Human hair growth
Encyclopedia
Human hair grows everywhere on the body except for the soles of the feet, the lips, the palms of the hands, and the eyelids, apart from eyelashes. Like skin, hair is a stratified squamous
, keratinized epithelium made of multi-layered, flat cells with overlying keratin
(a protein), whose rope-like filament
s provide structure and strength to the hair shaft.
Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.
The body has different types of hair, including vellus hair and androgenic hair
, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.
a person can follow that ensures a healthy head of hair. One's lifestyle, diet
and hair type should all be considered when attempting to adopt a routine to develop and maintain healthy hair.
hair. Apart from supplying the follicle with nutrient-rich blood, exercise also helps to alleviate stress
, which is known to cause hair loss.
and can last up to eight 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 faster and longer it will grow. During this phase, the cells in the papilla divide to produce new hair fibers, and the follicle buries itself into the dermal layer of the skin to nourish the strand. About 85% of the hairs on one's head are in the anagen phase at a given time.
.
drugs work by attacking fast replicating cells. Rapid cell replication is one of the hallmarks of cancers, however hair follicle cells also grow and divide quickly; consequently, the chemotherapy drugs usually inhibit hair growth. The dose and type of medicine determine how severe a person experiences hair loss, but once the chemotherapy has ended, one can begin to experience new hair growth after three to 10 months.
is a hair loss disease that can occur in anyone at any stage of life. Specifically Alopecia areata
is an 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, or active growth, phase and enter the resting, or telogen phase. The hair growth in the affected follicles is lessened or stopped completely.
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 will often replace the hair that is lost. 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.
Stratified squamous epithelium
A stratified squamous epithelium consists of squamous epithelial cells arranged in layers upon a basement membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity...
, keratinized epithelium made of multi-layered, flat cells with overlying keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
(a protein), whose rope-like filament
Protein filament
In biology, a filament is a "long chain of proteins, such as those found in hair, muscle, or in flagella". They are often bundled together for strength and rigidity. Some cellular examples include:*Actin filaments*Microtubules*Intermediate filaments...
s provide structure and strength to the hair shaft.
Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.
The body has different types of hair, including vellus hair and androgenic hair
Androgenic hair
Androgenic hair, colloquially body hair, is the terminal hair that develops on the body during and after puberty. It is differentiated from the head hair and less visible vellus hair, which are much finer and lighter in color. The growth of androgenic hair is related to the level of androgens in...
, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.
Normal growth
There is no one regimenRegimen
A regimen is a plan, a regulated course such as a diet, exercise or medical treatment, designed to give a positive result. A low-salt diet is a regimen. A course of penicillin is a regimen.- History :...
a person can follow that ensures a healthy head of hair. One's lifestyle, diet
Diet (nutrition)
In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...
and hair type should all be considered when attempting to adopt a routine to develop and maintain healthy hair.
Exercise
Exercise stimulates the blood to flow. This nutrient-rich blood nourishes each hair follicle, promoting healthy hair growth. Lack of this sufficient blood flow would yield the growth of duller, more brittleBrittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
hair. Apart from supplying the follicle with nutrient-rich blood, exercise also helps to alleviate stress
Stress (biology)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...
, which is known to cause hair loss.
Growth cycle
The three stages of hair growth are the anagen, 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 rate or speed of hair growth is about 1.25 centimeters or 0.5 inches per month, or about 15 centimeters or 6 inches per year.Anagen phase
The anagen phase is known as the growth phase. It begins in the papillaHair follicle
A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
and can last up to eight 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 faster and longer it will grow. During this phase, the cells in the papilla divide to produce new hair fibers, and the follicle buries itself into the dermal layer of the skin to nourish the strand. About 85% of the hairs on one's head are in the anagen phase at a given time.
Catagen phase
Signals sent out by the body determine when the anagen phase ends and the catagen phase begins. The catagen phase, also known as the transitional phase, allows the follicle to, in a sense, renew itself. 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. Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward. While hair is not growing during this phase, the length of the terminal fibers increase when the follicle pushes them upward.Telogen phase
During the telogen, or resting, phase the hair and follicle remain dormant anywhere from 1–4 months. Ten to fifteen percent of the hairs on one's head are in this phase of growth in any given time. The anagen phase begins again once the telogen phase is complete. The preceding hair strand is pushed up and out by the new, growing strand. The process causes the normal hair loss known as sheddingMoult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...
.
Chemotherapy
Most chemotherapyChemotherapy
Chemotherapy is the treatment of cancer with an antineoplastic drug or with a combination of such drugs into a standardized treatment regimen....
drugs work by attacking fast replicating cells. Rapid cell replication is one of the hallmarks of cancers, however hair follicle cells also grow and divide quickly; consequently, the chemotherapy drugs usually inhibit hair growth. The dose and type of medicine determine how severe a person experiences hair loss, but once the chemotherapy has ended, one can begin to experience new hair growth after three to 10 months.
Alopecia-related syndromes
AlopeciaAlopecia
Alopecia means loss of hair from the head or body. Alopecia can mean baldness, a term generally reserved for pattern alopecia or androgenic alopecia. Compulsive pulling of hair can also produce hair loss. Hairstyling routines such as tight ponytails or braids may induce Traction alopecia. Both...
is a hair loss disease that can occur in anyone at any stage of life. Specifically Alopecia areata
Alopecia areata
Alopecia areata is a medical condition in which hair is lost from some or all areas of the body, usually from the scalp. Because it causes bald spots on the scalp, especially in the first stages, it is sometimes called spot baldness. In 1–2% of cases, the condition can spread to the entire scalp ...
is an autoimmune disease
Autoimmune disease
Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...
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, or active growth, phase and enter the resting, or telogen phase. The hair growth in the affected follicles is lessened or stopped completely.
Traction alopecia
Traction alopecia
Traction alopecia is a form of alopecia, or gradual hair loss, caused primarily by pulling force being applied to the hair. This commonly results from the sufferer frequently wearing his/her hair in a particularly tight ponytail, pigtails, or braids...
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 will often replace the hair that is lost. 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.