Tracheal replacement: a critical review

Incompletely Decellularized Tracheal Matrix Scaffold for Tissue Engineering

BACKGROUND

Dense cartilaginous extracellular matrix makes decellularization and repopulation of tracheal cartilage difficult. However, the dense matrix isolates cartilaginous antigens from the recipient's immune system. Therefore, allorejection may be avoided by removing antigens from noncartilaginous tissues. In this study, incompletely decellularized tracheal matrix scaffolds were developed for tracheal tissue engineering.

METHODS

Brown Norway rat tracheae were decellularized with 4% sodium deoxycholate treatment. The cell and antigen removal efficacy, histoarchitecture, surface ultrastructure, glycosaminoglycan, collagen contents, mechanical properties, and chondrocyte viability of the scaffold were evaluated in vitro. Brown Norway rat tracheal matrix scaffolds ( n = 6) were implanted subcutaneously into Lewis rats and observed for 4 weeks. Brown Norway rat tracheae ( n = 6) and Lewis rat scaffolds ( n = 6) were implanted as controls. Histologic analysis of macrophage and lymphocyte infiltration was performed.

RESULTS

One decellularization cycle removed all cells and antigens from noncartilaginous tissue. Incomplete decellularization preserved the structural integrity of the tracheal matrix and chondrocyte viability. Except for 31% glycosaminoglycan loss, the scaffold had comparable collagen content and tensile and compressive mechanical properties to those of the native trachea. The allogeneic scaffold showed remarkably reduced CD68 + , CD8 + , and CD4 + cell infiltration compared with the allografts and demonstrated similar cell infiltration to the syngeneic scaffold. It also maintained the three-dimensional tracheal structure and cartilage viability in vivo.

CONCLUSIONS

Incompletely decellularized trachea did not induce immunorejection and maintained the integrity and viability of cartilage in vivo. Tracheal decellularization and repopulation can be simplified for urgent tracheal replacement.

CLINICAL RELEVANCE STATEMENT

The present study describes the development of an incomplete decellularization protocol that creates a decellularized matrix scaffold for tracheal tissue engineering, aiming to provide preliminary data that this method may generate suitable tracheal scaffolds for use in tracheal replacement.

De-Epithelialized Viable Tracheal Allotransplantation Without Immunosuppressants: 5-Year Follow-Up

OBJECTIVE

Tracheal transplantation could be a better option for patients with long segmental laryngotracheal stenosis or defects, but the need for immunosuppressants limits its widespread use due to the antigenicity of the tracheal epithelium. Chemically treated or cryopreserved nonviable tracheal allografts have no immunogenicity but lead to necrosis and stenosis in long-term outcomes. The present report describes the 5-year outcomes of de-epithelialized viable tracheal allotransplantation without immunosuppressants in a patient with severe laryngotracheal stenosis.

METHODS

The recipient was a 47-year-old female with relapsing polychondritis affecting the larynx and cervical trachea and producing a 5 cm long stenosis that could not be repaired using resection and anastomosis. A tracheal allograft was obtained from a 45-year-old male donor and treated with a combination of 3% sodium dodecyl sulfate (SDS) and organ preservation solution for 138 hours. The allograft was revascularized by heterotopical implantation in the infrahyoid muscles of the recipient for 3 months and then transplantation to the laryngotracheal defect with a split-thickness skin graft sutured to the lumen and a silicon T-tube. No immunosuppressants were used postoperatively.

RESULTS

The allograft was de-epithelialized, and most of the cartilage rings remained viable after the treatment. The allograft was revascularized, viable, and mechanically stable after 3 months of heterotopic implantation. No apparent signs of rejection or destruction were observed. The T-tube was removed, and the internal lining of the allograft was repopulated 4 months after orthotopic transplantation, despite the skin graft necrotizing at 2 weeks. Endoscopy and computed tomography showed a patent airway 5 years after orthotopic transplantation. The patient was able to resume her usual quality of life.

CONCLUSION

The present study demonstrates that transplantation of the de-epithelialized viable tracheal allograft without immunosuppressants is safe and promising for patients with long laryngotracheal stenosis or defects, especially for those with malignant tumor resections.

Global Bibliometric and Visualized Analysis of Tracheal Tissue Engineering Research

The development of tracheal tissue engineering (TTE) has seen a rapid growth in recent years. The purpose of this study was to investigate the global status, trends, and hotspots of TTE research based on bibliometrics and visualization analysis. Publications related to TTE were retrieved and included in the Web of Science Core Collection. VOSviewer and CiteSpace were used to generate knowledge maps. Six hundred fifty-five publications were identified, and the quantity of the annual publications worldwide was on the increase. International collaboration is a widespread reality. The United States led the world in the field of trachea tissue engineering, whereas University College London was the institution with the greatest contribution. In addition, Biomaterials had a great influence in this field, attracting the largest number of papers. Moreover, the topics of TTE research largely concentrated on the biomechanical scaffold preparation, the vascularization and epithelialization of scaffold, the tracheal cartilage regeneration, and the tissue-engineered tracheal transplantation. And the research on the application of decellularization and 3D printing for the construction of a tissue-engineered trachea was likely to receive more widespread attention in the future. Impact statement In recent years, tracheal tissue engineering (TTE) has experienced rapid growth. In this study, we investigated the worldwide status and trends of TTE research, and revealed the countries, institutions, journals, and authors that had made significant contributions to the field of TTE. Moreover, the possible research hotspots in the future were predicted. According to our research, researchers can gain a better understanding of the trends in this field, and stay informed of the most current research by tracking key journals, institutions, and authors.

Extended autologous tracheal replacement by a novel pedicled thoraco-chondro-costal flap: a cadaveric proof of concept

OBJECTIVES

Our aim was to report an anatomic model of an autologous flap based on the internal thoracic blood supply: the pedicled thoraco-chondro-costal flap; and establish the feasibility of various types of extended tracheal replacement with this novel flap, according to a newly proposed topographic classification.

METHODS

In a cadaveric model, a cervicotomy combined with median sternotomy was performed. The incision was extended laterally to expose the chest wall. The internal thoracic pedicle was freed from its origin down to the upper limit of the delineated flap to be elevated. The perichondria and adjacent periostea were incised longitudinally to remove cartilages and adjacent rib segments, preserving perichondria and periostea. A full-thickness quadrangular chest wall flap pedicled on internal thoracic vessels was then elevated and shaped into a neo conduit to replace the trachea with the pleura as an inner lining.

RESULTS

Various types of extended non-circumferential and full-circumferential tracheal replacements were achieved with this composite flap. No anastomosis tension was noticed despite the absence of release manoeuvres.

CONCLUSIONS

This model could represent a suitable autologous tracheal substitute, which is long, longitudinally flexible and eventually transversely rigid. No microsurgical vascular anastomoses are required. The technique is reproducible. The perichondria and periostea would regenerate vascularized neo-cartilaginous rings, potentially decreasing the need for long-term stenting. The inner pleural lining could potentially transform into ciliated epithelium as shown in previous preclinical studies.

Regeneration Process of an Autologous Tissue-Engineered Trachea (aTET) in a Rat Patch Tracheoplasty Model

The treatment of long-tracheal lesion is difficult because there are currently no viable grafts for tracheal replacement. To solve this problem, we have developed an autologous Tissue-Engineered Trachea (aTET), which is made up of collagenous tissues and cartilage-like structures derived from rat chondrocytes. This graft induced successful long-term survival in a small-animal experiment in our previous study. In this study, we investigated the regeneration process of an aTET to attain reproducible success. We prepared an aTET by using a specially designed mold and performed patch tracheoplasty with an aTET. We assigned twenty-seven rats to three groups according to the three types of patch grafts used: aTET patches (the aTET group), fresh tracheal autograft patches (the Ag group), or polylactic acid and polycaprolactone copolymer sheets (the PPc group). In each group, gross and histological evaluations were performed at 1 month (n = 3), 3 months (n = 3), and 6 months (n = 3) after implantation. We obtained high survival rates in all groups, but only the PPc group attained thick tracheal walls with granular tissues and no tracheal regeneration. On the other hand, the aTET and Ag groups reproducibly achieved complete tracheal regeneration in 6 months. So, an aTET could be a promising candidate for tracheal regeneration grafts.

Tracheal Tissue Engineering: Principles and State of the Art

Patients affected by long-segment tracheal defects or stenoses represent an unsolved surgical issue, since they cannot be treated with the conventional surgery of tracheal resection and consequent anastomosis. Hence, different strategies for tracheal replacement have been proposed (synthetic materials, aortic allografts, transplantation, autologous tissue composites, and tissue engineering), each with advantages and drawbacks. Tracheal tissue engineering, on the other hand, aims at recreating a fully functional tracheal substitute, without the need for the patient to receive lifelong immunosuppression or endotracheal stents. Tissue engineering approaches involve the use of a scaffold, stem cells, and humoral signals. This paper reviews the main aspects of tracheal TE, starting from the choice of the scaffold to the type of stem cells that can be used to seed the scaffold, the methods for their culture and expansion, the issue of graft revascularization at the moment of in vivo implantation, and experimental models of tracheal research. Moreover, a critical insight on the state of the art of tracheal tissue engineering is also presented.

Changes in the Proportion of Each Cell Type After hiPSC-Derived Airway Epithelia Transplantation

No radical treatment is available for the regeneration of dysfunction and defects in airway epithelia. Artificial tracheae made of polypropylene and collagen sponge were used in clinical studies to reconstitute tracheae after resection. For early epithelialization of the luminal surface of the artificial trachea, a model was established, that is, an artificial trachea covered with human-induced pluripotent stem cell-derived airway epithelial cells (hiPSC-AECs) was transplanted into a tracheal defect in an immunodeficient rat. Unlike the cell types of hiPSC-derived cells that are currently used in clinical studies, AECs maintain tissues by proliferation and differentiation of basal cells into various cell types that constitute AECs constantly. Therefore, post-transplantation, the proportion of each cell type, such as ciliated and goblet cells, may change; however, no studies have examined this possibility. In this study, using our hiPSC-AEC-transplanted rat model, we investigated changes in the proportion of each cell type in hiPSC-AECs pre-transplantation and post-transplantation. As a result, the proportion of each cell type changed post-transplantation. The proportion of ciliated, basal, and club cells increased, and the proportion of goblet cells decreased post-transplantation. In addition, the proportion of each cell type in engrafted hiPSC-AECs is more similar to the proportion of each cell type in normal proximal airway tissue than the proportion of each cell type pre-transplantation. The results of this study are useful for the development of therapeutic techniques using hiPSC-AEC transplantation.

Surgical Innovations in Tracheal Reconstruction: A Review on Synthetic Material Fabrication

Background and Objectives: The aim of this review is to explore the recent surgical innovations in tracheal reconstruction by evaluating the uses of synthetic material fabrication when dealing with tracheomalacia or stenotic pathologies, then discussing the challenges holding back these innovations. Materials and Methods: A targeted non-systematic review of published literature relating to tracheal reconstruction was performed within the PubMed database to help identify how synthetic materials are utilised to innovate tracheal reconstruction. Results: The advancements in 3D printing to aid synthetic material fabrication have unveiled promising alternatives to conventional approaches. Achieving successful tracheal reconstruction through this technology demands that the 3D models exhibit biocompatibility with neighbouring tracheal elements by encompassing vasculature, chondral foundation, and immunocompatibility. Tracheal reconstruction has employed grafts and scaffolds, showing a promising beginning in vivo. Concurrently, the integration of resorbable models and stem cell therapy serves to underscore their viability and application in the context of tracheal pathologies. Despite this, certain barriers hinder its advancement in surgery. The intricate tracheal structure has posed a challenge for researchers seeking novel approaches to support its growth and regeneration. Conclusions: The potential of synthetic material fabrication has shown promising outcomes in initial studies involving smaller animals. Yet, to fully realise the applicability of these innovative developments, research must progress toward clinical trials. These trials would ascertain the anatomical and physiological effects on the human body, enabling a thorough evaluation of post-operative outcomes and any potential complications linked to the materials or cells implanted in the trachea.

Evaluation of a decellularized bronchial patch transplant in a porcine model

Biological scaffolds for airway reconstruction are an important clinical need and have been extensively investigated experimentally and clinically, but without uniform success. In this study, we evaluated the use of a decellularized bronchus graft for airway reconstruction. Decellularized left bronchi were procured from decellularized porcine lungs and utilized as grafts for airway patch transplantation. A tracheal window was created and the decellularized bronchus was transplanted into the defect in a porcine model. Animals were euthanized at 7 days, 1 month, and 2 months post-operatively. Histological analysis, immunohistochemistry, scanning electron microscopy, and strength tests were conducted in order to evaluate epithelialization, inflammation, and physical strength of the graft. All pigs recovered from general anesthesia and survived without airway obstruction until the planned euthanasia timepoint. Histological and electron microscopy analyses revealed that the decellularized bronchus graft was well integrated with native tissue and covered by an epithelial layer at 1 month. Immunostaining of the decellularized bronchus graft was positive for CD31 and no difference was observed with immune markers (CD3, CD11b, myeloperoxidase) at two months. Although not significant, tensile strength was decreased after one month, but recovered by two months. Decellularized bronchial grafts show promising results for airway patch reconstruction in a porcine model. Revascularization and re-epithelialization were observed and the immunological reaction was comparable with the autografts. This approach is clinically relevant and could potentially be utilized for future applications for tracheal replacement.

Tracheal replacement with aortic grafts: Bench to clinical practice

Tracheal reconstruction following extensive resection for malignant or benign lesions remains a major challenge in thoracic surgery. Numerous studies have attempted to identify the optimal tracheal replacement with different biological or prosthetic materials, such as various homologous and autologous tissues, with no encouraging outcomes. Recently, a few clinical studies reported attaining favorable outcomes using in vitro or stem cell-based airway engineering and also with tracheal allograft implantation following heterotopic revascularization. However, none of the relevant studies offered a standardized technology for airway replacement. In 1997, a novel approach to airway reconstruction was proposed, which involved using aortic grafts as the biological matrix. Studies on animal models reported achieving in-vivo cartilage and epithelial regeneration using this approach. These encouraging results inspired the subsequent application of cryopreserved aortic allografts in humans for the first time. Cryopreserved aortic allografts offered further advantages, such as easy availability in tissue banks and no requirement for immunosuppressive treatments. Currently, stented aortic matrix-based airway replacement has emerged as a standard approach, and its effectiveness was also verified in the recently reported TRITON-01 study. In this context, the present review aims to summarize the current status of the application of aortic grafts in tracheal replacement, including the latest advancements in experimental and clinical practice.

Application of aortic allograft in trachea transplantation

BACKGROUND

The use of tracheal implants for tracheal reconstruction remains a challenge in thoracic medicine due to the complex structure of the trachea in mammalian organisms, including smooth muscles, cartilage, mucosa, blood vessels, cilia, and other tissues, and the difficulty in achieving tracheal regeneration using implants from either allografts or synthetic biomaterials.

METHODS

This project used the Lee-Sung strain pig, a swine breed local to Taiwan, as the experimental subject. The aorta of the pig was harvested, decellularized to form the scaffold, and transplanted into the trachea of allogeneic pigs together with growth factors. Postoperative physiological function and tissue changes were observed. The postoperative physiological parameters of the LSP were monitored, and they were sacrificed after a certain period to observe the pathological changes in the tracheal epithelial cells and cartilages.

RESULTS

Overall, six LSP tracheal transplantations were performed between March 4, 2020, and March 10, 2021. These included aortic patch anastomosis for pig 1 and aortic segmental anastomosis for pigs 2-6. The shortest and longest survival periods were 1 day and 147 days, respectively. Excluding the pig that survived for only 1 day due to a ruptured graft anastomosis, all other subjects survived for over 1 month on average.

CONCLUSION

In this study, we grafted a decellularized porcine aorta into a recipient pig with a tracheal defect. We found cryopreservation of the allogeneic aorta transplantation was a feasible and safe method for the management of airway disease, and immunosuppressants were unnecessary during the treatment course.

Evaluation of Structural Viability of Porcine Tracheal Scaffolds after 3 and 6 Months of Storage under Three Different Protocols

Tracheal replacement with a bioengineered tracheal substitute has been developed for long-segment tracheal diseases. The decellularized tracheal scaffold is an alternative for cell seeding. It is not defined if the storage scaffold produces changes in the scaffold's biomechanical properties. We tested three protocols for porcine tracheal scaffold preservation immersed in PBS and alcohol 70%, in the fridge and under cryopreservation. Ninety-six porcine tracheas (12 in natura, 84 decellularized) were divided into three groups (PBS, alcohol, and cryopreservation). Twelve tracheas were analyzed after three and six months. The assessment included residual DNA, cytotoxicity, collagen contents, and mechanical properties. Decellularization increased the maximum load and stress in the longitudinal axis and decreased the maximum load in the transverse axis. The decellularization of the porcine trachea produced structurally viable scaffolds, with a preserved collagen matrix suitable for further bioengineering. Despite the cyclic washings, the scaffolds remained cytotoxic. The comparison of the storage protocols (PBS at 4 °C, alcohol at 4 °C, and slow cooling cryopreservation with cryoprotectants) showed no significant differences in the amount of collagen and in the biomechanical properties of the scaffolds. Storage in PBS solution at 4 °C for six months did not change the scaffold mechanics.

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.

Decellularization compromises mechanical and structural properties of the native trachea

Tracheal replacement using tissue engineering technologies offers great potential to improve previously intractable clinical interventions, and interest in this area has increased in recent years. Many engineered airway constructs currently rely on decellularized native tracheas to serve as the scaffold for tissue repair. Yet, mechanical failure leading to airway narrowing and collapse remains a major cause of morbidity and mortality following clinical implantation of decellularized tracheal grafts. To understand better the factors contributing to mechanical failure in vivo, we characterized the histo-mechanical properties of tracheas following two different decellularization protocols, including one that has been used clinically. All decellularized tracheas deviated from native mechanical behavior, which may provide insights into observed in vivo graft failures. We further analyzed protein content by western blot and analyzed microstructure by histological staining and found that the specific method of decellularization resulted in significant differences in the depletion of proteoglycans and degradation of collagens I, II, III, and elastin. Taken together, this work demonstrates that the heterogeneous architecture and mechanical behavior of the trachea is severely compromised by decellularization. Such structural deterioration may contribute to graft failure clinically and limit the potential of decellularized native tracheas as viable long-term orthotopic airway replacements.

A facile 3D bio-fabrication of customized tubular scaffolds using solvent-based extrusion printing for tissue-engineered tracheal grafts

Tracheal implantation remains a major therapeutic challenge due to the unavailability of donors and the lack of biomimetic tubular grafts. Fabrication of biomimetic tracheal scaffolds of suitable materials with matched rigidity, enhanced flexibility and biocompatibility has been a major challenge in the field of tracheal reconstruction. In this study, customized tubular grafts made up of FDA-approved polycaprolactone ( PCL ) and polyurethane ( PU ) were fabricated using a novel solvent-based extrusion 3D printing. The printed scaffolds were investigated by various physical, thermal, and mechanical characterizations such as contact angle measurement, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), radial compression, longitudinal compression, and cyclic radial compression. In this study, the native goat trachea was used as a reference for the fabrication of different types of scaffolds (cylindrical, bellow-shaped, and spiral-shaped). The mechanical properties of the goat trachea were also compared to find suitable formulations of PCL / PU . Spiral-shaped scaffolds were found to be an ideal shape based on longitudinal compression and torsion load maintaining clear patency. To check the long-term implantation, in vitro degradation test was performed for all the 3D printed scaffolds and it was found that blending of PU with PCL reduced the degradation behavior. The printed scaffolds were further evaluated for biocompatibility assay, live/dead assay, and cell adhesion assay using bone marrow-derived human mesenchymal stem cells (hMSCs). From biomechanical and biological assessments, PCL 70 / PU 30 of spiral-shaped scaffolds could be a suitable candidate for the development of tracheal regenerative applications.

Perpendicular implantation of porcine trachea extracellular matrix for enhanced xenogeneic scaffold surface epithelialization in a canine model

Objective

The availability of clinically applied medical materials in thoracic surgery remains insufficient, especially materials for treating tracheal defects. Herein, the potential of porcine extracellular matrix (P-ECM) as a new airway reconstruction material was explored by xenotransplanting it into a canine trachea.

Methods

P-ECM was first transplanted into the buttocks of Narc Beagle dogs (n = 3) and its overall immuno-induced effects were evaluated. Subsequently, nine dogs underwent surgery to create a tracheal defect that was 1 × 2 cm. In group A, the P-ECM was implanted parallel to the tracheal axis (n = 3), whereas in group B the P-ECM was implanted perpendicular to the tracheal axis (n = 6). The grafts were periodically observed by bronchoscopy and evaluated postoperatively at 1 and 3 months through macroscopic and microscopic examinations. Immunosuppressants were not administered. Statistical evaluation was performed for Bronchoscopic stenosis rate, graft epithelialization rate, shrinkage rate and ECM live-implantation rate.

Results

No sign of P-ECM rejection was observed after its implantation in the buttocks. Bronchoscopic findings showed no improvement concerning stenosis in group A until 3 months after surgery; epithelialization of the graft site was not evident, and the ECM site appeared scarred and faded. In contrast, stenosis gradually improved in group B, with continuous epithelium within the host tissues and P-ECM. Histologically, the graft site contracted longitudinally and no epithelialization was observed in group A, whereas full epithelialization was observed on the P-ECM in group B. No sign of cartilage regeneration was confirmed in both groups. No statistically significant differences were found in bronchoscopic stenosis rate, shrinkage rate and ECM live-implantation rate, but graft epithelialization rate showed a statistically significant difference (G-A; sporadic (25%) 3, vs. G-B; full covered (100%) 3; p = 0.047).

Conclusions

P-ECM can support full re-epithelialization without chondrocyte regeneration, with perpendicular implantation facilitating epithelialization of the ECM. Our results showed that our decellularized tracheal matrix holds clinical potential as a biological xenogeneic material for airway defect repair.

Preclinical and clinical orthotopic transplantation of decellularized/engineered tracheal scaffolds: A systematic literature review

Severe tracheal injuries that cannot be managed by mobilization and end-to-end anastomosis represent an unmet clinical need and an urgent challenge to face in surgical practice; within this scenario, decellularized scaffolds (eventually bioengineered) are currently a tempting option among tissue engineered substitutes. The success of a decellularized trachea is expression of a balanced approach in cells removal while preserving the extracellular matrix (ECM) architecture/mechanical properties. Revising the literature, many Authors report about different methods for acellular tracheal ECMs development; however, only few of them verified the devices effectiveness by an orthotopic implant in animal models of disease. To support translational medicine in this field, here we provide a systematic review on studies recurring to decellularized/bioengineered tracheas implantation. After describing the specific methodological aspects, orthotopic implant results are verified. Furtherly, the only three clinical cases of compassionate use of tissue engineered tracheas are reported with a focus on outcomes.

Laparoscopic Transhiatal Esophagectomy With En Bloc Posterior Tracheal Wall Resection

Esophageal adenoid cystic carcinoma is a rare cancer that is a challenge to treat because the tumor often invades local structures. Complete resection with grossly negative margins is key to disease-free survival. We describe a case in which the esophageal tumor invaded a significant portion of the posterior trachea, making a tracheal resection with primary anastomosis impossible. Therefore, to resect the tumor completely, we performed a laparoscopic esophagectomy and posterior tracheal resection with tracheoplasty using a rotational flap while the patient was on venovenous extracorporeal membrane oxygenation.

Skin-derived epithelial lining facilitates orthotopic tracheal transplantation by protecting the tracheal cartilage and inhibiting granulation hyperplasia

Long-segment tracheal defects caused by tumours, inflammation or trauma can cause serious damage to the quality of life of patients. Although many novel neotracheas have been constructed, the therapeutic effect of orthotopic transplantation was compromised mainly because of the lack of an epithelial lining in those neotracheas. In this study, we aimed to investigate the therapeutic function of skin-derived epithelial lining for orthotopic tracheal transplantation. Strips of auricular cartilage with fixed interval were interrupted sutured on a silicone tube to mimic the cartilage rings of the native trachea. Neotrachea in the with epithelium group retained the unilateral skin as the epithelial lining in the lumen, whereas the neotrachea in the without epithelium group consisted solely of cartilage strips. After revascularized in the sternohyoid muscle, 2-cm-long tracheal defects were made and were reconstructed using these neotracheas. Our results showed that the skin-derived epithelial lining simultaneously protected the engineered tracheal cartilage and inhibited granulation hyperplasia in the tracheal lumen; further, compared with the without epithelium group, the group with epithelium showed a marked improvement in the tracheal lumen patency and the survival rate of rabbits. Our study provides a critical cue for improvements in the repair of tracheal defects via skin-derived epithelial lining and may significantly advance the clinical translation of tissue-engineered trachea.

A Review of Woven Tracheal Stents: Materials, Structures, and Application

The repair and reconstruction of tracheal defects is a challenging clinical problem. Due to the wide choice of materials and structures, weaving technology has shown unique advantages in simulating the multilayer structure of the trachea and providing reliable performance. Currently, most woven stent-based stents focus only on the effect of materials on stent performance while ignoring the direct effect of woven process parameters on stent performance, and the advantages of weaving technology in tissue regeneration have not been fully exploited. Therefore, this review will introduce the effects of stent materials and fabric construction on the performance of tracheal stents, focusing on the effects of weaving process parameters on stent performance. We will summarize the problems faced by woven stents and possible directions of development in the hope of broadening the technical field of artificial trachea preparation.

Evaluation of Biomechanical Properties and Morphometric Structures of the Trachea in Pigs and Rabbits

BACKGROUND/AIM

Animals differ in the biochemical composition, attachments, and mechanical properties of tracheal cartilage. This study examined the biomechanical properties and morphological structure of the trachea of pigs, and rabbits as preclinical models.

MATERIALS AND METHODS

The trachea in pigs and rabbits can be divided into four regions, cranial cervical, middle cervical, thoracic inlet, and intra-thoracic parts.

RESULTS

The total number of tracheal rings in pigs and rabbits was 32-35 and 34-38 rings, respectively. The pig bronchus first branches from the trachea, reaching the cranial lobe of the lungs before branching to the main bronchus, while the rabbit bronchus branched after the main bronchus. A comparison of the posterior region of the crosssectional trachea shows that the rabbit has a C-shape with cartilage connected to the tracheal muscle, and the pig has the tracheal muscle covered with cartilage. The trachea of pigs and rabbits decreased in tracheal thickness and size from the thoracic inlet toward the lungs. The stress-strain in the longitudinal and transverse tensile test was higher in rabbits than in pigs. The tensile stress of the four regions was significantly different in the transverse tensile test (p<0.001). In the bending test, more force was required to bend pig than rabbit tracheas. Microscopic and scanning electron microscopy showed no structural differences in tracheal cartilage between the two species.

CONCLUSION

These results suggest that there is great variation in morphology and physical properties of the trachea in pigs and rabbits. We found porcine tracheas have similar biomechanical properties to those of humans.

The Growing Medical Need for Tracheal Replacement: Reconstructive Strategies Should Overcome Their Limits

Breathing, being predominantly an automatic action, is often taken for granted. However, respiratory diseases affect millions of people globally, emerging as one of the major causes of disability and death overall. Among the respiratory dysfunctions, tracheal alterations have always represented a primary challenge for clinicians, biologists, and engineers. Indeed, in the case of wide structural alterations involving more than 50% of the tracheal length in adults or 30% in children, the available medical treatments are ineffective or inapplicable. So far, a plethora of reconstructive approaches have been proposed and clinically applied to face this growing, unmet medical need. Unfortunately, none of them has become a well-established and routinely applied clinical procedure to date. This review summarizes the main clinical reconstructive attempts and classifies them as non-tissue engineering and tissue engineering strategies. The analysis of the achievements and the main difficulties that still hinder this field, together with the evaluation of the forefront preclinical experiences in tracheal repair/replacement, is functional to promote a safer and more effective clinical translation in the near future.

Tissue Engineering for Tracheal Replacement: Strategies and Challenges

The critical feature in trachea replacement is to provide a hollow cylindrical framework that is laterally stable and longitudinally flexible, facilitating cartilage and epithelial tissue formation. Despite advanced techniques and sources of materials used, most inherent challenges are related to the complexity of its anatomy. Limited blood supply leads to insufficient regenerative capacity for cartilage and epithelium. Natural and synthetic scaffolds, different types of cells, and growth factors are part of tissue engineering approaches with varying outcomes. Pre-vascularization remains one of the crucial factors to expedite the regenerative process in tracheal reconstruction. This review discusses the challenges and strategies used in tracheal tissue engineering, focusing on scaffold implantation in clinical and preclinical studies conducted in recent decades.

Tracheal reconstruction with the scaffolded cartilage sheets in an orthotopic animal model

Tracheal reconstruction remains challenged in clinical. We aimed to fabricate scaffolded cartilage sheets with rigid and elastic supports for tracheal reconstruction. The chondrocyte cell-infiltration activity was examined in the polycaprolactone sheet scaffolds with various thicknesses and pore sizes after seeding cells on the top surface of the sheet scaffolds. The expression of cartilage-related genes and accumulation of sulfated glycosaminoglycans was elevated in the cells-scaffold composites upon the chondrogenic induction. Mechanical properties of the cartilage sheets were measured by the 3-point flexural test and vertical compression test. Two tracheal defects were replaced with and cartilage sheets implants in a rabbit model for 16 weeks. The formation of the cartilaginous tissues, fibrous tissues, and airway epithelium was observed by Safranin O, Masson trichrome, and hematoxylin & eosin Y histological stains, respectively. The generation of micro-vessels, granulation tissue, and adipose tissues in the tracheal explants were analyzed with immunohistochemistry staining. Finally, cartilage sheets could be a reconstructive therapy candidate applying in reconstructing defects in the trachea and other tissues composed of cartilage.

In vivo biocompatibility analysis of the recellularized canine tracheal scaffolds with canine epithelial and endothelial progenitor cells

Decellularized extracellular matrix (ECM) has frequently been applied as a biomaterial for tissue engineering purposes. When implanted, their role can be essential for partial trachea replacement in patients that require a viable transplant solution. Acellular canine tracheal scaffolds with preserved ECM structure, flexibility, and proteins were obtained by high pressure vacuum decellularization. Here, we aimed to evaluate the cell adhesion and proliferation of canine tracheal epithelial cells (EpC) and canine yolk sac endothelial progenitor cells (YS) cultivated on canine decellularized tracheal scaffolds and test the in vivo biocompatibility of these recellularized scaffolds implanted in BALB-c nude mice. In order to evaluate the recellularization efficiency, scaffolds were evaluated by scanning electron microscopy (SEM), immunofluorescence, DNA quantification, mycoplasma test, and in vivo biocompatibility. The scaffolds sterility was confirmed, and EpC and YS cells were cultured by 7 and 14 days. We demonstrated by SEM, immunofluorescence, and genomic DNA analyzes cell adhesion to tracheal ECM. Then, recellularized scaffolds were in vivo subcutaneously implanted in mice and after 45 days, the fragments were collected and analyzed by Hematoxylin-Eosin and Gömori Trichrome staining and PCNA, CD4, CD8, and CD68 immunohistochemistry. In vivo results confirmed that the implanted tissue remains preserved and proliferative, and no fibrotic tissue process was observed in animals. Finally, our results showed the recellularization success due the preserved ECM proteins, and that these may be suitable to future preclinical studies applications for partial trachea replacement in tissue engineering.

Development and clinical translation of tubular constructs for tracheal tissue engineering: a review

Effective restoration of extensive tracheal damage arising from cancer, stenosis, infection or congenital abnormalities remains an unmet clinical need in respiratory medicine. The trachea is a 10-11 cm long fibrocartilaginous tube of the lower respiratory tract, with 16-20 tracheal cartilages anterolaterally and a dynamic trachealis muscle posteriorly. Tracheal resection is commonly offered to patients suffering from short-length tracheal defects, but replacement is required when the trauma exceeds 50% of total length of the trachea in adults and 30% in children. Recently, tissue engineering (TE) has shown promise to fabricate biocompatible tissue-engineered tracheal implants for tracheal replacement and regeneration. However, its widespread use is hampered by inadequate re-epithelialisation, poor mechanical properties, insufficient revascularisation and unsatisfactory durability, leading to little success in the clinical use of tissue-engineered tracheal implants to date. Here, we describe in detail the historical attempts and the lessons learned for tracheal TE approaches by contextualising the clinical needs and essential requirements for a functional tracheal graft. TE manufacturing approaches explored to date and the clinical translation of both TE and non-TE strategies for tracheal regeneration are summarised to fully understand the big picture of tracheal TE and its impact on clinical treatment of extensive tracheal defects.

Decellularized tracheal scaffolds in tracheal reconstruction: An evaluation of different techniques

In humans, the trachea is a conduit for ventilation connecting the throat and lungs. However, certain congenital or acquired diseases may cause long-term tracheal defects that require replacement. Tissue engineering is considered a promising method to reconstruct long-segment tracheal lesions and restore the structure and function of the trachea. Decellularization technology retains the natural structure of the trachea, has good biocompatibility and mechanical properties, and is currently a hotspot in tissue engineering studies. This article lists various recent representative protocols for the generation of decellularized tracheal scaffolds (DTSs), as well as their validity and limitations. Based on the advancements in decellularization methods, we discussed the impact and importance of mechanical properties, revascularization, recellularization, and biocompatibility in the production and implantation of DTS. This review provides a basis for future research on DTS and its application in clinical therapy.

Successful Total Tracheal Replacement by Cryopreserved Aortic Allograft in a Patient Post-COVID-19 Infection

This is the first report to our knowledge of a successful total tracheal replacement in a post-COVID-19 patient by cryopreserved aortic allograft. The graft was anastomosed to the cricoid and carina; a silicon stent was inserted to ensure patency. The patient was extubated on the operative table and was immediately able to breathe, speak, and swallow. No immunosuppression was administered. Three weeks after surgery, the patient was discharged from hospital in excellent health, and was able to resume his normal lifestyle, work, and activity as an amateur cyclist. Two months after surgery, the patient assumes aerosol with saline solution three times per day and no other therapy; routine bronchoscopy to clear secretions is no longer needed.

Isolation of multipotent progenitor cells from pleura and pericardium for tracheal tissue engineering purposes

Tissue engineering (TE) of long tracheal segments is conceptually appealing for patients with inoperable tracheal pathology. In tracheal TE, stem cells isolated from bone marrow or adipose tissue have been employed, but the ideal cell source has yet to be determined. When considering the origin of stem cells, cells isolated from a source embryonically related to the trachea may be more similar. In this study, we investigated the feasibility of isolating progenitor cells from pleura and pericard as an alternative cells source for tracheal tissue engineering. Porcine progenitor cells were isolated from pleura, pericard, trachea and adipose tissue and expanded in culture. Isolated cells were characterized by PCR, RNA sequencing, differentiation assays and cell survival assays and were compared to trachea and adipose‐derived progenitor cells. Progenitor‐like cells were successfully isolated and expanded from pericard and pleura as indicated by gene expression and functional analyses. Gene expression analysis and RNA sequencing showed a stem cell signature indicating multipotency, albeit that subtle differences between different cell sources were visible. Functional analysis revealed that these cells were able to differentiate towards chondrogenic, osteogenic and adipogenic lineages. Isolation of progenitor cells from pericard and pleura with stem cell features is feasible. Although functional differences with adipose‐derived stem cells were limited, based on their gene expression, pericard‐ and pleura‐derived stem cells may represent a superior autologous cell source for cell seeding in tracheal tissue engineering.

Short-Term Preclinical Application of Functional Human Induced Pluripotent Stem Cell-Derived Airway Epithelial Patches

Airway pathologies including cancer, trauma, and stenosis lack effective treatments, meanwhile airway transplantation and available tissue engineering approaches fail due to epithelial dysfunction. Autologous progenitors do not meet the clinical need for regeneration due to their insufficient expansion and differentiation, for which human induced pluripotent stem cells (hiPSCs) are promising alternatives. Airway epithelial patches are engineered by differentiating hiPSC-derived airway progenitors into physiological proportions of ciliated (73.9 ± 5.5%) and goblet (2.1 ± 1.4%) cells on a silk fibroin-collagen vitrigel membrane (SF-CVM) composite biomaterial for transplantation in porcine tracheal defects ex vivo and in vivo. Evaluation of ex vivo tracheal repair using hiPSC-derived SF-CVM patches demonstrate native-like tracheal epithelial metabolism and maintenance of mucociliary epithelium to day 3. In vivo studies demonstrate SF-CVM integration and maintenance of airway patency, showing 80.8 ± 3.6% graft coverage with an hiPSC-derived pseudostratified epithelium and 70.7 ± 2.3% coverage with viable cells, 3 days postoperatively. The utility of bioengineered, hiPSC-derived epithelial patches for airway repair is demonstrated in a short-term preclinical survival model, providing a significant leap for airway reconstruction approaches.

Mechanical Properties and In Vitro Evaluation of Thermoplastic Polyurethane and Polylactic Acid Blend for Fabrication of 3D Filaments for Tracheal Tissue Engineering

Surgical reconstruction of extensive tracheal lesions is challenging. It requires a mechanically stable, biocompatible, and nontoxic material that gradually degrades. One of the possible solutions for overcoming the limitations of tracheal transplantation is a three-dimensional (3D) printed tracheal scaffold made of polymers. Polymer blending is one of the methods used to produce material for a trachea scaffold with tailored characteristics. The purpose of this study is to evaluate the mechanical and in vitro properties of a thermoplastic polyurethane (TPU) and polylactic acid (PLA) blend as a potential material for 3D printed tracheal scaffolds. Both materials were melt-blended using a single screw extruder. The morphologies (as well as the mechanical and thermal characteristics) were determined via scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, tensile test, and Differential Scanning calorimetry (DSC). The samples were also evaluated for their water absorption, in vitro biodegradability, and biocompatibility. It is demonstrated that, despite being not miscible, TPU and PLA are biocompatible, and their promising properties are suitable for future applications in tracheal tissue engineering.

Tracheal Repair with Human Umbilical Cord Mesenchymal Stem Cells Differentiated in Chondrocytes Grown on an Acellular Amniotic Membrane: A Pre-Clinical Approach

Acellular amniotic membrane (AM) has been studied, with promising results on the reconstruction of lesioned tissues, and has become an attractive approach for tracheal repair. This study aimed to evaluate the repair of the trachea with human umbilical cord mesenchymal stem cells (hucMSCs) differentiated in chondrocytes, grown on an experimental model. Tracheal defects were induced by surgical tracheostomy in 30 New Zealand rabbits, and the acellular amniotic membrane, with or without cells, was covering the defect. The hucMSCs were isolated and cultivated with chondrogenic differentiation over the culture of 14 days, and then grown on the AM. In this study, the AM was biocompatible and hucMSCs differentiated into chondrocytes. Our results demonstrated an important role for AM with cultured cells in the promotion of immature collagen, known to produce tissue regeneration. In addition, cartilaginous tissue was found at the tracheal defects, demonstrated by immunohistology results. This study suggests that this biomaterial implantation can be an effective future therapeutic alternative for patients with tracheal injury.

In vivo performance of decellularized tracheal grafts in the reconstruction of long length tracheal defects: Experimental study

BACKGROUND

The repair of long-segment tracheal lesions remains an important challenge. Nowdays no predictable and dependable substitute has been found. Decellularized tracheal scaffolds have shown to be a promising graft for tracheal transplantation, since it is non-immunogenic.

OBJECTIVE

Evaluate in vivo decellularized tracheal allografts performance to replace long tracheal segment.

METHODS

Forty-five swines underwent surgery as follows: Fifteen trachea donors and 30 receptors of decellularized trachea allografts. The receptors were randomly divided in five groups (n = 6). In groups I and II, donor trachea segment was decellularized by 15 cycles with sodium deoxycholate and deoxyribonuclease, in group II, the allograft was reinforced with external surgical steel wire. Groups, III, IV, and V decellularization was reduced to seven cycles, supplemented with cryopreservation in group IV and with glutaraldehyde in group V. A 10 rings segment was excised from the receptor swine and the decellularized trachea graft was implanted to re-establish trachea continuity.

RESULTS

Both decellularization cycles caused decreased stiffness. All trachea receptors underwent euthanasia before the third post-implant week due to severe dyspnea and trachea graft stenosis, necrosis, edema, inflammation, hemorrhage, and granulation tissue formation in anastomotic sites. Histologically all showed total loss of epithelium, separation of collagen fibers, and alterations in staining.

CONCLUSIONS

Both decellularization techniques severely damaged the structure of the trachea and the extracellular matrix of the cartilage, resulting in a no functional graft, in spite of the use of surgical wire, cryopreservation or glutaraldehyde treatment. An important drawback was the formation of fibrotic stenosis in both anastomosis.

The History of Engineered Tracheal Replacements: Interpreting the Past and Guiding the Future

The development of a tracheal graft to replace long-segment defects has thwarted clinicians and engineers alike for over 100 years. To better understand the challenges facing this field today, we have consolidated all published reports of engineered tracheal grafts used to repair long-segment circumferential defects in humans, from the first in 1898 to the most recent in 2018, totaling 290 clinical cases. Distinct trends emerge in the types of grafts used over time, including repair using autologous fascia, rigid tubes of various inert materials, and pretreated cadaveric allografts. Our analysis of maximum clinical follow-up, as a proxy for graft performance, revealed that the Leuven protocol has a significantly longer clinical follow-up time than all other methods of airway reconstruction. This method involves transplanting a cadaveric tracheal allograft that is first prevascularized heterotopically in the recipient. We further quantified graft-related causes of mortality, revealing failure modes that have been resolved, and those that remain a hurdle, such as graft mechanics. Finally, we briefly summarize recent preclinical work in tracheal graft development. In conclusion, we synthesized top clinical care priorities and design criteria to inform and inspire collaboration between engineers and clinicians toward the development of a functional tracheal replacement graft. Impact statement The field of tracheal engineering has floundered in recent years due to multiple article retractions. However, with recent advances in biofabrication and tissue analysis techniques, the field remains ripe for advancement through collaboration between engineers and clinicians. With a long history of clinical application of tracheal replacements, engineered tracheas are arguably the regenerative technology with the greatest potential for translation. This work describes the many phases of engineered tracheal replacements that have been applied in human patients over the past 100 years with the goal of carrying forward critical lessons into development of the next generation of engineered tracheal graft.

Decellularized tracheal prelamination implant: A proposed bilateral double organ technique

In tracheal replacement transplantation, prelamination is a critical stage. Nowadays, the most widely used prelamination technique is the prethoracic fascia flap with lateral thoracic artery. We propose a flap based on the internal thoracic artery, which allows a relatively non-aggressive double organ implant, and we have tested its efficacy in decellularized tracheas. Tracheas of albino New Zealand rabbits were decellularized following a protocol that uses detergents and cryogenization, sterilized with 1kGy gamma radiation, and tutorized with a stent. Bilateral pedicled flaps made of pectoral fascia and a muscular component were harvested through a longitudinal 3-cm central thoracic incision, wrapping the tracheas with them in 16 rabbits, remaining them implanted for 2, 4, 8, and 12 weeks. The tracheas were then studied histologically using standard stainings plus immunohistochemistry (CD31). The models were adjusted with Bayesian statistics using ordinal regression; results as odds ratios and credibility intervals. All analysis were performed using R software. Acute inflammatory cell invasion was observed at 2 weeks, which almost disappeared at week 8 after implant. Only macrophages and giant cells increased between Weeks 8 and 12 (OR 10.487, CI [1.603-97.327]). The cartilage maintained its structure, with slight signs of ischemia in a few cases. New CD31-positive vessels were observed from Week 2 and increasing thereafter, reaching a maximum peak at Week 8. We propose a bilateral implant technique that is viable and effective as a prelamination option for two concurrent tracheas, achieving perfect vascularization and integration of the organ with hardly any inflammatory response in the medium or long term.

Statistical modelling and optimization of print quality and mechanical properties of customized tubular scaffolds fabricated using solvent-based extrusion 3D printing process

Tissue-engineered tubular scaffolds offer huge potential to heal or replace the diseased organ parts like blood vessels, trachea, oesophagus and ureter. However, manufacturing these scaffolds in various scales and shapes is always challenging and requires progressive technology. Developing a flexible and accurate manufacturing method is a major developmental direction in the field of tubular scaffold fabrication. In this context, the present work presents a novel solvent-based extrusion 3D printing which allows extruding over a rotating mandrel to fabricate tubular scaffolds of polycaprolactone (PCL) and polyurethane (PU). Experimental runs were planned as per the central composite design (CCD) to evaluate the effects of input parameters like infill density, layer thickness, print speed and percentage of PU on the output responses like printing quality and mechanical characteristics. The printing quality was quantified by measuring average surface roughness of the printed scaffolds and mechanical properties were evaluated by measuring radial compressive load, and percentage of elongation. The experimental investigations revealed that printing quality was improved at higher infill densities and was deteriorated at higher print speeds and layer thicknesses. Similarly, the radial compressive load was improved with the increase in infill density and was decreased with an increase in layer thickness, print speed and percentage of PU. The percentage of elongation was found to improve at higher infill densities and percentages of PU and was reduced with an increase in layer thickness and print speed. Additionally, a multi-objective optimization using Genetic Algorithm was used to evaluate the optimum conditions to minimize surface roughness and maximizing radial compression load and percentage of elongation. Finally, a case study was performed by comparing the mechanical properties of tubular organs and scaffolds from the existing reports and results of the present work.

Replacement of a 5-cm intrathoracic trachea with a tissue-engineered prosthesis in a canine model

BACKGROUND

Critical obstacles must be addressed before clinical application of artificial tracheas. The major complications of long tracheal replacement include anastomotic dehiscence and stenosis owing to poor vascularity and incomplete re-epithelialization. The objective of this report was to clarify whether pre-incubation of the prosthesis in the omentum could be applicable for reconstruction of a long segment of the intrathoracic trachea in a canine model.

METHODS

The framework of an artificial trachea was fabricated from a polypropylene mesh tube and coated with 1% neutral atelocollagen inside and outside the lumen. The prosthesis was placed in the omentum of nine healthy male beagle dogs for 3 weeks. Then, the pedicled prosthesis was used to replace a 50 mm long section of intrathoracic trachea. Results were evaluated bronchoscopically, macroscopically, and histologically.

RESULTS

After 3 weeks of abdominal incubation, the prostheses were incorporated into the host tissue. None of the dogs showed dehiscence of the anastomosis or infection of the prostheses during the postoperative period. Seven of the nine dogs survived for more than 1 year. One dog died of a bowel obstruction resulting from a diaphragmatic hernia 3 months after replacement, and another died due to reasons unrelated to the prosthesis at 6 months. Bronchoscopic examination revealed no stenosis or dehiscence, and microscopic examination of all dogs showed that the luminal surface was covered by newly regenerated connective tissue and respiratory epithelium.

CONCLUSIONS

Pedicled omentum-prosthesis complexes may allow successful reconstruction of a long segment of the intrathoracic trachea.

Current Strategies for Tracheal Replacement: A Review

Airway cancers have been increasing in recent years. Tracheal resection is commonly performed during surgery and is burdened from post-operative complications severely affecting quality of life. Tracheal resection is usually carried out in primary tracheal tumors or other neoplasms of the neck region. Regenerative medicine for tracheal replacement using bio-prosthesis is under current research. In recent years, attempts were made to replace and transplant human cadaver trachea. An effective vascular supply is fundamental for a successful tracheal transplantation. The use of biological scaffolds derived from decellularized tissues has the advantage of a three-dimensional structure based on the native extracellular matrix promoting the perfusion, vascularization, and differentiation of the seeded cell typologies. By appropriately modulating some experimental parameters, it is possible to change the characteristics of the surface. The obtained membranes could theoretically be affixed to a decellularized tissue, but, in practice, it needs to ensure adhesion to the biological substrate and/or glue adhesion with biocompatible glues. It is also known that many of the biocompatible glues can be toxic or poorly tolerated and induce inflammatory phenomena or rejection. In tissue and organ transplants, decellularized tissues must not produce adverse immunological reactions and lead to rejection phenomena; at the same time, the transplant tissue must retain the mechanical properties of the original tissue. This review describes the attempts so far developed and the current lines of research in the field of tracheal replacement.

Partial Decellularization for Segmental Tracheal Scaffold Tissue Engineering: A Preliminary Study in Rabbits

In this study, we developed a new procedure for the rapid partial decellularization of the harvested trachea. Partial decellularization was performed using a combination of detergent and sonication to completely remove the epithelial layers outside of the cartilage ring. The post-decellularized tracheal segments were assessed with vital staining, which showed that the core cartilage cells remarkably remained intact while the cells outside of the cartilage were no longer viable. The ability of the decellularized tracheal segments to evade immune rejection was evaluated through heterotopic implantation of the segments into the chest muscle of rabbits without any immunosuppressive therapy, which demonstrated no evidence of severe rejection or tissue necrosis under H&E staining, as well as the mechanical stability under stress-pressure testing. Finally, orthotopic transplantation of partially decellularized trachea with no immunosuppression treatment resulted in 2 months of survival in two rabbits and one long-term survival (2 years) in one rabbit. Through evaluations of posttransplantation histology and endoscopy, we confirmed that our partial decellularization method could be a potential method of producing low-immunogenic cartilage scaffolds with viable, functional core cartilage cells that can achieve long-term survival after in vivo transplantation.

Experimental Tracheal Replacement: Angiogenesis and Null Apoptosis Promote Stenosis

Background

Tracheal replacement is a challenge for thoracic surgeons due to stenosis in the trachea-prosthesis anastomosis. We propose that stenosis occurs due to fibrosis as a result of an abnormal healing process, characterized by an increased expression of wound healing growth factors (vascular endothelial growth factor [VEGF], survivin, and CD31), which promote angiogenesis and decrease apoptosis. We analyzed the immunoreactivity of VEGF, survivin, CD31, and caspase-3 in the development of fibrotic stenosis in prosthetic tracheal replacement.

Methods

Fourteen dogs were operated on group I (n=7) received a 6-ring cervical tracheal segment autograft, while in group II (n=7), a 6-ring segment of the cervical trachea was resected and tracheal continuity was restored with a Dacron prosthesis. The follow-up was 3 months. Immunoreactivity studies for VEGF, survivin, CD31, and caspase-3 were performed. A statistical analysis was done using the Wilcoxon signed rank test.

Results

Four animals in group I were euthanized on the 10th postoperative day due to autograft necrosis. Three animals completed the study without anastomotic stenosis. Moderate expression of VEGF (p=0.038), survivin (p=0.038), and CD31 (p=0.038) was found. All group II animals developed stenosis in the trachea-prosthesis anastomotic sites. Microscopy showed abundant collagen and neovascularization vessels. Statistically significant immunoreactive expression of VEGF (p=0.015), survivin (p=0.017), and CD31 (p=0.011) was observed. No expression of caspase-3 was found.

Conclusion

We found a strong correlation between fibrosis in trachea-prosthesis anastomoses and excessive angiogenesis, moderate to intense VEGF, CD31, and survivin expression, and null apoptotic activity. These factors led to uncontrolled collagen production.

Insulin-like growth factor 1 promotes the extension of Tracheal Epithelium in an in Vitro Tracheal organ culture model

OBJECTIVE

Rapid epithelialization is crucial to maintain tracheal patency and prevent potential graft failure in tracheal reconstruction after tracheal resection for cancer with tracheal infiltration or tracheal stenosis. Insulin-like growth factor 1 is a liver-secreted endocrine molecule that controls cell proliferation, differentiation, and apoptosis and has been reported to promote epithelialization in several organs. Here, we utilized mouse tracheal organ cultures to examine the effect of insulin-like growth factor 1 on tracheal epithelialization.

METHODS

The trachea was resected from thirteen-week-old female ICR mice, and cut into small plate-shaped tracheal sections. First, the expression of insulin-like growth factor 1 receptor was assessed by immunohistochemistry. Secondly, the tracheal sections were cultured for seven days in the culture medium, and the morphological change during the seven-day culture was assessed by immunohistochemistry, hematoxylin and eosin staining, and scanning electron microscopy. Moreover, the tracheal sections were cultured for 48 h with different concentration of insulin-like growth factor 1 (0, 0.1, 1 and 10 µg/mL) in the culture medium, and the extension length of the tracheal epithelium during culture was measured in order to assess the effect of topical IGF1 on tracheal epithelialization.

RESULTS

Immunohistochemistry showed that insulin-like growth factor 1 receptor was expressed in tracheal epithelium. Immunohistochemistry, hematoxylin and eosin staining, and scanning electron microscopy showed that the tracheal organ cultures were stable for at least seven days without apparent morphological damage. The effect of insulin-like growth factor 1 on tracheal epithelialization was examined in plate-shaped tracheal sections cultured in medium supplemented with or without insulin-like growth factor 1 for 48 h. We also found that the epithelial edge of plate-shaped tracheal sections extended further along the surface of the tracheal section in culture medium containing insulin-like growth factor 1 compared with that in culture medium without insulin-like growth factor 1.

CONCLUSION

The current study using an in vitro mouse tracheal organ culture model demonstrated that topical insulin-like growth factor 1 treatment promoted the extension of tracheal epithelium, suggesting the potential utility of insulin-like growth factor 1 in aiding rapid tracheal epithelialization in patients requiring tracheal reconstruction using tissue-engineered tracheas.

Airway ciliated cells regenerated on collagen sponge implants acquire planar polarities towards nearby edges of implanted areas

Tissue-engineered tracheae have been developed to replace defective tracheae. However, the direction of ciliated cells in the regenerated epithelium remains unclear. We investigated planar polarity formed in the regenerated airway epithelium after tracheal graft implantation. We partially resected the rat trachea and implanted a collagen scaffold. The direction of the basal foot was assessed by transmission electron microscopy. Immunofluorescence staining was performed to examine the biased distribution of Vangl1 and Frizzled6 proteins. The direction of mucociliary transport was analyzed by video microscopy. Our results showed that the basal feet of cilia in the proximal and distal regions of the implanted areas were respectively oriented toward the proximal and distal directions. The biased distribution of Vangl1 and Frizzled6, and the directions of mucociliary transport showed that planar polarities formed in the regenerated epithelium were oriented toward the proximal, distal, left, and right directions in the proximal, distal, left, and right regions of the implanted area. These polarities persisted until nine months after implantation. Hence, the results suggest that planar polarities formed in epithelia regenerated on tracheal grafts are directed toward the nearby edges of implanted areas and are preserved for a prolonged period. The polarities can, at least partially, contribute to clearing external materials from the implanted areas by transporting them to a normal region.

Tracheal In Vitro Reconstruction Using a Decellularized Bio-Scaffold in Combination with a Rotating Bioreactor

Long-segment airway stenosis as well as their neoplastic transformation is life-threatening and still currently represent unsolved clinical problems. Indeed, despite several attempts, definitive surgical procedures are not presently available, and a suitable tracheal reconstruction or replacement remains an urgent clinical need. A possible innovative strategic solution to restore upper airway function may be represented by the creation of a bioprosthetic trachea, obtained through the combination of tissue engineering and regenerative medicine.Here we describe a two-step protocol for the ex vivo generation of tracheal segments. The first step involves the application of a decellularization technique that allows for the production of a naturally derived extracellular matrix (ECM)-based bio-scaffold, that maintains the macro- and micro-architecture as well as 9 the matrix-related signals distinctive of the original tissue. In the second step chondrocytes are seeded onto decellularized trachea, using a rotating bioreactor to ensure a correct scaffold repopulation.This multi-step approach represents a powerful tool for in vitro reconstruction of a bioengineered trachea that may constitute a promising solution to restore upper airway function. In addition, the procedures here described allow for the creation of a suitable 3D platform that may find useful applications, both for toxicological studies as well as organ transplantation strategies.

Hyaluronic Acid Coating on Hydrophobic Tracheal Scaffold Enhances Mesenchymal Stem Cell Adhesion and Tracheal Regeneration

BACKGROUND

Long segmental tracheal repair is challenging in regenerative medicine due to low adhesion of stem cells to tracheal scaffolds. Optimal transplantation of stem cells for tracheal defects has not been established. We evaluated the role of hyaluronic acid (HA) coating of tracheal scaffolds in mesenchymal stem cell (MSC) adhesion and tracheal regeneration in a rabbit model.

METHODS

A three-dimensionally printed tubular tracheal prosthesis was incubated with dopa-HA-fluorescein isothiocyanate in phosphate-buffered saline for 2 days. MSCs were incubated with an HA-coated scaffold, and their adhesion was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. HA coated scaffolds with or without MSC seeding were transplanted at the circumferential tracheal defect in rabbits, and survival, rigid bronchoscopy, radiologic findings, and histologic findings were compared between the two groups.

RESULTS

HA-coated scaffolds showed better MSC adhesion than non-coated scaffolds. The HA-coated scaffolds with MSC group showed a wider airway and greater mucosal regeneration compared to the HA-coated scaffolds without MSC group.

CONCLUSION

HA coating of scaffolds can promote MSC adhesion and tracheal regeneration.

Novel composite trachea grafts using 3-dimensional printing

Objective

Porcine-derived small intestine submucosa (SIS) extracellular matrix (ECM) surgical patches claim to have greater regenerative properties compared with dermal extracellular matrices. We hypothesized that using SIS-ECM in a bioengineered composite tracheal graft would allow better incorporation into the native tissue.

Methods

Two types of size-matched polycaprolactone support scaffolds were designed: rigid and flexible. The SIS-ECM was wrapped around the polycaprolactone supports lining the inside and outside of the graft. The grafts were implanted in 4 Yorkshire pigs, replacing an ∼2 cm segment of native trachea. Airway patency was evaluated with computed tomography scans and explanted grafts were examined grossly and histologically.

Results

All animals survived through the immediate postoperative period. Generally, extraluminal examination showed a smooth transition between native and graft without significant volumetric loss. Animals that received the flexible design survived ∼10 days longer than those that received the rigid design; however, severe perianastomotic intraluminal granulation tissue was observed. The rigid design had less significant intraluminal granulation tissue development at the distal anastomosis, but partial dehiscence had occurred at the proximal anastomosis interrupting graft incorporation.

Conclusions

The generally good extraluminal graft incorporation in our composite tracheal graft highlights some increased regenerative capabilities of SIS-ECM. However, the presence of intraluminal granulation tissue indicates that its use as an off-the-shelf, unaltered substrate in an airway graft is still not ideal. Further research must be conducted to determine whether a modification of the substrate is possible to enhance luminal airway incorporation and to exert control over the mechanisms responsible for granulation tissue development.

Mechanical properties of de-epithelialized tracheal allografts

Background

Many studies have shown that mucosa-treated tracheas can be used for allotransplantation without the use of immunosuppressants. However, no literature has reported whether the transplanted trachea has normal biomechanics. In this study, fresh dog trachea was used as a control to determine the mechanical properties of allograft trachea after mucous membrane removal.

Methods

Cervical tracheas (~10 cm in length, consisting of the cartilage rings down to the tracheal carina) and the surrounding connective tissues were removed from 20 mixed-breed dogs and then divided into 4 groups with 5 samples in each group. The groups were defined as follows: group A, fresh canine tracheas without any treatment; group B, canine tracheas after sodium dodecyl sulphate (SDS) de-epithelialization; group C, canine tracheas implanted into the dorsal fascia of beagle dogs for 1 month after SDS de-epithelialization; group D, canine tracheas implanted into the dorsal fascia of beagle dogs for 6 months after SDS de-epithelialization. All heterotopically implanted beagle dogs were free of any immunosuppressive treatment. Uniaxial tensile (n=20) and compressive (n=20) experiments were performed on the 4 groups of trachea using a universal testing machine; the tensile load was recorded at 150% of the original tracheal length (50% displacement), and the compressive load was recorded at 50% of the initial anteroposterior diameter (50% displacement). The average difference in tensile and compressive loads between groups was compared by analysis of variance.

Results

No significant difference was found in the mean compressive strength among groups (group A =2.38±0.87 N, group B =3.33±1.03 N, group C =5.39±2.44 N, group D =3.04±2.12 N; P=0.07 for all groups). The mean tensile strength of group C was 9.68±0.86 N, resulting in a significant difference in tensile strength compared to group A (22.51±3.91 N) and B (24.24±7.46 N) (P<0.001). No significant difference in tensile strength was observed among groups A, B, and D.

Conclusions

The de-epithelialized trachea showed no significant change in mechanical properties after 6 months of implantation.

Computational fluid dynamics for enhanced tracheal bioreactor design and long-segment graft recellularization

Successful re-epithelialization of de-epithelialized tracheal scaffolds remains a challenge for tracheal graft success. Currently, the lack of understanding of the bioreactor hydrodynamic environment, and its relation to cell seeding outcomes, serve as major obstacles to obtaining viable tracheal grafts. In this work, we used computational fluid dynamics to (a) re-design the fluid delivery system of a trachea bioreactor to promote a spatially uniform hydrodynamic environment, and (b) improve the perfusion cell seeding protocol to promote homogeneous cell deposition. Lagrangian particle-tracking simulations showed that low rates of rotation provide more uniform circumferential and longitudinal patterns of cell deposition, while higher rates of rotation only improve circumferential uniformity but bias cell deposition proximally. Validation experiments with human bronchial epithelial cells confirm that the model accurately predicts cell deposition in low shear stress environments. We used the acquired knowledge from our particle tracking model, as a guide for long-term tracheal repopulation studies. Cell repopulation using conditions resulting in low wall shear stress enabled enhanced re-epithelialization of long segment tracheal grafts. While our work focuses on tracheal regeneration, lessons learned in this study, can be applied to culturing of any tissue engineered tubular scaffold.