Histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

is the study of the minute structure, composition, and function of tissues. Mature human

vocal cords In humans, vocal cords, also known as vocal folds or voice reeds, are folds of throat tissues that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for speech ...

are composed of layered structures which are quite different at the histological level.

Structure

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 cartilage The arytenoid cartilages () are a pair of small three-sided pyramids which form part of the larynx. They are the site of attachment of the vocal cords. Each is pyramidal or ladle-shaped and has three surfaces, a base, and an apex. The arytenoid ...

. The anterior glottis is the

primary structure Protein primary structure is the linear sequence of amino acids in a peptide or protein. By convention, the primary structure of a protein is reported starting from the amino-terminal (N) end to the carboxyl-terminal (C) end. Protein biosynthes ...

of vocal fold vibration for phonation and the posterior glottis is the widest opening between the vocal folds for respiration. Thus,

voice disorders Voice disordersTitze, I.R. (1994). Principles of Voice Production, Prentice Hall, . are medical conditions involving abnormal pitch, loudness or quality of the sound produced by the larynx and thereby affecting speech production. These include: * ...

often involve lesions of the anterior glottis. There are gradual changes in stiffness between the pliable vocal fold and hard,

hyaline cartilage Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has ...

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 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. It does not completely encircle the larynx (only the cricoid cartilage ...

anteriorly by the macula flava and anterior commissure tendon, or Broyles' 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 (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

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 5Hirano, M., & Bless, D.M. (1993). ''Videostroboscopic Examination of the Larynx''. San Diego CA: Singular Publishing. 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 The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between Epithelium, epithelial tissues including mesot ...

zone), and the superficial layer of the

lamina propria The lamina propria is a thin layer of connective tissue that forms part of the moist linings known as mucous membranes or mucosae, which line various tubes in the body, such as the respiratory tract, the gastrointestinal tract, and the urogenita ...

. The transition is composed of the intermediate and deep layers of the lamina propria. The body is composed of the

thyroarytenoid muscle The thyroarytenoid muscle is a broad, thin muscle that forms the body of the vocal fold and that supports the wall of the ventricle and its appendix. It functions to shorten the vocal folds. Structure It arises in front from the lower half of the ...

. This layered structure of tissues is very important for vibration of the true vocal folds.

The cover

Mature humans' 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 The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike project ...

pseudostratified epithelium A pseudostratified epithelium is a type of epithelium that, though comprising only a single layer of cells, has its cell nuclei positioned in a manner suggestive of stratified epithelia. As it rarely occurs as squamous or cuboidal epithelia ...

. The inner-lining surface of this squamous epithelium is covered by a layer of mucus (acting as a

mucociliary clearance Mucociliary clearance (MCC), mucociliary transport, or the mucociliary escalator, describes the self-clearing mechanism of the airways in the respiratory system. It is one of the two protective processes for the lungs in removing inhaled particl ...

), 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 clearance keeps the vocal folds essentially moist and lubricated. The epidermis layer is secured to the deeper connective tissue by basement membrane. Due to the primarily amorphous fibrous and nonfibrous proteins in the

, the

applies strong anchoring-filaments like collagen IV and VII to secure the

hemidesmosome Hemidesmosomes are very small stud-like structures found in keratinocytes of the epidermis of skin that attach to the extracellular matrix. They are similar in form to desmosomes when visualized by electron microscopy, however, desmosomes attach t ...

of basal cell to the lamina propria. These attachments are strong enough to sustain beating and stretch, to which the vocal cords are subjected. The population density of some of the anchoring fibers in the basement membrane, such as collagen VII, is genetically determined, and these genetics may influence the health and pathogenesis of the vocal folds. Illu_larynx

The next three layers comprise lamina of lipopolysaccharides (LPs), which are stratified by their histological composition of elastin and collagen fibers, with

fibroblast A fibroblast is a type of cell (biology), biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework (Stroma (tissue), stroma) for animal Tissue (biology), tissues, and plays a critical role in wound ...

, myofibroblast and macrophages interspersed sparsely. The superficial layer lipopolysaccharides (SLLPs), also known as

Reinke's space Reinke's space is a potential space between the vocal ligament and the overlying mucosa. It is not an empty space, but contains cells, special fibers and extracellular matrix. It plays an important role in the vibration of the vocal cords. Edema of ...

, is composed of amorphous substance and microfibrils which allows this cover layer to "slide" over the deep layer easily.Linda Rammage; M D Morrison; Hamish Nichol, Management of the voice and its disorders, published by:: Singular/Thomson Learning, San Diego, CA ,2001, 269–270. The vibratory and viscoelastic characteristics of the human vocal cords are mainly attributed to the molecular composition of SLLPs. In normal vocal folds, 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

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide stru ...

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 L.P.s consists primarily of elastic fibre, while the deep layer L.P. consists of fewer elastin and more collagen fibres. These two layers have poor differentiated boundaries but are increasingly stiffer than SLLPs. The intermediate and deep layers of lipopolysaccharides compose the vocal ligaments which are enclosed within the vocal folds and are responsible for strain in

phonation The term phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, ''phonation'' is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the defini ...

. Within the extracellular matrix 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 the vocal folds 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

turnover during

tissue maturation Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem ce ...

and aging. Each vocal ligament is a band of yellow elastic tissue attached in front to the angle of the

, and behind to the

vocal process In the human larynx, the vocal process is the anterior angle of the base of the arytenoid cartilage, as it projects horizontally forward and gives attachment to the vocal ligament. The arytenoids are paired cartilages with a medial and a lateral ...

of the

Epithelium

The free edge of the vibratory portion of the vocal fold, the anterior glottis, is covered with stratified

squamous epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

. 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 or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...

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 tissue may refer to: Medicine * Scar, an area of fibrous tissue that replaces normal skin after injury * Granulation tissue, a product of healing in major wounds Film and television * ''Scar Tissue'' (1975 film), or ''Wanted: Babysitter'' ...

, 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 epithelium 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 cavity The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication bet ...

, 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 (or yellow fibers) are an essential component of the extracellular matrix composed of bundles of proteins (elastin) which are produced by a number of different cell types including fibroblasts, endothelial, smooth muscle, and air ...

, as well as glycoprotein and glycosaminoglycan. These fibers serve as scaffolds for structural maintenance, providing

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

and resilience so that the vocal folds may vibrate freely but still retain their shape.

The transition

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 body

The thyroarytenoid muscle

This muscle is variously described as being divided into the thyroarytenoid and vocalis muscles or the thyrovocalis and the thyromuscularis,Sanders, I. (2003). The Microanatomy of the Vocal Fold Musculature. In Rubin, J.S., Sataloff, R.T., & Korovin, G.S. (Eds.), ''Diagnosis and Treatment of Voice Disorders'' (pp. 49-68). Clifton Park, NY: Delmar Learning. depending on the source.

References

{{DEFAULTSORT:Histology Of The Vocal Folds Human throat Human voice