hair

The title of this article is ambiguous. Further meanings are listed under hair (disambiguation) .

This article is about mammalian hair. There are also a number of hair-like structures: for plant hair see trichome , for hair-like structures in arthropods see Seta (zoology) , for those in annelids, see Chaeta , for sticky hairs on the feet of geckos, see spatula , for hair-like sensory organs in arthropods and some Reptiles, see sensillum , for cilia on cells, see cilia (especially kinozilia), for hair-like extensions on sensory cells (hair cells) see stereocilia . Hair is a redirect to this article, for the desert of the same name in Saxony-Anhalt, see hair (desert) .

A human hair under the microscope
(image section: 0.6 × 0.4 millimeters)

Schematic cross section through the skin with hair follicles.

A hair ( lat. Pilus, capillus [head hair], crinis, coma ) is a long horny thread that grows on the skin of mammals . Hair is essentially made up of keratin . All mammals have at least some hair on their skin, the mucous membranes are always hairless. With a few exceptions ( palms , insides of the fingers , soles of the feet , nipples , red lips ), the entire outer skin of humans is hairy. When it comes to human hair (also known as hair as a collective ), a distinction is made between head hair (main hair, beard hair and eyebrows ) and body hair .

etymology

The designation Middle High German , Old High German hār , "hair", also said from animal fur, goes back to Germ. * Hēra- "hair", which to the idg. Root k̑er [s] "stare; to be rough, scruffy ”.

Animal hair / fur

The hair is the characteristic body covering of all mammal species and is usually referred to as fur or fur (with skin: follicle). In some species or in certain phases of life and body regions, however, hardly any hair is formed. In animals , a distinction is made between fur hair ( capilli , as guide and guard hair), bristle hair ( setae ), wool hair ( pili lanei ) and long hair . In addition, many mammals have whiskers (whiskers). In addition, further developed, horny hairs appear as spines (e.g. in hedgehogs ).

The hair organ

construction

Hierarchical structure of human hair in the cortex and cuticle.

The hair is roughly built up in three layers: cuticle, cortex and medulla.

The outermost layer, called the cuticle or scaly layer, consists of flat, overlapping, keratinized , dead cells , oriented similar to the tip of the hair like a pine cone. It consists of six to ten such cell layers. The cuticle shows the health of the hair. In healthy hair, the cuticle lies flat and creates a smooth, translucent surface. The light is optimally reflected and thus gives the hair the healthy shine. Alkaline environment opens the scales , acidic environment closes them.

The cortex ("bark") - also called the fiber layer or fiber trunk - makes up about 80% of the hair. All chemical processes that are relevant for the hairdresser take place here. The cortex consists of fiber bundles that consist of a large number of the finest keratin fibers, the fibrils. These are probably caused by the fact that cortical cells attach to each other. The connection between the two cells is established by the cell membrane complex, which can be thought of as a kind of cement substance . The tear resistance and elasticity of the hair are due to this cement.

In a few cases, and then only with thick hair, a strong loosening of the fiber structure in the center of the hair is noticeable. In its longitudinal direction, there is a channel-shaped mass, which varies in width depending on the hair diameter and is irregularly arranged. The otherwise ordered structure in the fiber trunk is missing here. Some cavities can be seen. This area of ​​the hair is called the medullary canal or simply the medulla.

Hair root

Schematic longitudinal section of the hair root

In the lower area of ​​the dermis , the hair is created on the hair papilla . In the formation area, the matrix, numerous melanocytes are stored , which release their pigments to the developing hair. The keratinous horny cells migrate upwards and form the hair shaft that pushes itself to the surface of the skin within the follicle.

Hair follicle (hair follicle)

The hair shaft ( Scapus ) lies in an elongated indentation of the epidermis, the hair follicle or hair follicle, at the lower end of which the hair is formed in the hair root ( radix pili ). A sebum gland opens into the follicle , sometimes also a scent gland.

The length of the hair follicle is surrounded by an inner and an outer epithelial hair root sheath.

The histological oblique section of the human scalp shows hair follicles cut at different heights (hematoxylin-eosin staining)

The outer hair root sheath can be seen as a continuation of the stratum basale epithelii (= stratum germinativum epithelii ) in the hair funnel. The hair funnel is the funnel-shaped depression of the skin at the point where the hair emerges from the skin. The outer hair root sheath surrounds the part of the hair that runs in the skin, the hair root, and forms a covering around the inner hair root sheath, which holds the growing hair in the follicle with its horny scales directed against the hair root (see Fig.).

Hair follicles. Mx: matrix cells, DP: hair papilla, HS: hair shaft, IRS: inner layer of the hair root, ORS: outer layer of the hair root

The inner hair root sheath surrounds both the hair root and the hair shaft in the follicle and is derived from matrix cells located on the outer edge of the hair root. It can be further subdivided into:

• Henle layer (outermost layer)
• Huxley layer (middle layer)
• Cuticle (innermost part of the inner hair root sheath attached to the hair root or the root cortex)

The cells of the inner epithelial hair root sheath also keratinize and anchor the hair in the follicle: Their horny scales are directed towards the root, while the scales of the hair are directed towards the tip of the hair, which means that they are interlocked.

The outermost covering, the connective tissue root sheath, anchors the follicle in the dermis . With the exception of primary hair (top and coat hair), the small hair straightening muscle ( arrector pili muscle , also known as the hair follicle muscle or pilomotor ) , consists of smooth muscles . It straightens the hair when it is cold or psychological influences such as excitement and anger. In humans, the skin structure caused by straightening the hair ( piloerection ) is also called goose bumps .

Finally, some nerve fibers wrap around the follicle and, as hair follicle receptors, perform tactile functions .

Hair follicle mites are found in the hair follicles of the face of many people (approx. 70%) and in animals .

Fine structure of the cortex

The cortex (fiber trunk) consists of elongated, approx. 5 µm thick cortex cells . The cortex cells contain 20–30 macrofibrils that give the hair its strength. A macrofibril (diameter 300 nm) contains hundreds of microfibrils (diameter 7–10 nm), these in turn protofibrils , consisting of helical keratin molecules . The fibers are connected to one another by sulfur bridges and twisted together mechanically. The cortical cells are embedded in a kind of cement (isotropic keratin).

Hair biochemistry

The substances involved in the structure of hair mainly consist of the elements carbon (50%), oxygen (23%), nitrogen (17%), hydrogen (6%) and sulfur (4%). Under normal conditions, human hair has a water content of 10%, which significantly influences its mechanical properties. Depending on the humidity of the surrounding air, the water content can rise to over 30% through diffusion of water vapor. On the other hand, hair has a water-repellent effect because the outer cuticle in particular also contains lipids such as fats , fatty acids , sphingolipids ( ceramides , sphingomyelins , cerebrosides and gangliosides ) and steroids such as cholesterol and its derivatives (especially cholesterol sulfate).

Schematic representation of disulfide bridges within a keratin molecule; the α-helical structure of keratin is not taken into account for the sake of clarity of the scheme.

The cortex, i.e. the main part of the hair, consists essentially of natural polymers : 90 percent of the dry weight are proteins , which are known as keratins . The conformation is predominantly helical (peptide spiral). The hair proteins are held together by covalent disulfide bridges between cysteine ​​residues , but also by weaker dipole-dipole interactions, as well as hydrogen bonds and van der Waals forces . The keratins form filaments , which in turn aggregate to form macrofibrils .

Keratin is chemically very stable - almost intact hair was found in Egyptian graves. A change in the cysteine ​​content leads to a change in the stiffness of the hair. The properties of keratin cause all chemically relevant processes that determine the shape or type of hairstyle . The disulfide bridges are z. B. split by permanent wave products such as thioglycolate , whereby the crosslinking is temporarily canceled. Hydrogen bridges in the keratin, on the other hand, are easily loosened and allow the hair to be reshaped, for example by blow-drying, twisting or by moistening and drying in shape.

Melanins are responsible for hair color . Eumelanin determines tones from brown to black. Phaeomelanin determines the color of blonde to red hair. The dilute gene is responsible for gray to isabel-colored hair colors. In people with albinism , the hair is white to light blonde due to the lack of melanins. The UV component in sunlight can, especially when exposed to salts (e.g. in sea water) and oxygen , bleach the melanin. Similar shades are achieved when bleaching with hydrogen peroxide . However, when the hair is bleached or bleached, the structure of the hair also changes.

German and British researchers published a study in March 2009 in which they found that the gray color of hair with age is the result of less breakdown of hydrogen peroxide in the hair.

Hair also contains a large number of trace elements and drug residues . Some substances are determined by environmental influences or are variable due to different diets and lifestyles.

Hair types and their location in humans

There are three types of hair:

• Lanugo hair
• Vellus hair
• Terminal hair

Figures for hair in humans

The growth rate and hair diameter, as well as the number of hairs, are genetic factors that can vary in each person. Nevertheless, the number of hairs fluctuates within certain ranges depending on the hair color . Blondes have an average of 150,000, black-haired 110,000, brunettes 100,000 and redheads 75,000.

• Number of hairs on the head: approx. 0–150,000
• Number of hairs on the entire body: approx. 5,000,000
• Hair density: approx. 200 hairs / cm²
• Daily head hair loss: approx. 60–100 pieces
• Growth rate: approx. 0.33 mm / day, therefore approx. 1 cm / month
• Hair diameter: 0.04 mm ( vellus hair ) to 0.12 mm ( terminal hair )
• Tensile strength : approx. 200 N / mm²
• Young's modulus : 125 N / mm²
• Lifespan of the hair roots: approx. 6–8 years

Hair grows all the time. (Hair growth ends with death, the apparent growth of whiskers in the deceased is solely due to the shrinking of the skin due to water loss.)

Hair shapes

San woman with typical peppercorn hair

The type of hair formation (straight, wavy, curly) depends largely on the hair shape, i.e. the hair cross-section. Hair of East Asians has a round cross-section, which means that it is usually very smooth. The European type mostly has a round to oval cross-section, which means that the hair is straight or tends to form curls. Africans, on the other hand, have hair with a strongly elliptical cross-section. This is why her hair usually forms very strong, small curls.

A special type of hair with what appear to be tufts of hair twisted together in a spiral, between which the scalp is visible, is known as filfil or "peppercorn hair". It occurs almost exclusively in the descendants of the oldest human population , which, according to studies by human geneticists, branched off from those of other populations at least 100,000 years ago and which is found today among the Khoisan and Mbuti pygmies .

Hair growth

Not everyone can get hair as long as Marianne Ernst (German "Long hair Model"). At the beginning of March 2016, her hair was 174 cm long. Hair lengths over one meter are rare because the life span of a hair ensures that they fall out beforehand and the new hair starts all over again in its development

Hair grows in cycles, a hair follicle goes through several phases, which are known as the hair cycle. Head hair grows 0.3 to 0.5 mm per day, about 15 cm in a year. In addition to the growth performance, the duration of the sustained growth is also decisive for the resulting hair length. While many animals experience hair loss once or twice a year due to the season, human hair grows over several years, longer in women than in men, until the (long) hair falls out. The size of the growth depends on individual factors as well as the point in time in the hair cycle.

The common assumption that body hair (beard hair, leg hair) would grow faster or more through regular shaving is wrong.

Hair cycle

• Anagen phase : In this growth phase a new hair root forms and the production of a hair begins. The anagen phase in human scalp hair lasts about two to six years, depending on age, gender and the specific location. Around 85–90% of the hair on the scalp is in this phase. Hair in the anagen phase is called "papillary hair".
• Catagen phase : In this transition phase, which lasts about 2 to 3 weeks, the matrix stops producing cells and the hair follicle narrows in the lower area. The hair becomes detached from the papilla and atrophies. The hair follicle shortens. About 1% of all hairs are in this phase, which is then referred to as "Beethaare".
• Telogen phase : With this end phase, in which up to 18% of the scalp hair is located, the hair papilla is renewed and the hair follicle regenerates. The matrix re-emerges and starts cell division, creating a new hair. This section of the hair cycle lasts for 2 to 4 months. The hair in the telogen phase is called "piston hair".

In addition, a kenogene phase is distinguished in animals . It begins after the hair falls out and ends with the start of a new hair cycle. The different fur densities in summer and winter fur are controlled via these hairless hair follicles.

Trichogram

If structural damage to the hair is suspected or to clarify hair loss , a trichogram is created. To do this, 50 to 100 hairs are plucked out with tweezers (after three days of not washing and only gently combed). The hair roots are then assessed under the microscope and assigned to the individual growth phases. Normal values: anagen hair 85 percent, catagen hair 1%, telogen hair 13%. The rest is due to defective hair. For some years now, a computer trichogram has often been carried out instead of the trichogram. The hair is shortened in a small area and recorded with a camera after two days. In this way, the percentage of anagen hairs (grown hair) can be determined without plucking the hair.

Development of head and body hair

With a few exceptions, the entire human body is covered with hair. A distinction must be made between the head hair , the pubic hair and the rest of the body hair , as they each follow different developmental modalities with regard to the sensitivity to androgens.

Hair on the head may be formed at birth, but this often does not remain. The hair on the head is then developed as terminal hair in early childhood. The body hair initially consists entirely of vellus hair and only later, during puberty , develops into terminal hair in certain places . Exceptions, however, are eyelashes and eyebrows , which, like scalp hair, are already present as terminal hair from childhood.

Puberty and armpit hair usually begin to grow more abundantly with puberty, as does the beard in men, and later mostly on the chest and shoulders.

With age, often prematurely, the growth of the main hair decreases, especially in many men. Conversely, hair growth on the upper lips of women increases. Hair growth also usually increases on the earlobes, feet and nose.

Hair diseases

Hair diseases can be attributed to a wide variety of causes. These include genetic, hormonal or environmental influences.

• Alopecia (hair loss)
• Hypotrichosis (lack of hair)
• Hypertrichosis (excessive hair growth)
• Monilethrix (brittle hair due to hair changes)
• Trichorrhexis nodosa ( knotty, thickened areas, brittle hair)
• Trichomycosis palmellina (bacterial colonization of the hair shafts)
• Hirsutism (male hair type in women)
• Albinism (colorlessness due to a lack of melanin, with a white coat)
• Pili annulati (ring hairs, hereditary streaking of the hair)
• Trichofolliculoma (benign tumor of the hair follicle)
• Pilomatrixoma (benign, calcifying skin tumor of the hair matrix)
• Trichotillomania (compulsive pulling out of the hair, in the narrower sense not a hair disease , but a mental illness)
• There are also some types of dermatomycosis (mainly dermatophytosis ) that can affect hair growth in various ways . The pathogens attack the hair follicles or follicles.

Laboratory animals with genetically determined hair abnormalities are bred for molecular biological education (nude mice or others).

Tasks and functions of hair

The hair fulfills several functions:

• Light protection: Hair absorbs UV radiation and infrared light (heat radiation) and thus protects against the harmful effects of sunlight.
• Thermal insulation: They prevent the body from cooling down too quickly ( hypothermia ).
• Moisture protection against rain and when swimming, in combination with greasing (e.g. beaver geil (Castoreum) of the beaver ). Missing sebum glands (e.g. in musk ox ) make the animals vulnerable to rain.
• Moisture regulation: The skin is provided with sweat glands . Excess moisture is absorbed by the hair.
• Camouflage: The hair mostly has color pigments and determines the appearance of an animal. In some animals, the coloring adapts to the season - it is lighter in winter, for example (see also mimicry ).
• Imposing and threatening functions: Apparent enlargement of an animal through temporary straightening of the fur or fixed distribution of the coat (manes, remains of the shoulder paulettes in adult males).
• Better dissemination of the body's own fragrances, such as B. Pheromones.
• For example In horses, the tail hair serves to drive away insects (especially flies) in the anus region (by swinging the tail beet ) .

For the polar bear it was postulated that the hair acts as a light guide to guide radiation onto the skin, but this thesis has been refuted.

Synthetic hair

In human culture, hair or fur also has other functions, as in the following products

• artificial eyelashes
• Extensions (hairpieces / real hair to synthetic fibers)
• Toupee , wig , artificial braid
• Tobacco products (furs and skins)

See also

• Depilation ( epilation or depilation )
• Split ends
• Hair extension
• Hair transplant

literature

Scientific literature

• H. Zahn: The hair from the chemist's point of view. In: Chemistry in Our Time. 23rd year 1989, No. 5, p. 141, ISSN  0009-2851
• Constantin E. Orfanos (ed.): Hair and hair diseases . Fischer, Stuttgart / New York 1979, ISBN 3-437-30282-5 .
• Arthur R. Rook, Rodney PR Dawber: Hair Diseases. Diagnosis and therapy . Blackwell, Berlin 1995, ISBN 3-89412-102-5 .
• Robert Sauer: Asian and European human hair - ethnic differences and their relevance for the permanent waving process . Dissertation . RWTH, Aachen 2001 ( full text ).
• Hans Geyer: hair. In: Salomon / Geyer / Gille (Hrsg.): Anatomy for veterinary medicine . 2nd ext. Edition. Enke-Verlag, Stuttgart 2008, ISBN 978-3-8304-1075-1 , pp. 637-640.

Other literature

• Stefanie Adomeit: Aspects of a literary obsession - hair as a fetish motif of the 19th century . Dissertation. University of Freiburg im Breisgau, 2007 ( full text ).
• Kim Bagus, Franz Josef Görtz (Ed.): Bald head, plait and perm. A hairy reader . Reclam, Leipzig 1996, ISBN 3-379-01560-1 .
• Christian Janecke (ed.): Wear hair. A cultural-scientific approach . Böhlau, Vienna / Cologne 2004, ISBN 3-412-19103-5 .
• Michel Odoul, Rémy Portrait: What Hair Reveals . Aurum, Braunschweig 2000, ISBN 3-591-08472-7 (psychological approach).
• Imke Barbara Peters: It grows on you. Entertaining, curious, amusing, interesting facts about hair . Stam, Cologne 1997, ISBN 3-8237-7438-7 .
• Ralph M. Trüeb, Doris Lier: The main thing is hair. The hair in the mirror of medicine and psychology . Rüffer and Rub, Zurich 2002, ISBN 3-907625-13-7 .

Web links

Wiktionary: Hair  - explanations of meanings, word origins, synonyms, translations

Commons : Hair  - collection of pictures, videos and audio files

Wikiquote: Hair  Quotes

• www.wissenschaft.de: Why hair grows constantly - Researcher: In the early days of human development, the hairstyle indicated social status (report on an article in New Scientist , November 4, 2006, p. 39).
• Hessischer Rundfunk: A Brief Cultural History of the Body: (1) The Hair (June 15, 2009) Audio file (MP3; 6.6 MB)

Individual evidence

1. ^ The dictionary of origin (=  Der Duden in twelve volumes . Volume 7 ). 5th edition. Dudenverlag, Berlin 2014 ( p. 359 ). See also DWDS ( "hair" ) and Friedrich Kluge , Alfred Götze : Etymological dictionary of the German language . 20th edition, ed. by Walther Mitzka . De Gruyter, Berlin / New York 1967; Reprint (“21st unchanged edition”) ibid 1975, ISBN 3-11-005709-3 , p. 278.
2. ↑ Hydrogen peroxide is to blame for gray hair. on: Spiegel-online. Science. March 4, 2009.
3. ↑ G. Sobottka, A. Weber: Geometric and physical properties of human hair. (PDF) University of Bonn, January 2003, accessed on February 12, 2019 . 
4. ↑ Duden - The great foreign dictionary: origin and meaning of foreign words. Dudenverlag, Mannheim / Leipzig / Vienna / Zurich 2013.
5. ↑ Gary Stix: How did humanity expand? In: Spectrum of Science. Spektrumverlag, Heidelberg September 2009.
6. ↑ Christoph Drösser: Shaving useless. on: ZEIT online. Knowledge. March 25, 1998.
7. ^ Yelva L Lynfield, Peter Macwilliams: Shaving and Hair Growth . In: Journal of Investigative Dermatology . 55, 1970, pp. 170-172, doi: 10.1111 / 1523-1747.ep12280667
8. ↑ Meike Horn: Post-clipping alopecia vs. Alopecia X - facts, diagnostics and therapeutic approaches. In: veterinärspiegel issue 4 2017, pp. 135–140.
9. ↑ Jörg Ehrhardt: Pathomorphological characterization of the new hypotric mouse mutant sht / sht. (PDF; 2.6 MB) Inaugural dissertation . University of Veterinary Medicine Hannover, 1997.
10. ↑ María Victoria de Gálvez, José Aguilera, Jean-Luc Bernabó, Cristina Sánchez ‐ Roldán, Enrique Herrera ‐ Ceballos: Human Hair as a Natural Sun Protection Agent: A Quantitative Study . In: Photochemistry and Photobiology . tape 91 , no. 4 , 2015, ISSN  1751-1097 , p. 966-970 , doi : 10.1111 / php.12433 .
11. ^ Daniel W. Koon: Is polar bear hair fiber optic? In: Applied optics. 37 (15) 1998, pp. 3198-3200.