Human hair growth

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The growth of human hair occurs everywhere on the body except for the soles of the feet, the palms of the hands, the inside of the mouth, the lips, the backs of the ears, some external genital areas, the navel, and, apart from eyelashes, the eyelids. [1] Hair is a stratified squamous keratinized epithelium made of multi-layered flat cells whose rope-like filaments provide structure and strength to the hair shaft. The protein called keratin makes up hair and stimulates hair growth. Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen. Each phase has specific characteristics that determine the length of the hair.

Contents

The body has different types of hair, including vellus hair and 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. [2] 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

Hair-follicle cycling
Hair grows at different speeds and different lengths. Its composition causes different colors and textures, which influence how long the hair strands grow. Blond long-haired young lady woman.jpg
Hair grows at different speeds and different lengths. Its composition causes different colors and textures, which influence how long the hair strands grow.
Marianne Ernst, a German "Long hair model". Marianne Ernst, Long hair model.jpg
Marianne Ernst, a German "Long hair model".

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 growth rate of hair varies from individual to individual depending on their age, genetic predisposition and a number of 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. [3] Thicker hair (>60 µm) grows generally faster (11.4 mm per month) than thinner (20–30 µm) hair (7.6 mm per month). [4]

It was previously thought that Caucasian hair grew faster than Asian hair and that women's grew faster than men's; [3] but newer research shows that the sexes do not differ significantly in speed of hair growth [5] and that the hair of Chinese people grows faster than that of French Caucasians and West and Central Africans. [6] The quantity of hairs depends on hair colour (before graying): [7] [8] an average blond-haired person has 150,000 hairs, a brown-haired person has 110,000, a black-haired person has 100,000, and a redhead has 90,000. [9] Hair growth stops growing at death; the illusion of growth after death is due to shrinkage of the skin by drying. [10]

Anagen phase

The anagen phase, known as the growth phase, is when the hair grows about 1 cm per month. [11] It begins in the papilla and can last from three to five years. [12] 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 in a germinative layer divide to produce new hair fibers, [13] and the follicle buries itself into the dermal 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 the end of melanin production in the hair bulb and apoptosis of follicular melanocytes. [14] Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward.

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. [15] [ better source needed ]

Chemotherapy

Most 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. [16] 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 is a hair loss disease that can occur in anyone at any stage of life. [17] [18] 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. Primary Cicatricial Alopecias (PCAs), commonly referred to as scarring alopecias, are a collective group of inflammatory disorders with distinctive pathological features that include the development of fibrosis, permanent destruction of the pilosebaceous unit of the hair follicle (HF), and irreversible hair loss. [19]

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.

Radiation therapy to the head

Human hair follicles are very sensitive to the effects of 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 range have been shown to inhibit hair growth, reduce hair melanin and damage hair follicles. [20]

See also

Related Research Articles

<span class="mw-page-title-main">Alopecia areata</span> Medical condition

Alopecia areata, also known as spot baldness, is a condition in which hair is lost from some or all areas of the body. It often results in a few bald spots on the scalp, each about the size of a coin. Psychological stress and illness are possible factors in bringing on alopecia areata in individuals at risk, but in most cases there is no obvious trigger. People are generally otherwise healthy. In a few cases, all the hair on the scalp is lost, or all body hair is lost. Hair loss can be permanent, or temporary.

<span class="mw-page-title-main">Hair loss</span> Loss of hair from the head or body

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 scarring is not usually present. Hair loss in some people causes psychological distress.

<span class="mw-page-title-main">Minoxidil</span> Medication used to treat hair loss

Minoxidil is a medication used for the treatment of high blood pressure and pattern hair loss. It is an antihypertensive and a vasodilator. It is available as a generic medication by prescription in oral tablet form and over the counter as a topical liquid or foam.

<span class="mw-page-title-main">Hair follicle</span> Organ found in the dermal layer of the mammalian skin that regulates hair growth

The hair follicle is an organ found in mammalian skin. It resides in the 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 complex interaction between hormones, neuropeptides, and immune cells. This complex interaction induces the hair follicle to produce different types of hair as seen on different parts of the body. For example, terminal hairs grow on the scalp and lanugo hairs are seen covering the bodies of fetuses in the uterus and in some newborn babies. The process of hair growth occurs in distinct sequential stages: anagen is the active growth phase, catagen is the regression of the hair follicle phase, telogen is the resting stage, exogen is the active shedding of hair phase and kenogen is the phase between the empty hair follicle and the growth of new hair.

<span class="mw-page-title-main">Telogen effluvium</span> Medical condition

Telogen effluvium is a scalp disorder characterized by the thinning or shedding of hair resulting from the early entry of hair in the telogen phase. It is in this phase that telogen hairs begin to shed at an increased rate, where normally the approximate rate of hair loss is 125 hairs per day.

<span class="mw-page-title-main">Vellus hair</span> Type of hair that is short, thin, slight-colored, and barely noticeable

Vellus hair is short, thin, light-colored, and barely noticeable hair that develops on most of a human's body during childhood. Exceptions include the lips, the back of the ear, the palm of the hand, the sole of the foot, some external genital areas, the navel, and scar tissue. The density of hair – the number of hair follicles per area of skin – varies from person to person. Each strand of vellus hair is usually less than 2 mm long and the follicle is not connected to a sebaceous gland.

The management of hair loss, includes prevention and treatment of alopecia, baldness, and hair thinning, and regrowth of hair.

<span class="mw-page-title-main">Body hair</span> Hair on the human body during and after puberty

Body hair or androgenic hair is terminal hair that develops on the human body during and after puberty. It is different from head hair and also from less visible vellus hair, which is much finer and lighter in color. Growth of androgenic hair is related to the level of androgens and the density of androgen receptors in the dermal papillae. Both must reach a threshold for the proliferation of hair follicle cells.

<span class="mw-page-title-main">Pattern hair loss</span> Medical condition

Pattern hair loss is a hair loss condition that primarily affects the top and front of the scalp. In male-pattern hair loss (MPHL), the hair loss typically presents itself as either a receding front hairline, loss of hair on the crown (vertex) of the scalp, or a combination of both. Female-pattern hair loss (FPHL) typically presents as a diffuse thinning of the hair across the entire scalp.

<span class="mw-page-title-main">Uncombable hair syndrome</span> Rare scalp hair shaft dysplasia

Uncombable hair syndrome (UHS) is a rare structural anomaly of the hair with a variable degree of effect. It is characterized by hair that is silvery, dry, frizzy, wiry, and impossible to comb. It was first reported in the early 20th century. It typically becomes apparent between the ages of 3 months and 12 years. UHS has several names, including pili trianguli et canaliculi (Latin), cheveux incoiffables (French), and "spun-glass hair". This disorder is believed to be autosomal recessive in most instances, but there are a few documented cases where multiple family members display the trait in an autosomal dominant fashion. Based on the current scientific studies related to the disorder, the three genes that have been causally linked to UHS are PADI3, TGM3, and TCHH. These genes encode proteins important for hair shaft formation. Clinical symptoms of the disorder arise between 3 months and 12 years of age. The quantity of hair on the head does not change, but hair starts to grow more slowly and becomes increasingly "uncombable". To be clinically apparent, 50% of all scalp hair shafts must be affected by UHS. This syndrome only affects the hair shaft of the scalp and does not influence hair growth in terms of quantity, textural feel, or appearance on the rest of the body.

<span class="mw-page-title-main">FGF5</span> Mammalian protein found in Homo sapiens

Fibroblast growth factor 5 is a protein that in humans is encoded by the FGF5 gene.

Madarosis is a condition that results in the loss of eyelashes, and sometimes eyebrows. The term "madarosis" is derived from the ancient Greek "madaros", meaning "bald". It originally was a disease of only losing eyelashes but it currently is the loss of both eyelashes and eyebrows. Eyebrows and eyelashes are both important in the prevention of bacteria and other foreign objects from entering the eye. A majority of patients with madarosis have leprosy, and it was reported that 76% of patients with varying types of leprosy had madarosis.

Anagen effluvium is the pathologic loss of anagen or growth-phase hairs. Classically, it is caused by radiation therapy to the head and systemic chemotherapy, especially with alkylating agents.

<span class="mw-page-title-main">Loose anagen syndrome</span> Medical condition

Loose anagen syndrome, also known as loose anagen hair syndrome, is a hair disorder related to dermatology. It is characterised by the easy and pain free detachment of anagen staged hairs from the scalp. This hair condition can be spontaneous or genetically inherited.

Scarring hair loss, also known as cicatricial alopecia, is the loss of hair which is accompanied with scarring. This is in contrast to non scarring hair loss.

Central centrifugal cicatricial alopecia (CCCA), is a type of alopecia first noticed in African Americans in the 1950s and reported by LoPresti et al. in 1968 as a result of application of petrolatum followed by a stove-heated iron comb. The original theory was that the hot petrolatum would travel down to the hair root, burn the follicle, and after repetitive injury scarring would result. Later CCCA was realized to affect men and women without a history significant for use of such styling techniques. Consequently, the terms "follicular degeneration syndrome" per Sperling and Sau in 1992 and then CCCA per Olsent et al. in 2003 were evolved. Plausible contributing factors may include other African-American styling techniques such as relaxers, tight braids, heavy extensions, certain oils, gels or pomades.

Non scarring hair loss, also known as noncicatricial alopecia is the loss of hair without any scarring being present. There is typically little inflammation and irritation, but hair loss is significant. This is in contrast to scarring hair loss during which hair follicles are replaced with scar tissue as a result of inflammation. Hair loss may be spread throughout the scalp (diffuse) or at certain spots (focal). The loss may be sudden or gradual with accompanying stress.

<span class="mw-page-title-main">Trichoscopy</span>

Trichoscopy is a method of hair and scalp evaluation and is used for diagnosing hair and scalp diseases. The method is based on dermoscopy. In trichoscopy hair and scalp structures may be visualized at many-fold magnification. Currently magnifications ranging from 10-fold to 70-fold are most popular in research and clinical practice.

<span class="mw-page-title-main">Frontal fibrosing alopecia</span> Medical condition

Frontal fibrosing alopecia is the frontotemporal hairline recession and eyebrow loss in postmenopausal women that is associated with perifollicular erythema, especially along the hairline. It is considered to be a clinical variant of lichen planopilaris.

<span class="mw-page-title-main">Prepubertal hypertrichosis</span> Medical condition

Prepubertal hypertrichosis, also known as childhood hypertrichosis, is a cutaneous condition characterized by increased hair growth, found in otherwise healthy infants and children. Prepubertal hypertrichosis is a cosmetic condition and does not affect any other health aspect. Individuals with this condition may suffer with low self esteem and mental health issues due to societal perceptions of what a "normal" appearance should be. The mechanism of prepubertal hypertrichosis is unclear, but causes may include genetics, systemic illnesses, or medications.

References

  1. ↑ Buffoli, Barbara; Rinaldi, Fabio; Labanca, Mauro; Sorbellini, Elisabetta; Trink, Anna; Guanziroli, Elena; Rezzani, Rita; Rodella, Luigi F. (2014). "The human hair: from anatomy to physiology". International Journal of Dermatology. 53 (3): 331–341. doi:10.1111/ijd.12362. PMID   24372228. S2CID   1310059.
  2. ↑ Schneider, Marlon R.; Schmidt-Ullrich, Ruth; Paus, Ralf (2009-02-10). "The hair follicle as a dynamic miniorgan". Current Biology. 19 (3): R132–142. doi: 10.1016/j.cub.2008.12.005 . ISSN   1879-0445. PMID   19211055.
  3. 1 2 Harkey, M. R. (December 1993). "Anatomy and physiology of hair". Forensic Science International. Hair Analysis as a Diagnostic Tool for Drugs of Abuse Investigation. 63 (1): 9–18. doi:10.1016/0379-0738(93)90255-9. ISSN   0379-0738. PMID   8138238.
  4. ↑ Van Neste DJ, Rushton DH (2016). "Gender differences in scalp hair growth rates are maintained but reduced in pattern hair loss compared to controls". Skin Res Technol. 22 (3): 363–9. doi:10.1111/srt.12274. PMID   26526232. S2CID   19060270.
  5. ↑ Joseph Castro (January 27, 2014). "How Fast Does Hair Grow?". Live Science. Retrieved July 3, 2020.
  6. ↑ Loussouarn, Geneviève; El Rawadi, Charles; Genain, Gilles (2005). "Diversity of hair growth profiles". International Journal of Dermatology. 44 (s1): 6–9. doi: 10.1111/j.1365-4632.2005.02800.x . PMID   16187948. S2CID   39103960.
  7. ↑ "How many hairs are on a human head in total and per square inch?". curlcentric.com. Retrieved April 18, 2022.
  8. ↑ "Number of hairs on human head". harvard.edu. Archived from the original on 14 December 2021. Retrieved April 18, 2022.
  9. ↑ "How much hair does a human have on their head?". mevolife.com. Retrieved April 18, 2022.
  10. ↑ "Do your hair and fingernails grow after death?". bbc.com. Retrieved April 18, 2022.
  11. ↑ Ruszczak, Zbigniew (2012). "Hair Disorders and Alopecia". In Elzouki, Abdelaziz Y.; Harfi, Harb A.; Nazer, Hisham M.; Stapleton, F. Bruder; Oh, William; Whitley, Richard J. (eds.). Textbook of Clinical Pediatrics. pp. 1489–508. doi:10.1007/978-3-642-02202-9_146. ISBN   978-3-642-02201-2.
  12. ↑ Braun-Falco, Otto (2000). Dermatology (2., completely rev. ed.). Berlin: Springer. p. 1101. ISBN   9783540594529.
  13. ↑ Nicolas, Jean-François; Sequeira, Inês (15 October 2012). "Redefining the structure of the hair follicle by 3D clonal analysis". Development. 139 (20): 3741–3751. doi: 10.1242/dev.081091 . ISSN   0950-1991. PMID   22991440.
  14. ↑ Araújo, Rita; Fernandes, Margarida; Cavaco-Paulo, Artur; Gomes, Andreia (2010). "Biology of Human Hair: Know Your Hair to Control It". Biofunctionalization of Polymers and their Applications. Advances in Biochemical Engineering/Biotechnology. Vol. 125. pp. 121–43. doi:10.1007/10_2010_88. hdl:1822/15299. ISBN   978-3-642-21948-1. PMID   21072698. S2CID   21502785.
  15. ↑ "Natural Hair Growth". Sunday, 11 April 2021
  16. ↑ Grevelman, E. G.; Breed, W. P. M. (2005-03-01). "Prevention of chemotherapy-induced hair loss by scalp cooling". Annals of Oncology. 16 (3): 352–358. doi: 10.1093/annonc/mdi088 . ISSN   0923-7534. PMID   15642703.
  17. ↑ Erjavec SO, Gelfman S, Abdelaziz AR, Lee EY, Monga I, Alkelai A, Ionita-Laza I, Petukhova L, Christiano AM (Feb 2022). "Whole exome sequencing in Alopecia Areata identifies rare variants in KRT82". Nat Commun. 13 (1): 800. doi:10.1038/s41467-022-28343-3. PMC   8831607 . PMID   35145093.
  18. ↑ "Hair loss (alopecia) - NHS Choices". 2013-09-27. Archived from the original on 2013-09-27. Retrieved 2021-11-26.
  19. ↑ Wang EHC, Monga I, Sallee BN, Chen JC, Abdelaziz AR, Perez-Lorenzo R, Bordone LA, Christiano AM (Jul 2022). "Primary cicatricial alopecias are characterized by dysregulation of shared gene expression pathways". PNAS Nexus. 1 (3): pgac111. doi:10.1093/pnasnexus/pgac111. PMC   9308563 . PMID   35899069.
  20. ↑ Lu, Zhongfa; Fischer, Tobias W; Hasse, Sybille; Sugawara, Koji; Kamenisch, York; Krengel, Sven; Funk, Wolfgang; Berneburg, Mark; Paus, Ralf (2009). "Profiling the Response of Human Hair Follicles to Ultraviolet Radiation". Journal of Investigative Dermatology. 129 (7): 1790–804. doi: 10.1038/jid.2008.418 . PMID   19158839.
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