Tracheal Replacement Therapy with a Stem Cell-Seeded Graft: Lessons from Compassionate Use Application of a GMP-Compliant Tissue-Engineered Medicine

Preparation of Decellularized Tissue as Dual Cell Carrier Systems: A Step Towards Facilitating Re-epithelization and Cell Encapsulation for Tracheal Reconstruction

Surgical treatment of tracheal diseases, trauma, and congenital stenosis has shown success through tracheal reconstruction coupled with palliative care. However, challenges in surgical-based tracheal repairs have prompted the exploration of alternative approaches for tracheal replacement. Tissue-based treatments, involving the cultivation of patient cells on a network of extracellular matrix (ECM) from donor tissue, hold promise for restoring tracheal structure and function without eliciting an immune reaction. In this study, we utilized decellularized canine tracheas as tissue models to develop two types of cell carriers: a decellularized scaffold and a hydrogel. Our hypothesis posits that both carriers, containing essential biochemical niches provided by ECM components, facilitate cell attachment without inducing cytotoxicity. Canine tracheas underwent vacuum-assisted decellularization (VAD), and the ECM-rich hydrogel was prepared through peptic digestion of the decellularized trachea. The decellularized canine trachea exhibited a significant reduction in DNA content and major histocompatibility complex class II, while preserving crucial ECM components such as collagen, glycosaminoglycan, laminin, and fibronectin. Scanning electron microscope and fluorescent microscope images revealed a fibrous ECM network on the luminal side of the cell-free trachea, supporting epithelial cell attachment. Moreover, the ECM-rich hydrogel exhibited excellent viability for human mesenchymal stem cells encapsulated for 3 days, indicating the potential of cell-laden hydrogel in promoting the development of cartilage rings of the trachea. This study underscores the versatility of the trachea in producing two distinct cell carriers-decellularized scaffold and hydrogel-both containing the native biochemical niche essential for tracheal tissue engineering applications.

Tracheal Replacement Techniques and Associated Mortality: A Systematic Review

OBJECTIVE

Tracheal replacement is a crucial operation to enhance the quality of life for patients with extensive tracheal lesions. The most suitable surgical techniques for different clinical conditions remain a topic of debate. Through a reviewing of the relevant literature, this study investigated the association between surgical techniques and mortality rate.

DATA SOURCES

Studies were collected from PubMed, Embase, the Web of Science, the Cochrane Center Register of Controlled Trials, and ClinicalTrials.gov.

METHODS

This systematic review encompassed literature from the inception of each database to May 10, 2023, focusing on tracheal replacement for patients who underwent circumferential resection of the trachea or partial resection with preservation of the posterior membranous wall. Non-human and non-clinical studies were excluded.

RESULTS

About 31 studies were included in the assessment comprising a combination of case reports and case series, and 118 patients underwent tracheal replacement through four underlying methodologies, including tracheal allotransplantation, autologous tissue reconstruction, bioprosthetic reconstruction, or tissue engineering surgery. Each modality exhibits unique advantages and disadvantages, leading to variable outcomes in clinical application.

CONCLUSION

Tracheal replacement is challenging due to the absence of an ideal substitution or graft material. Despite limited clinical successes observed across various modalities, we believe autologous tissue reconstruction for tracheal replacement has the advantage of broadest indications, low rejection rate, and avoidance of immunosuppressive agents. Future research should focus on achieving tracheal replacement that preserves mucociliary clearance, lateral rigidity, and longitudinal flexibility.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:1517-1522, 2024.

Pedicled pericardial patch in tracheal reconstruction in children - Novel technique to provide vascularized tissue for salvaging difficult airways

BACKGROUND

Management of large central airway defects are often complex. Children who present with these defects have multiple co-morbidities or have had previous surgeries. Surgical options include various tissue cover for these defects without longer term benefits. Vascularized autologous pericardial patch offers a better solution to these defects by providing vascularity and potential for remodelling in future.

METHODS

41 children (M:F of 24:17) were operated for large trachea-bronchial defects between January 2015 and August 2022. The median age of was 12 months with median weight of 8.9 kg (IQR 3.3 kg-17.7 kg) Causes leading to the central tracheal defect include failed repair of previous trachea-oesophageal fistula (TOF) (n = 21) and acquired fistula due to button battery injury (n = 11). Surgical repair consisted of autologous pedicled pericardial patch repair for the airway defect under cardiopulmonary bypass.

RESULTS

There were two operative deaths related to extensive sepsis and necrosis of reconstructed trachea. Four children had further reoperation with additional patch. Bronchoscopy was used as surveillance in all these children, with use of airway stents (biodegradable stent) in 9 children. The median ventilation time was 8 days, with tracheostomy being needed in 5 for long term support.

CONCLUSIONS

Autologous pericardial patch is a versatile technique and can be used to salvage large tracheal defects when other method have failed or not feasible. Tracheomalacia at the site of repair could be managed with biodegradable stents. Vascularity and ciliary function of the patch still needs to be evaluated.

I Walk the Line: Between Basic Science and Paediatric Surgery

The role of a basic scientist working with paediatric surgeons is not an obvious one. However, there are several levels at which science can contribute to the speciality, and also ways that scientists can learn useful lessons from paediatric surgery. As most conditions treated by paediatric surgeons have low case numbers, we need to find ways of defining optimal treatment and developing novel therapies within a challenging number of patients.

Airway disease decreases the therapeutic potential of epithelial stem cells

BACKGORUND

Tissue-engineered tracheal grafts (TETG) can be recellularized by the host or pre-seeded with host-derived cells. However, the impact of airway disease on the recellularization process is unknown.

METHODS

In this study, we determined if airway disease alters the regenerative potential of the human tracheobronchial epithelium (hTBE) obtained by brushing the tracheal mucosa during clinically-indicated bronchoscopy from 48 pediatric and six adult patients.

RESULTS

Our findings revealed that basal cell recovery and frequency did not vary by age or region. At passage 1, all samples produced enough cells to cellularize a 3.5 by 0.5 cm2 graft scaffold at low cell density (~ 7000 cells/cm2), and 43.75% could cellularize a scaffold at high cell density (~ 100,000 cells/cm2). At passage 2, all samples produced the number of cells required for both recellularization models. Further evaluation revealed that six pediatric samples (11%) and three (50%) adult samples contained basal cells with a squamous basal phenotype. These cells did not form a polarized epithelium or produce differentiated secretory or ciliated cells. In the pediatric population, the squamous basal cell phenotype was associated with degree of prematurity (< 28 weeks, 64% vs. 13%, p = 0.02), significant pulmonary history (83% vs. 34%, p = 0.02), specifically with bronchopulmonary dysplasia (67% vs. 19%, p = 0.01), and patients who underwent previous tracheostomy (67% vs. 23%, p = 0.03).

CONCLUSIONS

In summary, screening high-risk pediatric or adult population based on clinical risk factors and laboratory findings could define appropriate candidates for airway reconstruction with tracheal scaffolds.

LEVEL OF EVIDENCE

Level III Cohort study.

External Esophageal Stenting Technique in Palliation for Tracheal Agenesis in a Case of Esophageal Lung: A Lesson Learned from the Experience for Tracheomalacia

Tracheal agenesis (TA) is a rare congenital anomaly with an incidence of 1 per 50,000 newborns. It appears at birth with severe respiratory distress, cyanosis, and inaudible crying. Prompt esophageal intubation and long-term management of the esophageal airway are essential to overcome this catastrophic condition. In the long-term management, external stenting of the esophageal airway has been reported as promising to support the fragile esophageal wall; this technique was taken from the surgery for tracheomalacia. We experienced a case of an infant with tracheal agenesis whose respiratory status was stabilized after external esophageal stenting. The stenting was performed based on a lesson learned in the extensive experience in the surgical treatment for tracheomalacia, and the surgical techniques for successful stenting are herein described.

Application of tissue engineering techniques in tracheal repair: a bibliometric study

Transplantation of tissue-engineered trachea is an effective treatment for long-segment tracheal injury. This technology avoids problems associated with a lack of donor resources and immune rejection, generating an artificial trachea with good biocompatibility. To our knowledge, a systematic summary of basic and clinical research on tissue-engineered trachea in the last 20 years has not been conducted. Here, we analyzed the development trends of tissue-engineered trachea research by bibliometric means and outlined the future perspectives in this field. The Web of Science portal was selected as the data source. CiteSpace, VOSviewer, and the Bibliometric Online Analysis Platform were used to analyze the number of publications, journals, countries, institutions, authors, and keywords from 475 screened studies. Between 2000 and 2023, the number of published studies on tissue-engineered trachea has been increasing. Biomaterials published the largest number of papers. The United States and China have made the largest contributions to this field. University College London published the highest number of studies, and the most productive researcher was an Italian scholar, Paolo Macchiarini. However, close collaborations between various researchers and institutions from different countries were generally lacking. Despite this, keyword analysis showed that manufacturing methods for tracheal stents, hydrogel materials, and 3D bioprinting technology are current popular research topics. Our bibliometric study will help scientists in this field gain an in-depth understanding of the current research progress and development trends to guide their future work, and researchers in related fields will benefit from the introduction to transplantation methods of tissue-engineered trachea.

Material matters: exploring the interplay between natural biomaterials and host immune system

Biomaterials are widely used for various medical purposes, for instance, implants, tissue engineering, medical devices, and drug delivery systems. Natural biomaterials can be obtained from proteins, carbohydrates, and cell-specific sources. However, when these biomaterials are introduced into the body, they trigger an immune response which may lead to rejection and failure of the implanted device or tissue. The immune system recognizes natural biomaterials as foreign substances and triggers the activation of several immune cells, for instance, macrophages, dendritic cells, and T cells. These cells release pro-inflammatory cytokines and chemokines, which recruit other immune cells to the implantation site. The activation of the immune system can lead to an inflammatory response, which can be beneficial or detrimental, depending on the type of natural biomaterial and the extent of the immune response. These biomaterials can also influence the immune response by modulating the behavior of immune cells. For example, biomaterials with specific surface properties, such as charge and hydrophobicity, can affect the activation and differentiation of immune cells. Additionally, biomaterials can be engineered to release immunomodulatory factors, such as anti-inflammatory cytokines, to promote a tolerogenic immune response. In conclusion, the interaction between biomaterials and the body's immune system is an intricate procedure with potential consequences for the effectiveness of therapeutics and medical devices. A better understanding of this interplay can help to design biomaterials that promote favorable immune responses and minimize adverse reactions.

Multi-disciplinary Insights from the First European Forum on Visceral Myopathy 2022 Meeting

Visceral myopathy is a rare, life-threatening disease linked to identified genetic mutations in 60% of cases. Mostly due to the dearth of knowledge regarding its pathogenesis, effective treatments are lacking. The disease is most commonly diagnosed in children with recurrent or persistent disabling episodes of functional intestinal obstruction, which can be life threatening, often requiring long-term parenteral or specialized enteral nutritional support. Although these interventions are undisputedly life-saving as they allow affected individuals to avoid malnutrition and related complications, they also seriously compromise their quality of life and can carry the risk of sepsis and thrombosis. Animal models for visceral myopathy, which could be crucial for advancing the scientific knowledge of this condition, are scarce. Clearly, a collaborative network is needed to develop research plans to clarify genotype-phenotype correlations and unravel molecular mechanisms to provide targeted therapeutic strategies. This paper represents a summary report of the first 'European Forum on Visceral Myopathy'. This forum was attended by an international interdisciplinary working group that met to better understand visceral myopathy and foster interaction among scientists actively involved in the field and clinicians who specialize in care of people with visceral myopathy.

Development of tissue-engineered tracheal scaffold with refined mechanical properties and vascularisation for tracheal regeneration

Introduction: Attempted tracheal replacement efforts thus far have had very little success. Major limiting factors have been the inability to efficiently re-vascularise and mimic the mechanical properties of native tissue. The major objective of this study was to optimise a previously developed collagen-hyaluronic acid scaffold (CHyA-B), which has shown to facilitate the growth of respiratory cells in distinct regions, as a potential tracheal replacement device. Methods: A biodegradable thermoplastic polymer was 3D-printed into different designs and underwent multi-modal mechanical assessment. The 3D-printed constructs were incorporated into the CHyA-B scaffolds and subjected to in vitro and ex vivo vascularisation. Results: The polymeric backbone provided sufficient strength to the CHyA-B scaffold, with yield loads of 1.31-5.17 N/mm and flexural moduli of 0.13-0.26 MPa. Angiogenic growth factor release (VEGF and bFGF) and angiogenic gene upregulation (KDR, TEK-2 and ANG-1) was detected in composite scaffolds and remained sustainable up to 14 days. Confocal microscopy and histological sectioning confirmed the presence of infiltrating blood vessel throughout composite scaffolds both in vitro and ex vivo. Discussion: By addressing both the mechanical and physiological requirements of tracheal scaffolds, this work has begun to pave the way for a new therapeutic option for large tracheal defects.

Current and future developments in the pharmacology of asthma and COPD: ERS seminar, Naples 2022

Pharmacological management of airway obstructive diseases is a fast-evolving field. Several advances in unravelling disease mechanisms as well as intracellular and molecular pathways of drug action have been accomplished. While the clinical translation and implementation of in vitro results to the bedside remains challenging, advances in comprehending the mechanisms of respiratory medication are expected to assist clinicians and scientists in identifying meaningful read-outs and designing clinical studies. This European Respiratory Society Research Seminar, held in Naples, Italy, 5-6 May 2022, focused on current and future developments of the drugs used to treat asthma and COPD; on mechanisms of drug action, steroid resistance, comorbidities and drug interactions; on prognostic and therapeutic biomarkers; on developing novel drug targets based on tissue remodelling and regeneration; and on pharmacogenomics and emerging biosimilars. Related European Medicines Agency regulations are also discussed, as well as the seminar's position on the above aspects.

Regenerative medicine: current research and perspective in pediatric surgery

The field of regenerative medicine, encompassing several disciplines including stem cell biology and tissue engineering, continues to advance with the accumulating research on cell manipulation technologies, gene therapy and new materials. Recent progress in preclinical and clinical studies may transcend the boundaries of regenerative medicine from laboratory research towards clinical reality. However, for the ultimate goal to construct bioengineered transplantable organs, a number of issues still need to be addressed. In particular, engineering of elaborate tissues and organs requires a fine combination of different relevant aspects; not only the repopulation of multiple cell phenotypes in an appropriate distribution but also the adjustment of the host environmental factors such as vascularisation, innervation and immunomodulation. The aim of this review article is to provide an overview of the recent discoveries and development in stem cells and tissue engineering, which are inseparably interconnected. The current status of research on tissue stem cells and bioengineering, and the possibilities for application in specific organs relevant to paediatric surgery have been specifically focused and outlined.

Current Status and Future Outlook of Additive Manufacturing Technologies for the Reconstruction of the Trachea

Tracheal stenosis and defects occur congenitally and in patients who have undergone tracheal intubation and tracheostomy due to long-term intensive care. Such issues may also be observed during tracheal removal during malignant head and neck tumor resection. However, to date, no treatment method has been identified that can simultaneously restore the appearance of the tracheal skeleton while maintaining respiratory function in patients with tracheal defects. Therefore, there is an urgent need to develop a method that can maintain tracheal function while simultaneously reconstructing the skeletal structure of the trachea. Under such circumstances, the advent of additive manufacturing technology that can create customized structures using patient medical image data provides new possibilities for tracheal reconstruction surgery. In this study, the three-dimensional (3D) printing and bioprinting technologies used in tracheal reconstruction are summarized, and various research results related to the reconstruction of mucous membranes, cartilage, blood vessels, and muscle tissue, which are tissues required for tracheal reconstruction, are classified. The prospects for 3D-printed tracheas in clinical studies are also described. This review serves as a guide for the development of artificial tracheas and clinical trials using 3D printing and bioprinting.

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.

De-Epithelialization Protocol with Tapered Sodium Dodecyl Sulfate Concentrations Enhances Short-Term Chondrocyte Survival in Porcine Chimeric Tracheal Allografts

Background: Tracheal transplantation is indicated in cases where injury exceeds 50% of the organ in adults and 30% in children. However, transplantation is not yet considered a viable treatment option partly due to high morbidity and mortality associated with graft rejection. Recently, decellularization (decell) has been explored as a technique for creating bioengineered tracheal grafts. However, risk of post-operative stenosis increases due to the death of chondrocytes, which are critical to maintain the biochemical and mechanical integrity of tracheal cartilage. In this project, we propose a new de-epithelialization protocol that adequately removes epithelial, mucosal, and submucosal cells while maintaining a greater proportion of viable chondrocytes. Methods: The trachea of adult male outbred Yorkshire pigs were extracted, decontaminated, and decellularized according to the original and new protocols before incubation at 37 °C in DMEM for 10 days. Chondrocyte viability was quantified immediately following post-decellularization and on days 1, 4, 7, and 10. Histology was performed pre-decellularization, post-decellularization, and post-incubation. Results: The new protocol showed a significant (p < 0.05) increase in chondrocyte viability up to four days after de-ep when compared to the original protocol. We also found that the new protocol preserves ECM composition to a similar degree as the original protocol. When scaffolds created using the new protocol were re-epithelialized, cell growth curves were near identical to published data from the original protocol. Conclusion: While not without limitations, our new protocol may be used to engineer chimeric tracheal allografts without the need for cartilage regeneration.

A Multimodal Approach to Quantify Chondrocyte Viability for Airway Tissue Engineering

OBJECTIVES/HYPOTHESIS

Partially decellularized tracheal scaffolds have emerged as a potential solution for long-segment tracheal defects. These grafts have exhibited regenerative capacity and the preservation of native mechanical properties resulting from the elimination of all highly immunogenic cell types while sparing weakly immunogenic cartilage. With partial decellularization, new considerations must be made about the viability of preserved chondrocytes. In this study, we propose a multimodal approach for quantifying chondrocyte viability for airway tissue engineering.

METHODS

Tracheal segments (5 mm) were harvested from C57BL/6 mice, and immediately stored in phosphate-buffered saline at -20°C (PBS-20) or biobanked via cryopreservation. Stored and control (fresh) tracheal grafts were implanted as syngeneic tracheal grafts (STG) for 3 months. STG was scanned with micro-computed tomography (μCT) in vivo. STG subjected to different conditions (fresh, PBS-20, or biobanked) were characterized with live/dead assay, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and von Kossa staining.

RESULTS

Live/dead assay detected higher chondrocyte viability in biobanked conditions compared to PBS-20. TUNEL staining indicated that storage conditions did not alter the proportion of apoptotic cells. Biobanking exhibited a lower calcification area than PBS-20 in 3-month post-implanted grafts. Higher radiographic density (Hounsfield units) measured by μCT correlated with more calcification within the tracheal cartilage.

CONCLUSIONS

We propose a strategy to assess chondrocyte viability that integrates with vivo imaging and histologic techniques, leveraging their respective strengths and weaknesses. These techniques will support the rational design of partially decellularized tracheal scaffolds.

LEVEL OF EVIDENCE

N/A Laryngoscope, 133:512-520, 2023.

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.

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.

Airway replacement using stented aortic matrices: Long-term follow-up and results of the TRITON-01 study in 35 adult patients

Over the past 25 years, we have demonstrated the feasibility of airway bioengineering using stented aortic matrices experimentally then in a first-in-human trial (n = 13). The present TRITON-01 study analyzed all the patients who had airway replacement at our center to confirm that this innovative approach can be now used as usual care. For each patient, the following data were prospectively collected: postoperative mortality and morbidity, late airway complications, stent removal and status at last follow-up on November 2, 2021. From October 2009 to October 2021, 35 patients had airway replacement for malignant (n = 29) or benign (n = 6) lesions. The 30-day postoperative mortality and morbidity rates were 2.9% (n = 1/35) and 22.9% (n = 8/35) respectively. At a median follow-up of 29.5 months (range 1-133 months), 27 patients were alive. There have been no deaths directly related to the implanted bioprosthesis. Eighteen patients (52.9%) had stent-related granulomas requiring a bronchoscopic treatment. Ten among 35 patients (28.6%) achieved a stent free survival. The actuarial 2- and 5-year survival rates (Kaplan-Meier estimates) were respectively 88% and 75%. The TRITON-01 study confirmed that airway replacement using stented aortic matrices can be proposed as usual care at our center. Clinicaltrials.gov Identifier: NCT04263129.

Bioengineering lungs: An overview of current methods, requirements, and challenges for constructing scaffolds

Chronic respiratory diseases remain a significant health burden worldwide. The only option for individuals with end-stage lung failure remains Lung Transplantation. However, suitable organ donor shortages and immune rejection following transplantation remain a challenge. Since alternative options are urgently required to increase tissue availability for lung transplantation, researchers have been exploring lung bioengineering extensively, to generate functional, transplantable organs and tissue. Additionally, the development of physiologically-relevant artificial tissue models for testing novel therapies also represents an important step toward finding a definite clinical solution for different chronic respiratory diseases. This mini-review aims to highlight some of the most common methodologies used in bioengineering lung scaffolds, as well as the benefits and disadvantages associated with each method in conjunction with the current areas of research devoted to solving some of these challenges in the area of lung bioengineering.

Effects of Laser Application on Alveolar Bone Mesenchymal Stem Cells and Osteoblasts: An In Vitro Study

Mesenchymal stem cells isolated from the bone marrow have a great differentiation potential, being able to produce many cell lines, including osteoblasts. Osteoblasts have an important role in bone remodeling by actively participating in the maturation and mineralization of the extracellular matrix. The aim of this study was to determine the effect of laser application on the viability and proliferation of osteoblasts. Methods: Alveolar bone was harvested from 8 patients and placed into a culture medium to induce proliferation of mesenchymal stem cells. These were differentiated into osteoblasts in special conditions. The cells from each patient were split into two groups, one was treated using a 980 nm laser (1W output power, pulsed mode, 20 s, 50 mm distance) (laser “+”) and the other one did not receive laser stimulation (laser “-”). Results: Using the confocal microscope, we determined that the cells from the laser “+” group were more active when compared to the laser “-” group. The number of cells in the laser “+” group was significantly greater compared to the laser “-” group as the ImageJ-NIH software showed (p = 0.0072). Conclusions: Laser application increases the proliferation rate of osteoblasts and intensifies their cellular activity.

Novel Therapy for Acquired Tracheomalacia with a Tissue-Engineered Extraluminal Tracheal Splint and Autologous Mesenchymal-Derived Chondrocytes

Introduction Acquired tracheomalacia (ATM) is characterized by a loss of structural strength of the tracheal framework, resulting in airway collapse during breathing. Near half of the patients undergoing prolonged invasive mechanical ventilation will suffer tracheal lesions. Treatment for ATM includes external splinting with rib grafts, prosthetic materials, and tracheal resection. Failure in the use of prosthetic materials has made reconsidering natural origin scaffolds and tissue engineering as a suitable alternative.

Objective To restore adequate airway patency in an ovine model with surgically-induced ATM employing a tissue-engineered extraluminal tracheal splint (TE-ETS).

Methods In the present prospective pilot study, tracheal rings were partially resected to induce airway collapse in 16 Suffolk sheep (Ovis aries). The TE-ETS was developed with autologous mesenchymal-derived chondrocytes and allogenic decellularized tracheal segments and was implanted above debilitated tracheal rings. The animals were followed-up at 8, 12, and 16 weeks and at 1-year postinsertion. Flexible tracheoscopies were performed at each stage. After sacrifice, a histopathological study of the trachea and the splint were performed.

Results The TE-ETS prevented airway collapse for 16 weeks and up to 1-year postinsertion. Tracheoscopies revealed a noncollapsing airway during inspiration. Histopathological analyses showed the organization of mesenchymal-derived chondrocytes in lacunae, the proliferation of blood vessels, and recovery of epithelial tissue subjacent to the splint. Splints without autologous cells did not prevent airway collapse.

Conclusion It is possible to treat acquired tracheomalacia with TE-ETS without further surgical removal since it undergoes physiological degradation. The present study supports the development of tissue-engineered tracheal substitutes for airway disease.

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.

Enhancing Stem Cell-Based Therapeutic Potential by Combining Various Bioengineering Technologies

Stem cell-based therapeutics have gained tremendous attention in recent years due to their wide range of applications in various degenerative diseases, injuries, and other health-related conditions. Therapeutically effective bone marrow stem cells, cord blood- or adipose tissue-derived mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), and more recently, induced pluripotent stem cells (iPSCs) have been widely reported in many preclinical and clinical studies with some promising results. However, these stem cell-only transplantation strategies are hindered by the harsh microenvironment, limited cell viability, and poor retention of transplanted cells at the sites of injury. In fact, a number of studies have reported that less than 5% of the transplanted cells are retained at the site of injury on the first day after transplantation, suggesting extremely low (<1%) viability of transplanted cells. In this context, 3D porous or fibrous national polymers (collagen, fibrin, hyaluronic acid, and chitosan)-based scaffold with appropriate mechanical features and biocompatibility can be used to overcome various limitations of stem cell-only transplantation by supporting their adhesion, survival, proliferation, and differentiation as well as providing elegant 3-dimensional (3D) tissue microenvironment. Therefore, stem cell-based tissue engineering using natural or synthetic biomimetics provides novel clinical and therapeutic opportunities for a number of degenerative diseases or tissue injury. Here, we summarized recent studies involving various types of stem cell-based tissue-engineering strategies for different degenerative diseases. We also reviewed recent studies for preclinical and clinical use of stem cell-based scaffolds and various optimization strategies.

Primary MSCs for Personalized Medicine: Ethical Challenges, Isolation and Biocompatibility Evaluation of 3D Electrospun and Printed Scaffolds

Autologous cell therapy uses patients’ own cells to deliver precise and ideal treatment through a personalized medicine approach. Isolation of patients’ cells from residual tissue extracted during surgery involves specific planning and lab steps. In the present manuscript, a path from isolation to in vitro research with human mesenchymal stem cells (MSCs) obtained from residual bone tissues is described as performed by a medical unit in collaboration with a research center. Ethical issues have been addressed by formulating appropriate harvesting protocols according to European regulations. Samples were collected from 19 patients; 10 of them were viable and after processing resulted in MSCs. MSCs were further differentiated in osteoblasts to investigate the biocompatibility of several 3D scaffolds produced by electrospinning and 3D printing technologies; traditional orthopedic titanium and nanostructured titanium substrates were also tested. 3D printed scaffolds proved superior compared to other substrates, enabling significantly improved response in osteoblast cells, indicating that their biomimetic structure and properties make them suitable for synthetic tissue engineering. The present research is a proof of concept that describes the process of primary stem cells isolation for in vitro research and opens avenues for the development of personalized cell platforms in the case of patients with orthopedic trauma. The demonstration model has promising perspectives in personalized medicine practices.

Tissue-engineered composite tracheal grafts create mechanically stable and biocompatible airway replacements

We tested composite tracheal grafts (CTG) composed of a partially decellularized tracheal graft (PDTG) combined with a 3-dimensional (3D)-printed airway splint for use in long-segment airway reconstruction. CTG is designed to recapitulate the 3D extracellular matrix of the trachea with stable mechanical properties imparted from the extraluminal airway splint. We performed segmental orthotopic tracheal replacement in a mouse microsurgical model. MicroCT was used to measure graft patency. Tracheal neotissue formation was quantified histologically. Airflow dynamic properties were analyzed using computational fluid dynamics. We found that CTG are easily implanted and did not result in vascular erosion, tracheal injury, or inflammation. Graft epithelialization and endothelialization were comparable with CTG to control. Tracheal collapse was absent with CTG. Composite tracheal scaffolds combine biocompatible synthetic support with PDTG, supporting the regeneration of host epithelium while maintaining graft structure.

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.

Recellularization of Bioengineered Scaffolds for Vascular Composite Allotransplantation

Traumatic injuries or cancer resection resulting in large volumetric soft tissue loss requires surgical reconstruction. Vascular composite allotransplantation (VCA) is an emerging reconstructive option that transfers multiple, complex tissues as a whole subunit from donor to recipient. Although promising, VCA is limited due to side effects of immunosuppression. Tissue-engineered scaffolds obtained by decellularization and recellularization hold great promise. Decellularization is a process that removes cellular materials while preserving the extracellular matrix architecture. Subsequent recellularization of these acellular scaffolds with recipient-specific cells can help circumvent adverse immune-mediated host responses and allow transplantation of allografts by reducing and possibly eliminating the need for immunosuppression. Recellularization of acellular tissue scaffolds is a technique that was first investigated and reported in whole organs. More recently, work has been performed to apply this technique to VCA. Additional work is needed to address barriers associated with tissue recellularization such as: cell type selection, cell distribution, and functionalization of the vasculature and musculature. These factors ultimately contribute to achieving tissue integration and viability following allotransplantation. The present work will review the current state-of-the-art in soft tissue scaffolds with specific emphasis on recellularization techniques. We will discuss biological and engineering process considerations, technical and scientific challenges, and the potential clinical impact of this technology to advance the field of VCA and reconstructive surgery.

Emerging Paradigms in Bioengineering the Lungs

In end-stage lung diseases, the shortage of donor lungs for transplantation and long waiting lists are the main culprits in the significantly increasing number of patient deaths. New strategies to curb this issue are being developed with the help of recent advancements in bioengineering technology, with the generation of lung scaffolds as a steppingstone. There are various types of lung scaffolds, namely, acellular scaffolds that are developed via decellularization and recellularization techniques, artificial scaffolds that are synthesized using synthetic, biodegradable, and low immunogenic materials, and hybrid scaffolds which combine the advantageous properties of materials in the development of a desirable lung scaffold. There have also been advances in the design of bioreactors in terms of providing an optimal regenerative environment for the maturation of functional lung tissue over time. In this review, the emerging paradigms in the field of lung tissue bioengineering will be discussed.

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.

Planar organization of airway epithelial cell morphology using hydrogel grooves during ciliogenesis fails to induce ciliary alignment

Topographical cues are known to influence cell organization both in native tissues and in vitro. In the trachea, the matrix beneath the epithelial lining is composed of collagen fibres that run along the long axis of the airway. Previous studies have shown that grooved topography can induce morphological and cytoskeletal alignment in epithelial cell lines. In the present work we assessed the impact of substrate topography on the organization of primary human tracheal epithelial cells (HTECs) and human induced pluripotent stem cell (hiPSC)-derived airway progenitors and the resulting alignment of cilia after maturation of the airway cells under Air-Liquid-Interface (ALI) culture. Grooves with optimized dimensions were imprinted into collagen vitrigel membranes (CVM) to produce gel inserts for ALI culture. Grooved CVM substrates induced cell alignment in HTECs and hiPSC airway progenitors in submerged culture. Further, both cell types were able to terminally differentiate into a multi-ciliated epithelium on both flat and groove CVM substrates. When exposed to ALI conditions, HTECs lost alignment after 14 days. Meanwhile, hiPSC-derived airway progenitors maintained their alignment throughout 31 days of ALI culture. Interestingly, neither initial alignment on the grooves, nor maintained alignment on the grooves induced alignment of cilia basal bodies, an indication of the direction of ciliary beating direction in the airway cells. Planar organization of airway cells during or prior to ciliogenesis therefore does not appear to be a feasible strategy to control cilia organization and subsequent airway epithelial function and additional cues are likely necessary to produce cilia alignment.

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.

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.

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.

State of the art of clinical applications of Tissue Engineering in 2021

Tissue engineering (TE) was introduced almost 30 years ago as a potential technique for regenerating human tissues. However, despite promising laboratory findings, the complexity of the human body, scientific hurdles, and lack of persistent long-term funding still hamper its translation towards clinical applications. In this report, we compile an inventory of clinically applied TE medical products relevant to surgery. A review of the literature, including articles published within the period from 1991 to 2020, was performed according to the PRISMA protocol, using databanks PubMed, Cochrane Library, Web of Science, and Clinicaltrials.gov. We identified 1039 full-length articles as eligible; due to the scarcity of clinical, randomised, controlled trials and case studies, we extended our search towards a broad surgical spectrum. Forty papers involved clinical TE studies. Amongst these, 7 were related to TE protocols for cartilage applied in the reconstruction of nose, ear, and trachea. Nine papers reported TE protocols for articular cartilage, 9 for urological purposes, 7 described TE strategies for cardiovascular aims, and 8 for dermal applications. However, only two clinical studies reported on three-dimensional (3D) and functional long-lasting TE constructs. The concept of generating 3D TE constructs and organs based on autologous molecules and cells is intriguing and promising. The first translational tissue-engineered products and techniques have been clinically implemented. However, despite the 30 years of research and development in this field, TE is still in its clinical infancy. Multiple experimental, ethical, budgetary, and regulatory difficulties hinder its rapid translation. Nevertheless, the first clinical applications show great promise and indicate that the translation towards clinical medical implementation has finally started.

Stem Cell-derived Respiratory Epithelial Cell Cultures as Human Disease Models

Advances in stem cell biology and the understanding of factors that determine lung stem cell self-renewal have enabled long-term in vitro culture of human lung cells derived from airway basal and alveolar type II cells. Improved capability to expand and study primary cells long term, including in clonal cultures that are recently derived from a single cell, will allow experiments that address fundamental questions about lung homeostasis and repair, as well as translational questions in asthma, chronic obstructive pulmonary disease, pulmonary fibrosis, and lung cancer research. Here, we provide a brief history of postnatal lung epithelial cell culture and describe recent methodological advances. We further discuss the applications of primary cultures in defining "normal" epithelium, in modeling lung disease, and in future cell therapies.

Regenerative medicine for end-stage fibrosis and tissue loss in the upper aerodigestive tract: a twenty-first century review

OBJECTIVE

This review assesses regenerative medicine of the upper aerodigestive tract during the first two decades of the twenty-first century, focusing on end-stage fibrosis and tissue loss in the upper airways, salivary system, oropharynx and tongue.

METHOD

PubMed, Embase, Google Scholar, Cochrane Library, Medline and clinicaltrials.org were searched from 2000 to 2019. The keywords used were: bioengineering, regenerative medicine, tissue engineering, cell therapy, regenerative surgery, upper aerodigestive tract, pharynx, oropharynx, larynx, trachea, vocal cord, tongue and salivary glands. Original studies were subcategorised by anatomical region. Original human reports were further analysed. Articles on periodontology, ear, nose and maxillofacial disorders, and cancer immunotherapy were excluded.

RESULTS

Of 716 relevant publications, 471 were original studies. There were 18 human studies included, within which 8 reported airway replacements, 5 concerned vocal fold regeneration and 3 concerned salivary gland regeneration. Techniques included cell transplantation, injection of biofactors, bioscaffolding and bioengineered laryngeal structures.

CONCLUSION

Moderate experimental success was identified in the restoration of upper airway, vocal fold and salivary gland function. This review suggests that a shift in regenerative medicine research focus is required toward pathology with a higher disease burden.

Regenerative Medicine of Epithelia: Lessons From the Past and Future Goals

This article explores examples of successful and unsuccessful regenerative medicine on human epithelia. To evaluate the applications of the first regenerated tissues, the analysis of the past successes and failures addresses some pending issues and lay the groundwork for developing new therapies. Research should still be encouraged to fill the gap between pathologies, clinical applications and what regenerative medicine can attain with current knowledge.

In vivo regeneration of rat laryngeal cartilage with mesenchymal stem cells derived from human induced pluripotent stem cells via neural crest cells

The laryngotracheal cartilage is a cardinal framework for the maintenance of the airway for breathing, which occasionally requires reconstruction. Because hyaline cartilage has a poor intrinsic regenerative ability, various regenerative approaches have been attempted to regenerate laryngotracheal cartilage. The use of autologous mesenchymal stem cells (MSCs) for cartilage regeneration has been widely investigated. However, long-term culture may limit proliferative capacity. Human-induced pluripotent stem cell-derived MSCs (iMSCs) can circumvent this problem due to their unlimited proliferative capacity. This study aimed to investigate the efficacy of iMSCs in the regeneration of thyroid cartilage in immunodeficient rats. Herein, we induced iMSCs through neural crest cell intermediates. For the relevance to prospective future clinical application, induction was conducted under xeno-free/serum-free conditions. Then, clumps fabricated from an iMSC/extracellular matrix complex (C-iMSC) were transplanted into thyroid cartilage defects in immunodeficient rats. Histological examinations revealed cartilage-like regenerated tissue and human nuclear antigen (HNA)-positive surviving transplanted cells in the regenerated lesion. HNA-positive cells co-expressed SOX9, and type II collagen was identified around HNA-positive cells. These results indicated that the transplanted C-iMSCs promoted thyroid cartilage regeneration and some of the iMSCs differentiated into chondrogenic lineage cells. Induced MSCs may be a promising candidate cell therapy for human laryngotracheal reconstruction.

Regeneration of partially decellularized tracheal scaffolds in a mouse model of orthotopic tracheal replacement

Decellularized tracheal scaffolds offer a potential solution for the repair of long-segment tracheal defects. However, complete decellularization of trachea is complicated by tracheal collapse. We created a partially decellularized tracheal scaffold (DTS) and characterized regeneration in a mouse model of tracheal transplantation. All cell populations except chondrocytes were eliminated from DTS. DTS maintained graft integrity as well as its predominant extracellular matrix (ECM) proteins. We then assessed the performance of DTS in vivo. Grafts formed a functional epithelium by study endpoint (28 days). While initial chondrocyte viability was low, this was found to improve in vivo. We then used atomic force microscopy to quantify micromechanical properties of DTS, demonstrating that orthotopic implantation and graft regeneration lead to the restoration of native tracheal rigidity. We conclude that DTS preserves the cartilage ECM, supports neo-epithelialization, endothelialization and chondrocyte viability, and can serve as a potential solution for long-segment tracheal defects.

Tracheal Agenesis: Vertical Division of the Native Esophagus - A Novel Surgical Approach and Review of the Literature

INTRODUCTION

Tracheal agenesis (TA) is rare and usually fatal. Few survivors with concomitant tracheoesophageal fistulae (TEF) who underwent ligation of the distal esophagus with creation of a spit-fistula and neo-trachea from the proximal esophagus exist. We report a novel surgical technique whereby the esophagus is divided longitudinally to preserve a functional alimentation tract and a parallel neo-trachea. We review the literature of reported cases, including survivors beyond 12 months.

METHODS

Case report and literature review.

RESULTS

A female infant with prenatal polyhydramnios was born at 35 weeks gestation with immediate respiratory distress and absent cry. Oxygenation was maintained with a laryngeal mask airway. Despite a normal appearing larynx, she could not be intubated and emergent neck exploration disclosed no cervical trachea. The patient was placed on extra corporeal membranous oxygenation (ECMO), and later diagnosed with TA, Floyd Type I. Parental desire for reconstruction but refusal of a spit-fistula necessitated a novel procedure. The esophagus was divided longitudinally via a microstapler to preserve the original alimentary tract and create a parallel neo-trachea originating from the TEF and terminating as a cervical stoma. The healing process was complicated but the baby was ultimately discharged to home where she developed normally neurologically until succumbing one night to accidental decannulation at 16 months of age.

CONCLUSION

We describe a novel surgical approach to manage TA. This includes avoiding creation of a spit fistula and preserving the native esophagus. We then survey the literature, reporting the survivorship duration and operative management of 174 reported cases of TA.

Advances in regenerative medicine for otolaryngology/head and neck surgery

Head and neck structures govern the vital functions of breathing and swallowing. Additionally, these structures facilitate our sense of self through vocal communication, hearing, facial animation, and physical appearance. Loss of these functions can lead to loss of life or greatly affect quality of life. Regenerative medicine is a rapidly developing field that aims to repair or replace damaged cells, tissues, and organs. Although the field is largely in its nascence, regenerative medicine holds promise for improving on conventional treatments for head and neck disorders or providing therapies where no current standard exists. This review presents milestones in the research of regenerative medicine in head and neck surgery.

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.

Trachea Engineering Using a Centrifugation Method and Mouse-Induced Pluripotent Stem Cells

The outcomes of tracheal transplantation for the treatment of airway stenosis are unsatisfactory. We investigated the feasibility of regeneration of the trachea using a rat decellularized tracheal scaffold and mouse-induced pluripotent stem (iPS) cells for in vivo transplantation. The rat trachea was first decellularized using a detergent/enzymatic treatment method. We successfully established a centrifugation method that can transplant cells onto the luminal surface of the decellularized rat tracheal scaffold circumferentially. Two types of mouse iPS cells were differentiated into definitive endoderm cells and transplanted onto the luminal surface of the decellularized tracheal matrix scaffold using this centrifugation method. For in vivo study, normal rat tracheas, no-cell rat tracheal scaffolds, or rat tracheal scaffolds recellularized with rat tracheal epithelial cells (EGV-4T) were orthotopically transplanted on F344 rats, and rat tracheal scaffolds recellularized with mouse iPS cells were transplanted on F344/NJc1-rnu/rnu rats. Rats transplanted with no-cell scaffolds or scaffolds recellularized with EGV-4T survived for 1 month, although airway stenosis was observed. One of the F344/NJc1-rnu/rnu rats transplanted with rat trachea regenerated using mouse iPS cells survived over 5 weeks. Histological analysis indicated the cause of death was airway stenosis due to colonic cellular proliferation of undifferentiated iPS cells. Re-epithelialization with numerous ciliated epithelial cells was observed in one of the rats transplanted with trachea bioengineered using iPS cells. In this study, we present a simple and efficient tracheal tissue engineering model using a centrifugation method in a small-animal model. Tissue-engineered trachea using decellularized tracheal scaffolds and iPS cells is potentially applicable for tracheal transplantation.

State of the Art: Tissue Engineering in Congenital Heart Surgery

Congenital heart disease is the leading cause of death secondary to congenital abnormalities in the United States and the incidence has increased significantly over the last 50 years. For those defects requiring surgical repair, bioprosthetic xenografts, allografts, and synthetic materials have traditionally been used. However, none of these modalities offer the potential for growth and accommodation within the pediatric population. Tissue engineering has been an area of great interest in a variety of cardiac applications as an innovative solution to create a product that can grow and regenerate within the body over time. Over the last 30 years, the original tissue engineering paradigm of a scaffold seeded with cells and cultured in a bioreactor has been expanded upon to include innovative methods of decellularization and production of "off-the-shelf" tissue-engineered products capable of in situ host cell repopulation. Despite progress in conceptual design and promising clinical results, widespread use of tissue-engineered products remains limited due to both regulatory and ongoing scientific challenges. Here, we describe the current state of the art with regards to in vitro, in vivo, and in situ tissue engineering as applicable within the field of congenital heart surgery and provide a brief overview of challenges and future directions.

Challenges With the Development of Biomaterials for Sustainable Tissue Engineering

The field of tissue engineering has tantalizingly offered the possibility of regenerating new tissue in order to treat a multitude of diseases and conditions within the human body. Nevertheless, in spite of significant progress with in vitro and small animal studies, progress toward realizing the clinical and commercial endpoints has been slow and many would argue that ultimate goals, especially in treating those conditions which, as yet, do not have acceptable conventional therapies, may never be reached because of flawed scientific rationale. In other words, sustainable tissue engineering may not be achievable with current approaches. One of the major factors here is the choice of biomaterial that is intended, through its use as a "scaffold," to guide the regeneration process. For many years, effective specifications for these biomaterials have not been well-articulated, and the requirements for biodegradability and prior FDA approval for use in medical devices, have dominated material selection processes. This essay argues that these considerations are not only wrong in principle but counter-productive in practice. Materials, such as many synthetic bioabsorbable polymers, which are designed to have no biological activity that could stimulate target cells to express new and appropriate tissue, will not be effective. It is argued here that a traditional 'scaffold' represents the wrong approach, and that tissue-engineering templates that are designed to replicate the niche, or microenvironment, of these target cells are much more likely to succeed.