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The growth of human hair occurs everywhere on the body except for the soles of the feet, the inside of the mouth, the lips, the backs of the ears, the palms of the hands, some external
genital A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (such as a sp ...

genital
areas, the
navel The navel (clinically known as the umbilicus, commonly known as the belly button) is a protruding, flat, or hollowed area on the abdomen The abdomen (colloquially called the belly, tummy, midriff or stomach) is the part of the body between th ...

navel
,
scar A scar is an area of fibrosis, fibrous tissue that replaces normal skin after an injury. Scars result from the biological process of wound repair in the skin, as well as in other Organ (anatomy), organs, and biological tissue, tissues of the b ...
tissue, and, apart from
eyelash An eyelash (also called lash) (Latin: ''Cilia'') is one of the hairs that grows at the edge of the eyelids. It grows in one layer on the edge of the upper and lower eyelids. Eyelashes protect the eye from debris, dust and small particles and pe ...
es, the
eyelid An eyelid is a thin fold of skin that covers and protects an eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photorecepto ...

eyelid
s. Hair is a
stratified squamous
stratified squamous
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprot ...

keratin
ized
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
made of multi-layered flat cells whose rope-like
filament The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: In commerce * Fiber Fiber or fibre (from la, fibra, links=no) is a natural or #Man-made f ...

filament
s provide structure and strength to the
hair shaft Hair is a protein filament that grows from hair follicle, follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick ...
. The protein called
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprot ...

keratin
makes up hair and stimulates hair growth. Hair follows a specific growth cycle with three distinct and concurrent phases:
anagen The hair follicle is an Organ (anatomy), organ found in mammalian skin. It resides in the Dermis, dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a comple ...
, catagen, and telogen. Each phase has specific characteristics that determine the length of the hair. The body has different types of hair, including
vellus hair Vellus hair is short, thin, light-colored, and barely noticeable hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of g ...
and
androgenic hair Body hair, or androgenic hair, is the terminal hair In humans, terminal hair is thick and long, such as what grows on the scalp, as compared with vellus hair Vellus hair is short, thin, light-colored, and barely noticeable hair Hair is ...

androgenic hair
, each with its own type of cellular construction. This varied construction gives the hair unique characteristics, serving specific purposes, mainly warmth (redundant in modern humans) and physical protection. Most humans develop the longest thickest hair on their scalps and (mostly observed in males) faces. This hair will usually grow to several feet before terminating, but many humans develop much longer hair.


Growth cycle

The three stages of hair growth are the
anagen The hair follicle is an Organ (anatomy), organ found in mammalian skin. It resides in the Dermis, dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a comple ...
, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The growth rate of hair varies from individual to individual depending on their age, genetic predisposition and countless environmental factors. It is commonly stated that hair grows about 1 cm per month on average; however reality is more complex, since not all hair grows at once. Scalp hair was reported to grow between 0.6 cm and 3.36 cm per month. The growth rate of scalp hair somewhat depends on age (hair tends to grow more slowly with age), sex, and ethnicity. Thicker hair (>60 µm) grows generally faster (11.4 mm per month) than thinner (20-30 µm) hair (7.6 mm per month). It was previously thought that Caucasian hair grew more quickly than Asian hair and that the growth rate of women's hair was faster than that of men. However, more recent research has shown that the growth rate of hair in men and women does not significantly differ and that the hair of Chinese people grew more quickly than the hair of French Caucasians and West and Central Africans.


Anagen phase

The anagen phase, known as the growth phase, is when the hair physically grows approximately 1 cm per month. It begins in the
papilla Papilla (Latin, 'nipple The nipple is a raised region of tissue on the surface of the breast The breast is one of two prominences located on the upper region of the of s. In females, it serves as the , which produces and secretes mil ...

papilla
and can last from three to five years. The span at which the hair remains in this stage of growth is determined by genetics. The longer the hair stays in the anagen phase, the longer it will grow. During this phase, cells neighboring the
papilla Papilla (Latin, 'nipple The nipple is a raised region of tissue on the surface of the breast The breast is one of two prominences located on the upper region of the of s. In females, it serves as the , which produces and secretes mil ...
in a germinative layer divide to produce new hair fibers, and the follicle buries itself into the
dermal The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
layer of the skin to nourish the strand. About 85%–90% of the hairs on one's head are in the anagen phase at any given time.


Catagen phase

The catagen phase, or the transitional phase, allows the follicle to renew itself (in a sense). During this time, which lasts about two weeks, the hair follicle shrinks due to disintegration and the papilla detaches and "rests," cutting the hair strand off from its nourishing blood supply. Signals sent out by the body (that only selectively affect 1 percent of all hair of one's body at any given time) determine when the on of melanin production in the hair bulb and apoptosis of follicular melanocytes. Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward.


Telogen phase

During the telogen or resting phase (also known as shedding phase) the follicle remains dormant for one to three months. Ten to fifteen percent of the hairs on one's head are in this phase of growth at any given time. In this phase, the epidermal cells lining the follicle channel continue to grow as normal and may accumulate around the base of the hair, temporarily anchoring it in place and preserving the hair for its natural purpose without taxing the body's resources needed during the growth phase. At some point, the follicle will begin to grow again, softening the anchor point of the shaft initially. The hair base will break free from the root and the hair will be shed. Within two weeks, the new hair shaft will begin to emerge once the telogen phase is complete. The process results in normal hair loss known as
shedding Shedding may refer to: * Shedding or moulting of body parts * Desquamation, pathologic or non-pathologic skin shedding * Peeling of the skin * Shedding game, a family of card games where the objective is to get rid of one's hand first * Natural hai ...

shedding
.


Growth inhibitors and disorders

In most people, scalp hair growth will halt due to follicle devitalization after reaching a length of generally two or three feet. Exceptions to this rule can be observed in individuals with hair development abnormalities, which may cause an unusual length of hair growth.


Chemotherapy

Most
chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment Cancer Cancer is a group of diseases A disease is a particular abnormal condition that negatively affects the structure or f ...

chemotherapy
drugs work by attacking rapidly dividing cells. Rapid cell replication is one of the hallmarks of cancer; however, hair follicle cells also grow and divide quickly. Consequently, the chemotherapy drugs usually inhibit hair growth. The dose and type of medicine will determine the severity of hair loss. Once the course of chemotherapy has ended, new hair growth may begin after three to 10 weeks.


Hair loss

Alopecia Hair loss, also known as alopecia or baldness, refers to a loss of hair from part of the head or body. Typically at least the head is involved. The severity of hair loss can vary from a small area to the entire body. Inflammation or scarrin ...

Alopecia
is a hair loss disease that can occur in anyone at any stage of life. Specifically
alopecia areata Alopecia areata, also known as spot baldness, is a condition in which hair is lost from some or all areas of the body. Often it results in a few bald spots on the scalp, each about the size of a coin. The disease may cause psychological stress ...

alopecia areata
is an
autoimmune disease An autoimmune disease is a condition arising from an abnormal immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of differe ...

autoimmune disease
that causes hair to spontaneously fall out. It is mainly characterized by bald patches on the scalp or other parts of the body, and can ultimately cause baldness across the entire body. This disease interferes with the hair growth cycle by causing a follicle to prematurely leave the
anagen The hair follicle is an Organ (anatomy), organ found in mammalian skin. It resides in the Dermis, dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a comple ...
, or active growth, phase and enter the resting, or telogen, phase. The hair growth in the affected follicles is lessened or stopped completely.
Traction alopeciaTraction alopecia is a form of alopecia Hair loss, also known as alopecia or baldness, refers to a loss of hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of m ...

Traction alopecia
is caused by adding too much strain on the hair on one's head. Tight ponytails and other styles that require added tension to the hair are often what cause this disease. It can also occur on the face in areas where the hair is often styled. Plucking or waxing one's eyebrows frequently, for example, can yield suppressed hair growth in the area. On the scalp, the hair is usually known to be lost around the hair line, leaving the densest amount of hair at the crown. Small
vellus hair Vellus hair is short, thin, light-colored, and barely noticeable hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of g ...
will often replace the hair that is lost.


Radiation therapy to the head

Human hair follicles are very sensitive to the effects of
radiation therapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of s or s that have sufficient to s or s by detachi ...

radiation therapy
administered to the head, most commonly used to treat cancerous growths within the brain. Hair shedding may start as soon as two weeks after the first dose of radiation and will continue for a couple of weeks. Hair follicles typically enter the telogen phase, and regrowth should commence 2.5 to 3 months after the hair begins to shed. Regrowth may be sparser after treatment.


UV-B

Ultraviolet light levels of either 20 or 50 mJ cm−2 in the
UV-B Ultraviolet (UV) is a form of with from 10 (with a corresponding frequency around 30 ) to 400 nm (750 ), shorter than that of , but longer than s. UV radiation is present in , and constitutes about 10% of the total electromagnetic radi ...
range have been shown to inhibit hair growth, reduce hair
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
and damage
hair follicles The hair follicle is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given orga ...

hair follicles
.


See also

* *
Hair transplantation Hair transplantation is a surgical Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical or dental specialty that uses operative manual and instrumental techn ...


References

{{Human hair Human hair