Hair follicle

Last updated
Hair follicle
Hair follicle-en.svg
Hair follicle
ArmHair.jpg
A photograph of hair on a human arm emerging from follicles
Details
System Integumentary system
Artery Supratrochlear, supraorbital, superficial temporal, occipital
Vein Superficial temporal, posterior auricular, occipital
Nerve Supratrochlear, supraorbital, greater occipital, lesser occipital
Lymph Occipital, mastoid
Identifiers
Latin Folliculus pili
MeSH D018859
TA98 A16.0.00.023
TA2 7064
TH H3.12.00.3.01034
FMA 70660
Anatomical terminology

The hair follicle is an organ found in mammalian skin. [1] 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. [1] 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. [1] 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. [1]

Contents

The function of hair in humans has long been a subject of interest and continues to be an important topic in society, developmental biology and medicine. Of all mammals, humans have the longest growth phase of scalp hair compared to hair growth on other parts of the body. [1] For centuries, humans have ascribed esthetics to scalp hair styling and dressing and it is often used to communicate social or cultural norms in societies. In addition to its role in defining human appearance, scalp hair also provides protection from UV sun rays and is an insulator against extremes of hot and cold temperatures. [1] Differences in the shape of the scalp hair follicle determine the observed ethnic differences in scalp hair appearance, length and texture.

There are many human diseases in which abnormalities in hair appearance, texture or growth are early signs of local disease of the hair follicle or systemic illness. Well known diseases of the hair follicle include alopecia [2] or hair loss, hirsutism or excess hair growth and lupus erythematosus . [3] [2]

Structure

Structure of a hair follicle. Blausen 0438 HairFollicleAnatomy 02.png
Structure of a hair follicle.

The position and distribution of hair follicles varies over the body. For example, the skin of the palms and soles does not have hair follicles whereas skin of the scalp, forearms, legs and genitalia has abundant hair follicles. [1] There are many structures that make up the hair follicle. Anatomically, the triad of hair follicle, sebaceous gland and arrector pili muscle make up the pilosebaceous unit. [1]

A hair follicle consists of :

Other structures associated with the hair follicle include the cup in which the follicle grows known as the infundibulum, [8] the arrector pili muscles, the sebaceous glands, and the apocrine sweat glands. Hair follicle receptors sense the position of the hair.

Attached to the follicle is a tiny bundle of muscle fiber called the arrector pili . This muscle is responsible for causing the follicle lissis to become more perpendicular to the surface of the skin, and causing the follicle to protrude slightly above the surrounding skin (piloerection) and a pore encased with skin oil. This process results in goose bumps (or goose flesh).

Also attached to the follicle is a sebaceous gland, which produces the oily or waxy substance sebum. The higher the density of the hair, the more sebaceous glands that are found.

Variation

There are ethnic differences in several different hair characteristics. The differences in appearance and texture of hair are due to many factors: the position of the hair bulb relative to the hair follicle, size and shape of the dermal papilla, and the curvature of the hair follicle. [1] The scalp hair follicle in Caucasians is elliptical in shape and, therefore, produces straight or wavy hair, whereas the scalp hair follicle of people of African descent is more curvy, resulting in the growth of tightly curled hair. [1]

Terminal Scalp Hair Characteristics by Ethnicity [1]
ethnicitydiameter

(micrometers)

cross-sectional shapeappearance
Blonde-haired Caucasian40–80ellipticalstraight or wavy
Dark brown/black haired/red haired Caucasian50–90ellipticalstraight or wavy
Black60–100elliptical and ribbon-likecurly
Asian80–100circularstraight
Terminal Scalp Hair Characteristics by Taxon [9] [10]
Animaldiameter

(micrometers)

cross-sectional shapeappearance
Chimpanzee101-113circularstraight
Orangutan140-170circularstraight
Buffalo110circularstraight

Development

In utero, the epithelium and underlying mesenchyme interact to form hair follicles. [11] [12]

Aging

A key aspect of hair loss with age is the aging of the hair follicle. Ordinarily, hair follicle renewal is maintained by the stem cells associated with each follicle. Aging of the hair follicle appears to be primed by a sustained cellular response to the DNA damage that accumulates in renewing stem cells during aging. [13] This damage response involves the proteolysis of type XVII collagen by neutrophil elastase in response to the DNA damage in the hair follicle stem cells. Proteolysis of collagen leads to elimination of the damaged cells and then to terminal hair follicle miniaturization.

Hair growth

Hair-follicle cycling
Hair follicle

Hair grows in cycles of various phases: [14] anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase (names derived using the Greek prefixes ana-, kata-, and telos- meaning up, down, and end respectively). Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one or several hairs that might arise from a single follicle exits. Normally up to 85% of the hair follicles are in anagen phase, while 10–14% are in telogen and 1–2% in catagen. The cycle's length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hair have a much shorter length limit compared to hair on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor. DLX3 is a crucial regulator of hair follicle differentiation and cycling. [15] [16]

Anagen phase

Anagen is the active growth phase of hair follicles [17] during which the root of the hair is dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. A hair pulled out in this phase will typically have the root sheath attached to it which appears as a clear gel coating the first few mm of the hair from its base; this may be misidentified as the follicle, the root or the sebaceous gland by non-health care professionals. Scalp hair stays in this active phase of growth for 2–7 years; this period is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.

Catagen phase

The catagen phase is a short transition stage that occurs at the end of the anagen phase. [18] It signals the end of the active growth of a hair. This phase lasts for about 2–3 weeks while the hair converts to a club hair. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. A bulb of keratin attaches to the bottom tip of the hair and keeps it in place while a new hair begins to grow below it. A hair pulled out in this phase will have the bulb of keratin attached to it which appears as a small white ball on the end of the hair. This process cuts the hair off from its blood supply and from the cells that produce new hair. When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase.

Telogen phase

The telogen phase is the resting phase of the hair follicle, about three months. [19] When the body is subjected to extreme stress, as much as 70 percent of hair can prematurely enter the telogen phase and begin to fall, causing a noticeable loss of hair. This condition is called telogen effluvium. [20] The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair. [11] Fifty to one-hundred club hairs are shed daily from a normal scalp. [11]

Timeline

Clinical significance

Disease

There are many human diseases in which abnormalities in hair appearance, texture or growth are early signs of local disease of the hair follicle or systemic illness. Well known diseases of the hair follicle include alopecia or hair loss, hirsutism or excess hair growth, and lupus erythematosus . [3] Therefore, understanding the function of the normal hair follicle is fundamental to diagnosing and treating many dermatologic and systemic diseases with hair abnormalities. [3] Studies of Witka et al. 2020 has shown the role of microbiome in the biology, immunology and diseases of scalp hair follicle. Studies further shown that change in hair follicle microbiome result into scalp disease like; Seborrheic dermatitis of the scalp and dandruff, Folliculitis decalvans, Androgenetic alopecia, Scalp psoriasis and Alopecia areata. [21]

Hair restoration

Hair follicles form the basis of the two primary methods of hair transplantation in hair restoration, Follicular Unit Transplantation (FUT) and follicular unit extraction (FUE). In each of these methods, naturally occurring groupings of one to four hairs, called follicular units, are extracted from the hair restoration patient and then surgically implanted in the balding area of the patient's scalp, known as the recipient area. These follicles are extracted from donor areas of the scalp, or other parts of the body, which are typically resistant to the miniaturization effects of the hormone DHT. It is this miniaturization of the hair shaft that is the primary predictive indicator of androgenetic alopecia, [22] commonly referred to as male pattern baldness or male hair loss. When these DHT-resistant follicles are transplanted to the recipient area, they continue to grow hair in the normal hair cycle, thus providing the hair restoration patient with permanent, naturally-growing hair.

While hair transplantation dates back to the 1950s, [23] and plucked human hair follicle cell culture in vitro to the early 1980s, [24] it was not until 1995 when hair transplantation using individual follicular units was introduced into medical literature. [25]

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">Sebaceous gland</span> Gland to lubricate the hair and skin

A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest number on the face and scalp, but also on all parts of the skin except the palms of the hands and soles of the feet. In the eyelids, meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum into tears. Surrounding the female nipple, areolar glands are specialized sebaceous glands for lubricating the nipple. Fordyce spots are benign, visible, sebaceous glands found usually on the lips, gums and inner cheeks, and genitals.

<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.

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

The arrector pili muscles, also known as hair erector muscles, are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end, known colloquially as goose bumps (piloerection).

<span class="mw-page-title-main">Hair care</span> Hygiene and cosmetology involving human hair

Hair care or haircare is an overall term for hygiene and cosmetology involving the hair which grows from the human scalp, and to a lesser extent facial, pubic and other body hair. Hair care routines differ according to an individual's culture and the physical characteristics of one's hair. Hair may be colored, trimmed, shaved, plucked or otherwise removed with treatments such as waxing, sugaring and threading. Hair care services are offered in salons, barbershops and day spas, and products are available commercially for home use. Laser hair removal and electrolysis are also available, though these are provided by licensed professionals in medical offices or speciality spas.

Trichology is the study of the hair and scalp. The term derives from Ancient Greek θρίξ (thríx), "hair" and -λογία -logia. In most jurisdictions the title of a trichologist, not the field of trichology, is considered a para-medical discipline.

<span class="mw-page-title-main">Hair transplantation</span> Surgical operation to relocate hair follicles

Hair transplantation is a surgical technique that removes hair follicles from one part of the body, called the 'donor site', to a bald or balding part of the body known as the 'recipient site'. The technique is primarily used to treat male pattern baldness. In this minimally invasive procedure, grafts containing hair follicles that are genetically resistant to balding are transplanted to the bald scalp.

<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.

Follicular unit transplantation (FUT) is a hair restoration technique, also known as the strip procedure, where a patient's hair is transplanted in naturally occurring groups of 1 to 4 hairs, called follicular units. Follicular units also contain sebaceous (oil) glands, nerves, a small muscle, and occasional fine vellus hairs. In follicular unit transplantation, these small units allow the surgeon to safely transplant thousands of grafts in a single session, which maximizes the cosmetic impact of the procedure.

<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.

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. 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.

References

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  2. 1 2 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. Bibcode:2022NatCo..13..800E. doi:10.1038/s41467-022-28343-3. PMC   8831607 . PMID   35145093.
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  4. ↑ Pawlina, Wojciech; Ross, Michael W.; Kaye, Gordon I. (2003). Histology: a text and atlas: with cell and molecular biology. Hagerstown, Maryland: Lippincott Williams & Wilkins. ISBN   978-0-683-30242-4.
  5. ↑ Ma DR; Yang EN; Lee ST (2004). "A review: the location, molecular characterisation and multipotency of hair follicle epidermal stem cells". Ann. Acad. Med. Singap. 33 (6): 784–8. doi: 10.47102/annals-acadmedsg.STlee . PMID   15608839. S2CID   20267123.
  6. ↑ Cotsarelis G (2006). "Epithelial stem cells: a folliculocentric view". J. Invest. Dermatol. 126 (7): 1459–68. doi: 10.1038/sj.jid.5700376 . PMID   16778814.
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  9. ↑ Chernova, Olga (May 2014). "Scanning electron microscopy of the hair medulla of orangutan, chimpanzee, and man". Journal of Biological Sciences. 456 (1): 199–202. doi:10.1134/S0012496614030065. PMID   24985515. S2CID   6597226.
  10. ↑ S.V, Kshirsagar (July 2009). "Comparative Study of Human and Animal Hair in Relation with Diameter and Medullary Index" (PDF). Indian Journal of Forensic Medicine and Pathology. 2 (3). Archived from the original (PDF) on 2021-12-22. Retrieved 2022-02-21.
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  12. ↑ "Hair Follicle". 2017-11-27. Archived from the original on 2018-12-16. Retrieved 2018-12-18. Wednesday, 19 December 2018
  13. ↑ Matsumura H, Mohri Y, Binh NT, Morinaga H, Fukuda M, Ito M, Kurata S, Hoeijmakers J, Nishimura EK (2016). "Hair follicle aging is driven by transepidermal elimination of stem cells via COL17A1 proteolysis". Science. 351 (6273): aad4395. doi:10.1126/science.aad4395. PMID   26912707. S2CID   5078019.
  14. ↑ K. S. Stenn & R. Paus (1 January 2001). "Controls of Hair Follicle Cycling". Physiological Reviews. 81 (1): 449–494. doi:10.1152/physrev.2001.81.1.449. PMID   11152763. S2CID   35922975. (comprehensive topic review, successor to landmark review of 1954 by HB Chase)
  15. ↑ Hwang, J.; Mehrani, T.; Millar, S. E.; Morasso, M. I. (2008). "Dlx3 is a crucial regulator of hair follicle differentiation and cycling". Development. 135 (18): 3149–3159. doi:10.1242/dev.022202. PMC   2707782 . PMID   18684741.
  16. ↑ Park, G. T.; Morasso, M. I. (2002). "Bone morphogenetic protein-2 (BMP-2) transactivates Dlx3 through Smad1 and Smad4: Alternative mode for Dlx3 induction in mouse keratinocytes". Nucleic Acids Research. 30 (2): 515–522. doi:10.1093/nar/30.2.515. PMC   99823 . PMID   11788714.
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  18. ↑ Brannon, Maryland, Heather. "Categen Phase". The New York Times. Archived from the original on 2015-09-06. Retrieved 2007-11-14.
  19. ↑ Brannon, Maryland, Heather. "Telogen Phase". The New York Times. Archived from the original on 2009-02-08. Retrieved 2007-11-14.
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  21. ↑ Witka, Katarzyna Polak; Rudnicka, Lidia; Peytavi, Ulrike Blume; Vogt, Annika (2020). "The role of the microbiome in scalp hair follicle biology and disease". Experimental Dermatology. 29 (3): 286–294. doi: 10.1111/exd.13935 . PMID   30974503. S2CID   109939740.
  22. ↑ Bernstein RM; Rassman WR (September 1997). "Follicular transplantation. Patient evaluation and surgical planning". Dermatol Surg. 23 (9): 771–84, discussion 801–5. doi:10.1111/j.1524-4725.1997.tb00417.x. PMID   9311372. S2CID   32873299.
  23. ↑ Orentreich N (November 1959). "Autografts in alopecias and other selected dermatological conditions". Ann. N. Y. Acad. Sci. 83 (3): 463–79. doi:10.1111/j.1749-6632.1960.tb40920.x. PMID   14429008. S2CID   35356631.
  24. ↑ Wells J; Sieber VK (December 1985). "Morphological characteristics of cells derived from plucked human hair in vitro". Br. J. Dermatol. 113 (6): 669–75. doi:10.1111/j.1365-2133.1985.tb02402.x. PMID   2420350. S2CID   31137232.
  25. ↑ Bernstein RM; Rassman WR; Szaniawski W; Halperin A (1995). "Follicular Transplantation". Intl J Aesthetic Restorative Surgery. 3: 119–32. ISSN   1069-5249. OCLC   28084113.
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