The tracheoesophageal region has many tissues with simple structures, such as cartilage, mucosa, nerves, and muscles. For regeneration of these tissues, new technologies are expected to be developed based on the three elements of organ regeneration. Basic research in the tracheoesophageal region varies from in vitro to in vivo. Studies of new scaffolding materials and cell transplantation of induced pluripotent stem cells, mesenchymal stem cells, fibroblasts, adipose tissue-derived stem cells, etc. have been carried out ; however, few have reached clinical application. The cervical trachea and cricoid cartilage have been partially regenerated by in situ tissue regeneration-inducing artificial trachea. Vocal fold scar was treated with basic fibroblast growth factor and hepatocyte growth factor. Physician-led clinical trials are being conducted on these medical technologies, and their practical application is awaited in the near future. A nerve induction tube is available on the market and is used for recurrent laryngeal nerve reconstruction. In addition, an oral mucosal epithelial cell sheet has been clinically applied to prevent postoperative stenosis of the esophagus, and clinical trials are currently underway. Through translational research that links the results of basic research to clinical application, it is expected that regenerative medicine in the tracheoesophageal region will be put into practical use as a safer and more effective treatment.

The reconstruction of a large tracheal defect requires invasive procedures followed by difficult post-operative management. As such, establishment of a safe and easy procedure for tracheal reconstruction has long been a challenge for head and neck surgeons and thoracic surgeons. With recent developments in regenerative and biomedical engineering, various therapeutic strategies have been tried to reconstruct/regenerate the trachea. Moreover, procedures such as implantation of decellularized allograft and tracheal transplantation have actually been applied in clinical settings. In addition, an in situ tissue regeneration-inducing artificial trachea made of polypropylene and collagen and implant-type tissue-engineered cartilage made of auricular chondrocytes and a polylactic acid scaffold have been developed in Japan, and their safety and efficacy have been examined by clinical trials. Although no procedure for tracheal reconstruction has yet been established, the recent remarkable advances in biomedical engineering raise expectations of early establishment of tracheal regenerative medicine.

The larynx is a complex organ consisting of cartilage, muscle, and vocal fold mucosa which has multi-functions including respiration, voice, and swallowing. Severe damage to the larynx deteriorates the quality of life, and requires a treatment strategy of tissue engineering and regenerative medicine. Basic and clinical researches on regeneration of the larynx have rapidly emerged since the beginning of the 21st century. Regeneration of the vocal fold for cases with vocal fold scarring and atrophy has received widespread attention. Basic fibroblast growth factor (bFGF) and hepatocyte growth factor (HGF) have proven to have regenerative effects on the vocal fold by simulating production and deposition of hyaluronic acid (HA) in the lamina propria of the vocal fold, in mean time by modulating collagen turnover. Mesenchymal stem cells (MSC) derived from bone marrow or fat tissue have also shown great potential in regeneration of the vocal fold. Early-phase clinical trials using bFGF, HGF, and MSCs have been completed showing promising effects on vocal fold regeneration with no severe adverse events. Basic FGF has also proven to have regenerative effects on the degenerative vocalis muscle in paralyzed vocal folds. Large defects of the larynx, including cartilage through the mucosa in cases of hemilaryngectomy, were treated with implant of bio-scaffolds in animal studies, which revealed simultaneous regeneration of the complex tissues. Decellularization technology has enabled creation of a decellularized whole larynx which has no immune systems. Such bio-scaffold may be used for transplantation after total laryngectomy with no concern of immune rejection. More basic and clinical researches may achieve clinical translation for several severe diseases of the larynx.

The basic research on laryngeal function recovery after recurrent laryngeal nerve injury was reviewed. Problems after recurrent nerve injury include (1) loss of motoneurons in the nucleus ambiguus, (2) degeneration and poor regeneration of neural fibers, (3) degeneration of the motor neural endplate, (4) atrophy of laryngeal muscle, and (5) inappropriate or misdirected reinnervation. For (1)-(4), novel therapies including gene therapies have been reported to be of value. However, even when morphological regeneration is achieved, the recovery of laryngeal motor function is difficult due to misdirection. We investigated (1) the usefulness of a neural regeneration tube (polyglycolic acid [PGA] tube filled with collagen fibers : PGA-C tube) as a scaffold, and (2) a novel therapy for misdirection by inhibiting regeneration of sensory and autonomic nerve fibers. In (1), we confirmed that the PGA-C tube has a drug sustained-release property and that it could be used not only as a scaffold for nerve regeneration but also as a drug delivery system. In (2), a relatively high recovery rate of vocal fold movement and motor nerve regeneration were observed. These effects derived from inhibiting regeneration between the motor nerve fibers and sensory/autonomic nerve fibers. Enhancement of glottic adduction by inhibition of neural regeneration to the abductor muscle, in combination with (1) and (2), might have potential as a therapeutic strategy for laryngeal function recovery.

The esophageal mucosal epithelium is composed of stratified squamous epithelial cells and regenerates from progenitor cells present on the basal layer of the esophagus. In endoscopic submucosal dissection (ESD), which is an endoscopic treatment for early-stage esophageal cancer, these progenitor cells are removed with the specimen. As a result, at the site resected by extensive ESD, inflammation and stricture occur frequently during regeneration. By applying cell sheet engineering, we have developed a treatment to regenerate the esophageal mucosal epithelium by endoscopically transplanting cell sheets, which are prepared from oral mucosal epithelial cells instead of the esophageal mucosa, to the ulcer surface. Cell sheet engineering utilities a “temperature-responsive culture dish” to harvest and transfer cell sheets. The processes can be performed easily by just reducing the temperature of the dish. Cell sheets are more advantageous for transplanting squamous cell stem cells and progenitor cells compared to injection of cell suspensions. The efficacy and safety have already been confirmed by conducting a first-in-human (FIH) test in which a cell sheet is endoscopically transplanted. This technology allows not only squamous cell transplantation but also transplantation of various types of cells from inside the gastrointestinal tract. Future development of this technology is promising.