Histology of the Vocal Folds
Encyclopedia
Histology
is the study of the minute structure, composition, and function of tissues. The histology of the vocal folds
is the reason for vocal fold vibration.
. The anterior glottis is the primary structure
of vocal fold vibration for phonation and the posterior glottis is the widest opening between the vocal folds for respiration. Thus, voice disorders often involve lesions of the anterior glottis. There are gradual changes in stiffness between the pliable vocal fold and hard, hyaline cartilage
of the arytenoid. The vocal processes of the arytenoid cartilages form a firm framework for the glottis but are made of elastic cartilage at the tip. Therefore, the vocal process of the arytenoid bends at the elastic cartilage portion during adduction and abduction of the vocal folds.
anteriorly by the macula flava and anterior commissure
tendon, or Broyle's ligament. Posteriorly, this vibratory portion is connected to the vocal process of the arytenoid cartilage by the posterior macula flava. The macula flava in newborn vocal folds is important for the growth and development of the vocal ligament and layered structure of the vocal folds. In the adult, the macula flavae are probably required for metabolism
of the extracellular matrices of the vocal fold mucosa, replacing damaged fibers in order to maintain the integrity and elasticity of the vocal fold tissues. Age-related changes in the macula flava influence the fibrous components of the vocal folds and are partially responsible for the differences in the acoustics of the adult and aged voice.
The cover is composed of the epithelium (mucosa), basal lamina (or basement membrane
zone), and the superficial layer of the lamina propria
.
The transition is composed of the intermediate and deep layers of the lamina propria.
The body is composed of the thyroarytenoid muscle
.
This layered structure of tissues is very important for vibration of the true vocal folds.
. This epithelium is five to twenty-five cells thick with the most superficial layer consisting of one to three cells that are lost to abrasion of the vocal folds during the closed phase of vibration. The posterior glottis is covered with pseudostratified ciliated epithelium. On the surfaces of the epithelial cells
are microridges and microvilli. Lubrication of the vocal folds through adequate hydration is essential for normal phonation to avoid excessive abrasion, and the microridges and microvilli help to spread and retain a mucous coat on the epithelium. Surgery of the vocal folds can disturb this layer with scar tissue
, which can result in the inability of the epithelium to retain an adequate mucous coat, which will in turn impact lubrication of the vocal folds. The epithelium has been described as a thin shell, the purpose of which is to maintain the shape of the vocal fold.
, it is a potential space. If there really is a space, there is a problem. The superficial layer of the lamina propria is a structure that vibrates a great deal during phonation, and the viscoelasticity needed to support this vibratory function depends mostly on extracellular matrices. The primary extracellular matrices of the vocal fold cover are reticular, collagenous and elastic fibers
, as well as glycoprotein and glycosaminoglycan. These fibers serve as scaffolds for structural maintenance, providing tensile strength
and resilience so that the vocal folds may vibrate freely but still retain their shape.
(F°) during phonation.
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.
. In females during puberty, the vocal muscle thickens slightly, but remains very supple and narrow. The squamous mucosa also differentiates into three distinct layers (the lamina propria) on the free edge of the vocal folds. The sub- and supraglottic glandular mucosa becomes hormone-dependent to estrogens and progesterone. For women, 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 hormone
s also affect dynamic function of the vocal folds (Hashimoto’s Thyroiditis affects the fluid balance in 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. In women, androgens are secreted principally by the adrenal cortex
and the ovaries and can have irreversible masculinizing effects if present in high enough concentration. In men, they are essential to male sexuality
. In muscles, they cause a hypertrophy of striated muscles with a reduction in the fat cells in skeletal muscle
s, 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 man voice, and the change is irreversible. 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.
) resulting in a decrease in the ability of the lamina propria to expand caused by cross-branching of the elastin fibers. Among other things, this leads to the mature voice being better suited to the rigors of opera.
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...
is the study of the minute structure, composition, and function of tissues. The histology of the vocal folds
Vocal folds
The vocal folds, also known commonly as vocal cords, are composed of twin infoldings of mucous membrane stretched horizontally across the larynx...
is the reason for vocal fold vibration.
Histoanatomy of the Glottis
The glottis is defined as the true vocal folds and the space between them. It is composed of an intermembranous portion or anterior glottis, and an intercartilaginous portion or posterior glottis. The border between the anterior and posterior glottises is defined by an imaginary line drawn across the vocal fold at the tip of the vocal process of the arytenoid cartilageArytenoid cartilage
The arytenoid cartilages are a pair of small three-sided pyramids which form part of the larynx, to which the vocal folds are attached...
. The anterior glottis is the primary structure
Primary structure
The primary structure of peptides and proteins refers to the linear sequence of its amino acid structural units. The term "primary structure" was first coined by Linderstrøm-Lang in 1951...
of vocal fold vibration for phonation and the posterior glottis is the widest opening between the vocal folds for respiration. Thus, voice disorders often involve lesions of the anterior glottis. There are gradual changes in stiffness between the pliable vocal fold and hard, hyaline cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
of the arytenoid. The vocal processes of the arytenoid cartilages form a firm framework for the glottis but are made of elastic cartilage at the tip. Therefore, the vocal process of the arytenoid bends at the elastic cartilage portion during adduction and abduction of the vocal folds.
Attachments of the Vocal Fold
The vibratory portion of the vocal fold in the anterior glottis is connected to the thyroid cartilageThyroid cartilage
The thyroid cartilage is the largest of the nine cartilages that make up the laryngeal skeleton, the cartilage structure in and around the trachea that contains the larynx....
anteriorly by the macula flava and anterior commissure
Anterior commissure
The anterior commissure is a bundle of nerve fibers , connecting the two cerebral hemispheres across the midline, and placed in front of the columns of the fornix...
tendon, or Broyle's ligament. Posteriorly, this vibratory portion is connected to the vocal process of the arytenoid cartilage by the posterior macula flava. The macula flava in newborn vocal folds is important for the growth and development of the vocal ligament and layered structure of the vocal folds. In the adult, the macula flavae are probably required for metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
of the extracellular matrices of the vocal fold mucosa, replacing damaged fibers in order to maintain the integrity and elasticity of the vocal fold tissues. Age-related changes in the macula flava influence the fibrous components of the vocal folds and are partially responsible for the differences in the acoustics of the adult and aged voice.
Layered Structure of the Adult Vocal Fold
The histological structure of the vocal fold can be separated into 5 or 6 tissues, depending on the source, which can then be grouped into three sections as the cover, the transition, and the body.The cover is composed of the epithelium (mucosa), basal lamina (or basement membrane
Basal lamina
The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....
zone), and the superficial layer of the lamina propria
Lamina propria
The lamina propria is a constituent of the moist linings known as mucous membranes or mucosa, which line various tubes in the body ....
.
The transition is composed of the intermediate and deep layers of the lamina propria.
The body is composed of the thyroarytenoid muscle
Thyroarytenoid muscle
The thyroarytenoid is a broad, thin muscle that lies parallel with and lateral to the vocal fold and that supports the wall of the ventricle and its appendix...
.
This layered structure of tissues is very important for vibration of the true vocal folds.
Epithelium
The free edge of the vibratory portion of the vocal fold, the anterior glottis, is covered with stratified squamous epitheliumSquamous epithelium
In anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
. This epithelium is five to twenty-five cells thick with the most superficial layer consisting of one to three cells that are lost to abrasion of the vocal folds during the closed phase of vibration. The posterior glottis is covered with pseudostratified ciliated epithelium. On the surfaces of the epithelial cells
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
are microridges and microvilli. Lubrication of the vocal folds through adequate hydration is essential for normal phonation to avoid excessive abrasion, and the microridges and microvilli help to spread and retain a mucous coat on the epithelium. Surgery of the vocal folds can disturb this layer with scar tissue
Scar
Scars are areas of fibrous tissue that replace normal skin after injury. A scar results from the biological process of wound repair in the skin and other tissues of the body. Thus, scarring is a natural part of the healing process. With the exception of very minor lesions, every wound results in...
, which can result in the inability of the epithelium to retain an adequate mucous coat, which will in turn impact lubrication of the vocal folds. The epithelium has been described as a thin shell, the purpose of which is to maintain the shape of the vocal fold.
Basal Lamina or Basement Membrane Zone (BMZ)
This is transitional tissue composed of two zones, the lamina lucida and lamina densa. The lamina lucida appears as a low density clear zone medial to the epithelial basal cells. The lamina densa has a greater density of filaments and is adjacent to the lamina propria. The basal lamina or BMZ mainly provides physical support to the epithelim through anchoring fibers and is essential for repair of the epithelium.Superficial Layer of the Lamina Propria
This layer consists of loose fibrous components and extracellular matrices that can be compared to soft gelatin. This layer is also known as Reinke’s space but it is not a space at all. Like the pleural cavityPleural cavity
In human anatomy, the pleural cavity is the potential space between the two pleura of the lungs. The pleura is a serous membrane which folds back onto itself to form a two-layered, membrane structure. The thin space between the two pleural layers is known as the pleural cavity; it normally...
, it is a potential space. If there really is a space, there is a problem. The superficial layer of the lamina propria is a structure that vibrates a great deal during phonation, and the viscoelasticity needed to support this vibratory function depends mostly on extracellular matrices. The primary extracellular matrices of the vocal fold cover are reticular, collagenous and elastic fibers
Elastic fibers
Elastic fibres are bundles of proteins found in extracellular matrix of connective tissue and produced by fibroblasts and smooth muscle cells in arteries. These fibers can stretch up to 1.5 times their length, and snap back to their original length when relaxed...
, as well as glycoprotein and glycosaminoglycan. These fibers serve as scaffolds for structural maintenance, providing tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
and resilience so that the vocal folds may vibrate freely but still retain their shape.
Intermediate and Deep Layers of the Lamina Propria
The intermediate layer of the lamina propria is primarily made up of elastic fibers while the deep layer of the lamina propria is primarily made up of collagenous fibers. These fibers run roughly parallel to the vocal fold edge and these two layers of the lamina propria comprise the vocal ligament. The transition layer is primarily structural, giving the vocal fold support as well as providing adhesion between the mucosa, or cover, and the body, the thyroarytenoid muscle.The Thyroarytenoid Muscle
This muscle is variously described as being divided into the thyroarytenoid and vocalis muscles or the thyrovocalis and the thyromuscularis, depending on the source.Vocal Fold Lesions
The majority of vocal fold lesions primarily arise in the cover of the folds. Since the basal lamina secures the epithelium to the superficial layer of the lamina propria with anchoring fibers, this is a common site for injury. If a person has a phonotrauma or habitual vocal hyperfunction, also known as pressed phonation, the proteins in the basal lamina can shear, causing vocal fold injury, usually seen as nodules or polyps, which increase the mass and thickness of the cover. The squamous cell epithelium of the anterior glottis are also a frequent site of layrngeal cancer caused by smoking.Reinke’s Edema
A voice pathology called Reinke’s edema, swelling due to abnormal accumulation of fluid, occurs in the superficial lamina propria or Reinke’s space. This causes the vocal fold mucosa to appear floppy with excessive movement of the cover that has been described as looking like a loose sock. The greater mass of the vocal folds due to increased fluid lowers the fundamental frequencyFundamental frequency
The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the lowest frequency of a periodic waveform. In terms of a superposition of sinusoids The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the...
(F°) during phonation.
Histological Changes From Birth to Old Age
The histologic structure of the vocal fold differs from the pediatric to the adult and old-age populations.Pediatrics
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
Puberty usually lasts from 2–5 years, and typically occurs between the ages of 12 to 17. During puberty, voice change is controlled by sex hormonesSex steroid
Sex steroids, also known as gonadal steroids, are steroid hormones that interact with vertebrate androgen or estrogen receptors. Their effects are mediated by slow genomic mechanisms through nuclear receptors as well as by fast nongenomic mechanisms through membrane-associated receptors and...
. In females during puberty, the vocal muscle thickens slightly, but remains very supple and narrow. The squamous mucosa also differentiates into three distinct layers (the lamina propria) on the free edge of the vocal folds. The sub- and supraglottic glandular mucosa becomes hormone-dependent to estrogens and progesterone. For women, 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 hormone
Thyroid hormone
The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...
s also affect dynamic function of the vocal folds (Hashimoto’s Thyroiditis affects the fluid balance in 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
Extracellular fluid
Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...
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. In women, androgens are secreted principally by the adrenal cortex
Adrenal cortex
Situated along the perimeter of the adrenal gland, the adrenal cortex mediates the stress response through the production of mineralocorticoids and glucocorticoids, including aldosterone and cortisol respectively. It is also a secondary site of androgen synthesis.-Layers:Notably, the reticularis in...
and the ovaries and can have irreversible masculinizing effects if present in high enough concentration. In men, they are essential to male sexuality
Human male sexuality
Human male sexuality covers physiological, psychological, social, cultural, and political aspects of the human male sexual response and related phenomenon...
. In muscles, they cause a hypertrophy of striated muscles with a reduction in the fat cells in skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
s, 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 man voice, and the change is irreversible. 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.
Adulthood
There is a steady increase in the elastin content of the lamina propria as we age (elastin is a yellow scleroprotein, the essential constituent of the elastic connective tissueConnective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
) resulting in a decrease in the ability of the lamina propria to expand caused by cross-branching of the elastin fibers. Among other things, this leads to the mature voice being better suited to the rigors of opera.