Human Vocal Fold Anatomy and Its Biomechanical Consideration
Mature human VFs are composed of layered structures which are quite different at the histological level. The top most layer comprises stratified squamous epithelium which is bordered by pseudostratified ciliated epithelium. The luminal surface of this squamous epithelium is covered by a layer of mucus (mucociliary blanket) which is composed of two layers: a mucinous layer and serous layer. Both mucus layers provide viscous and watery environment for cilia beating posteriorally and superiorly. The mucociliary blanket which keeps the vocal fold moist and lubricated is indispensable in vocal health. The epidermis layer is secured to the deeper connective tissue by basement membrane. Due to the primarily amorphous fibrous and nonfibrous proteins in lamina propria (LPs), the basement membrane applies strong anchoring filaments like collagenⅣ and Ⅶ to secure the hemidesmosome of basal cell to LPs. These attachments are strong enough to sustain beating and stretch to which VFs are subjected. The population density of some of the anchoring fibers in basal membrane, collagen Ⅶ, for instance, is genetically determined, which points out that genetics may influence vocal fold health and pathogenesis.
The next three layers comprise lamina LPs, which is stratified by their histological composition of elastin and collagen fibers, with fibroblast, myofibroblast and macrophages interspersed sparsely. The superficial layer LPs (SLLPs), also known as Reinke’s space, is composed of amorphous substance and microfibrils which allows this cover layer to “slide” over the deep layer easily. The vibratory and viscoelastic characteristics of human VFs are mainly attributed to the molecular composition of SLLPs. In normal vocal fold, the jelly-like “Reinke’s space” is very loose and abundant with interstitial proteins such as hyaluronic acid, fibronectin, proteoglycan like fibromodulin, decorin and versican. All these ECM components together regulate the water content of vocal fold and render the viscous shear property for it. The squamous epithelium and superficial lamina propria form the vocal mucosa which serves as vibratory component in phonation. The mucosa layer vibrates at a frequency range of 100–1000 Hz and displacement at 1mm approximately. The intermediate layer of LPs consists primarily of elastic fiber while the deep layer LP consists of fewer elastin and more collagen fibers. These two layers have poor differentiated boundary but are increasingly stiffer than SLLPs. The intermediate and deep layers of LPs compose the vocal ligament which is responsible for strain in phonation. Within the ECM community of vocal ligament, fibrous proteins such as elastin and collagen are pivotal in maintaining the proper elastic biomechanical property of vocal fold. Elastin fibers impart the flexibility and elasticity of VFs and, collagen is responsible for the resistance and resiliece to tensile strength. The normal strain level of vocal ligament ranges from 0-15% during phonation These fibrous proteins exhibit distribution variations spatially and temporally due to fibroblast turnover during tissue maturation and aging.
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