materials ex vivo and allowed to either mature in bioreactors or be implanted without undergoing any maturation; and regeneration per se, where the diseased tissue or organ is regenerated by recapitulation of its multi-step ontogenesis. This latter avenue may be induced either in vivo or ex vivo. While bioengineering technology has already manufactured segments of the digestive tract and sphincters, pure regeneration of any segment of the digestive tract has not yet been described. However, models of regeneration extrapolated from simple organisms are elucidating the complex yet fascinating mechanisms that regulate the ontogenesis of the digestive tract and are paving the way for the development of new regenerative technologies and methods.

The body as a living bioreactor: a feasibility study of pedicle flaps for tracheal transplantation

Reconstruction of long-segment tracheal stenosis remains problematic. Ex vivo transplantation of stem cell-derived tracheas has been established in humans using external tissue bioreactors. These bioreactors, however, are not widely accessible. Thus, we are developing a rotational flap-based "internal bioreactor" to allow in vivo stem cell engraftment in a pre-vascularized recipient bed. This muscle will also then serve as a carrier for the transplanted trachea during rotation into position for airway reconstruction. Herein, we present a study investigating the feasibility of two pedicle muscle flaps for implantation and subsequent tracheal transplantation. Trapezius and latissimus flaps were raised using established surgical techniques. The length and width of each flap, along with the distance from the pedicle takeoff to the trachea, were measured. The overall ability of the flaps to reach the trachea was assessed. Twelve flaps were raised in 5 fresh adult human cadavers. For the trapezius flap, averages were: flap length of 16.4 cm, flap width of 5.95 cm at the tip, and distance from the pedicle takeoff to the trachea of 11.1 cm. For the latissimus dorsi flap, averages were: flap length of 35.4 cm, flap width of 7.25 cm at the tip, and distance from the pedicle takeoff to the trachea of 27.3 cm. All flaps showed sufficient durability and rotational ability. Our results show that both trapezius and latissimus dorsi flaps can be transposed into the neck to allow tension-free closure of tracheal defects. For cervical tracheal transplantation, both flaps are equally adequate. We believe that trapezius and latissimus dorsi muscle flaps are potential tracheal implantation beds in terms of vascular supply, durability, and rotational ability.

Silk fibroin in tissue engineering

Tissue engineering (TE) is a multidisciplinary field that aims at the in vitro engineering of tissues and organs by integrating science and technology of cells, materials and biochemical factors. Mimicking the natural extracellular matrix is one of the critical and challenging technological barriers, for which scaffold engineering has become a prime focus of research within the field of TE. Amongst the variety of materials tested, silk fibroin (SF) is increasingly being recognized as a promising material for scaffold fabrication. Ease of processing, excellent biocompatibility, remarkable mechanical properties and tailorable degradability of SF has been explored for fabrication of various articles such as films, porous matrices, hydrogels, nonwoven mats, etc., and has been investigated for use in various TE applications, including bone, tendon, ligament, cartilage, skin, liver, trachea, nerve, cornea, eardrum, dental, bladder, etc. The current review extensively covers the progress made in the SF-based in vitro engineering and regeneration of various human tissues and identifies opportunities for further development of this field.

Regenerative medicine as applied to general surgery

The present review illustrates the state of the art of regenerative medicine (RM) as applied to surgical diseases and demonstrates that this field has the potential to address some of the unmet needs in surgery. RM is a multidisciplinary field whose purpose is to regenerate in vivo or ex vivo human cells, tissues, or organs to restore or establish normal function through exploitation of the potential to regenerate, which is intrinsic to human cells, tissues, and organs. RM uses cells and/or specially designed biomaterials to reach its goals and RM-based therapies are already in use in several clinical trials in most fields of surgery. The main challenges for investigators are threefold: Creation of an appropriate microenvironment ex vivo that is able to sustain cell physiology and function in order to generate the desired cells or body parts; identification and appropriate manipulation of cells that have the potential to generate parenchymal, stromal and vascular components on demand, both in vivo and ex vivo; and production of smart materials that are able to drive cell fate.

The concept of in vivo airway tissue engineering

We investigated whether decellularized pig tracheas could regenerate in vivo, without being recellularized before transplantation, using the own body as bioreactor. Decellularized pig tracheal scaffolds were intraoperative conditioned with mononuclear cells and growth and differentiation factors. During the postoperative period, the in situ regeneration was boosted by administering bioactive molecules to promote peripheral mobilization and differentiation of stem/progenitor cells and ultimately the regenerative process. Results revealed, after 2 weeks, a nearly normal trachea, with respiratory epithelium and a double-banded cartilage but without any mechanical differences compared to the native tissue. The growth factor administration resulted in a mobilization of progenitor and stem cells into the peripheral circulation and in an up-regulation of anti-apoptotic genes. Isolated stem/progenitor cells could be differentiated in vitro into several cell types, proving their multipotency. We provide evidence that the own body can be used as bioreactor to promote in vivo tissue engineering replacement. Moreover, we demonstrated the beneficial effect of additional pharmaceutical intervention for an improved engraftment of the transplant.