Hair

HairFollicle

 

Hair is a filamentous biomaterial 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 keratin. Attitudes towards hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person’s personal beliefs or social position, such as their age, gender, or religion.[1]

 

Overview

The word “hair” often refers to two distinct structures:

  1. the part beneath the skin, called the hair follicle or when pulled from the skin, called the bulb. This organ is located in the dermis and maintains stem cells which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound;[2]
  2. the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones.

Hair fibers have a structure consisting of several layers. Starting from the outside:

  1. the cuticle which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles,
  2. the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like; and in some cases,
  3. the medulla, a disorganized and open area at the fiber’s center.[3]

Description

Strand of human hair at 200x magnification

Each strand of hair is made up of the medulla, cortex, and cuticle.[4] The innermost region, the medulla, is not always present and is an open, unstructured region.[5][6] The highly structural and organized cortex, or middle layer of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. 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. Asian hair typically has a round fiber and is quite straight. Oval and irregularly shaped fibers are generally more wavy or even curly.[5] The cuticle 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.[4] The diameter of human hair varies from 17 to 180 micrometers (0.00067 to 0.0071 in).[7] There are two million small, tubular glands and sweat glands that produce watery fluids that cool the body by evaporation. The glands at the opening of the hair produce a fatty secretion that lubricates the hair and prevents it from dying.[8]

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”.[5] The base of the root is called the bulb, which contains the cells that produce the hair shaft.[9] Other structures of 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.[10]

 

Natural color

A girl with reddish-brown hair

All natural hair colors are the result of two types of hair pigment. Both of these pigments are melanin types, produced inside the hair follicle and packed into granules found in the fibers. Eumelanin is the dominant pigment in dark-blond, brown hair, and black hair, while pheomelanin is dominant in red hair.[5] Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin production decreases or stops, while poliosis, typically in spots is hair (and often the skin to which the hair is attached to) that never possessed melanin at all in the first place, or ceased for natural genetic reasons, generally in the first years of life.

Human hair growth

Hair grows everywhere on the external body except for mucus membranes and glabrous skin, such as that found on the palms of the hands, soles of the feet, and on the lips.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each has specific characteristics that determine the length of the hair. All three occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

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.

Eyebrows and eyelashes

Eyelashes and eyebrows help to protect the eyes from dust, dirt, and sweat

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, surprisal and excitement.[15] In many other mammals, they contain much longer, whisker-like hairs that act as tactile sensors.

The eyelash grows at 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.[16] The eye reflexively closes as a result of this sensation.

Human hairlessness

The hairlessness of humans compared to related species may be due to loss of functionality in the pseudogene KRTHAP1 (which helps produce keratin) in the human lineage about 240,000 years ago.[21] Mutations in the gene HR can lead to complete hair loss, though this is not typical in humans.[22]

In order to comprehend why humans are essentially hairless, it is essential to understand that mammalian body hair is not merely an aesthetic characteristic; it protects the skin from wounds, bites, heat, cold, and UV radiation.[23] Additionally, it can be used as a communication tool and as a camouflage.[24] To this end, it can be concluded that benefits stemming from the loss of human body hair must be great enough to outweigh the loss of these protective functions by nakedness.

Humans are the only primate species that have undergone significant hair loss and of the approximately 5000 extant species of mammal, only a handful are effectively hairless. This list includes elephants, rhinoceroses, hippopotamuses, walruses, pigs, whales and other cetaceans, and naked mole rats.[24] Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as would be experienced in Africa. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.

It was expected that dating the split of the ancestral human louse into two species, the head louse and the pubic louse, would date the loss of body hair in human ancestors. However, it turned out that the human pubic louse does not descend from the ancestral human louse, but from the gorilla louse, diverging 3.3 million years ago. This suggests that humans had lost body hair (but retained head hair) and developed thick pubic hair prior to this date, were living in or close to the forest where gorillas lived, and acquired pubic lice from butchering gorillas or sleeping in their nests.[25][26] The evolution of the body louse from the head louse, on the other hand, places the date of clothing much later, some 100,000 years ago.[27][28]

Balding, where terminal hair switches to vellus hair, usually occurs at around thirty to forty years of age. In prehistoric times, most individuals did not survive to adulthood, let alone reaching their fourth decade[29] and therefore balding tends to act as a signal of maturity. In women survival to such an advanced age is usually coupled with a decrease in a fertility (see menopause), but in men fertility is retained beyond middle-age. The persistence (but non-ubiquity) of balding in men, coupled with its general absence in women, suggests that there was a selection pressure against balding in women (perhaps in the form of a pressure against signals of advancing age), but variations in hair patterns among men did not prevent their reproductive success leading to stable polymorphisms (perhaps representing different mating strategies); for example some men could have benefitted from baldness by signalling advanced maturity and social status; while other men simulated the appearance of youth and vigor by retaining their hair.[citation needed]

The soft, fine hair found on many nonhuman mammals is typically called fur.

Most species evolved as the climate in Africa changed, to adjust their thermoregulation to the intense UV and sunlight at the equator, mostly by panting. Early hominids likely possessed fur similar to other large apes, but about 2.5 million years ago they developed a greater distribution of sweat glands[citation needed] that enabled them to perspire over most of the body. It is not clear whether the change in body hair appearance occurred before or after the development of sweat glands. Humans have eccrine sweat glands all over their bodies.[30] Aside from the mammary glands that produce a specialized sweat called milk, most mammals just have apocrine sweat glands on their armpits and loin. The rest of their body is covered in eccrine glands. There is a trend in primates to have increased eccrine sweat glands over the general surface of the body.[30] It is unclear to what degree other primates sweat in response to heat, however.

The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating on most of their body, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the arrector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection.

Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.

Several hypotheses explained hairlessness as a thermoregulatory adaptation to hot and dry savanna. The most known thermoregulatory hypothesis in modern paleoanthropology was proposed by Peter Wheeler (1984, 1985). He suggests that a need for decreased body hair originated as a response to climate change that began approximately 3 million years ago.[31][32] At this time, the earth entered a period of global cooling that had a dehumidifying effect on the main early human habitats in East and Central Africa. Lush, wooded forests gave way to dry, grassland savannah; because of this, early humans were required to travel farther in search of food and water. As early humans diverged from their chimpanzee-lineage, they also became omnivorous in order to maximize calorie intake, an important distinction in a nutrient-scarce environment. Prey, however, are moving targets, and though early humans changed the traditionally ape-like appearance of the australopithecines and adapted long, strong legs to facilitate sustained running, dense, hairy coats still posed a potentially fatal risk of causing overheating during the chase.[24] It is posited that thick hair got in the way of the sweat evaporating, so humans evolved a sparser coat of fur. Although hair provides protection against harmful UV radiation, since our hominin ancestors were bipedal, only our heads were exposed to the noonday sun.[33] Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. The rise in eccrine glands occurred on the genes that determine the fate of epidermal stem cells in human embryonic development.

Another hypothesis for the thick body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), (see types of hair and vellus hair), as well as a much larger role of testosterone in men. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of men and women. On average, men have more body hair than women. Males have more terminal hair, especially on the face, chest, abdomen, and back, and females have more vellus hair, which is less visible. The halting of hair development at a juvenile stage, vellus hair, would also be consistent with the neoteny evident in humans, especially in females, and thus they could have occurred at the same time.[34] This theory, however, has significant holdings in today’s cultural norms. There is no evidence that sexual selection would proceed to such a drastic extent over a million years ago when a full, lush coat of hair would most likely indicate health and would therefore be more likely to be selected for, not against, and not all human populations today have sexual dimorphism in body hair.

A further hypothesis is that human hair was reduced in response to ectoparasites.[35][36] The “ectoparasite” explanation of modern human nakedness is based on the principle that a hairless primate would harbor fewer parasites. When our ancestors adopted group-dwelling social arrangements roughly 1.8 mya, ectoparasite loads increased dramatically. Early humans became the only one of the 193 primate species to have fleas, which can be attributed to the close living arrangements of large groups of individuals. While primate species have communal sleeping arrangements, these groups are always on the move and thus are less likely to harbor ectoparasites. Because of this, selection pressure for early humans would favor decreasing body hair because those with thick coats would have more lethal-disease-carrying ectoparasites and would thereby have lower fitness. However, early humans were not able to compensate for the loss of warmth and protection provided by body hair with clothing, and no other mammal lost body hair to reduce parasite loads.

A final view is the naked love theory proposed by James Giles.[37][38] According to this account, human hairlessness has its origin in the ancestral mother-infant relationship. In the naked love theory our hairlessness is ultimately the result of bipedalism. Because of bipedalism, ancestral infants lost the ability to grasp the mother’s fur with their feet, as do other primate infants. They thus could no longer hold onto the mother themselves. Early bipedal mothers were therefore under evolutionary pressure to carry their infants. Consequently, infants survived only if mothers had a strong desire to hold them. Because of the pleasure of skin-to-skin contact, especially in breast-feeding, the desire to hold the infant would have been stronger in less hair-covered mothers who passed their hairlessness onto their infants. These infants would have had a greater chance of survival than did hair-covered infants. The selection process here has been called maternal selection. This theory explains why females and children have less body hair than do adult males; for it was in the mother-infant relationship that hairlessness was first selected for. Although sexual selection probably played a role later (males would have preferred hairless females who resembled their hairless mothers), the theory is the only one that does not depend solely on sexual selection to explain hairlessness. It also explains why sexual selection for naked skin would have would have started in the first place.[original research?]

Texture

Curly hair

Jablonski[39] asserts head hair 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 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[citation needed] (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 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 (1998) findings 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 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 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.

Some conversely believe that tightly coiled hair that grows into a typical Afro-like formation would have greatly reduced the ability of the head and brain to cool because although African peoples is much less dense than its European counterpart, in the intense sun the effective ‘woolly hat’ that such hair produced would have been a disadvantage. However, such anthropologists as Nina Jablonski oppositely argue about this hair texture. Specifically, Jablonski’s assertions [39] 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,[39] more likely facilitates an increase in 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 the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair).

Further, some interpret the ideas of Charles Darwin as suggesting that some traits, such as hair texture, were so arbitrary to human survival that the role natural selection played was trivial. Hence, they argue in favor of his suggestion that sexual selection may be responsible for such traits.[42] However, inclinations towards deeming hair texture “adaptively trivial” may root in certain cultural value judgments more than objective logic. In this sense the possibility that hair texture may have played an adaptively significant role cannot be completely eliminated from consideration. In fact, while the sexual selection hypothesis cannot be ruled out, the asymmetrical distribution of this trait vouches for environmental influence. Specifically, if hair texture were simply the result of adaptively arbitrary human aesthetic preferences, one would expect that the global distribution of the various hair textures would be fairly random.[dubious ] Instead, the distribution of Afro-hair is strongly skewed toward the equator. 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 the most genetically diverse continental group on Earth, Afro-textured hair approaches ubiquity in this region.[citation needed] 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.

Straight hair

Straight black hair

According to the recent single origin hypothesis, anatomically modern humans arose in East Africa approximately 200,000 years ago. Then, ~150,000 years later (i.e. around 50,000 years ago), sub-groups of this population began to expand our species’ range to regions outside of, and (later) within, this continent (Tishkoff, 1996). For those members of this group who migrated far north (i.e. to northern Eurasia, etc.), the UV light of these regions was too weak to penetrate the highly pigmented skin of the initially (relatively) dark-skinned migrants so as to provide enough vitamin D for healthy bone development.[39] Malformed bones in the pelvic area were especially deadly for women because they interfered with the successful delivery of babies, leading to the death of both the mother and the infant during labor. Hence, those with less pigmented skin survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D.[39] Thus, the skin of those in the group that left the African continent and went far north gradually developed adaptations for relatively greater translucence compared to equatorial hues. This enabled the passage of more UV light into the body at high latitudes, facilitating the natural human body-process of manufacturing vitamin D (which is essential for bone development) in response to said light.[39]

In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski’s suggestions [39] that the need for vitamin D triggered the transition from dark to pale, translucent skin among modern humans. Specifically, the distribution of this trait suggests that this need may have (initially) grown so intense at certain (early) points that those among said (initially more deeply pigmented skinned) Northern-migrants with mutations for straighter hair survived and had children at (somewhat) higher rates. This early change in texture was likely subsequently followed by the accumulation of adaptively advantageous genetic changes that led to the above-mentioned skin-translucence. This argument is made based on the principle that straight fibers better facilitate the passage of UV light into the body relative to curly hair. It is substantiated by Iyengar’s (1998) findings that UV light can pass through straight human hair roots in a manner similar to the way that light passes through fiber optic tubes (Iyengar, 1998).

Man with straight hair

Nonetheless, some argue[who?] against this stance because straighter hair ends tend to point downward while fiber optics requires that light be transmitted at a high angle to the normal of the inner reflective surface. In light of this, they suggest that only light reflected from the ground could successfully enter the hair follicle and be transmitted down the shaft. Even this process, they argue, is hindered by the curvature at the base of the hair. Therefore, coupled with the amount of skin covered by long head hair, these factors seem to militate against the adaptive usefulness of straight hair at northern latitudes. They further argue[who?] that UV light also is poorly reflected from soil and dull surfaces. These ideas can be countered by the fact that during the winter, the time of year in which UV light is most scarce at northern latitudes, the ground is often covered with white snow. Given that white is the most effective color in terms of facilitating the reflection of ground light, the hypothesis that straight hair could have been adaptively favorable, cannot be fully discounted in this regard.[citation needed] In addition, as mentioned in the previous section, straight hair also may have contributed to enhanced comfort levels in the north. This is evident in the extent to which, relative to curly hair, it tends to provide a layer of protection for ears and necks against the cold.[citation needed]

The latter hypothesis seems the more plausible evolution determinant as the surface area of the head is minute compared to the remainder of the body, thus the energy required in producing long hair for the express purpose of “optical” amplification of UV light reflected from the snow seems counterproductive (however, it’s very likely that the trait was sustained due to a nuanced combination of multiple influences, given that human hunting-skills and ingenuity were such by 50,000 years ago that said benefits in terms of ‘comfort’ could have alternatively been derived from constructing head and ear warmers of fur from prey, etc.). Scientists point to the fact that straight hair found in many ethnic groups is denser as well and has a greater ability to “show” as it does not coil, hence providing more warmth as the likely deterministic factor for the evolution of straight long hair. Some scientists argue that since the head and appendages are the greatest areas for heat loss from the body, the ability to grow long hair on the crown of the head as well as the face provides a distinct advantage in a cold climate. Since the main sensory organs are anatomically located on the head, long hair provides the necessary warmth and protection in a cold climate that allows the use of these organs by exposing them to the elements to “sense”, in for example a hunt, yet still providing necessary warmth and protection to sustain prolonged exposure. It may be argued, therefore, that the ability to grow long, straight, densely packed hair provides a distinct evolutionary advantage in cold climate; however, it would be a distinct disadvantage in a hot climate, when compared to loosely packed, spongy, closely cropped hair.

Removal practices

Depilation is the removal of hair from the surface of the skin. This can be achieved through methods such as shaving. Epilation is the removal of the entire hair strand, including the part of the hair that has not yet left the follicle. A popular way to epilate hair is through waxing.

Shaving

Many razors have multiple blades purportedly to ensure a close shave. While shaving initially will leave skin feeling smooth and hair free, new hair growth will appear a few hours after hair removal.

Shaving is accomplished with bladed instruments, such as razors. The blade is brought close to the skin and stroked over the hair in the desired area to cut the terminal hairs and leave the skin feeling smooth. Depending upon the rate of growth, one can begin to feel the hair growing back within hours of shaving. This is especially evident in men who develop a five o’clock shadow after having shaved their faces. This new growth is called stubble. Stubble typically appears to grow back thicker because the shaved hairs are blunted instead of tapered off at the end, although the hair never actually grows back thicker.

Waxing

Waxing involves using a sticky wax and strip of paper or cloth to pull hair from the root. Waxing is the ideal hair removal technique to keep an area hair-free for long periods of time. It can take five to nine weeks for waxed hair to begin to resurface again. Hair in areas that have previously been waxed also is known to grow back finer and thinner, especially compared to hair that has been shaved with a razor.

Laser Removal

Laser hair removal is a cosmetic method where a small laser beam pulses selective heat on dark target matter in the area that causes hair growth without harming the skin tissue. This process is repeated several times over the course of many months to a couple of years with hair regrowing less frequently until it finally stops; this is used as a more permanent solution to waxing or shaving. Laser removal is practiced in many clinics along with many at-home products.

Cutting and trimming

Because the hair on the head is normally longer than other types of body hair, it is cut with scissors or clippers. People with longer hair will most often use scissors to cut their hair, whereas shorter hair is maintained using a trimmer. Depending on the desired length and overall health of the hair, periods without cutting or trimming the hair can vary.

Many people will confuse what a haircut is versus what a trim is. A haircut is usually performed in order to change one’s hairstyle, while a trim helps to keep away split ends and keep the hair well-groomed.[original research?] Cutting hair tends to take off more hair than trimming hair does. When hair is trimmed, only the first few centimeters need to be removed, whereas haircuts can sometimes result in the loss of many inches of hair.

 

Afro-textured hair is a term used to refer to the natural texture of African American hair that has not been altered by hot combs, flat irons, or chemicals (through perming, relaxing, or straightening).

The contemporary term is Natural hair.

Each strand of this hair type grows in a tiny spring-like, helix shape. The overall effect is such that, despite relatively fewer hair shafts compared to straight hair,[1] Afro-textured hair appears and feels denser than its straight counterparts.

For several reasons, possibly including its relatively flat cross section,[2] this hair type conveys a dry or matte appearance.[3][4] Its unique shape makes it very prone to breakage when combed or brushed.[4]

 

 

 

 

 

 

 

 

 
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