The vocal folds, also known commonly as vocal cords or voice reeds, are composed of twin infoldings of mucous membrane stretched horizontally, from back to front, across the larynx. They vibrate, modulating the flow of air being expelled from the lungs during phonation. Open when breathing and images hyaluronic acid essence for speech or singing, the folds are controlled via the vagus nerve.
The vocal folds are located within the larynx at the top of the trachea. They are attached posteriorly to the arytenoid cartilages, and anteriorly to the thyroid cartilage. Situated above the larynx, the epiglottis acts as a flap which closes off the trachea during the act of swallowing to direct food into the esophagus. Males and females have different vocal fold sizes. Adult male voices are usually lower pitched due to longer and thicker folds. The male vocal folds are between 1.
The vocal folds are sometimes called ‘true vocal folds’ to distinguish them from the ‘false vocal folds’ known as vestibular folds or ventricular folds. These are a pair of thick folds of mucous membrane that protect and sit slightly superior to the more delicate true folds. Mature human vocal folds are composed of layered structures which are quite different at the histological level. The topmost layer comprises stratified squamous epithelium which is bordered by ciliated pseudostratified epithelium. Newborns have a uniform monolayered lamina propria, which appears loose with no vocal ligament. HA is a bulky, negatively charged glycosaminoglycan, whose strong affinity with water procures HA its viscoelastic and shock absorbing properties essential to vocal biomechanics.
Viscosity and elasticity are critical to voice production. The fibroblasts in the newborn Reinke’s space are immature, showing an oval shape, and a large nucleus-cytoplasm ratio. The rough endoplasmic reticulum and Golgi apparatus, as shown by electron micrographs, are not well developed, indicating that the cells are in a resting phase. In the infant, many fibrous components were seen to extend from the macula flava towards the Reinke’s space.
Fibronectin is very abundant in the Reinke’s space of newborn and infant. Fibronectin is a glycoprotein that is believed to act as a template for the oriented deposition of the collagen fibers, stabilizing the collagen fibrils. Fibronectin also acts as a skeleton for the elastic tissue formation. Human VF are paired structures located in the larynx, just above the trachea, which vibrate and are brought in contact during phonation. Histologically, the human VF are a laminated structure composed of five different layers.
The vocalis muscle, main body of the VF, is covered by the mucosa, which consists of the epithelium and the lamina propria. The extracellular matrix of the VF LP is composed of fibrous proteins such as collagen and elastin, and interstitial molecules such as HA, a non-sulfated glycosaminoglycan. Vocal fold structure in adults is quite different from that in newborns. Exactly how the VF mature from an immature monolayer in newborns to a mature three layer tissue in adults is still unknown, however a few studies have investigated the subjects and brought some answers. Maculae flavae are located at the anterior and posterior ends of the membranous parts of the VF. The histological structure of the macula flava is unique, and Sato and Hirano speculated that it could play an important role in growth, development and aging of VF. Adult VF have a layered structure which is based on the layers differential in ECM distribution.
Newborns on the other hand, do not have this layered structure. Interestingly, the results show that the vocal fold mucosae were hypoplastic, and rudimentary, and like newborns, did not have any vocal ligament, Reinke’s space, or layered structure. Furthermore, using a specially designed bioreactor, Titze et al. ECM production from fibroblasts that are not exposed to mechanical stimulation. The gene expression levels of ECM constituents such as fibronectin, MMP1, decorin, fibromodulin, HA synthase 2, and CD44 were altered.
Other studies suggest that hormones play also an important role in vocal fold maturation. Hormones are molecules secreted into the blood stream to be delivered at different targeted sites. They usually promote growth, differentiation and functionality in different organs or tissues. Vocal fold phonatory functions are known to change from birth to old age. The most significant changes occur in development between birth and puberty, and in old age. To oscillate, the vocal folds are brought near enough together such that air pressure builds up beneath the larynx.
The folds are pushed apart by this increased subglottal pressure, with the inferior part of each fold leading the superior part. The perceived pitch of a person’s voice is determined by a number of different factors, most importantly the fundamental frequency of the sound generated by the larynx. The fundamental frequency is influenced by the length, size, and tension of the vocal folds. This frequency averages about 125 Hz in an adult male, 210 Hz in adult females, and over 300 Hz in children. The vocal folds generate a sound rich in harmonics.