Hyaluronan-based scaffolds to tissue-engineer cartilage implants for laryngotracheal reconstruction

A Pilot Study of Decellularized Cartilage for Laryngotracheal Reconstruction in a Neonatal Pig Model

OBJECTIVE

Severe subglottic stenosis develops as a response to intubation in 1% of the >200,000 neonatal intensive care unit infants per year and may require laryngotracheal reconstruction (LTR) with autologous hyaline cartilage. Although effective, LTR is limited by comorbidities, severity of stenosis, and graft integration. In children, there is a significant incidence of restenosis requiring revision surgery. Tissue engineering has been proposed to develop alterative grafting options to improve outcomes and eliminate donor-site morbidity. Our objective is to engineer a decellularized, channel-laden xenogeneic cartilage graft, that we deployed in a proof-of-concept, neonatal porcine LTR model.

METHODS

Meniscal porcine cartilage was freeze-thawed and washed with pepsin/elastase to decellularize and create microchannels. A 6 × 10-mm decellularized cartilage graft was then implanted in 4 infant pigs in an anterior cricoid split. Airway patency and host response were monitored endoscopically until sacrifice at 12 weeks, when the construct phenotype, cricoid expansion, mechanics, and histomorphometry were evaluated.

RESULTS

The selective digestion of meniscal components yielded decellularized cartilage with cell-size channels. After LTR with decellularized meniscus, neonatal pigs were monitored via periodic endoscopy observing re-epithelization, integration, and neocartilage formation. At 12 weeks, the graft appeared integrated and exhibited airway expansion of 4 mm in micro-CT and endoscopy. Micro-CT revealed a larger lumen compared with age-matched controls. Finally, histology showed significant neocartilage formation.

CONCLUSION

Our neonatal porcine LTR model with a decellularized cartilage graft is a novel approach to tissue engineered pediatric LTR. This pilot study sets the stage for "off-the-shelf" graft procurement and future optimization of MEND for LTR.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:807-814, 2024.

Insoluble hyaluronan films obtained by heterogeneous crosslinking with iron(III) as resorbable implants

We present water-insoluble hyaluronan films crosslinked by trivalent iron developed as potential resorbable implants. The films were crosslinked by sorption of ferric salt into solid HA films in water/2-propanol bath. These heterogeneously crosslinked films (het-FeHA) remained tough and dimensionally stable when rehydrated in saline. In contrast, films prepared by drying the well-known homogeneous ferric hyaluronate gels (hom-FeHA) softened upon rehydration and expanded rapidly. Differences between hom-FeHA and het-FeHA result from polymer network topology (dominant inter- or intra-molecular crosslink, respectively). Moreover, Mössbauer spectroscopy of het-FeHA revealed diiron complexes, while iron in the hom-FeHA was present exclusively as γ-FeOOH nanoparticles or amorphous FeOOH. The biocompatibility tests of het-FeHA did not show any adverse effect and the sample disintegrated within one day when implanted in mice peritoneum. In conclusion, we developed implantable hyaluronan-based free-standing film with minimal swelling that can be resorbed quickly enough to avoid induction of foreign-body reaction.

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.

High-Throughput, Temporal and Dose Dependent, Effect of Vitamins and Minerals on Chondrogenesis

Tissue engineered hyaline cartilage is plagued by poor mechanical properties largely due to inadequate type II collagen expression. Of note, commonly used defined chondrogenic media lack 14 vitamins and minerals, some of which are implicated in chondrogenesis. Type II collagen promoter-driven Gaussia luciferase was transfected into ATDC5 cells to create a chondrogenic cell with a secreted-reporter. The reporter cells were used in an aggregate-based chondrogenic culture model to develop a high-throughput analytic platform. This high-throughput platform was used to assess the effect of vitamins and minerals, alone and in combination with TGFβ1, on COL2A1 promoter-driven expression. Significant combinatorial effects between vitamins, minerals, and TGFβ1 in terms of COL2A1 promoter-driven expression and metabolism were discovered. An “optimal” continual supplement of copper and vitamin K in the presence of TGFβ1 gave a 2.5-fold increase in COL2A1 promoter-driven expression over TGFβ1 supplemented media alone in ATDC5 cells.

A systematic review of simulated laryngotracheal reconstruction animal models

OBJECTIVES

Review of the literature to identify practical, high-fidelity, commercially available animal models for simulation training and surgical skills maintenance in laryngotracheal reconstruction (LTR).

METHODS

A systematic review of PubMed and Embase databases was conducted independently by two authors, according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Search terms included "laryngotracheal reconstruction," "laryngotracheoplasty," "pig and larynx," "sheep and larynx," and "rabbit and larynx." Articles were then assessed, identifying model cost and availability, model validation, feasibility as a training tool, and verisimilitude to pediatric LTR.

RESULTS

In total, 79 articles were considered suitable for inclusion in the study, incorporating both in vitro and in vivo models. Models utilized included rabbit (n = 69), pig (n = 7), sheep (n = 1), and goat (n = 2). The rabbit model was similar in size to the neonate, but differences in laryngeal anatomy and cartilage texture made graft insertion difficult. The anatomy of the pig, sheep, and goat larynges more closely resembled the pediatric patient, allowing improved grafting, but corresponded more in size to that of an older child. Commercial availability of the pig and sheep was considered greatest, and was reflected in cost. None of the animal models identified in the literature have been validated as a simulation tool.

CONCLUSIONS

The rabbit, sheep and pig models seemed to demonstrate the greatest potential for use as advanced pediatric airway surgery simulation models, with the rabbit model being most utilized in the literature. However, as yet there have been no models formally validated as a simulation training tool. Laryngoscope, 129:235-243, 2019.

A Modular Strategy to Engineer Complex Tissues and Organs

Currently, there are no synthetic or biologic materials suitable for long-term treatment of large tracheal defects. A successful tracheal replacement must (1) have radial rigidity to prevent airway collapse during respiration, (2) contain an immunoprotective respiratory epithelium, and (3) integrate with the host vasculature to support epithelium viability. Herein, biopolymer microspheres are used to deliver chondrogenic growth factors to human mesenchymal stem cells (hMSCs) seeded in a custom mold that self-assemble into cartilage rings, which can be fused into tubes. These rings and tubes can be fabricated with tunable wall thicknesses and lumen diameters with promising mechanical properties for airway collapse prevention. Epithelialized cartilage is developed by establishing a spatially defined composite tissue composed of human epithelial cells on the surface of an hMSC-derived cartilage sheet. Prevascular rings comprised of human umbilical vein endothelial cells and hMSCs are fused with cartilage rings to form prevascular-cartilage composite tubes, which are then coated with human epithelial cells, forming a tri-tissue construct. When prevascular- cartilage tubes are implanted subcutaneously in mice, the prevascular structures anastomose with host vasculature, demonstrated by their ability to be perfused. This microparticle-cell self-assembly strategy is promising for engineering complex tissues such as a multi-tissue composite trachea.

Use of autologous adipose-derived mesenchymal stem cells for creation of laryngeal cartilage

OBJECTIVES/HYPOTHESIS

Adipose-derived mesenchymal stem cells (ASCs) are an exciting potential cell source for tissue engineering because cells can be derived from the simple excision of autologous fat. This study introduces a novel approach for tissue-engineering cartilage from ASCs and a customized collagen oligomer solution, and demonstrates that the resultant cartilage can be used for laryngeal cartilage reconstruction in an animal model.

STUDY DESIGN

Basic science experimental design.

METHODS

ASCs were isolated from F344 rats, seeded in a customized collagen matrix, and cultured in chondrogenic differentiation medium for 1, 2, and 4 weeks until demonstrating cartilage-like characteristics in vitro. Large laryngeal cartilage defects were created in the F344 rat model, with the engineered cartilage used to replace the cartilage defects, and the rats followed for 1 to 3 months. Staining examined cellular morphology and cartilage-specific features.

RESULTS

In vitro histological staining revealed rounded chondrocyte-appearing cells evenly residing throughout the customized collagen scaffold, with positive staining for cartilage-specific markers. The cartilage was used to successfully repair large cartilaginous defects in the rat model, with excellent functional results.

CONCLUSIONS

This study is the first study to demonstrate, in an animal model, that ASCs cultured in a unique form of collagen oligomer can create functional cartilage-like grafts that can be successfully used for partial laryngeal cartilage replacement.

LEVEL OF EVIDENCE

NA. Laryngoscope, 128:E123-E129, 2018.

Reconstruction of defects of the trachea

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

Applications of Chondrocyte-Based Cartilage Engineering: An Overview

Chondrocytes are the exclusive cells residing in cartilage and maintain the functionality of cartilage tissue. Series of biocomponents such as different growth factors, cytokines, and transcriptional factors regulate the mesenchymal stem cells (MSCs) differentiation to chondrocytes. The number of chondrocytes and dedifferentiation are the key limitations in subsequent clinical application of the chondrocytes. Different culture methods are being developed to overcome such issues. Using tissue engineering and cell based approaches, chondrocytes offer prominent therapeutic option specifically in orthopedics for cartilage repair and to treat ailments such as tracheal defects, facial reconstruction, and urinary incontinence. Matrix-assisted autologous chondrocyte transplantation/implantation is an improved version of traditional autologous chondrocyte transplantation (ACT) method. An increasing number of studies show the clinical significance of this technique for the chondral lesions treatment. Literature survey was carried out to address clinical and functional findings by using various ACT procedures. The current study was conducted to study the pharmacological significance and biomedical application of chondrocytes. Furthermore, it is inferred from the present study that long term follow-up studies are required to evaluate the potential of these methods and specific positive outcomes.

Pediatric laryngotracheal reconstruction with tissue-engineered cartilage in a rabbit model

OBJECTIVES/HYPOTHESIS

To develop an effective rabbit model of in vitro- and in vivo-derived tissue-engineered cartilage for laryngotracheal reconstruction (LTR).

STUDY DESIGN

1) Determination of the optimal scaffold 1% hyaluronic acid (HA), 2% HA, and polyglycolic acid (PGA) and in vitro culture time course using a pilot study of 4 by 4-mm in vitro-derived constructs analyzed on a static culture versus zero-gravity bioreactor for 4, 8, and 12 weeks, with determination of compressive modulus and histology as outcome measures. 2) Three-stage survival rabbit experiment utilizing autologous auricular chondrocytes seeded in scaffolds, either 1% HA or PGA. The constructs were cultured for the determined in vitro time period and then cultured in vivo for 12 weeks. Fifteen LTRs were performed using HA cartilage constructs, and one was performed with a PGA construct. All remaining specimens and the final reconstructed larynx underwent mechanical testing, histology, and glycosaminoglycan (GAG) content determination, and then were compared to cricoid control specimens (n = 13) and control LTR using autologous thyroid cartilage (n = 18).

METHODS

1) One rabbit underwent an auricular punch biopsy, and its chondrocytes were isolated and expanded and then encapsulated in eight 4 by 4-mm discs of 1% HA, 2% HA, PGA either in rotary bioreactor or static culture for 4, 8, and 12 weeks, respectively, with determination of compressive modulus, GAG content, and histology. 2) Sixteen rabbits underwent ear punch biopsy; chondrocytes were isolated and expanded. The cells were seeded in 13 by 5 by 2.25-mm UV photopolymerized 1% HA (w/w) or calcium alginate encapsulated synthetic PGA (13 × 5 × 2 mm); the constructs were then incubated in vitro for 12 weeks (the optimal time period determined above in paragraph 1) on a shaker. One HA and one PGA construct from each animal was tested mechanically and histologically, and the remaining eight (4 HA and 4 PGA) were implanted in the neck. After 12 weeks in vivo, the most optimal-appearing HA construct was used as a graft for LTR in 15 rabbits and PGA in one rabbit. The seven remaining specimens underwent hematoxylin and eosin, Safranin O, GAG content determination, and flexural modulus testing. At 12 weeks postoperative, the animals were euthanized and underwent endoscopy. The larynges underwent mechanical and histological testing. All animals that died underwent postmortem examination, including gross and microhistological analysis of the reconstructed airway.

RESULTS

Thirteen of the 15 rabbits that underwent LTR with HA in vitro- and in vivo-derived tissue-engineered cartilage constructs survived. The 1% HA specimens had the highest modulus and GAG after 12 weeks in vitro. The HA constructs became well integrated in the airway, supported respiration for the 12 weeks, and were histologically and mechanically similar to autologous cartilage.

CONCLUSIONS

The engineering of in vitro- and in vivo-derived cartilage with HA is a novel approach for laryngotracheal reconstruction. The data suggests that the in vitro- and in vivo-derived tissue-engineered approaches may offer a promising alternative to current strategies used in pediatric airway reconstruction, as well as other head and neck applications.

LEVEL OF EVIDENCE

NA. Laryngoscope, 126:S5-S21, 2016.

Introducing a 3-dimensionally Printed, Tissue-Engineered Graft for Airway Reconstruction

Objective:

To use 3-dimensional (3D) printing and tissue engineering to create a graft for laryngotracheal reconstruction (LTR).

Study Design:

In vitro and in vivo pilot animal study.

Setting:

Large tertiary care academic medical center.

Subjects and Methods:

A 3D computer model of an anterior LTR graft was designed. That design was printed with polylactic acid on a commercially available 3D printer. The scaffolds were seeded with mature chondrocytes and collagen gel and cultured in vitro for up to 3 weeks. Scaffolds were evaluated in vitro for cell viability and proliferation. Anterior graft LTR was performed on 9 New Zealand white rabbits with the newly created scaffolds. Three animals were sacrificed at each time point (4, 8, and 12 weeks). The in vivo graft sites were assessed via bronchoscopy and histology.

Results:

The in vitro cell proliferation assay demonstrated initial viability of 87.5%. The cells proliferated during the study period, doubling over the first 7 days. Histology revealed that the cells retained their cartilaginous properties during the 21-day study period. In vivo testing showed that all animals survived for the duration of the study. Bronchoscopy revealed a well-mucosalized tracheal lumen with no evidence of scarring or granulation tissue. Histology indicated the presence of newly formed cartilage in the region where the graft was present.

Conclusions:

Our results indicate that it is possible to produce a custom-designed, 3D-printed, tissue-engineered graft for airway reconstruction.

An educational review of cartilage repair: precepts & practice--myths & misconceptions--progress & prospects

OBJECTIVE

The repair of cartilaginous lesions within synovial joints is still an unresolved and weighty clinical problem. Although research activity in this area has been indefatigably sustained, no significant progress has been made during the past decade. The aim of this educational review is to heighten the awareness amongst students and scientists of the basic issues that must be tackled and resolved before we can hope to escape from the whirlpool of stagnation into which we have fallen: cartilage repair redivivus!

DESIGN

Articular-cartilage lesions may be induced traumatically (e.g., by sports injuries and occupational accidents) or pathologically during the course of a degenerative disease (e.g., osteoarthritis). This review addresses the biological basis of cartilage repair and surveys current trends in treatment strategies, focussing on those that are most widely adopted by orthopaedic surgeons [viz., abrasive chondroplasty, microfracturing/microdrilling, osteochondral grafting and autologous-chondrocyte implantation (ACI)]. Also described are current research activities in the field of cartilage-tissue engineering, which, as a therapeutic principle, holds more promise for success than any other experimental approach.

RESULTS AND CONCLUSIONS

Tissue engineering aims to reconstitute a tissue both structurally and functionally. This process can be conducted entirely in vitro, initially in vitro and then in vivo (in situ), or entirely in vivo. Three key constituents usually form the building blocks of such an approach: a matrix scaffold, cells, and signalling molecules. Of the proposed approaches, none have yet advanced beyond the phase of experimental development to the level of clinical induction. The hurdles that need to be surmounted for ultimate success are discussed.

Tissue engineered scaffolds for an effective healing and regeneration: reviewing orthotopic studies

It is commonly stated that tissue engineering is the most promising approach to treat or replace failing tissues/organs. For this aim, a specific strategy should be planned including proper selection of biomaterials, fabrication techniques, cell lines, and signaling cues. A great effort has been pursued to develop suitable scaffolds for the restoration of a variety of tissues and a huge number of protocols ranging from in vitro to in vivo studies, the latter further differentiating into several procedures depending on the type of implantation (i.e., subcutaneous or orthotopic) and the model adopted (i.e., animal or human), have been developed. All together, the published reports demonstrate that the proposed tissue engineering approaches spread toward multiple directions. The critical review of this scenario might suggest, at the same time, that a limited number of studies gave a real improvement to the field, especially referring to in vivo investigations. In this regard, the present paper aims to review the results of in vivo tissue engineering experimentations, focusing on the role of the scaffold and its specificity with respect to the tissue to be regenerated, in order to verify whether an extracellular matrix-like device, as usually stated, could promote an expected positive outcome.

Tissue-engineered tracheal reconstruction using three-dimensionally printed artificial tracheal graft: preliminary report

Three-dimensional printing has come into the spotlight in the realm of tissue engineering. We intended to evaluate the plausibility of 3D-printed (3DP) scaffold coated with mesenchymal stem cells (MSCs) seeded in fibrin for the repair of partial tracheal defects. MSCs from rabbit bone marrow were expanded and cultured. A half-pipe-shaped 3DP polycaprolactone scaffold was coated with the MSCs seeded in fibrin. The half-pipe tracheal graft was implanted on a 10 × 10-mm artificial tracheal defect in four rabbits. Four and eight weeks after the operation, the reconstructed sites were evaluated bronchoscopically, radiologically, histologically, and functionally. None of the four rabbits showed any sign of respiratory distress. Endoscopic examination and computed tomography showed successful reconstruction of trachea without any collapse or blockage. The replaced tracheas were completely covered with regenerated respiratory mucosa. Histologic analysis showed that the implanted 3DP tracheal grafts were successfully integrated with the adjacent trachea without disruption or granulation tissue formation. Neo-cartilage formation inside the implanted graft was sufficient to maintain the patency of the reconstructed trachea. Scanning electron microscope examination confirmed the regeneration of the cilia, and beating frequency of regenerated cilia was not different from those of the normal adjacent mucosa. The shape and function of reconstructed trachea using 3DP scaffold coated with MSCs seeded in fibrin were restored successfully without any graft rejection.

Polysaccharides as Cell Carriers for Tissue Engineering: the Use of Cellulose in Vascular Wall Reconstruction

Polysaccharides are long carbohydrate molecules of monosaccharide units joined together by glycosidic bonds. These biological polymers have emerged as promising materials for tissue engineering due to their biocompatibility, mostly good availability and tailorable properties. This complex group of biomolecules can be classified using several criteria, such as chemical composition (homo- and heteropolysaccharides), structure (linear and branched), function in the organism (structural, storage and secreted polysaccharides), or source (animals, plants, microorganisms). Polysaccharides most widely used in tissue engineering include starch, cellulose, chitosan, pectins, alginate, agar, dextran, pullulan, gellan, xanthan and glycosaminoglycans. Polysaccharides have been applied for engineering and regeneration of practically all tissues, though mostly at the experimental level. Polysaccharides have been tested for engineering of blood vessels, myocardium, heart valves, bone, articular and tracheal cartilage, intervertebral discs, menisci, skin, liver, skeletal muscle, neural tissue, urinary bladder, and also for encapsulation and delivery of pancreatic islets and ovarian follicles. For these purposes, polysaccharides have been applied in various forms, such as injectable hydrogels or porous and fibrous scaffolds, and often in combination with other natural or synthetic polymers or inorganic nanoparticles. The immune response evoked by polysaccharides is usually mild, and can be reduced by purifying the material or by choosing appropriate crosslinking agents.

An ectopic approach for engineering a vascularized tracheal substitute

Tissue engineering can provide alternatives to current methods for tracheal reconstruction. Here we describe an approach for ectopic engineering of vascularized trachea based on the implantation of co-cultured scaffolds surrounded by a muscle flap. Poly(L-lactic-co-glycolic acid) (PLGA) or poly(ε-caprolactone) (PCL) scaffolds were seeded with chondrocytes, bone marrow stem cells and co-cultured both cells respectively (8 groups), wrapped in a pedicled muscle flap, placed as an ectopic culture on the abdominal wall of rabbits (n = 24), and harvested after two and four weeks. Analysis of the biochemical and mechanical properties demonstrated that the PCL scaffold with co-culture cells seeding displayed the optimal chondrogenesis with adequate rigidity to maintain the cylindrical shape and luminal patency. Histological analysis confirmed that cartilage formed in the co-culture groups contained a more homogeneous and higher extracellular matrix content. The luminal surfaces appeared to support adequate epithelialization due to the formation of vascularized capsular tissue. A prefabricated neo-trachea was transferred to the defect as a tracheal replacement and yielded satisfactory results. These encouraging results indicate that our co-culture approach may enable the development of a clinically applicable neo-trachea.

Overexpression of CXCR4 in tracheal epithelial cells promotes their proliferation and migration to a stromal cell-derived factor-1 gradient

Tracheal reconstruction has been an important issue in clinic, but it is limited for the ability of epithelial regeneration. Several reports have shown that stromal cell-derived factor-1 (SDF-1) and chemokine receptor CXCR4 play an important role in cell proliferation and migration of multiple cell types. But there is no report of SDF-1 and CXCR4 in tracheal cells. In this paper, the rat tracheal epithelial cells covered with cilium were isolated and cultured using two enzyme digestions, and CXCR4 lentivirus was constructed and infected to the tracheal cells successfully. The results showed that the expression of CXCR4 which was covered on cellular membrane majorly was low in normal cells, and the cell proliferation was increased accompanied with the increase in SDF-1 concentration. The cell proliferation, migration and intracellular free calcium were increased significantly in CXCR4 lentivirus infected groups in a dose-dependent manner, and these effects could be inhibited after CXCR4 inhibitor AMD3100 treated because the expression of CXCR4 was decreased. Our findings indicate that the activation of CXCR4 may promote tracheal cell proliferation and migration to the sites of airway injury where SDF-1 is regulated.

Optimization of the Expansion and Differentiation of Rabbit Chondrocytes In Vitro

OBJECTIVE

To develop a tissue culture expansion method for rabbit chondrocytes that promotes robust expansion while preserving chondrogenic potential.

DESIGN

Rabbit chondrocytes isolated from articular or auricular chondrocytes were assessed for chondrogenic differentiation potential versus population doubling using different expansion and differentiation conditions. Expansion conditions included serum alone, serum plus basic fibroblast growth factor 2 (FGF-2), and serum plus insulin-like growth factor 1 (IGF-1) and FGF-2. Differentiation conditions consisted of defined medium with and without bone morphogenetic protein 2 (BMP-2).

RESULTS

Nonsupplemented chondrocytes showed limited expandability, whereas supplementation with FGF-2 allowed articular chondrocytes to be expanded past 10 population doublings (PDs) and allowed auricular chondrocytes to expand past 15 population doublings. Differentiation, as measured by glycosaminoglycan production in aggregate cultures, was minimal in articular chondrocytes without BMP-2 supplementation and diminished to less than 50% maximal in auricular chondrocytes by PD 20. However, when FGF-2 was used during expansion and BMP-2 used during differentiation, both articular and auricular chondrocytes retained greater than 50% maximal differentiation for more than 25 PDs. The addition of IGF-1 to FGF-2 during expansion decreased chondrogenicity of auricular chondrocytes exposed to BMP-2, whereas for articular chondrocytes, chondrogenic expression increased.

CONCLUSION

These results demonstrate that FGF-2, for expansion, and BMP-2, for differentiation, dramatically increase the functional expansion of auricular and articular chondrocytes and provide a methodology to expand sufficient numbers of chondrocytes for tissue engineering applications.

Endogenous and exogenous stem cells: a role in lung repair and use in airway tissue engineering and transplantation

Rapid repair of the denuded alveolar surface after injury is a key to survival. The respiratory tract contains several sources of endogenous adult stem cells residing within the basal layer of the upper airways, within or near pulmonary neuroendocrine cell rests, at the bronchoalveolar junction, and within the alveolar epithelial surface, which contribute to the repair of the airway wall. Bone marrow-derived adult mesenchymal stem cells circulating in blood are also involved in tracheal regeneration. However, an organism is frequently incapable of repairing serious damage and defects of the respiratory tract resulting from acute trauma, lung cancers, and chronic pulmonary and airway diseases. Therefore, replacement of the tracheal tissue should be urgently considered. The shortage of donor trachea remains a major obstacle in tracheal transplantation. However, implementation of tissue engineering and stem cell therapy-based approaches helps to successfully solve this problem. To date, huge progress has been achieved in tracheal bioengineering. Several sources of stem cells have been used for transplantation and airway reconstitution in animal models with experimentally induced tracheal defects. Most tracheal tissue engineering approaches use biodegradable three-dimensional scaffolds, which are important for neotracheal formation by promoting cell attachment, cell redifferentiation, and production of the extracellular matrix. The advances in tracheal bioengineering recently resulted in successful transplantation of the world's first bioengineered trachea. Current trends in tracheal transplantation include the use of autologous cells, development of bioactive cell-free scaffolds capable of supporting activation and differentiation of host stem cells on the site of injury, with a future perspective of using human native sites as micro-niche for potentiation of the human body's site-specific response by sequential adding, boosting, permissive, and recruitment impulses.

Esterified hyaluronic acid improves cartilage viability in experimental tracheal reconstruction with an auricular graft

OBJECTIVE: The aim of this study was to determine the efficacy of esterified hyaluronic acid (HYAFF) on the vitality of auricular cartilage grafts used for tracheoplasty, with respect to macroscopic and microscopic parameters.

STUDY DESIGN: Prospective, controlled.

SETTING: Academic research laboratory.

SUBJECTS AND METHODS: The study included 14 New Zealand rabbits acquired specifically for the study. The rabbits were divided into two groups: the control group, in which free cartilage grafts were not exposed to any materials or additional procedures (n = 7), and the hyaluronic acid (HA) treatment group, in which auricular grafts and anastomosis lines were covered with HYAFF (n = 7). Free auricular cartilage grafts used for the reconstruction of experimentally created tracheal defects were anastomosed extraluminally. All the rabbits were sacrificed two months post surgery. Samples were collected and examined histopathologically. The sections were stained with hematoxylineosin, Masson's trichrome, and inducible nitric oxide synthase (iNOS) and examined under a light microscope.

RESULTS: Fibrosis and cartilage mass significantly differed between the control and HYAFF group ( P < 0.05). Immunohistochemical examination showed that more chondrocytes stained with iNOS in the control group than in the HYAFF group, according to histologists' observations.

CONCLUSION: HYAFF catalyzed wound healing with less fibrous tissue formation, had chondroprotective and stimulatory effects on chondrocyte metabolism, and decreased nitric oxide production and apoptosis via improving the nourishment of free auricular cartilage grafts, subsequently preventing hypoxia and oxidative stress, particularly in the early postimplantation period.

Scaffold-free tissue-engineered cartilage implants for laryngotracheal reconstruction

OBJECTIVES/HYPOTHESIS

Donor site morbidity, including pneumothorax, can be a considerable problem when harvesting cartilage grafts for laryngotracheal reconstruction (LTR). Tissue engineered cartilage may offer a solution to this problem. This study investigated the feasibility of using autologous chondrocytes to tissue-engineer scaffold-free cartilage grafts for LTR in rabbits to avoid degradation that often arises from an inflammatory reaction to scaffold carrier matrix.

STUDY DESIGN

Animal study.

METHODS

Auricular cartilage was harvested from seven New Zealand white rabbits, the chondrocytes expanded and loaded onto a custom-made bioreactor for 7 to 8 weeks to fabricate autologous scaffold-free cartilage sheets. The sheets were cut to size and used for LTR, and the rabbits were sacrificed 4, 8, and 12 weeks after the LTR and prepared for histology.

RESULTS

None of the seven rabbits showed signs of respiratory distress. A smooth, noninflammatory scar was visible intraluminally; the remainder of the tracheal lumen was unremarkable. Histologically, the grafts showed no signs of degradation or inflammatory reaction, were covered with mucosal epithelium, but did show signs of mechanical failure at the implantation site.

CONCLUSIONS

These results show that autologous chondrocytes can be used to fabricate an implantable sheet of cartilage that retains a cartilage phenotype, becomes integrated, and does not produce a significant inflammatory reaction. These findings suggest that with the design of stronger implants, these implants can be successfully used as a graft for LTR.

Tissue-engineered tracheal transplantation

Regenerative medicine offers new tools with which to tackle disorders for which there is currently no good therapeutic option. The trachea is an ideal organ in which to explore the clinical potential of tissue engineering because severe large airway disease is poorly managed by conventional treatments, and the success of a graft is determined only by its ability to conduct air lifelong: that is, whether it can become a sustainable biological conduit. We define the component parts of tissue engineering and review the experimental methods used to produce airway implants to date, including a recent successful, first-in-man experience.

Subglottic stenosis examined as a fibrotic response to airway injury characterized by altered mucosal fibroblast activity

OBJECTIVE

To investigate the association between mucosal fibroblast activity and subglottic stenosis (SGS) development.

DESIGN

Prospective study of an animal model of SGS.

SETTING

Academic research laboratory.

SUBJECTS

New Zealand white rabbits were assigned to either the cricothyroidotomy and carbon dioxide laser injury group or the cricothyroidotomy and silver nitrate injury group. Airways were excised for histologic analysis and the establishment of primary fibroblast cultures. Lesions from surgical excision of established SGS and subglottic tissue were used to analyze SGS recurrence.

INTERVENTIONS

The subglottis was approached via cricothyroidotomy and was subjected to either carbon dioxide laser or silver nitrate injury before closure. The SGS lesions were excised at 8 to 10 weeks and were used to establish explants for fibroblast culture. The animals underwent recovery for an additional 14 days to follow recurrence of SGS. After 14 days, all the animals were killed humanely, and subglottic tissue was harvested for histologic evaluation. Rates of migration and contraction of SGS and normal airway fibroblasts were assayed using established in vitro methods under basal conditions and with prostaglandin E(2) treatment.

MAIN OUTCOME MEASURES

For in vivo studies, injury, healing, and scarring of the mucosa and cartilage were the primary measures. For cultured fibroblast experiments, cellular responses of fibroblasts from normal and stenosed mucosa were compared and contrasted.

RESULTS

Mucosal injury resulted in acute fibroplasia and chronic SGS, surgical excision of mature SGS at 8 weeks resulted in rapid recurrence of stenosis, and SGS-derived fibroblasts were relatively refractory to the effects of prostaglandin E(2) on migration and contraction.

CONCLUSIONS

Subglottic stenosis represents a fibrotic airway repair process that involves fibroblasts that produce recurrent, excessive scar formation. We suggest that SGS development and recurrence may be partially dictated by altered fibroblast responsiveness to antifibroplastic signals during mucosal repair.

Tissue-engineered trachea for airway reconstruction

OBJECTIVES/HYPOTHESIS

Scaffold-free cartilage has been used to engineer biocompatible and mechanically stable neotracheas in vivo. The purpose of this animal study was to determine if neotracheal constructs, implanted paratracheally, could successfully be used for segmental tracheal reconstruction.

STUDY DESIGN

Animal study.

METHODS

Culture-expanded auricular rabbit chondrocytes were used to engineer scaffold-free cartilage sheets. Cartilage and a strap muscle flap were wrapped around a tube and implanted paratracheally. At 12 to 14 weeks postimplantation neotracheas were used to reconstruct 20 mm tracheal defects. Surgical technique was modified several times in an attempt to decrease the amount of neotracheal obstruction and fibrosis. In one of the six rabbits, neotrachea with its intact strap muscle flap was dropped into the defect followed by an end-to-end anastomosis; in two animals the muscle flap was partially, and in one rabbit completely removed. In two animals the muscle flap was partially removed, the tube reinserted, and the construct reimplanted for 5 weeks to allow formation of a fibrous lining over the exposed cartilage followed by tracheal reconstruction.

RESULTS

All implants developed into vascularized and mechanically sound neotracheas. Following reconstruction, none of the animals showed immediate signs of respiratory distress; however, one died after 24 hours due to extensive endotracheal muscle flap edema, whereas rabbits who had undergone partial or complete muscle flap removal survived up to 39 days before developing cicatricial stenosis.

CONCLUSIONS

Tissue-engineered neotracheas proved to have excellent biocompatibility and stability to function under physiologic conditions, but lacked adequate endotracheal lining resulting in neotracheal stenosis.

The elusive path to cartilage regeneration

Numerous attempts have been made to develop an efficacious strategy for the repair of articular cartilage. These endeavours have been undaunted, if not spurred, by the challenge of the task and by the largely disappointing outcomes in animal models. Of the strategies that have been lately applied in a clinical setting, the autologous-chondrocyte-transplantation technique is the most notorious example. This methodology, which was prematurely launched on the clinical scene, was greeted with enthusiasm and has been widely adopted. However, a recent prospective and randomized clinical trial has revealed the approach to confer no advantage over conventional microfracturing. Why is the repair of articular cartilage such a seemingly intractable problem? The root of the evil undoubtedly lies in the tissue's poor intrinsic healing capacity. But the failure of investigators to tackle the biological stumbling blocks systematically rather than empirically is hardly a less inauspicious circumstance. Moreover, it is a common misbelief that the formation of hyaline cartilage per se suffices, whereas to be durable and functionally competent, the tissue must be fully mature. An appreciation of this necessity, coupled with a thorough understanding of the postnatal development of articular cartilage, would help to steer investigators clear of biological cul-de-sacs.

Cartilage repair: past and future--lessons for regenerative medicine

Since the first cell therapeutic study to repair articular cartilage defects in the knee in 1994, several clinical studies have been reported. An overview of the results of clinical studies did not conclusively show improvement over conventional methods, mainly because few studies reach level I of evidence for effects on middle or long term. However, these explorative trials have provided valuable information about study design, mechanisms of repair and clinical outcome and have revealed that much is still unknown and further improvements are required. Furthermore, cellular and molecular studies using new technologies such as cell tracking, gene arrays and proteomics have provided more insight in the cell biology and mechanisms of joint surface regeneration. Besides articular cartilage, cartilage of other anatomical locations as well as progenitor cells are now considered as alternative cell sources. Growth Factor research has revealed some information on optimal conditions to support cartilage repair. Thus, there is hope for improvement. In order to obtain more robust and reproducible results, more detailed information is needed on many aspects including the fate of the cells, choice of cell type and culture parameters. As for the clinical aspects, it becomes clear that careful selection of patient groups is an important input parameter that should be optimized for each application. In addition, the study outcome parameters should be improved. Although reduced pain and improved function are, from the patient's perspective, the most important outcomes, there is a need for more structure/tissue-related outcome measures. Ideally, criteria and/or markers to identify patients at risk and responders to treatment are the ultimate goal for these more sophisticated regenerative approaches in joint surface repair in particular, and regenerative medicine in general.

Fabrication of a neotrachea using engineered cartilage

OBJECTIVES

Surgical management of long-segment tracheal stenosis is an ongoing problem. Many types of tracheal prostheses have been tried but with limited success because of immune rejection, graft ischemia, or restenosis. Tissue engineered cartilage may offer a solution to this problem, although scaffolds, which are currently often used for support, can lead to biocompatibility problems. This study investigated the feasibility of scaffold-free cartilage to tissue engineer a vascularized neotrachea in rabbits.

STUDY DESIGN

Animal study.

METHODS

Autologous neotracheal constructs were implanted in the abdomen of six New Zealand white rabbits. Auricular chondrocytes were used to engineer scaffold-free cartilage sheets. A muscle flap raised from the external abdominal oblique muscle and the engineered cartilage were wrapped around a silicone stent to fabricate a vascularized neotrachea in vivo. In two of the six rabbits, a full thickness skin graft was used to create an epithelial lining. The constructs were harvested after either 6 or 10 weeks.

RESULTS

All neotracheal constructs were healthy with well-vascularized and integrated layers. The implanted engineered cartilage underwent a remodeling process, forming a solid tracheal framework. Constructs harvested after 10 weeks proved to have significantly better mechanical properties than after 6 weeks and were comparable with the rabbit's native trachea.

CONCLUSION

Scaffold-free engineered cartilage can successfully fabricate a well-vascularized, autologous neotrachea with excellent mechanical properties. The results suggest that this approach can be used to reconstruct tracheal defects in rabbits.