Animal models for tracheal research

Establishing Benchmarks for Airway Replacement: Long-Term Outcomes of Tracheal Autografts in a Large Animal Model

OBJECTIVE

Airway replacement is a challenging surgical intervention and remains an unmet clinical need. Due to the risk of airway stenosis, anastomotic separation, poor vascularization, and necrosis, it is necessary to establish the gold-standard outcomes of tracheal replacement. In this study, we use a large animal autograft model to assess long-term outcomes following tracheal replacement.

METHODS

Four New Zealand White rabbits underwent tracheal autograft surgery and were observed for 6 months. Clinical and radiographic surveillance were recorded, and grafts were analyzed histologically and radiographically at endpoint.

RESULTS

All animals survived to the endpoint with minimal respiratory symptoms and normal growth rates. No complications were observed. Computed tomography scans of the post-surgical airway demonstrated graft patency at all time points. Histological sections showed no sign of stenosis or necrosis with preservation of the native structure of the trachea.

CONCLUSION

We established benchmarks for airway replacement. Our findings suggest that a rabbit model of tracheal autograft with direct reimplantation is feasible and does not result in graft stenosis or airway collapse.

Tracheal Tissue Engineering: Principles and State of the Art

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

Engineering Large Airways

A key issue facing trachea replacement attempts has been the discrepancy of the mechanical properties between the native tracheal tissue and that of the replacement construct; this difference is often one of the major causes for implant failure in vivo and within clinical efforts. The trachea is composed of distinct structural regions, with each component fulfilling a different role in maintaining overall tracheal stability. The trachea's horseshoe-shaped hyaline cartilage rings, smooth muscle and annular ligament collectively produce an anisotropic tissue that allows for longitudinal extensibility and lateral rigidity. Therefore, any tracheal substitute must be mechanically robust in order to withstand intra-thoracic pressure changes that occur during respiration. Conversely, they must also be able to deform radially to allow for changes in the cross-sectional area during coughing and swallowing. These complicated native tissue characteristics, coupled with a lack of standardised protocols to accurately quantify tracheal biomechanics as guidance for implant design, constitute a significant hurdle for tracheal biomaterial scaffold fabrication. This chapter aims to highlight the pressure forces exerted on the trachea and how they can influence tracheal construct design and also the biomechanical properties of the three main components of the trachea and how to mechanically assess them.

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.

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.

Biomechanical properties of the ex vivo porcine trachea: A benchmark for three-dimensional bioprinted airway replacements

PURPOSE

Combining tissue engineering and three-dimensional (3D) printing may allow for the introduction of a living functional tracheal replacement graft. However, defining the biomechanical properties of the native trachea is a key prerequisite to clinical translation. To achieve this, we set out to define the rotation, axial stretch capacity, and positive intraluminal pressure capabilities for ex vivo porcine tracheas.

STUDY DESIGN

Animal study.

MATERIALS AND METHODS

Six full-length ex vivo porcine tracheas were bisected into 5.5 cm segments. Maximal positive intraluminal pressure was measured by sealing segment ends with custom designed 3D printed caps through which a pressure transducer was introduced. Axial stretch capacity and rotation were evaluated by stretching and rotating the segments along their axis between two clamps, respectively.

RESULTS

Six segments were tested for axial lengthening and the average post-stretch length percentage was 148.92% (range 136.81-163.48%, 95% CI 153-143%). The mean amount of length gain achieved per cartilaginous ring was 7.82% (range 4.71-10.95%, 95% CI 6.3-9.35%). Four tracheal segments were tested for maximal positive intraluminal pressure, which was over 400 mmHg. Degree of rotation testing found that the tracheal segments easily transformed 180° in anterior-posterior bending, lateral bending, and axial rotational twisting.

CONCLUSIONS

We define several biomechanical properties of the ex vivo porcine trachea by reporting the rotation, axial stretch capacity, and positive intraluminal pressure capabilities. We hope that this will aid future work in the clinical translation of 3D bioprinted airway replacement grafts and ensure their compatibility with native tracheal properties.

"Cannot intubate, cannot oxygenate": A novel 2-operator technique for cannula tracheotomy in an infant animal model-a feasibility study

BACKGROUND

Evidence regarding optimal management of the "Cannot Intubate, Cannot Oxygenate" (CICO) scenario in infants is scarce. When inserting a transtracheal cannula for front of neck access direct aspiration to confirm intratracheal location is standard practice. This postmortem "infant airway" animal model study describes a novel technique for cannula tracheotomy.

AIMS

To compare a novel technique of cannula tracheotomy to an accepted technique to assess success and complication rates.

METHODS

Two experienced proceduralists repeatedly performed tracheotomy using an 18-gauge BD InsyteTM cannula (BD, Franklin Lakes, NJ, USA) in 6 postmortem White New Zealand rabbits. Cannulas were attached either directly to a 5ml syringe (Direct Aspiration) or via a 25 cm length minimum volume extension tubing set (TUTA Healthcare Lidcombe, NSW, Australia) (Indirect Aspiration, 2 operator technique). Each technique was attempted a maximum of 12 times per rabbit with an ENT surgeon assessing success and complication rates endoscopically for each attempt.

RESULTS

72 tracheotomy attempts were made in total, 36 for each technique. Initial aspiration through the needle was achieved in 93% (97.2% direct versus 89% indirect). Advancement of the cannula and continued aspiration (success) into the trachea occurred in 67% for direct compared with 64% for indirect aspiration. Direct aspiration was associated with higher rates of lateral (10.3% versus 5.6%) and posterior (19.4% versus 13.9%) wall injury compared with the indirect 2-operator technique.

CONCLUSION

Cannula tracheotomy in infant-sized airways is technically difficult and seems frequently associated with tracheal wall injury. The reduced incidence of injury in the indirect group warrants further investigation in preclinical and clinical trials.

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.

Argon Plasma Coagulation (APC) for the Treatment of Excessive Dynamic Airway Collapse (EDAC): An Animal Pilot Study

BACKGROUND

Surgical stabilization of the airway through tracheobronchoplasty (TBP) is the current treatment modality for patients with severe symptomatic excessive dynamic airway collapse. However, TBP is associated with increased morbidity and mortality. Bronchoscopic treatment of the posterior membrane using argon plasma coagulation (APC) may be a safer alternative to TBP in highly selected patients. This study aimed to evaluate the effect of APC in the tracheobronchial tree of a sheep animal model.

PATIENTS AND METHODS

Two adult sheep were used for this study. Under flexible bronchoscopy, the posterior tracheal membrane was treated with precise APC using different power settings. Chest computed tomography was done at 2 days and bronchoscopy was performed at 30 days following initial procedure, before euthanasia. The airways were assessed for the presence of treatment-related histopathologic changes along with expression of genes associated with fibrosis.

RESULTS

There was no perioperative or postoperative morbidity or mortality. Chest computed tomography showed no signs of pneumomediastinum or pneumothorax. Flexible bronchoscopy showed adequate tracheobronchial tissue healing process, independent of the power settings used. Histologic changes demonstrated an increased extent of fibroblastic collagen deposition in the treated posterior membrane when higher power settings were used. In a similar manner, APC treatment managed to activate fibrosis-associated gene transcription factors, with higher settings achieving a higher level of expression.

CONCLUSION

APC at high-power settings achieved higher levels of fibroblast collagen deposition at the posterior membrane and higher expression of fibrosis-associated gene transcription factors, when compared with lower settings.

Evaluation of Antigenicity of Components of Tracheal Allotransplant and Effect of Immunosuppressant Regime in a Rodent Model

Background  Tracheal transplantation seems to be the logical step in the process of reconstruction of the trachea following a long-segment resection, which is usually done to treat malignant disease or benign stenosis of the airway caused by a traumatic, congenital, inflammatory, or iatrogenic lesion. Immunosuppression following transplant is essential but not ideal after oncoresection. Methods  The tracheal allografts, harvested from Sprague Dawley rats, were implanted in the Wistar strain rat. The harvested tracheal grafts were divided into groups and subgroups, based on the layers of trachea, method of decellularization, and immunosuppression. The antigenicity of different layers of trachea and the effect of various decellularization methods were studied within three time frames, that is, day 3, 9, and 15. Result  On structural analysis, the day 3 and day 15 samples showed no meaningful comparison could be made, due to extensive neutrophil infiltration in all three layers. The day 9 tracheal grafts showed loss of epithelium, with no signs of regeneration in most of the allografts. The subepithelial lymphoid infiltration was found to be severe in nonimmunosuppressed allografts. The group in which both inner and outer layers were removed showed moderate-to-severe infiltrate of lymphoid cells in all the allografts, but there was no cartilage loss, irrespective of the method of decellularization. The irradiated specimens retained the cartilage but showed extensive ischemic damage. Conclusion  Rat trachea is a good model for tracheal transplant research but not adequately sturdy to sustain mechanical debridement. Irradiation and chemical decellularization eliminates the immune response but causes intense ischemic damage. Out of the three time frames, day 9 seemed to be the best to study the immune response. To substantiate the results obtained in this study, the immunohistochemical study of the allografts is needed to be performed among a larger group of animals.

In-vivo efficacy of biodegradable ultrahigh ductility Mg-Li-Zn alloy tracheal stents for pediatric airway obstruction

Pediatric laryngotracheal stenosis is a complex congenital or acquired airway injury that may manifest into a potentially life-threatening airway emergency condition. Depending on the severity of obstruction, treatment often requires a combination of endoscopic techniques, open surgical repair, intraluminal stenting, or tracheostomy. A balloon expandable biodegradable airway stent maintaining patency while safely degrading over time may address the complications and morbidity issues of existing treatments providing a less invasive and more effective management technique. Previous studies have focused on implementation of degradable polymeric scaffolds associated with potentially life-threatening pitfalls. The feasibility of an ultra-high ductility magnesium-alloy based biodegradable airway stents was demonstrated for the first time. The stents were highly corrosion resistant under in vitro flow environments, while safely degrading in vivo without affecting growth of the rabbit airway. The metallic matrix and degradation products were well tolerated by the airway tissue without exhibiting any noticeable local or systemic toxicity.

3D Bioprinting Strategies for the Regeneration of Functional Tubular Tissues and Organs

It is difficult to fabricate tubular-shaped tissues and organs (e.g., trachea, blood vessel, and esophagus tissue) with traditional biofabrication techniques (e.g., electrospinning, cell-sheet engineering, and mold-casting) because these have complicated multiple processes. In addition, the tubular-shaped tissues and organs have their own design with target-specific mechanical and biological properties. Therefore, the customized geometrical and physiological environment is required as one of the most critical factors for functional tissue regeneration. 3D bioprinting technology has been receiving attention for the fabrication of patient-tailored and complex-shaped free-form architecture with high reproducibility and versatility. Printable biocomposite inks that can facilitate to build tissue constructs with polymeric frameworks and biochemical microenvironmental cues are also being actively developed for the reconstruction of functional tissue. In this review, we delineated the state-of-the-art of 3D bioprinting techniques specifically for tubular tissue and organ regeneration. In addition, this review described biocomposite inks, such as natural and synthetic polymers. Several described engineering approaches using 3D bioprinting techniques and biocomposite inks may offer beneficial characteristics for the physiological mimicry of human tubular tissues and organs.

Doxycycline-Eluting Core-Shell Type Nanofiber-Covered Trachea Stent for Inhibition of Cellular Metalloproteinase and Its Related Fibrotic Stenosis

In this study, we fabricated a doxycycline (doxy)-eluting nanofiber-covered endotracheal stent for the prevention of stent intubation-related tissue fibrosis and re-stenosis. The nanofiber was deposited directly on the outer surface of the stent using a coaxial electrospinning method to form a doxy-eluting cover sleeve. Poly(d,l-lactide) was used as the shell-forming polymer and dedicated drug release-control membrane. Polyurethane was selected as the drug-loading core polymer. The compositional ratio of the core to shell was adjusted to 1:0, 1:2, and 1:4 by changing the electro-spray rate of each polymeric solution and microscopic observation of nanofibers using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the fluorescence microscopy proved core-shell structure of nanofibers. The in vitro release study suggested that the release of doxy could be controlled by increasing the compositional ratio of the shell. The growth of HT1080 fibrosarcoma cells was inhibited by the 10% doxy-containing nanofiber. The real-time polymerase chain reaction (PCR) in HT1080 cells and xenografted tissue models indicated that the doxy-releasing nanofiber inhibited mRNA expression of metalloproteinases (MT1-MMP, MMP-2, and MMP-9). Overall, our study demonstrates that a doxy-eluting core-shell nanofiber stent can be successfully fabricated using coaxial electrospinning and displays the potential to prevent fibrotic re-stenosis, which is the most problematic clinical complication of tracheal stent intubation.

Engineered tissue micro-rings fabricated from aggregated fibroblasts and microfibres for a bottom-up tissue engineering approach

Tissue rings with incorporated microscaffolds have been engineered as promising building blocks for constructing biological tubes from the bottom up. However, the microscaffolds available for incorporation are very limited at present. In this paper we provide an efficient strategy to first incorporate microfluidic spun Ca-alginate microfibres encapsulating magnetic nanoparticles into self-assembled fibroblast micro-rings. Based on the surface modification, microfibres with a size of ∼40 μm allowed fibroblasts to spread and proliferate along the long axis. The optimal cell seeding density was obtained by evaluating the degree of coverage of fibroblasts on microfibres after 3 days of culture. Then we designed a magnetically guided culture apparatus with multiple annular micro-wells to facilitate cell-driven assembly of microfibres. A manipulation strategy dependent on surface tension was used to pattern microfibres along the micro-wells prior to cell seeding, and magnetic attraction further kept the patterned microfibres from being deposited in the micro-wells during cultivation. Within 3 days of culture, microfibre-incorporated tissue micro-rings were formed in the micro-wells. Quantitative analysis of the formation process revealed liquid-like aggregating behaviours, and incorporated microfibres showed the potential to promote the directed organization of cells in tissue micro-rings. Furthermore, magnetically driven manipulation was used robotically to assemble the micro-rings on a micropillar inserted into the centre of the culture apparatus. After 5 days of culture to allow cell fusion, a biological tubular microstructure was achieved. Microfluidic spinning can generate fibres with a variety of shapes, geometries, and compositions; therefore, our proposed method greatly enriches the variety of microscaffolds available for incorporation into tissue rings to engineer complex artificial organs for tissue engineering and regenerative medicine.

Replacement of Rat Tracheas by Layered, Trachea-Like, Scaffold-Free Structures of Human Cells Using a Bio-3D Printing System

Current scaffold-based tissue engineering approaches are subject to several limitations, such as design inflexibility, poor cytocompatibility, toxicity, and post-transplant degradation. Thus, scaffold-free tissue-engineered structures can be a promising solution to overcome the issues associated with classical scaffold-based materials in clinical transplantation. The present study seeks to optimize the culture conditions and cell combinations used to generate scaffold-free structures using a Bio-3D printing system. Human cartilage cells, human fibroblasts, human umbilical vein endothelial cells, and human mesenchymal stem cells from bone marrow are aggregated into spheroids and placed into a Bio-3D printing system with dedicated needles positioned according to 3D configuration data, to develop scaffold-free trachea-like tubes. Culturing the Bio-3D-printed structures with proper flow of specific medium in a bioreactor facilitates the rearrangement and self-organization of cells, improving physical strength and tissue function. The Bio-3D-printed tissue forms small-diameter trachea-like tubes that are implanted into rats with the support of catheters. It is confirmed that the tubes are viable in vivo and that the tracheal epithelium and capillaries proliferate. This tissue-engineered, scaffold-free, tubular structure can represent a significant step toward clinical application of bioengineered organs.

Stable Tracheal Regeneration Using Organotypically Cultured Tissue Composed of Autologous Chondrocytes and Epithelial Cells in Beagles

OBJECTIVES

In tracheal regeneration, the slow process of epithelialization is often a barrier to the stability and safety of the transplanted trachea. The aim of this study was to examine a new tracheal regeneration technique using organotypically cultured tissue composed of autologous cells.

METHODS

Nine beagles were prepared. Chondrocytes from auricular cartilage and epithelial cells from buccal mucosa were isolated and cultured. Tissue-engineered cartilages were fabricated with chondrocytes at a density of 1 × 10⁷ cells/mL (high-density group) and 1 × 10⁶ cells/mL (low-density group). A fabricated epithelial cell sheet was laid on a poly(lactic-co-glycolic acid) block in atelocollagen gel containing the chondrocytes, and the organotypically cultured tissues were transplanted into a partially resected trachea. The control group had only the block transplanted.

RESULTS

The tissue-engineered cartilages in the high-density group contained many viable chondrocytes and many cartilage matrices. The low-density group had abundant collagen fibers and no chondrocytes. Tracheal endoscopy revealed no deformation or atrophy at the transplant site in the high-density group. Histologically, partially hyaline cartilages covered with epithelium and lamina propria were found in the high-density group but not in the low-density and control groups.

CONCLUSIONS

Stable tracheal regeneration was achieved using organotypically cultured tissue fabricated with autologous high-density chondrocytes and epithelial cells.

A 3-dimensional bioprinted tracheal segment implant pilot study: Rabbit tracheal resection with graft implantation

OBJECTIVES

Surgical reconstruction of tracheal disease has expanded to include bioengineering and three dimensional (3D) printing. This pilot study investigates the viability of introducing a living functional tracheal replacement graft in a rabbit animal model.

METHODS

Seven New Zealand White rabbits were enrolled and six completed participation (one intraoperative mortality). Tracheal replacement grafts were created by impregnating 3D printed biodegradable polycaprolactone (PCL) tracheal scaffolds with rabbit tracheal hyaline chondrocytes. 2 cm of native trachea was resected and the tracheal replacement graft implanted. Subjects were divided into two equal groups (n = 3) that differed in their time of harvest following implantation (three or six weeks). Tracheal specimens were analyzed with intraluminal telescopic visualization and histopathology.

RESULTS

The two groups did not significantly differ in histopathology or intraluminal diameter. All sections wherein the implant telescoped over native trachea (anastomotic ends) contained adequate hyaline cartilage formation (i.e. chondrocytes within lacuna, surrounding extracellular matrix, and strong Safranin O staining). Furthermore, the PCL scaffold was surrounded by a thin layer of fibrous tissue. All areas without membranous coverage contained inadequate or immature cartilage formation with inflammation. The average intraluminal stenosis was 83.4% (range 34.2-95%).

CONCLUSIONS

We report normal cartilage growth in a tracheal replacement graft when chondrocytes are separated from the tracheal lumen by an intervening membrane. When no such membrane exists there is a propensity for inflammation and stenosis. These findings are important for future construction and implantation of tracheal replacement grafts.

LEVEL OF EVIDENCE

Not applicable: this is an in vivo animal trial.

Large animal models for long-segment tracheal reconstruction: a systematic review

BACKGROUND

The reconstruction of extensive tracheal defects is an unresolved problem. Despite decades of research, a reliable and practical substitute remains to be found. While there have been clinical reports of successful long-segment tracheal reconstruction, reproducibility and widespread applicability of these techniques have yet to be achieved. Large animals such as the dog, pig, sheep, and goat have comparable tracheal morphology and physiology to humans making them useful preclinical models to screen potential therapeutic strategies.

MATERIALS AND METHODS

The literature was reviewed to identify large animal models commonly used for tracheal reconstruction. A systematic search of PubMed and EMBASE was performed for large animal studies reporting on the reconstruction of long-segment tracheal and carinal defects. Fifty-seven studies were identified for analysis.

RESULTS

There is no standard large animal model available for tracheal research. In recent years, livestock species have gained favor over dogs as animal models in this field. The minimum requirements for successful tracheal replacement are rigidity, vascularity, and epithelial lining. Early attempts with synthetic prostheses were met with disappointing results. An autologous tracheal substitute is ideal but hindered by limited donor site availability and the lack of a dominant vascular pedicle for microsurgical reconstruction. Although tracheal allotransplantation enables like-for-like replacement, there are unresolved issues relating to graft vascularity, immunosuppression, and graft preservation. Tissue engineering holds great promise; however, the optimal combination of scaffold, cells, and culture conditions is still indeterminate.

CONCLUSIONS

Despite impressive advances in tracheal reconstruction, a durable substitute for extended tracheal defects continues to be elusive.

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.

Tracheal growth assessment in mongrel puppies (dogs) through multidetector CT

The aim of this study was to describe the tracheal growth pattern and its zoometric relationship in related medium-sized mongrel puppies through adulthood. Fourteen puppies were studied. CT monitoring was performed monthly, starting in the 1st month of life through the 7th month and subsequently at the 9th and 12th months. Additionally, six zoometric measurements were performed. Dorsoventral (DV) and transverse (TV) diameters and the luminal area from C1 to T2 were obtained. The global tracheal growth pattern revealed an increase up to 13 times its initial size, reaching a plateau phase during the last trimester. The relationship between the DV and the TV internal diameters of the tracheal lumen did not change during growth. As previously reported, the cranial tracheal area was wider, while the caudal part gradually decreased towards T1–T2; this consideration is important since the more distal an endotracheal tube is inserted, the greater the risk that injury may occur. The linear correlation between the zoometric measurements and the tracheal ring areas was positive. This study provides evidence for the evaluation of the morphometry of the canine trachea during physiological growth using helicoidal CT as a non-invasive, accurate tool.

Trachea Mechanics for Tissue Engineering Design

Trachea replacement for nonoperable defects remains an unsolved problem due to complications with stenosis and mechanical insufficiency. While native trachea has anisotropic mechanical properties, the vast majority of engineered constructs focus on uniform cartilaginous-like conduits. These conduits often lack quantitative mechanical analysis at the construct level, which limits analysis of functional outcomes in vivo, as well as comparisons across studies. This review aims to present a clear picture of native tracheal mechanics at the tissue and organ level, as well as loading conditions to establish design criteria for trachea replacements. We further explore the implications of failing to match native properties with regards to implant collapse, stenosis, and infection.

Regeneration of Tracheal Tissue in Partial Defects Using Porcine Small Intestinal Submucosa

Background

Surgical correction of tracheal defects is a complex procedure when the gold standard treatment with primary end-to-end anastomosis is not possible. An alternative treatment may be the use of porcine small intestinal submucosa (SIS). It has been used as graft material for bioengineering applications and to promote tissue regeneration. The aim of this study was to evaluate whether SIS grafts improved tracheal tissue regeneration in a rabbit model of experimental tracheostomy.

Methods

Sixteen rabbits were randomized into two groups. Animals in the control group underwent only surgical tracheostomy, while animals in the SIS group underwent surgical tracheostomy with an SIS graft covering the defect. We examined tissues at the site of tracheostomy 60 days after surgery using histological analysis with hematoxylin and eosin (H&E) staining and analyzed the perimeter and area of the defect with Image-Pro® PLUS 4.5 (Media Cybernetics).

Results

The average perimeter and area of the defects were smaller by 15.3% (p = 0.034) and 21.8% (p = 0.151), respectively, in the SIS group than in the control group. Histological analysis revealed immature cartilage, pseudostratified ciliated epithelium, and connective tissue in 54.5% (p = 0.018) of the SIS group, while no cartilaginous regeneration was observed in the control group.

Conclusions

Although tracheal SIS engraftment could not prevent stenosis in a rabbit model of tracheal injury, it produced some remarkable changes, efficiently facilitating neovascularization, reepithelialization, and neoformation of immature cartilage.

Prolonged in vitro precultivation alleviates post-implantation inflammation and promotes stable subcutaneous cartilage formation in a goat model

Synthetic biodegradable scaffolds such as polylactic acid coated polyglycolic acid (PLA-PGA) are especially suitable for engineering shaped cartilage such as auricle, but they induce a serious inflammatory reaction particularly in the immunologically aggressive subcutaneous site, leading to resorption of the engineered autologous cartilage. Our previous study in a rabbit model has demonstrated 2 weeks of in vitro precultivation could significantly alleviate the post-implantation inflammation induced by PLA-PGA engineered cartilaginous grafts, but reproduction of this result failed in a preclinical goat model. The aims of the current study were to investigate whether prolonged in vitro precultivation could form a mature cartilaginous graft to resist the acute host response and promote stable subcutaneous cartilage formation in a preclinical goat model. Goat chondrocytes were seeded onto PLA-PGA scaffolds, in vitro precultivated for 2, 4, 8, and 12 weeks, and then implanted subcutaneously in autologous goats for 1 and 8 weeks. The in vitro engineered cartilage (vitro-EC) was examined histologically (hematoxylin and eosin, safranin-O, collagen II). The 1 week explants were examined histologically and stained for CD3, CD68, collagen I, and apoptosis. The 8 week explants were evaluated by histology, wet weight, volume, glycosaminoglycan (GAG) quantification and Young's modulus. With prolonged in vitro time, the quality of vitro-EC improved and the amount of scaffold residue decreased; more pronounced cartilage formation with fewer immune cells (CD3 and CD68 positive), apoptotic cells, and less collagen I expression were observed in explants that had been in vitro precultivated for a longer period. The subcutaneously regenerated neocartilage became more mature after prolonged implantation. These results suggested that prolonged in vitro precultivation allowed formation of a mature cartilaginous graft to resist the acute host response and promoted stable subcutaneous cartilage formation in autologous goats. These findings may provide useful reference for engineering auricle, trachea, nose, and eyelid shaped cartilage, for example.

Tissue Engineered Airways: A Prospects Article

An ideal tracheal scaffold must withstand luminal collapse yet be flexible, have a sufficient degree of porosity to permit vascular and cellular ingrowth, but also be airtight and must facilitate growth of functional airway epithelium to avoid infection and aid in mucocilliary clearance. Finally, the scaffold must also be biocompatible to avoid implant rejection. Over the last 40 years, efforts to design and manufacture the airway have been undertaken worldwide but success has been limited and far apart. As a result, tracheal resection with primary repair remains the Gold Standard of care for patients presenting with airway disorders and malignancies. However, the maximum resectable length of the trachea is restricted to 30% of the total length in children or 50% in adults. Attempts to provide autologous grafts for human application have also been disappointing for a host of different reasons, including lack of implant integration, insufficient donor organs, and poor mechanical strength resulting in an unmet clinical need. The two main approaches researchers have taken to address this issue have been the development of synthetic scaffolds and the use of decellularized organs. To date, a number of different decellularization techniques and a variety of materials, including polyglycolic acid (PGA) and nanocomposite polymers have been explored. The findings thus far have shown great promise, however, there remain a significant number of caveats accompanying each approach. That being said, the possibilities presented by these two approaches could be combined to produce a highly successful, clinically viable hybrid scaffold. This article aims to highlight advances in airway tissue engineering and provide an overview of areas to explore and utilize in accomplishing the aim of developing an ideal tracheal prosthesis. J. Cell. Biochem. 117: 1497-1505, 2016. © 2016 Wiley Periodicals, Inc.

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

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

Patterned, tubular scaffolds mimic longitudinal and radial mechanics of the neonatal trachea

Tracheal damage, abnormality or absence can result from the growth of tumors or from Congenital High Airway Obstruction Syndrome. No optimal or routine treatment has been established for tracheal repair, despite numerous attempts with natural and artificial prostheses. The fetal trachea is comprised of cartilaginous rings connected by an elastomeric tissue. In an effort to design an engineered trachea replacement, we have synthesized 2-hydroxyethyl methacrylate hydrogels with moduli of 67 ± 3.1 kPa (soft) and 13.0 ± 1.8 MPa (hard). Given the criteria for longitudinal extensibility and lateral rigidity applied during respiration, we evaluated a series of patterned hydrogels with different sizes of hard and soft segments to mimic fetal tracheas. A 1:2 ratio of soft:hard segments resulted in a construct capable of 11.0 ± 1% extension within the elastic range. Tubular constructs with this ratio required similar load/length for cyclic compression as ovine trachea samples. Achieving biomimetic mechanical properties in a trachea replacement may be essential for achieving normal respiration in recipient patients.

STATEMENT OF SIGNIFICANCE

Fetal abnormalities or tumors can result in tracheal absence or damage. Despite numerous attempts with natural and artificial replacements, there is still no routine treatment for tracheal repair. The literature recognizes the importance of tracheal lateral rigidity and longitudinal extensibility for normal respiration. Achieving closely matched mechanical properties may provide proper function and help decrease implant fibrosis and subsequent occlusion. In this study, we evaluated the mechanics of a series of patterned, tubular hydrogels with different ratios of hard and soft segments to mimic alternating cartilage and ligament sections in fetal tracheas. We compared our results to that of sheep trachea. This is the first report to assess both radial rigidity and longitudinal extensibility in an engineered trachea construct.

Aerosol-Based Cell Therapy for Treatment of Lung Diseases

Aerosol-based cell delivery technique via intratracheal is an effective route for delivering transplant cells directly into the lungs. An aerosol device known as the MicroSprayer(®) Aerosolizer is invented to transform liquid into an aerosol form, which then can be applied via intratracheal administration for drug delivery. The device produces a uniform and concentrated distribution of aerosolized liquid. Using the capability of MicroSprayer(®) Aerosolizer to transform liquid into aerosol form, our group has designed a novel method of cell delivery using an aerosol-based technique. We have successfully delivered skin-derived fibroblast cells and airway epithelial cells into the airway of a rabbit with minimum risk of cell loss and have uniformly distributed the cells into the airway. This chapter illustrates the application of aerosol device to deliver any type of cells for future treatment of lung diseases.

Functional Reconstruction of Tracheal Defects by Protein-Loaded, Cell-Seeded, Fibrous Constructs in Rabbits

Tracheal stenosis is a life-threatening disease and current treatments include surgical reconstruction with autologous rib cartilage and the highly complex slide tracheoplasty surgical technique. We propose using a sustainable implant, composed of a tunable, fibrous scaffold with encapsulated chondrogenic growth factor (transforming growth factor-beta3 [TGF-β3]) or seeded allogeneic rabbit bone marrow mesenchymal stromal cells (BMSCs). In vivo functionality of these constructs was determined by implanting them in induced tracheal defects in rabbits for 6 or 12 weeks. The scaffolds maintained functional airways in a majority of the cases, with the BMSC-seeded group having an improved survival rate and the Scaffold-only group having a higher occurrence of more patent airways as determined by microcomputed tomography. The BMSC group had a greater accumulation of inflammatory cells over the graft, while also exhibiting normal epithelium, subepithelium, and cartilage formation. Overall, it was concluded that a simple, acellular scaffold is a viable option for tracheal tissue engineering, with the intraoperative addition of cells being an optional variation to the scaffolds.

The 'Can't intubate can't oxygenate' scenario in pediatric anesthesia: a comparison of the Melker cricothyroidotomy kit with a scalpel bougie technique

BACKGROUND

While the majority of pediatric intubations are uncomplicated, the 'Can't intubate, Can't Oxygenate' scenario (CICO) does occur. With limited management guidelines available, CICO is still a challenge even to experienced pediatric anesthetists.

OBJECTIVES

To compare the COOK Melker cricothyroidotomy kit (CM) with a scalpel bougie (SB) technique for success rate and complication rate in a tracheotomy on a cadaveric 'infant airway' animal model.

METHODS

Two experienced proceduralists repeatedly attempted tracheotomy in eight rabbits, alternately using CM and SB (4 fr) technique. The first attempt was performed at the level of the first tracheal cartilage with subsequent experimental trials of insertion progressively more caudad. Success was defined as intratracheal placement of cannula as seen on bronchoscope. Complications were assessed both by bronchoscopic and macropathological appearance.

RESULTS

32 attempts were made at tracheotomy. CM had an overall success rate of 100% compared to a 75% success rate for SB. Success rate for the first attempt was dependent on the level of the tracheotomy (Level 1 100%, level 2 62.5% and level 3 & 4 25%). While CM was associated with lateral and/or posterior wall damage on bronchoscopy/macropathology in 6% of 19% and 25% of 50% respectively, the damage observed was greater and more frequent with SB (19%/44% and 31%/50%, respectively).

CONCLUSIONS

At level 1, the first attempt success rate was 100% for both devices. Overall CM showed a better success rate than SB; however, both techniques were associated with significant complication rates, which were more pronounced following the scalpel bougie technique.

Engineered cartilaginous tubes for tracheal tissue replacement via self-assembly and fusion of human mesenchymal stem cell constructs

There is a critical need to engineer a neotrachea because currently there are no long-term treatments for tracheal stenoses affecting large portions of the airway. In this work, a modular tracheal tissue replacement strategy was developed. High-cell density, scaffold-free human mesenchymal stem cell-derived cartilaginous rings and tubes were successfully generated through employment of custom designed culture wells and a ring-to-tube assembly system. Furthermore, incorporation of transforming growth factor-β1-delivering gelatin microspheres into the engineered tissues enhanced chondrogenesis with regard to tissue size and matrix production and distribution in the ring- and tube-shaped constructs, as well as luminal rigidity of the tubes. Importantly, all engineered tissues had similar or improved biomechanical properties compared to rat tracheas, which suggests they could be transplanted into a small animal model for airway defects. The modular, bottom up approach used to grow stem cell-based cartilaginous tubes in this report is a promising platform to engineer complex organs (e.g., trachea), with control over tissue size and geometry, and has the potential to be used to generate autologous tissue implants for human clinical applications.

Defining the biomechanical properties of the rabbit trachea

OBJECTIVES/HYPOTHESIS

Surgical advancements rely heavily on validated animal models. The New Zealand White (NZW) rabbit is a widely used model for airway research, including regenerative medicine applications. Currently, the biomechanical properties of the normal rabbit trachea are not known. Our objective was to define these properties to assist in the standardization and understanding of future airway research using this model.

STUDY DESIGN

Laboratory-based study.

METHODS

Fresh tracheas from four adult NZW rabbits were dissected into 20 segments. To examine the biomechanical properties, segments were subjected to uniaxial tension (n = 9) and compression (n = 11) testing. Yield and maximum load (tension) and force at 50% displacement (compression) were recorded, and differences between segments were examined using analysis of covariance.

RESULTS

Normative data for native rabbit trachea show mean maximum load = 6.44 newtons (N), yield load = 5.93 N, and compressive strength = 2.10 N. In addition to establishing the baseline measurements, statistically significant differences in tensile measures based on location along the trachea and diameter were identified. Proximal segments had significantly higher maximum load (P = .0029) and yield load (P = .0062) than distal segments. Association between diameter and both maximum load (P = .0139) and yield load (P = .0082) was observed.

CONCLUSIONS

The adult NZW rabbit trachea is intrinsically less able to withstand tensile and compressive forces, compared to other airway models such as sheep or cadaveric human. Establishment of normative values will enable future research into changes in tracheal biomechanical properties during regenerative medicine manipulation and processing.

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.

A Quantitative Study on the Trachea of the Red Sokoto (Maradi) Goat (Capra hircus)

The trachea forms the part of the conducting system which transports air from the external environment to the lungs. The aim of this study was to provide quantitative dimensions of the trachea of Red Sokoto goat (Capra hircus). Quantitative analysis was conducted on nine tracheas from goats (ages were ranged between eight months and three years) without sex variation in this study. The results showed that tracheas were extended from the cricoid cartilage of larynx to the hilus of the lungs, where they were divided into the right and left bronchi. They were structurally composed of the cartilaginous rings that were incomplete dorsally but bridged by tracheal muscles at the ends of the tracheal cartilages. The mean length of the trachea from the first to the last ring was 257 ± 7.11 mm and the number of tracheal rings varied from 35 to 57, with a mean value of 49.33 ± 2.78. The left bronchial mean length (19.78 ± 2.66 mm) was significantly longer than the right (10.44 ± 1.79 mm). The cross-sectional area (CSA) was wider at the intrathoracic area (221.5 ± 0.2 mm²) than cervical area (176 ± 0.1 mm²).

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

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

Modification of macroporous titanium tracheal implants with biodegradable structures: tracking in vivo integration for determination of optimal in situ epithelialization conditions

Previously, we showed that macroporous titanium implants, colonized in vivo together with an epithelial graft, are viable options for tracheal replacement in sheep. To decrease the number of operating steps, biomaterial-based replacements for epithelial graft and intramuscular implantation were developed in the present study. Hybrid microporous PLLA/titanium tracheal implants were designed to decrease initial stenosis and provide a surface for epithelialization. They have been implanted in New Zealand white rabbits as tracheal substitutes and compared to intramuscular implantation samples. Moreover, a basement membrane like coating of the implant surface was also designed by Layer-by-Layer (LbL) method with collagen and alginate. The results showed that the commencement of stenosis can be prevented by the microporous PLLA. For determination of the optimum time point of epithelialization after implantation, HPLC analysis of blood samples, C-reactive protein (CRP), and Chromogranin A (CGA) analyses and histology were carried out. Following 3 weeks the implant would be ready for epithelialization with respect to the amount of tissue integration. Calcein-AM labeled epithelial cell seeding showed that after 3 weeks implant surfaces were suitable for their attachment. CRP readings were steady after an initial rise in the first week. Cross-linked collagen/alginate structures show nanofibrillarity and they form uniform films over the implant surfaces without damaging the microporosity of the PLLA body. Human respiratory epithelial cells proliferated and migrated on these surfaces which provided a better alternative to PLLA film surface. In conclusion, collagen/alginate LbL coated hybrid PLLA/titanium implants are viable options for tracheal replacement, together with in situ epithelialization.

Tracheobronchial bio-engineering: biotechnology fulfilling unmet medical needs

The development of substitutes for the human trachea or its bronchial tree represents a niche application in the rapidly advancing scientific field of Regenerative Medicine. Despite a comparatively small research foundation in the field of tracheo-bronchial bioengineering, four different approaches have already been translated into clinical settings and applied in patients. This can be attributed to the lack of established treatment options for a small group of patients with extensive major airway disease. In this review, the clinical background and tissue-specific basics of tracheo-bronchial bioengineering will be evaluated. Focusing on the clinical applications of bioengineered tracheal tissues, a "top-down" or "bedside-to-bench" analysis is performed in order to guide future basic and clinical research activities for airway bioengineering.

Human nasal turbinates as a viable source of respiratory epithelial cells using co-culture system versus dispase-dissociation technique

OBJECTIVE

To compare a co-culture system with a conventional dispase-dissociation method for obtaining functional human respiratory epithelial cells from the nasal turbinates for tissue engineering application.

METHODS

Human respiratory epithelial cells were serially passaged using a co-culture system and a conventional dispase-dissociation technique. The growth kinetics and gene expression levels of the cultured respiratory epithelial cells were compared. Four genes were investigated, namely cytokeratin-18, a marker for ciliated and secretory epithelial cells; cytokeratin-14, a marker for basal epithelial cells; MKI67, a proliferation marker; and MUC5B, a marker for mucin secretion. Immunocytochemical analysis was performed using monoclonal antibodies against the high molecular-weight cytokeratin 34 beta E12, cytokeratin 18, and MUC5A to investigate the protein expression from cultured respiratory epithelial cells.

RESULTS

Respiratory epithelial cells cultured using both methods maintained polygonal morphology throughout the passages. At passage 1, co-cultured respiratory epithelial showed a 2.6-times higher growth rate compared to conventional dispase dissociation technique, and 7.8 times higher at passage 2. Better basal gene expression was observed by co-cultured respiratory epithelial cells compared to dispase dissociated cells. Immunocytochemical analyses were positive for the respiratory epithelial cells cultured using both techniques.

CONCLUSION

Co-culture system produced superior quality of cultured human respiratory epithelial cells from the nasal turbinates as compared to dispase dissociation technique.

[Tissue engineering of respiratory epithelium. Regenerative medicine for reconstructive surgery of the upper airways]

Reconstruction of long tracheal defects remains an unsolved surgical problem. Tissue engineering of respiratory epithelium is therefore of utmost surgical and scientific interest. Successful cultivation and reproduction of respiratory epithelium in vitro is crucial to seed scaffolds of various biomaterials with functionally active respiratory mucosa. Most frequently, the suspension culture as well as the tissue or explant cultures are used. Collagenous matrices, synthetic and biodegradable polymers, serve as carriers in studies. It is essential for clinical practice that mechanically stable biomaterials be developed that are resorbable in the long term or that cartilaginous constructs produced in vitro be employed which are seeded with respiratory epithelium before implantation. Vascularization of a bioartificial matrix for tracheal substitution is also prerequisite for integration of the constructs produced in vitro into the recipient organism. Here, the state of the art of research, perspectives and limitations of tracheal tissue engineering are reviewed.

Tracheal remodeling: comparison of different composite cultures consisting of human respiratory epithelial cells and human chondrocytes

The reconstruction of extensive tracheal defects is still an unsolved challenge for thoracic surgery. Tissue engineering is a promising possibility to solve this problem through the generation of an autologous tracheal replacement from patients' own tissue. Therefore, this study investigated the potential of three different coculture systems, combining human respiratory epithelial cells and human chondrocytes. The coculture systems were analyzed by histological staining with alcian blue, immunohistochemical staining with the antibodies, 34betaE12 and CD44v6, and scanning electron microscopy. The first composite culture consisted of human respiratory epithelial cells seeded on human high-density chondrocyte pellets. For the second system, we used native articular cartilage chips as base for the respiratory epithelial cells. The third system consisted of a collagen membrane, seeded with respiratory epithelial cells and human chondrocytes onto different sides of the membrane, which achieved the most promising results. In combination with an air-liquid interface system and fibroblast-conditioned medium, an extended epithelial multilayer with differentiated epithelial cells could be generated. Our results suggest that at least three factors are necessary for the development towards a tracheal replacement: (1) a basal lamina equivalent, consisting of collagen fibers for cell-cell interaction and cell polarization, (2) extracellular factors of mesenchymal fibroblasts, and (3) the presence of an air-liquid interface system for proliferation and differentiation of the epithelial cells.