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Human hair growth

From Wikipedia, the free encyclopedia
For regrowth of hair and hair loss prevention, seemanagement of hair loss.

Thegrowth of human hairoccurs 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 externalgenitalareas, thenavel,and, apart fromeyelashes,theeyelids.[1]Hair is astratified squamouskeratinizedepitheliummade of multi-layered flat cells whose rope-likefilamentsprovide structure and strength to thehair shaft.The protein calledkeratinmakes up hair and stimulates hair growth. Hair follows a specific growth cycle with three distinct and concurrent phases:anagen,catagen,andtelogen.Each phase has specific characteristics that determine the length of the hair.

The body has different types of hair, includingvellus hairandandrogenic 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[edit]

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.
Marianne Ernst, a German "Long hair model".

The three stages of hair growth are theanagen,catagen,andtelogenphases. 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 averageblond-hairedperson has 150,000 hairs, abrown-hairedperson has 110,000, ablack-hairedperson has 100,000, and aredheadhas 90,000.[9]Hair growth stops at death; the illusion of growth after death is due to shrinkage of the skin by drying.[10]

Anagen phase[edit]

The anagen phase, known as the growth phase, is when the hair grows about 1 cm per month.[11]It begins in thepapillaand 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 thepapillain a germinative layer divide to produce new hair fibers,[13]and the follicle buries itself into thedermallayer 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[edit]

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[edit]

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[edit]

Mostchemotherapydrugs 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[edit]

Main article:Hair loss

Alopeciais a hair loss disease that can occur in anyone at any stage of life.[17][18]Specificallyalopecia areatais anautoimmune diseasethat 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 theanagen,or active growth, phase and enter the resting, ortelogen,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 alopeciais 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 wa xing 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. Smallvellus hairwill often replace the hair that is lost.

Radiation therapy to the head[edit]

Human hair follicles are very sensitive to the effects ofradiation therapyadministered 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 thetelogenphase, and regrowth should commence 2.5 to 3 months after the hair begins to shed. Regrowth may be sparser after treatment.

UV-B[edit]

Ultraviolet light levels of either 20 or 50 mJ cm−2in theUV-Brange have been shown to inhibit hair growth, reduce hairmelaninand damagehair follicles.[20]

See also[edit]

References[edit]

  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.PMID24372228.S2CID1310059.
  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.ISSN1879-0445.PMID19211055.
  3. ^abHarkey, 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.ISSN0379-0738.PMID8138238.
  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.PMID26526232.S2CID19060270.
  5. ^Joseph Castro (January 27, 2014)."How Fast Does Hair Grow?".Live Science.RetrievedJuly 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.PMID16187948.S2CID39103960.
  7. ^"How many hairs are on a human head in total and per square inch?".curlcentric.RetrievedApril 18,2022.
  8. ^"Number of hairs on human head".harvard.edu.Archived fromthe originalon 14 December 2021.RetrievedApril 18,2022.
  9. ^"How much hair does a human have on their head?".mevolife.RetrievedApril 18,2022.
  10. ^"Do your hair and fingernails grow after death?".bbc.RetrievedApril 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.ISBN978-3-642-02201-2.
  12. ^Braun-Falco, Otto (2000).Dermatology(2., completely rev. ed.). Berlin: Springer. p. 1101.ISBN9783540594529.
  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.ISSN0950-1991.PMID22991440.
  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.ISBN978-3-642-21948-1.PMID21072698.S2CID21502785.
  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.ISSN0923-7534.PMID15642703.
  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.PMC8831607.PMID35145093.
  18. ^"Hair loss (alopecia) - NHS Choices".2013-09-27. Archived fromthe originalon 2013-09-27.Retrieved2021-11-26.
  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.PMC9308563.PMID35899069.
  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.PMID19158839.
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