Vocal cords
The vocal cords, also known as vocal folds, are folds of throat tissues that are key in creating sounds through vocalization. The length of the vocal cords affects the pitch of voice, similar to a violin string. Open when breathing and vibrating for speech or singing, the folds are controlled via the recurrent laryngeal branch of the vagus nerve. They 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.
The 'true vocal cords' are distinguished from the 'false vocal folds', known as vestibular folds or ventricular folds, which sit slightly superior to the more delicate true folds. These have a minimal role in normal phonation, but can produce deep sonorous tones, screams and growls.
The length of the vocal fold at birth is approximately six to eight millimeters and grows to its adult length of eight to sixteen millimeters by adolescence. DHT, an androgen metabolite of testosterone which is secreted by the gonads, causes changes in the cartilages and musculature of the larynx when present in high enough concentrations, such as during an adolescent boy's puberty: The thyroid prominence appears, the vocal folds lengthen and become rounded, and the epithelium thickens with the formation of three distinct layers in the lamina propria.. These changes are only partially reversible via reconstructive surgery such as chondrolaryngoplasty, feminization laryngoplasty, and laser tuning of the vocal cords.
Structure
Location
The vocal folds are located within the larynx at the top of the trachea. They are attached at the back to the arytenoid cartilages, and at the front to the thyroid cartilage via Broyles ligament. They are part of the glottis. Their outer edges are attached to muscle in the larynx while their inner edges form an opening called the rima glottidis. They are constructed from epithelium, but they have a few muscle-fibres in them, namely the vocalis muscle which tightens the front part of the ligament near to the thyroid cartilage. They are flat triangular bands and are pearly white in color. Above both sides of the glottis are the two vestibular folds or false vocal folds which have a small sac between them.False vocal folds
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 higher to the more delicate true folds. They have a minimal role in normal phonation, but are often used to produce deep sonorous tones in Tibetan chant and Tuvan throat singing, as well as in musical screaming and the death growl vocal style.Microanatomy
The vocal cords are composed of twin infoldings of 3 distinct tissues: an outer layer of flat cells that do not produce keratin. Below this is the superficial layer of the lamina propria, a gel-like layer, which allows the vocal fold to vibrate and produce sound. The vocalis and thyroarytenoid muscles make up the deepest portion. These vocal folds are covered with a mucous membrane and are stretched horizontally, from back to front, across the larynx.Variation
Males and females have different vocal fold sizes. Adult male voices are usually lower-pitched due to longer and thicker folds. The male's vocal folds are between 1.75 cm and 2.5 cm in length, while females' vocal folds are between 1.25 cm and 1.75 cm in length. The vocal folds of children are much shorter than those of adult males and females. The difference in vocal fold length and thickness between males and females causes a difference in vocal pitch. Additionally, genetic factors cause variations between members of the same sex, with males' and females' voices being categorized into voice types.Development
In newborns
Newborns have a uniform single layered lamina propria, which appears loose with no vocal ligament. The monolayered lamina propria is composed of ground substances such as hyaluronic acid and fibronectin, fibroblasts, elastic fibers, and collagenous fibers. While the fibrous components are sparse, making the lamina propria structure loose, the hyaluronic acid content is high.HA is a bulky, negatively charged glycosaminoglycan, whose strong affinity with water procures hyaluronic acid its viscoelastic and shock absorbing properties essential to vocal biomechanics. Viscosity and elasticity are critical to voice production. Chan, Gray and Titze, quantified the effect of hyaluronic acid on both the viscosity and the elasticity of vocal folds by comparing the properties of tissues with and without HA. The results showed that removal of hyaluronic acid decreased the stiffness of the vocal cords by an average of 35%, but increased their dynamic viscosity by an average of 70% at frequencies higher than 1 Hz. Newborns have been shown to cry an average of 6.7 hours per day during the first 3 months, with a sustained pitch of 400–600 Hz, and a mean duration per day of 2 hours. Similar treatment on adult vocal cords would quickly result in edema, and subsequently aphonia. Schweinfurth and al. presented the hypothesis that high hyaluronic acid content and distribution in newborn vocal cords is directly associated with newborn crying endurance. These differences in newborn vocal fold composition would also be responsible for newborns inability to articulate sounds, besides the fact that their lamina propria is a uniform structure with no vocal ligament. The layered structure necessary for phonation will start to develop during the infancy and until the adolescence.
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. The collagenous and reticular fibers in the newborn the vocal cords are fewer than in the adult one, adding to the immaturity of the vocal fold tissue.
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. Reticular and collagenous fibers were seen to run along the edges of the vocal cords throughout the entire lamina propria. Fibronectin in the Reinke's space appeared to guide those fibers and orient the fibril deposition. The elastic fibers remained sparse and immature during infancy, mostly made of microfibrils. The fibroblasts in the infant Reinke's space were still sparse but spindle-shaped. Their rough endoplasmic reticulum and Golgi apparatus were still not well developed, indicating that despite the change in shape, the fibroblasts still remained mostly in a resting phase. Few newly released materials were seen adjacent to the fibroblasts. The ground substance content in the infant Reinke's space seemed to decrease over time, as the fibrous component content increased, thus slowly changing the vocal fold structure.
Children
The infant lamina propria is composed of only one layer, as compared to three in the adult, and there is no vocal ligament. The vocal ligament begins to be present in children at about four years of age. Two layers appear in the lamina propria between the ages of six and twelve, and the mature lamina propria, with the superficial, intermediate and deep layers, is only present by the conclusion of adolescence. As vocal fold vibration is a foundation for vocal formants, this presence or absence of tissue layers influences a difference in the number of formants between the adult and pediatric populations. In females, the voice is three tones lower than the child's and has five to twelve formants, as opposed to the pediatric voice with three to six. The length of the vocal fold at birth is approximately six to eight millimeters and grows to its adult length of eight to sixteen millimeters by adolescence. The infant vocal fold is half membranous or anterior glottis, and half cartilaginous or posterior glottis. The adult fold is approximately three-fifths membranous and two-fifths cartilaginous.Puberty
usually lasts from 2 to 5 years, and typically occurs between the ages of 12 and 17. During puberty, voice change is controlled by sex hormones. In females during puberty, the vocal muscle thickens slightly, but remains very supple and narrow. The squamous mucosa also differentiates into three distinct layers on the free edge of the vocal folds. The sub- and supraglottic glandular mucosa becomes hormone-dependent to estrogens and progesterone. For females, the actions of estrogens and progesterone produce changes in the extravascular spaces by increasing capillary permeability which allows the passage of intracapillary fluids to the interstitial space as well as modification of glandular secretions. Estrogens have a hypertrophic and proliferative effect on mucosa by reducing the desquamating effect on the superficial layers. The thyroid hormones also affect dynamic function of the vocal folds;. Progesterone has an anti-proliferative effect on mucosa and accelerates desquamation. It causes a menstrual-like cycle in the vocal fold epithelium and a drying out of the mucosa with a reduction in secretions of the glandular epithelium. Progesterone has a diuretic effect and decreases capillary permeability, thus trapping the extracellular fluid out of the capillaries and causing tissue congestion.Testosterone, an androgen secreted by the testes, will cause changes in the cartilages and musculature of the larynx for males during puberty, and to a lesser extent to females assigned at birth and others such as intersex individuals as well as those who are androgen deficient if they are given masculinizing hormone therapy. In females, androgens are secreted principally by the adrenal cortex and the ovaries and can have irreversible masculinizing effects if present in high enough concentration. In males, they are essential to male sexuality. In muscles, they cause a hypertrophy of striated muscles with a reduction in the fat cells in skeletal muscles, and a reduction in the whole body fatty mass. Androgens are the most important hormones responsible for the passage of the boy-child voice to adult male voice, and the change is irreversible without reconstructive surgery such as feminization laryngoplasty. The thyroid prominence, which contains the vocal cords appears, the vocal folds lengthen and become rounded, and the epithelium thickens with the formation of three distinct layers in the lamina propria. These changes are also irreversible without surgery, albeit the thyroid/laryngeal prominence, also known as an Adam's apple can be potentially diminished via a tracheal shave or feminization laryngoplasty.