Purpose Vocal fold epithelium is composed of layers of individual epithelial cells joined by junctional complexes constituting a unique interface with the external environment. This barrier provides structural stability to the vocal folds and protects underlying connective tissue from injury while being nearly continuously exposed to potentially hazardous insults including environmental or systemic-based irritants such as pollutants and reflux, surgical procedures, and vibratory trauma. Small disruptions in the epithelial barrier may have a large impact on susceptibility to injury and overall vocal health. The purpose of this article is to provide a broad-based review of our current knowledge of the vocal fold epithelial barrier. Methods A comprehensive review of the literature was conducted. Details of the structure of the vocal fold epithelial barrier are presented and evaluated in the context of function in injury and pathology. The importance of the epithelial-associated vocal fold mucus barrier is also introduced. Results/Conclusions Information presented in this review is valuable for clinicians and researchers as it highlights the importance of this understudied portion of the vocal folds to overall vocal health and disease. Prevention and treatment of injury to the epithelial barrier is a significant area awaiting further investigation.

Vocal fold surface liquid homeostasis contributes to optimal vocal physiology. In this paper we review emerging evidence that vocal fold surface liquid is maintained in part by salt and water fluxes across the epithelium. Based on recent immunolocalization and electrophysiological findings, we describe a transcellular pathway as one mechanism for regulating superficial vocal fold hydration. We propose that the pathway includes the sodium-potassium pump, sodium-potassium-chloride cotransporter, epithelial sodium channels, cystic fibrosis transmembrane regulator chloride channels, and aquaporin water channels. By integrating knowledge of the regulating mechanisms underlying ion and fluid transport with observations from hydration challenges and treatments using in vitro and in vivo studies, we provide a theoretical basis for understanding how environmental and behavioral challenges and clinical interventions may modify vocal fold surface liquid composition. We present converging evidence that clinical protocols directed at facilitating vocal fold epithelial ion and fluid transport may benefit healthy speakers, those with voice disorders, and those at risk for voice disorders.

Purpose of review-Increased vocal fold hydration is a popular target in the prevention and management of voice disorders. Current intervention strategies focus on enhancing both systemic (internal) and superficial (surface) hydration. This paper will review relevant bench and human subject research on the role of hydration in vocal fold physiology.Recent findings-Bench and human subject studies provide converging evidence that systemic and superficial dehydration are detrimental to vocal fold physiology. Dehydration challenges increase the viscous properties of excised vocal fold tissue. Systemic, superficial, and combined drying challenges increase aerodynamic and acoustic measures of voice production in speakers. Emerging theoretical and clinical evidence suggest that increasing both systemic and superficial hydration levels may benefit voice production, however, robust evidence for positive outcomes of hydration treatments is lacking.Summary-Increased systemic and superficial vocal fold hydration as a component of vocal hygiene may improve overall health and efficiency of the vocal apparatus. However, continued exploration of biological mechanisms regulating vocal fold hydration is needed to optimize clinical hydration interventions. Specifically, the development of hydration treatments that maximize positive phonatory outcomes will necessitate understanding of the signaling pathways linking systemic and superficial hydration.