Successful in vitro growth of human respiratory epithelium on a tracheal prosthesis

Tissue engineering of a composite trachea construct using autologous rabbit chondrocytes

The repair of large tracheal segmental defects remains an unsolved problem. The goal of this study is to apply tissue engineering principles for the fabrication of large segmental trachea replacements. Engineered tracheal replacements composed of autologous cells (neotracheas) were tested in a New Zealand White rabbit model. Neotracheas were formed in the rabbit neck by wrapping a silicone tube with consecutive layers of skin epithelium, platysma muscle, and an engineered cartilage sheet and allowing the construct to mature for 8-12 weeks. In total, 28 rabbits were implanted and the neotracheas assessed for tissue morphology. In 11 cases, neotracheas deemed sufficiently strong were used to repair segmental tracheal defects. Initially, the success rate of producing structurally sound neotracheas was impeded by physical disruption of the cartilage sheets during animal handling, but by the end of the study, 15 of 18 neotracheas (83.3%) were structurally sound. Of the 15 structurally sound neotracheas, 11 were used for segmental reconstruction and were left in place for up to 21 days. Histological examination showed the presence of variable amounts of viable epithelium, a vascularized platysma flap, and a layer of safranin O-positive cartilage along with evidence of endochondral ossification. Rabbits that had undergone segmental reconstruction showed good tracheal integration, had a viable epithelium with vascular support, and the cartilage was sufficiently strong to maintain a lumen when palpated. The results demonstrated that viable, trilayered, scaffold-free neotracheas could be constructed from autologous cells and could be integrated into native trachea to repair a segmental defect.

Re-epithelialization: a key element in tracheal tissue engineering

Trachea-tissue engineering is a thriving new field in regenerative medicine that is reaching maturity and yielding numerous promising results. In view of the crucial role that the epithelium plays in the trachea, re-epithelialization of tracheal substitutes has gradually emerged as the focus of studies in tissue-engineered trachea. Recent progress in our understanding of stem cell biology, growth factor interactions and transplantation immunobiology offer the prospect of optimization of a tissue-engineered tracheal epithelium. In addition, advances in cell culture technology and successful applications of clinical transplantation are opening up new avenues for the construction of a tissue-engineered tracheal epithelium. Therefore, this review summarizes current advances, unresolved obstacles and future directions in the reconstruction of a tissue-engineered tracheal epithelium.

Patient-specific carbon nanocomposite tracheal prosthesis

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Cell sources for trachea tissue engineering: past, present and future

Trachea tissue engineering has been one of the most promising approaches to providing a potential clinical application for the treatment of long-segment tracheal stenosis. The sources of the cells are particularly important as the primary factor for tissue engineering. The use of appropriate cells seeded onto scaffolds holds huge promise as a means of engineering the trachea. Furthermore, appropriate cells would accelerate the regeneration of the tissue even without scaffolds. Besides autologous mature cells, various stem cells, including bone marrow-derived mesenchymal stem cells, adipose tissue-derived stem cells, umbilical cord blood-derived mesenchymal stem cells, amniotic fluid stem cells, embryonic stem cells and induced pluripotent stem cells, have received extensive attention in the field of trachea tissue engineering. Therefore, this article reviews the progress on different cell sources for engineering tracheal cartilage and epithelium, which can lead to a better selection and strategy for engineering the trachea.

Prefabricated tracheal prosthesis with partial biodegradable materials: a surgical and tissue engineering evaluation in vivo

Large and circumferential tracheal defects remain at this time an unsolved problem for reconstructive surgery. Many types of prosthetic and tissue grafts have been used but with limited comfortable results. Major complications are anastomotic dehiscence, graft ischemia and stenosis due to the poor vascularization of the prosthetic complex. We studied the in vivo behaviour of a prefabricated flap composed of a partially bioresorbable tracheal prosthesis and an arterio-venous vascular carrier. The prosthesis was made of a tubular skeleton of knitted Dacron (20 microm porosity) embedded within a bioresorbable poly-lactic-co-glycolic acid polymer (PLA(75)GA(25)) covering both sides. Fifteen New Zealand White rabbits were divided in three groups, depending on the time of examination (30, 90 and 180 days post-implantation). The prosthesis was implanted in the visceral space of the neck using the common carotid trunk and the internal jugular vein as vascular pedicle. The histological, immunohistochemical, and ESEM analyses of collected samples, showed a time-dependent process of tissue neoformation and neovascularization on the prosthetic material with a significant increase from 30 to 90 days post-implantation. In contrast, there was no statistically significant difference in the fibrovascular connective deposition from 90 to 180 days. This finding indicated the three months time as the best period for the tissue deposition and consequent hypothetical orthotopic transplantation of the prosthesis. Further in vivo studies are intended to confirm the results.

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.

[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 replacement in rabbits with a new composite silicone-metallic prosthesis

A new composite silicone-metallic prosthesis was tested, studying the potential for respiratory epithelial covering over the biocompatible inner lining, in a rabbit survival model. Seven New Zealand White rabbits underwent near-total excision of their trachea and implantation of a sterile prosthesis. After 2 months, they were sacrificed and the prostheses were retrieved. Specimens were fixed and histologically examined for tissue reaction around the prosthesis, at the anastomotic lines, and particularly for the presence or absence of epithelialization of the inner lumen over the biocompatible surface. All rabbits survived the operation. At 2 months, the outer layer of the prosthesis was consistently covered with fibrosis and neutrophils. The inner layer showed necrotic cells and scant re-epithelialization over the biocompatible lining, up to 5 mm beyond the anastomosis, with no evidence of organized respiratory epithelium in the middle sections. The new prosthesis is a viable temporary solution for airway replacement in rabbits. Granulation tissue was not observed at the anastomosis, and re-epithelialization did occur, but failed to achieve full-length luminal covering. The potential for granulation tissue does not yet make this an ideal long-term solution. Improvements in prosthesis design or biocompatibility are required, and need to be re-evaluated before applicability for chronic use.

Tissue engineering in otorhinolaryngology

Tissue engineering is a field of research with interdisciplinary cooperation between clinicians, cell biologists, and materials research scientists. Many medical specialties apply tissue engineering techniques for the development of artificial replacement tissue. Stages of development extend from basic research and preclinical studies to clinical application. Despite numerous established tissue replacement methods in otorhinolaryngology, head and neck surgery, tissue engineering techniques opens up new ways for cell and tissue repair in this medical field. Autologous cartilage still remains the gold standard in plastic reconstructive surgery of the nose and external ear. The limited amount of patient cartilage obtainable for reconstructive head and neck surgery have rendered cartilage one of the most important targets for tissue engineering in head and neck surgery. Although successful in vitro generation of bioartificial cartilage is possible today, these transplants are affected by resorption after implantation into the patient. Replacement of bone in the facial or cranial region may be necessary after tumor resections, traumas, inflammations or in cases of malformations. Tissue engineering of bone could combine the advantages of autologous bone grafts with a minimal requirement for second interventions. Three different approaches are currently available for treating bone defects with the aid of tissue engineering: (1) matrix-based therapy, (2) factor-based therapy, and (3) cell-based therapy. All three treatment strategies can be used either alone or in combination for reconstruction or regeneration of bone. The use of respiratory epithelium generated in vitro is mainly indicated in reconstructive surgery of the trachea and larynx. Bioartificial respiratory epithelium could be used for functionalizing tracheal prostheses as well as direct epithelial coverage for scar prophylaxis after laser surgery of shorter stenoses. Before clinical application animal experiments have to prove feasability and safety of the different experimental protocols. All diseases accompanied by permanently reduced salivation are possible treatment targets for tissue engineering. Radiogenic xerostomia after radiotherapy of malignant head and neck tumors is of particular importance here due to the high number of affected patients. The number of new diseases is estimated to be over 500,000 cases worldwide. Causal treatment options for radiation-induced salivary gland damage are not yet available; thus, various study groups are currently investigating whether cell therapy concepts can be developed with tissue engineering methods. Tissue engineering opens up new ways to generate vital and functional transplants. Various basic problems have still to be solved before clinically applying in vitro fabricated tissue. Only a fraction of all somatic organ-specific cell types can be grown in sufficient amounts in vitro. The inadequate in vitro oxygen and nutrition supply is another limiting factor for the fabrication of complex tissues or organ systems. Tissue survival is doubtful after implantation, if its supply is not ensured by a capillary network.

A comparative study of primary and immortalized cell adhesion characteristics to modified polymer surfaces: toward the goal of effective re-epithelialization

Facilitating tissue regeneration or replacement requires development of synthetic surfaces that promote cell adhesion, migration, and proliferation. Two successful approaches have been to incorporate minimal cell adhesion recognition sequences at the biomaterial surface and to integrate the entire adhesion molecule into a compatible synthetic matrix. While adhesion assays using immortalized cell lines are important in evaluating synthetic materials, cell type and source play a significant role in the ability of such models to mimic real tissues. Models that utilize multiple cell types or primary cells are more representative of native tissues than models that use single cell types or primary cells. In this study we investigated primary respiratory epithelial cell (REC) adhesion to modified fluoropolymers incorporating simple functional groups and minimal peptide recognition sequences, and we evaluated the potential of hybrid biopolymer materials to support adhesion and proliferation. X-ray photoelectron spectroscopy (XPS) was used to verify substrate surface composition. Significant differences were found in the adhesion characteristics of primary REC and in the A549 lung carcinoma cell line. Model systems composed of multiple cell types and/or primary cells necessarily represent increased levels of complexity for an investigation of cellular responses to synthetic surfaces. When evaluating biomaterials, adhesion studies using immortalized cell lines cannot necessarily be extrapolated to normal cell behavior.

Biocompatible hydrogel supports the growth of respiratory epithelial cells: possibilities in tracheal tissue engineering

Extensive tracheal defect reconstruction is a major challenge in plastic and reconstructive surgery. The lack of an epithelial lining on the luminal surfaces of tracheal prostheses is among the major causes of their failure. Chitosan-gelatin hydrogels were synthesized for the development of biocompatible, growth-supportive substrata for respiratory epithelial cells. We employed J774 macrophages to test the immunocompatibility of this gel. The hydrogel did not exert a cytotoxic effect on macrophages, as confirmed by tetrazolium reduction and neutral red uptake assay. Flow cytometric analysis of macrophages cultured on the hydrogel showed a comparable expression of activation markers CD11b/CD18, CD45, and CD14 to the control. Semiquantitative RT-PCR results showed an absence of upregulation of interleukin-6 (IL-6) and TNF-alpha in these macrophages with respect to the controls. Primary human respiratory epithelial cells cultured on the hydrogel showed proper attachment, normal morphology, and growth. A small proportion of cells on the hydrogel showed synchronously beating cilia. RT-PCR analysis showed that cells on the hydrogel expressed mucins 2 and 5 and cytokeratin 13, which are markers for secretory goblet and squamous cells, respectively. All these results demonstrate that the hydrogel supports the growth of a mixed population of differentiated epithelial cells. This hydrogel is suitable as a culture substratum for respiratory epithelial cells and could be used as a potential candidate for coating tracheal prostheses.

Prefabrication of a free flap for tracheal reconstruction: an experimental study. Preliminary report

The development of a prefabricated free flap that could have potential for tracheal reconstruction has been investigated in the goat model. Through a staged procedure, a composite cutaneous-chondromucosal premolded, prevascularized flap was obtained by prefabrication techniques. The procedure comprised three surgical stages. In the first stage, on day 0, the cartilaginous frame-work was constructed, along with the vascular pedicle (implantation of an arteriovenous fistula as a vascular carrier). In the second stage, on day 50, the inner surface of the neotrachea was lined with nasal mucosa. In the third stage, on day 60, the flap was elevated and free transferred to reconstruct a 15-cm circumferential defect in the cervical trachea. Ten animals were operated on, and the results were one infection, three early deaths, one free-flap failure with early tracheal stenosis, and five long-term survivors without significant stenosis. The structure of the neotracheal flap closely resembled that of the native trachea: internal respiratory epithelial lining, cartilage rings, and fibrovascular tissue. Fiberoptic bronchoscopy was done to all the animals at 10 and 60 days, revealing no significant stenosis in the long-term survivors.

Mucociliary clearance following segmental tracheal reversal

Ciliated tracheal epithelium is arranged in a polarized pattern oriented according to the inferior-superior axis of the trachea and is responsible for the transport of mucus toward the larynx. In this study, ciliary beat orientation and the influence of external factors on mucociliary clearance direction were studied in rabbit inverted cervical tracheas. The animals displayed normal respiration postoperatively. After 16 weeks, airway clearance was studied by observation of the movement of silicone particles placed in the inverted segment and in normal parts of the ciliated epithelium. Cilia exhibited unidirectional and coordinated movement within inverted tracheal segments. As shown by the direction of effective flow produced by beating cilia and by scanning electron microscopy, the cilia in the inverted segment beat in the opposite direction from the cilia in the remainder of the trachea. This study demonstrated that ciliary orientation is irreversibly determined, but the reversal of ciliary beating within the cervical trachea had no adverse effects on the survival of the animals.

Experimental in vitro and in vivo studies of epithelium formation on biomaterials seeded with isolated respiratory cells

Extensive tracheal defects after intensive care medicine, trauma, or large resections in tumor surgery remain a major challenge in plastic and reconstructive surgery. Defects that cannot be satisfactorily treated by complicated and costly reconstructive techniques reveal a need for an alloplastic tracheal replacement. Recent experimental and clinical studies in the development of alloplastic tracheal prostheses proved that the lack of an epithelial lining on the luminal surfaces and inadequate biophysical properties and shapes of the prostheses were the main causes for failure of these prostheses. In this study a cell-seeding technique has been used. Adhesion, spreading, and differentiation of seeded mucosa cells on biomaterials in vitro were observed by scanning electron microscopy (SEM). Chemical properties and surface structure of the material influenced the differentiation process. Epithelium formation of incorporated tracheal prostheses was tested in animal experiments. Isolated respiratory cells were seeded into implanted tubular prostheses of porous polyurethane or expanded polytetrafluorethylene. Light microscopy and SEM showed the tendency of epithelium formation on the surface of the lumen. Vigorous cell layers, predominantly as multiple cell layers of squamous epithelium, were observed. Ciliated or mucus cells were not detected. It can be stated that the epithelium formation on incorporated porous implants is possible. Further studies of the stability and the differentiation process of the epithelium on such implants is needed before an introduction of tracheal replacements into the clinical practice can be considered.