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The hair papilla, which supplies the hair root with blood, is found inside the bottom of the hair bulb. New hair cells are constantly being made in the hair bulb, close to the papilla. The unwanted loss of hair, known as alopecia, is a widespread condition affecting both sexes, occurring in numerous patterns, and classified into non-scarring and scarring (cicatricial) subtypes. The sebaceous gland produces sebum, or oil, which is the body’s natural conditioner. More sebum is produced during puberty, which is why acne is common during the teen years.
The Biology, Structure, and Function of Hair
The follicle is the primary structure from which hair can grow. The histological arrangement of the follicle is divided into outer and inner root sheaths. The follicle is lined by an inner and outer sheath that protects and molds the growing hair. The inner sheath follows the hair and ends just before the opening of the oil gland, or sebaceous gland. 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 the other great apes, is about a century old.
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The external hair is completely dead and composed entirely of keratin. Furthermore, you can cut your hair or shave without damaging the hair structure because the cut is superficial. Most chemical hair removers also act superficially; however, electrolysis and yanking both attempt to destroy the hair bulb so hair cannot grow.
Structure of Hair
Human hair grows at a rate of about 0.5 inch (13 mm) per month. 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. 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 the epidermis. The opposite actions occur when the body is too warm; the arrector muscles make the hair lie flat on the skin which allows heat to leave.
The follicle also contains the germinal matrix, where cells produce new hairs. By week 22 of pregnancy, babies already have all of their hair follicles, or openings in the skin where hair grows. This totals about five million hair follicles, with about one million on the head and 100,000 on the scalp. This is the largest number of hair follicles you will ever have. New hair cells then start to multiply at the base of the “empty” hair follicle to form a new hair, and the growth phase of the hair growth cycle starts all over again.
The glassy membrane is a thick, clear connective tissue sheath covering the hair root, connecting it to the tissue of the dermis. The full strand of hair develops from this group of hardened hair cells. Because new hardened cells keep on attaching to the hair from below, it is gradually pushed up out of the skin. In this way, a single hair on your head grows at a rate of about 1 cm per month. Facial hair, and especially eyelashes, eyebrows and body hair grows at a slower pace.
Definition of hair – Learner’s Dictionary
The catagen phase begins with the end of the anagen phase and is characterized by a transition into quiescence. During this phase, which can last a few weeks, the hair follicle undergoes apoptosis-driven regression and loses about one-sixth of its standard diameter. The formation of a club hair, an important prognostic indicator in assessing hair pathology, also occurs at this time. If many hairs form club hair at once and are subsequently shed, it can give the appearance of thinning.
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This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface. This is visible in humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as dogs and cats. Just as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft as the hair grows. The medulla forms the central core of the hair, which is surrounded by the cortex, a layer of compressed, keratinized cells that is covered by an outer layer of very hard, keratinized cells known as the cuticle. These layers are depicted in a longitudinal cross-section of the hair follicle (Figure 2), although not all hair has a medullary layer.
Some people have curly corkscrew hair, while others have thick, straight, shiny hair. A piece of hair may look simple, but it’s one of the body's most complicated structures. The hair follicle is the part below the skin, and the hair shaft is what you see above your skin. It protects your skin and traps particles like dust around your eyes and ears. If your hair gets damaged, it can renew itself without scarring. Female art students known as the Cropheads also adopted the style, notably at the Slade School in London.
The color of the hair is determined by the amount of melanin in the hardened cells. This can vary a lot from person to person, and it changes over the course of a lifetime. The amount of melanin typically decreases as people get older, and more air gets trapped inside the hair – it then loses its color and turns white.
The rate of hair loss may increase noticeably if the hair roots are damaged during the growth phase or if a lot of hairs go into the resting phase at the same time. If no new hair grows and replaces the hair, that part of the skin becomes bald. This type of hair loss is referred to as alopecia – regardless of how large the bald spot is or whether it affects the scalp or body hair. But baldness can also be permanent – one typical example is gradual hair loss in men (male pattern hair loss). Hair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example, hair on the head protects the skull from the sun.
At and following puberty, this hair is supplemented by longer, coarser, more heavily pigmented hair called terminal hair that develops in the armpits, genital regions, and, in males, on the face and sometimes on parts of the trunk and limbs. The hairs of the scalp, eyebrows, and eyelashes are of separate types from these others and develop fairly early in life. On the scalp, where hair is usually densest and longest, the average total number of hairs is between 100,000 and 150,000.
It is surrounded by the hair follicle (a sheath of skin and connective tissue), which is also connected to a sebaceous gland. Anagen growth is the active phase in which the hair follicle takes on its onion-like shape and works to produce the hair fiber. The anagen phase can be further broken down into proanagen and metanagen phases. Proanagen sees the follicle proliferating hair progenitor cells and begins the differentiation process. The new hair shaft appears on the skin's surface to mark the metanagen phase.
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,[52] 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. 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. All natural hair colors are the result of two types of hair pigments. Both of these pigments are melanin types, produced inside the hair follicle and packed into granules found in the fibers.
The hair in the nose and ears, and around the eyes (eyelashes) defends the body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into and bothering the eyes. Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin surface.
An Expert Guide to Getting Your Most Defined Curls Ever - Fashionista
An Expert Guide to Getting Your Most Defined Curls Ever.
Posted: Wed, 15 Nov 2023 08:00:00 GMT [source]
Sensory information from hair stimulation enhances tactile ability. Autonomic nervous innervation primarily provides control of the arrector pili muscle. Contraction of these tiny muscles makes the hair "stand on end." This is likely a vestigial function related to fur; erecting the shafts served to trap air, conserve heat in cold climates, and cut a larger silhouette to intimidate rivals or would-be predators. Because hairs continue to enter the resting phase and then fall out, we are constantly losing hair. A healthy adult may lose about 70 to 100 hairs on their head per day.
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