In biology , a protein filament is a long chain of protein monomers, such as those found in hair, muscle, or in flagella . Protein filaments form together to make the cytoskeleton of the cell. They are often bundled together to provide support, strength, and rigidity to the cell. When the filaments are packed up together, they are able to form three different cellular parts. The three major classes of protein filaments that make up the cytoskeleton include: actin filaments , microtubules and intermediate filaments .
95-616: Hair is a protein filament that grows from follicles found in the dermis . Hair is one of the defining characteristics of mammals . The human body , apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair . Most common interest in hair is focused on hair growth , hair types, and hair care , but hair is also an important biomaterial primarily composed of protein , notably alpha-keratin . Attitudes towards different forms of hair, such as hairstyles and hair removal , vary widely across different cultures and historical periods, but it
190-539: A brown-haired person has 110,000, a black-haired person has 100,000, and a redhead has 90,000. Hair growth stops after a human's death. Visible growth of hair on the dead body happens only because of skin drying out due to water loss. The world record for longest hair on a living person stands with Smita Srivastava of Uttar Pradesh, India . At 7 feet and 9 inches long, she broke a Guinness World Record in November 2023, having grown her hair for 32 years. Hair exists in
285-913: A cotton thread, but is not stiff or rough. It is neither fine nor coarse. Thick strands whose shed strands usually are easily identified. Coarse hair feels hard and wiry. Hair is mainly composed of keratin proteins and keratin-associated proteins (KRTAPs). The human genome encodes 54 different keratin proteins which are present in various amounts in hair. Similarly, humans encode more than 100 different KRTAPs which crosslink keratins in hair. The content of KRTAPs ranges from less than 3% in human hair to 30–40% in echidna quill. Many mammals have fur and other hairs that serve different functions. Hair provides thermal regulation and camouflage for many animals; for others it provides signals to other animals such as warnings, mating, or other communicative displays; and for some animals hair provides defensive functions and, rarely, even offensive protection. Hair also has
380-429: A different from these other two forms of orientation. In an axon nerve cell, microtubules will arrange with their negatively charged end toward the cell body and positively charged end away from the cell body. However, in dendrites, microtubules can have a different orientation. In dendrites , microtubules can have their positively charged end toward the cell body, but their negatively charged end will likely be away from
475-434: A flat cross-section, while people of mostly European or Middle Eastern ancestry tend to have hair with a diameter of 70–100 μm and an oval cross-section, and people of mostly Asian or Native American ancestry tend to have hair with a diameter of 90–120 μm and a round cross-section. There are roughly two million small, tubular glands and sweat glands that produce watery fluids that cool the body by evaporation. The glands at
570-419: A protein called tubulin. The tubulin consists of dimers, named either "αβ-tubulin" or "tubulin dimers", which polymerize to form the microtubules. These microtubules are structurally quantified into three main groups: singlets, doublets, and triplets. Singlets are microtubule structures that are known to be found in the cytoplasm . Doublets are structures found in the cilia and flagella . Triplets are found in
665-404: A role in cell communication in a process known as crosstalk. This cross talk has the potential to help with the mechanosensing. This mechanosensing can help protect the cell during cellular migration within the body. They can also help with the linkage of actin and microtubules to the cytoskeleton which will lead to the eventual movement and division of cells. Lastly these intermediate filaments have
760-447: A sensory function, extending the sense of touch beyond the surface of the skin. Guard hairs give warnings that may trigger a recoiling reaction. While humans have developed clothing and other means of keeping warm, the hair found on the head serves primarily as a source of heat insulation and cooling (when sweat evaporates from soaked hair) as well as protection from ultra-violet radiation exposure. The function of hair in other locations
855-418: A significant role in the organization of organelles and vesicles, beating of cilia and flagella, nerve and red blood cell structure, and alignment/ separation of chromosomes during mitosis and meiosis. When a cell is in the interphase process, microtubules tend to all orient the same way. Their negatively charged end will be close to the nucleus of the cell, while their positively end will be oriented away from
950-474: A similar composition to human sebaceous glands, are androgen responsive, and have been used as a basis for study. Some species of bat, including the Mexican free-tailed , have a specialized sebaceous gland occurring on the throat called a "gular gland". This gland is present more frequently in males than females, and it is hypothesized that the secretions of the gland are used for scent-marking. Sebaceous adenitis
1045-429: A variety of cells which include smooth muscle cells, fibroblasts, and white blood cells. Type 4 intermediate filaments are the neurofilaments found in neurons. They can be found in many different motor axons supporting these cells. Type 5 intermediate filaments are composed of nuclear lamins which can be found in the nuclear envelope of many eukaryotic cells. They will help to assemble an orthogonal network in these cells in
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#17328522723041140-400: A variety of textures. Three main aspects of hair texture are the curl pattern, volume, and consistency. All mammalian hair is composed of keratin , so the make-up of hair follicles is not the source of varying hair patterns. There are a range of theories pertaining to the curl patterns of hair. Scientists have come to believe that the shape of the hair shaft has an effect on the curliness of
1235-423: Is a method which classifies the hair by curl pattern, hair-strand thickness and overall hair volume. Curliness Strands Thin strands that sometimes are almost translucent when held up to the light. Shed strands can be hard to see even against a contrasting background. Fine hair is difficult to feel or it feels like an ultra-fine strand of silk. Strands are neither fine nor coarse. Medium hair feels like
1330-486: Is a trait that may be associated with neoteny . Primates are relatively hairless compared to other mammals, and Hominini such as chimpanzees, have less dense hair than would be expected given their body size for a primate. Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2 million years ago. Part of this evolution was the development of endurance running and venturing out during
1425-404: Is an epidermal invagination known as a pilosebaceous unit . Sebaceous glands are also found in hairless areas ( glabrous skin ) of the eyelids , nose , penis , labia minora , the inner mucosal membrane of the cheek , and nipples . Some sebaceous glands have unique names. Sebaceous glands on the lip and mucosa of the cheek, and on the genitalia, are known as Fordyce spots , and glands on
1520-435: Is cognate with terms such as Old Norse and Norwegian : fax . Each strand of hair is made up of the medulla , cortex , and cuticle . The innermost region, the medulla , is an open and unstructured region that is not always present. The highly structural and organized cortex , or second of three layers of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin , which colors
1615-422: Is composed of three bands and one disk. The A band is the part of the actin that will bind to the myosin during muscle contraction. The I band is the part of the actin that is not bound to the myosin, but it will still move during muscle contraction. The H zone is the space in between two adjacent actin that will shrink when the muscle begins to contract. The Z disk is the part of the microfilament that characterizes
1710-419: Is debated. Hats and coats are still required while doing outdoor activities in cold weather to prevent frostbite and hypothermia , but the hair on the human body does help to keep the internal temperature regulated. When the body is too cold, the arrector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. These hairs then form a heat-trapping layer above
1805-467: Is interpreted in the neocortex , a section of the brain that expanded markedly in animals like Morganucodon and Hadrocodium . The more advanced therapsids could have had a combination of naked skin, whiskers , and scutes . A full pelage likely did not evolve until the therapsid-mammal transition. The more advanced, smaller therapsids could have had a combination of hair and scutes, a combination still found in some modern mammals, such as rodents and
1900-475: 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. Thicker hair (>60 μm) grows generally faster (11.4 mm per month) than thinner (20-30 μm) hair (7.6 mm per month). It was previously thought that Caucasian hair grew more quickly than Asian hair and that
1995-455: Is not just one method to discovering one's hair type. Additionally it is possible, and quite normal to have more than one kind of hair type, for instance having a mixture of both type 3a & 3b curls. The Andre Walker Hair Typing System is the most widely used system to classify hair. The system was created by Oprah Winfrey 's hairstylist, Andre Walker . According to this system there are four types of hair: straight, wavy, curly, kinky. This
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#17328522723042090-447: Is often used to indicate a person's personal beliefs or social position, such as their age, gender , or religion . The word "hair" usually refers to two distinct structures: Hair fibers have a structure consisting of several layers, starting from the outside: The word "hair" is derived from Middle English : heer and hêr , in turn derived from Old English : hǽr and hér , with influence from Old Norse : hár . Both
2185-427: Is partly composed of sebum produced by glands in the ear canal. These secretions are viscous and have a high lipid content, which provides good lubrication. Sebaceous glands are involved in skin problems such as acne and keratosis pilaris . In the skin pores, sebum and keratin can create a hyperkeratotic plug called a comedo . Acne is a common occurrence, particularly during puberty in teenagers , and
2280-407: Is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water. The diameter of human hair varies from 0.017 to 0.18 millimeters (0.00067 to 0.00709 in). Some of these characteristics in humans' head hair vary by race: people of mostly African ancestry tend to have hair with a diameter of 60–90 μm and
2375-430: Is the steady state. At this state the addition of monomers will equal the subtraction of monomers causing the microfilament to no longer grow. This is known as the critical concentration of actin. There are several toxins that have been known to limit the polymerization of actin. Cytochalasin is a toxin that will bind to the actin polymer, so it can no longer bind to the incoming actin monomers. Actin originally attached in
2470-445: Is thought to relate to an increased production of sebum due to hormonal factors. The increased production of sebum can lead to a blockage of the sebaceous gland duct. This can cause a comedo (commonly called a blackhead or a whitehead ), which can lead to infection, particularly by the bacteria Cutibacterium acnes . This can inflame the comedones, which then change into the characteristic acne lesions. Comedones generally occur on
2565-413: Is very mild, balsamic, and intended to wet and lubricate the mouth". In The Principles of Physiology 1834, Andrew Combe noted that the glands were not present in the palms of the hands or soles of the feet. The preputial glands of mice and rats are large modified sebaceous glands that produce pheromones used for territorial marking. These and the scent glands in the flanks of hamsters have
2660-544: The Upper Permian , shows smooth, hairless skin with what appears to be glandular depressions, though as a semi-aquatic species it might not have been particularly useful to determine the integument of terrestrial species. The oldest undisputed known fossils showing unambiguous imprints of hair are the Callovian (late middle Jurassic ) Castorocauda and several contemporary haramiyidans , both near-mammal cynodonts , giving
2755-412: The alkaline nature of contaminants. Sebaceous lipids help maintain the integrity of the skin barrier and supply vitamin E to the skin. During the last three months of fetal development , the sebaceous glands of the fetus produce vernix caseosa , a waxy white substance that coats the skin to protect it from amniotic fluid . The areolar glands are in the areola that surrounds the nipple in
2850-440: 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
2945-408: The epidermis called the hair follicle . The bulb of hair consists of fibrous connective tissue, glassy membrane, external root sheath, internal root sheath composed of epithelium stratum ( Henle's layer ) and granular stratum ( Huxley's layer ), cuticle, cortex and medulla. All natural hair colors are the result of two types of hair pigments. Both of these pigments are melanin types, produced inside
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3040-516: The epidermis . This process is formally called piloerection , derived from the Latin words 'pilus' ('hair') and 'erectio' ('rising up'), but is more commonly known as 'having goose bumps ' in English. This is more effective in other mammals whose fur fluffs up to create air pockets between hairs that insulate the body from the cold. The opposite actions occur when the body is too warm; the arrector muscles make
3135-417: The fiber based on the number, distribution and types of melanin granules. The melanin may be evenly spaced or cluster around the edges of the hair. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. People with straight hair have round hair fibers. Oval and other shaped fibers are generally more wavy or curly. The cuticle
3230-420: The hygiene and cosmetology of hair including hair on the scalp , facial hair ( beard and moustache ), pubic hair 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. Protein filament Compared to
3325-456: The lips , gums and inner cheeks , and genitals . In humans, sebaceous glands are found throughout all areas of the skin, except the palms of the hands and soles of the feet . There are two types of sebaceous glands: those connected to hair follicles and those that exist independently. Sebaceous glands are found in hair-covered areas, where they are connected to hair follicles . One or more glands may surround each hair follicle, and
3420-403: The opossum . The high interspecific variability of the size, color, and microstructure of hair often enables the identification of species based on single hair filaments. In varying degrees most mammals have some skin areas without natural hair. On the human body, glabrous skin is found on the ventral portion of the fingers , palms , soles of feet and lips , which are all parts of
3515-458: The EDAR locus, a region of the modern human genome that contributes to hair texture variation among most individuals of East Asian descent, support the hypothesis that (East Asian) straight hair likely developed in this branch of the modern human lineage subsequent to the original expression of tightly coiled natural afro-hair . Specifically, the relevant findings indicate that the EDAR mutation coding for
3610-459: The Old English and Old Norse words derive from Proto-Germanic : * hēran and are related to terms for hair in other Germanic languages such as Swedish : här , Dutch and German : haar , and Old Frisian : her . The now broadly obsolete word "fax" refers specifically to head hair and is found in compounds such as Fairfax and Halifax . It is derived from Old English : feax and
3705-575: The ability to help with vascular permeability through organizing continuous adherens junctions through plectin cross-linking. Intermediate filaments are composed of several proteins unlike microfilaments and microtubules which are composed of primarily actin and tubulin. These proteins have been classified into 6 major categories based on their similar characteristics. Type 1 and 2 intermediate filaments are those that are composed of keratins, and they are mainly found in epithelial cells. Type 3 intermediate filaments contain vimentin. They can be found in
3800-414: The actin limiting muscle contraction. Titin is another protein, but it binds to the myosin rather than the actin microfilament. Titin will help stabilize the contraction and myosin-actin structure. Microtubules are the largest type of filament, with a diameter of 25 nm wide, in the cytoskeleton. A single microtubule consists of 13 linear microfilaments. Unlike microfilaments, microtubules are composed of
3895-441: The age as no later than ≈220 ma based on the modern phylogenetic understanding of these clades. More recently, studies on terminal Permian Russian coprolites may suggest that non-mammalian synapsids from that era had fur. If this is the case, these are the oldest hair remnants known, showcasing that fur occurred as far back as the latest Paleozoic . Some modern mammals have a special gland in front of each orbit used to preen
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3990-524: The amount of sebum produced by the sebaceous glands. Should the usual treatments fail, the presence of the Demodex mite could be looked for as the possible cause. Other conditions that involve the sebaceous glands include: The word sebaceous , meaning 'consisting of sebum', was first termed in 1728 and comes from the Latin for 'tallow'. Sebaceous glands have been documented since at least 1746 by Jean Astruc , who defined them as "...the glands which separate
4085-504: The areas with more sebaceous glands, particularly the face, shoulders, upper chest and back. Comedones may be "black" or "white" depending on whether the entire pilosebaceous unit, or just the sebaceous duct, is blocked. Sebaceous filaments —innocuous build-ups of sebum—are often mistaken for whiteheads . There are many treatments available for acne from reducing sugars in the diet, to medications that include antibiotics, benzoyl peroxide, retinoids, and hormonal treatments. Retinoids reduce
4180-409: The basal bodies and centrioles. There are two main populations of these microtubules. There are unstable short-lived microtubules that will assemble and disassemble rapidly. The other population are stable long-lived microtubules. These microtubules will remain polymerized for longer periods of time and can be found in flagella, red blood cells, and nerve cells. Microtubules have the ability to play
4275-413: The belly and lower tail of a pelycosaur , possibly Haptodus shows the basal synapsid stock bore transverse rows of rectangular scutes , similar to those of a modern crocodile , so the age of acquirement of hair logically could not have been earlier than ≈299 ma, based on the current understanding of the animal's phylogeny. An exceptionally well-preserved skull of Estemmenosuchus , a therapsid from
4370-490: The body most closely associated with interacting with the world around us, as are the labia minora and glans penis . There are four main types of mechanoreceptors in the glabrous skin of humans: Pacinian corpuscles , Meissner's corpuscles , Merkel's discs , and Ruffini corpuscles . The naked mole-rat ( Heterocephalus glaber ) has evolved skin lacking in general, pelagic hair covering, yet has retained long, very sparsely scattered tactile hairs over its body. Glabrousness
4465-423: The body. Sapienic acid is a sebum fatty acid that is unique to humans, and is implicated in the development of acne. Sebum is odorless, but its breakdown by bacteria can produce strong odors. Sex hormones are known to affect the rate of sebum secretion; androgens such as testosterone have been shown to stimulate secretion, and estrogens have been shown to inhibit secretion. Dihydrotestosterone acts as
4560-439: The cell body. Colchicine is an example of a drug that has been known to be used as a microtubule inhibitor. It binds to both the α and β tubulin on dimers in microtubules. At low concentrations this can cause stabilization of microtubules, but at high concentrations it can lead to depolymerization of microtubules. Taxol is another drug often times used to help treat breast cancer through targeting microtubules. Taxol binds to
4655-552: The cell body. The basal body found within the cell helps the microtubule to orient in this specific fashion. In mitotic cells, they will see similar orientation as the positively charged end will be orientated away from the cell while the negatively charged end will be towards the Microtubule Organizing Center (MTOC). The positive end of these microtubules will attach to the kinetochore on the chromosome allowing for cellular division when applicable. Nerve cells tend to be
4750-419: The cellular cortex they can help with the structural integrity of the cell. Microfilament polymerization is divided into three steps. The nucleation step is the first step, and it is the rate limiting and slowest step of the process. Elongation is the next step in this process, and it is the rapid addition of actin monomers at both the plus and minus end of the microfilament. The final step
4845-411: The cellular division process, the overall microtubule length will not change. It will however produce a tread-milling effect that can cause the separation of these chromosomes. Intermediate filaments are part of the cytoskeleton structure found in most eukaryotic cells. An example of an intermediate filament is a Neurofilament . They provide support for the structure of the axon and are a major part of
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#17328522723044940-425: The circulation of cool air onto the scalp. Further, wet Afro-hair does not stick to the neck and scalp unless totally drenched and instead tends to retain its basic springy puffiness because it less easily responds to moisture and sweat than straight hair does. In this sense, the trait may enhance comfort levels in intense equatorial climates more than straight hair (which, on the other hand, tends to naturally fall over
5035-450: The condition of the strand. Fine hair has the smallest circumference, coarse hair has the largest circumference, and medium hair is anywhere between the other two. Coarse hair has a more open cuticle than thin or medium hair causing it to be the most porous. There are various systems that people use to classify their curl patterns. Being knowledgeable of an individual's hair type is a good start to knowing how to take care of one's hair. There
5130-430: The cytoskeleton. Intermediate filaments contain an average diameter of 10 nm, which is smaller than that of microtubules, but larger than that of microfilaments. These 10 nm filaments are made up of polypeptide chains, which belong to the same family as intermediate filaments. Intermediate filaments are not involved with the direct movement of cells unlike microtubules and microfilaments. Intermediate filaments can play
5225-431: The depolymerization of the actin polymer in the cell. There are several different proteins that interact with actin in the body. However, one of the most famous types of motor proteins is myosin . Myosin will bind to these actins causing the movement of actin. This movement of myosin along the microfilament can cause muscle contraction, membrane association, endocytosis , and organelle transport. The actin microfilament
5320-506: The ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair). Further, it is notable that the most pervasive expression of this hair texture can be found in sub-Saharan Africa; a region of the world that abundant genetic and paleo-anthropological evidence suggests, was the relatively recent (≈200,000-year-old) point of origin for modern humanity. In fact, although genetic findings (Tishkoff, 2009) suggest that sub-Saharan Africans are
5415-456: The edges of the eyelid and protects the eye from dirt. The eyelash is to humans, camels, horses, ostriches etc., what whiskers are to cats ; they are used to sense when dirt, dust , or any other potentially harmful object is too close to the eye. The eye reflexively closes as a result of this sensation . Eyebrows and eyelashes do not grow beyond a certain length (eyelashes are rarely more than 10 mm long). However, trichomegaly can cause
5510-558: The equator, straight hair may have (initially) evolved to aid the entry of UV light into the body during the transition from dark, UV-protected skin to paler skin. Jablonski's assertions suggest that the adjective "woolly" in reference to Afro-hair is a misnomer in connoting the high heat insulation derivable from the true wool of sheep. Instead, the relatively sparse density of Afro-hair, combined with its springy coils actually results in an airy, almost sponge-like structure that in turn, Jablonski argues, more likely facilitates an increase in
5605-413: The eyelids are known as meibomian glands . Sebaceous glands of the breast are also known as Montgomery's glands . Sebaceous glands are first visible from the 13th to the 16th week of fetal development , as bulgings off hair follicles. Sebaceous glands develop from the same tissue that gives rise to the epidermis of the skin. Overexpression of the signalling factors Wnt , Myc and SHH all increase
5700-407: The eyes are closed, and their lipid quality also prevents the eyelids from sticking together. They attach directly to the follicles of the eyelashes, which are arranged vertically within the tarsal plates of the eyelids. Fordyce spots , or Fordyce granules, are ectopic sebaceous glands found on the genitals and oral mucosa . They show themselves as yellowish-white milia (milk spots). Earwax
5795-432: The fat." He describes them in the oral cavity and on the head , eyelids , and ears , as "universally" acknowledged. Astruc describes them being blocked by "small animals" that are "implanted" in the excretory ducts and attributes their presence in the oral cavity to apthous ulcers , noting that "these glands naturally [secrete] a viscous humour, which puts on various colours and consistencies... in its natural state
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#17328522723045890-465: The female breast. These glands secrete an oily fluid that lubricates the nipple, and also secrete volatile compounds that are thought to serve as an olfactory stimulus for the newborn. During pregnancy and lactation these glands, also called Montgomery's glands, become enlarged. Meibomian glands , in the eyelids , secrete a form of sebum called meibum onto the eye , that slows the evaporation of tears . They also serve to create an airtight seal when
5985-491: The first years of life. Hair grows everywhere on the external body except for mucous membranes and glabrous skin, such as that found on the palms of the hands, soles of the feet, and lips. 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. The three stages of hair growth are
6080-424: The fur, called the harderian gland . Imprints of this structure are found in the skull of the small early mammals like Morganucodon , but not in their cynodont ancestors like Thrinaxodon . The hairs of the fur in modern animals are all connected to nerves, and so the fur also serves as a transmitter for sensory input. Fur could have evolved from sensory hair (whiskers). The signals from this sensory apparatus
6175-402: The glands themselves are surrounded by arrector pili muscles , forming a pilosebaceous unit. The glands have an acinar structure (like a many-lobed berry), in which multiple glands branch off a central duct. The glands deposit sebum on the hairs and bring it to the skin surface along the hair shaft . The structure, consisting of hair, hair follicles, arrector pili muscles, and sebaceous glands,
6270-469: 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 nipples , areolar glands are specialized sebaceous glands for lubricating the nipples. Fordyce spots are benign, visible, sebaceous glands found usually on
6365-415: The growth rate of women's hair was faster than that of men. However, more recent research has shown that the growth rate of hair in men and women does not significantly differ and that the hair of Chinese people grew more quickly than the hair of French Caucasians and West and Central Africans. The quantity of hair hovers in a certain range depending on hair colour. An average blonde person has 150,000 hairs,
6460-649: The hair and skin, rain is effectively repelled. Sebum is produced in a holocrine process , in which cells within the sebaceous gland rupture and disintegrate as they release the sebum and the cell remnants are secreted together with the sebum. The cells are constantly replaced by mitosis at the base of the duct. Sebum is secreted by the sebaceous gland in humans. It is primarily composed of triglycerides (≈41%), wax esters (≈26%), squalene (≈12%), and free fatty acids (≈16%). The composition of sebum varies across species. Wax esters and squalene are unique to sebum and not produced as final products anywhere else in
6555-514: The hair follicle and packed into granules found in the fibers. Eumelanin is the dominant pigment in brown hair and black hair , while pheomelanin is dominant in red hair . Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin production decreases or stops, while poliosis is white hair (and often the skin to which the hair is attached), typically in spots that never possessed melanin at all, or ceased for natural reasons, generally genetic, in
6650-415: The hair follicle include the oil producing sebaceous gland which lubricates the hair and the arrector pili muscles, which are responsible for causing hairs to stand up. In humans with little body hair, the effect results in goose bumps . The root of the hair ends in an enlargement, the hair bulb , which is whiter in color and softer in texture than the shaft and is lodged in a follicular involution of
6745-423: The hair follicle shape determines curl pattern, the hair follicle size determines thickness. While the circumference of the hair follicle expands, so does the thickness of the hair follicle. An individual's hair volume, as a result, can be thin, normal, or thick. The consistency of hair can almost always be grouped into three categories: fine, medium, and coarse. This trait is determined by the hair follicle volume and
6840-430: The hair follicle, sebaceous glands are composed of huge cells with many large vesicles that contain the sebum. These cells express Na and Cl ion channels, ENaC and CFTR (see Fig. 6 and Fig. 7 in reference ). Sebaceous glands secrete the oily, waxy substance called sebum ( Latin : fat, tallow ) that is made of triglycerides , wax esters , squalene , and metabolites of fat-producing cells. Sebum lubricates
6935-520: The hair lie flat on the skin which allows heat to leave. In some mammals, such as hedgehogs and porcupines , the hairs have been modified into hard spines or quills. These are covered with thick plates of keratin and serve as protection against predators. Thick hair such as that of the lion's mane and grizzly bear's fur do offer some protection from physical damages such as bites and scratches. Displacement and vibration of hair shafts are detected by hair follicle nerve receptors and nerve receptors within
7030-499: The hot times of the day that required efficient thermoregulation through perspiration . The loss of heat through heat of evaporation by means of sweat glands is aided by air currents next to the skin surface, which are facilitated by the loss of body hair. Another factor in human evolution that also occurred in the prehistoric past was a preferential selection for neoteny , particularly in females. The idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in
7125-436: The individual's hair. A very round shaft allows for fewer disulfide bonds to be present in the hair strand. This means the bonds present are directly in line with one another, resulting in straight hair. The flatter the hair shaft becomes, the curlier hair gets, because the shape allows more cysteines to become compacted together resulting in a bent shape that, with every additional disulfide bond, becomes curlier in form. As
7220-429: The lashes to grow remarkably long and prominent (in some cases the upper lashes grow to 15 mm long). Hair has its origins in the common ancestor of mammals, the synapsids , about 300 million years ago. It is currently unknown at what stage the synapsids acquired mammalian characteristics such as body hair and mammary glands , as the fossils only rarely provide direct evidence for soft tissues. Skin impression of
7315-449: The likelihood of sebaceous gland presence. The sebaceous glands of a human fetus secrete a substance called vernix caseosa , a waxy, translucent white substance coating the skin of newborns . After birth, activity of the glands decreases until there is almost no activity during ages two–six years, and then increases to a peak of activity during puberty , due to heightened levels of androgens . Relative to keratinocytes that make up
7410-410: The middle of the cell and overlap the equator where the cell is dividing. Kinetochore microtubules will extend and bind to the kinetochore on the chromosomes assisting in the division of a cell. These microtubules will attach to the kinetochore at their positive end. NDC80 is a protein found at this binding point that will help with the stabilization of this interaction during cellular division. During
7505-510: The most genetically diverse continental group on Earth, Afro-textured hair approaches ubiquity in this region. This points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-Saharan groups, left little room for genetic variation at the determining loci. Such a pattern, again, does not seem to support human sexual aesthetics as being the sole or primary cause of this distribution. A group of studies have recently shown that genetic patterns at
7600-606: The most successful species on the planet (and which also is very vulnerable at birth) was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). Sometime during the gradual process by which Homo erectus began a transition from furry skin to the naked skin expressed by Homo sapiens , hair texture putatively gradually changed from straight hair (the condition of most mammals, including humanity's closest cousins—chimpanzees) to Afro-textured hair or 'kinky' (i.e. tightly coiled). This argument assumes that curly hair better impedes
7695-427: The nuclear membrane. Type 6 intermediate filaments are involved with nestin that interact with the stem cells of central nervous system. Sebaceous gland 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
7790-404: The opening of the hair produce a fatty secretion that lubricates the hair. Hair growth begins inside the hair follicle . The only "living" portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered "dead". The base of a hair's root (the "bulb") contains the cells that produce the hair shaft. Other structures of
7885-591: The other great apes, is about a century old. Louis Bolk made a long list of such traits, and Stephen Jay Gould published a short list in Ontogeny and Phylogeny . In addition, paedomorphic characteristics in women are often acknowledged as desirable by men in developed countries. For instance, vellus hair is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation , women do not, leaving their vellus hair visible. Jablonski asserts head hair
7980-461: The other parts of the cytoskeletons, the microfilaments contain the thinnest filaments, with a diameter of approximately 7 nm. Microfilaments are part of the cytoskeleton that are composed of protein called actin . Two strands of actin intertwined together form a filamentous structure allowing for the movement of motor proteins. Microfilaments can either occur in the monomeric G-actin or filamentous F-actin. Microfilaments are important when it comes to
8075-438: The overall end of each side of the sarcomere , a structural unit of a myofibril . These microfilaments have the potential to be limited by several factors or proteins. Tropomodulin is a protein that will cap the ends of the actin filaments causing the overall stability of the structure. Nebulin is another protein that can bind to the sides of the actin preventing the attachment of myosin to them. This causes stabilization of
8170-418: The overall organization of the plasma membrane. Actin filaments are considered to be both helical and flexible. They are composed of several actin monomers chained together which add to their flexibility. They are found in several places in the body including the microvilli, contractile rings, stress fibers, cellular cortex, etc. In a contractile ring, actin have the ability to help with cellular division while in
8265-423: The passage of UV light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for light-skinned hominids living at the equator). It is substantiated by Iyengar's findings (1998) that UV light can enter into straight human hair roots (and thus into the body through the skin) via the hair shaft. Specifically, the results of that study suggest that this phenomenon resembles
8360-426: The passage of light through fiber optic tubes (which do not function as effectively when kinked or sharply curved or coiled). In this sense, when hominids (i.e. Homo erectus ) were gradually losing their straight body hair and thereby exposing the initially pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. By inverse logic, later, as humans traveled farther from Africa and/or
8455-415: The polymer is still leaving the microfilament causing depolymerization. Phalloidin is a toxin that will bind to actin locking the filament in place. Monomers are neither adding or leaving this polymer which causes the stabilization of the molecule. Latrunculin is similar to cytochalasin, but it is a toxin which will bind to the actin monomers preventing it from adding onto the actin polymer. This will cause
8550-610: The predominant East Asian 'coarse' or thick, straight hair texture arose within the past ≈65,000 years, which is a time frame that covers from the earliest of the 'Out of Africa' migrations up to now. Ringworm is a fungal disease that targets hairy skin. Premature greying of hair is another condition that results in greying before the age of 20 years in Europeans, before 25 years in Asians, and before 30 years in Africans. Hair care involves
8645-459: The primary androgen in the prostate and in hair follicles. Sebaceous glands are part of the body's integumentary system and serve to protect the body against microorganisms. Sebaceous glands secrete acids that form the acid mantle . This is a thin, slightly acidic film on the surface of the skin that acts as a barrier to microbes that might penetrate the skin. The pH of the skin is between 4.5 and 6.2, an acidity that helps to neutralize
8740-448: The side of a tubule and can lead to disruption in cell division. There are three main type of microtubules involved with cellular division . Astral microtubules are those extending out of the centrosome toward the cell cortex. They can connect to the plasma membrane via cortical landmark deposits. These deposits are determined via polarity cues, growth and differentiation factors, or adhesion contacts. Polar microtubules will extend toward
8835-432: The skin and hair of mammals. Sebaceous secretions in conjunction with apocrine glands also play an important thermoregulatory role. In hot conditions, the secretions emulsify the sweat produced by the eccrine sweat glands and this produces a sheet of sweat that is not readily lost in drops of sweat. This is of importance in delaying dehydration. In colder conditions, the nature of sebum becomes more lipid, and in coating
8930-597: The skin. Hairs can sense movements of air as well as touch by physical objects and they provide sensory awareness of the presence of ectoparasites . Some hairs, such as eyelashes , are especially sensitive to the presence of potentially harmful matter. The eyebrows provide moderate protection to the eyes from dirt , sweat and rain . They also play a key role in non-verbal communication by displaying emotions such as sadness, anger, surprise and excitement. In many other mammals, they contain much longer, whisker-like hairs that act as tactile sensors. The eyelash grows at
9025-410: Was evolutionarily advantageous for pre-humans to retain because it protected the scalp as they walked upright in the intense African (equatorial) UV light . While some might argue that, by this logic, humans should also express hairy shoulders because these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of
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