hair
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What is hair made of?
Hair consists mainly of keratin, a fibrous structural protein that is insoluble in hot and cold water and is not easily broken down by proteolytic enzymes. Melanins, synthesized from the amino acid tyrosine, are responsible for the color of hair.
Each hair grows from a follicle, which is a pit in the skin. The hair shaft comprises a pigmented, horny, fibrous material made of impacted fibrillar cells. A cuticle covers the cortex, and a medulla containing air bubbles runs through the center of many hairs.
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Where does hair grow on the human body?
Humans are among the most hairless of all mammals, yet hair grows profusely in certain areas. Though seemingly absent on most of the body, hair is present in differing degrees on all mammals.
Hair grows longer, thicker, and more pigmented on the scalp, in the armpits (axillae), and in the pubic area during adolescence. In males, hair also appears on the face at this time.
Scalp hair is usually densest, with an average total of 100,000 to 150,000 hairs. Hair also appears inside the external ears and nostrils, as well as on the eyelids as eyelashes and eyebrows.
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What are the main parts of a single strand of hair?
A single strand of hair consists of the hair follicle and the hair shaft.
- Hair follicle The follicle is a tubular indentation in the epidermis. The epidermal components of an active hair follicle consist of an outer layer of polyhedral cells, forming the outer root sheath, and an inner horny stratum, the inner root sheath.
- Hair shaft The hair shaft is made of a pigmented, horny, fibrous material of impacted fibrillar cells. The cortex is covered by imbricated scales, which form the cuticle. Many hairs feature a medulla, which contains air bubbles.
Hair is mainly composed of keratin, a fibrous structural protein.
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What is the function of the hair follicle?
The hair follicle is a tubular sheath in the skin that surrounds the lower part of the hair shaft. It is essentially a pit in the skin from which hair grows.
The follicle has several functions:
* It nourishes the growing basal part of the hair with a vascular papilla.
* It contains sensory nerves that transmit pressure on the hair shaft.
* It manufactures hairs via cell division in the bulb region at the base of the follicle, incorporating pigment from melanocytes.
* It goes through cycles of activity, including active (anagen), transition (catagen), and resting (telogen) phases, which determine hair length and molting.
* It anchors the arrector pili muscle, which, when contracted, makes the hair stand erect and causes goosebumps.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does hair grow and what affects its growth?
Hair grows from follicles, which are pockets of epidermis that extend into the skin's depth and enclose a dermal papilla at their base. Hair develops through cell division in the follicle's bulb, incorporating pigment from melanocytes.
Follicles cycle through activity periods of growth, transition, and rest. During the active phase, hair grows for an extended time, determining its length. The activity of hair follicles isn't synchronized, leading to constant hair loss of around 50 to 100 hairs daily.
Several factors affect hair growth:
* Hormones Androgens stimulate hair growth, with male-pattern baldness occurring only when androgens are present.
* Nutrition Malnutrition and protein starvation can cause hair to become sparse and brittle, while iron deficiency may lead to hair loss.
* Drugs Cytotoxic drugs, like those used in chemotherapy, can cause acute hair loss.4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What determines the color and texture of hair?
Hair color and texture are determined by a combination of genetic and biological factors.
- Melanin The pigment melanin, produced by melanocytes in hair follicles, is a primary determinant of hair color. Variations in the amount, distribution, and type of melanin result in different hair colors.
- Genetics Genes influence hair color, and multiple genes can interact to produce a range of colors. For instance, the gene for albinism can mask the expression of other color-related genes.
- Follicle activity Hair follicles cycle through active (anagen), transitional (catagen), and resting (telogen) phases, influencing hair length and texture.
- Other factors Stem cells within hair follicles, secretions from sebaceous glands, and levels of lipids and growth factors also affect hair texture. Gray hair, for example, results from reduced melanin and lower levels of these factors, leading to coarser texture.
- Hair shape Straight hair is round, while wavy hair is oval and round. Very curly hair is shaped like twisted ribbons.
- Keratin Hair is composed of keratin, a fibrous protein that provides structure. The arrangement and properties of keratin contribute to hair's texture.
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hair, in mammals, the characteristic threadlike outgrowths of the outer layer of the skin (epidermis) that form an animalās coat, or pelage. Hair is present in differing degrees on all mammals. On adult whales, elephants, sirenians, and rhinoceroses body hair is limited to scattered bristles. In most other mammals the hair is abundant enough to form a thick coat, while humans are among the most hairless of all mammals.
The most important function of hair in mammals is that of insulating against cold by conserving body heat. The differing colours and colour patterns in hair coats can also serve purposes of camouflage and of sexual recognition and attraction among the members of a species. Specialized hairs called vibrissae, or whiskers, serve as sensory organs for certain nocturnal animals. The specially modified hairs of the porcupine are called quills and serve defensive purposes.
Human beings have several different types of hairs. The first to develop is the lanugo, a layer of downy, slender hairs that begin growing in the third or fourth month of fetal life and are entirely shed either before or shortly after birth. During the first few months of infancy there grow fine, short, unpigmented hairs called down hair, or vellus. Vellus covers every part of the body except the palms of the hands, the soles of the feet, undersurfaces of the fingers and toes, and a few other places. 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. Human hair grows at a rate of about 0.5 inch (13 mm) per month.
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The typical mammalian hair consists of the shaft, protruding above the skin, and the root, which is sunk in a pit (follicle) beneath the skin surface. Except for a few growing cells at the base of the root, the hair is dead tissue, composed of keratin and related proteins. The hair follicle is a tubelike pocket of the epidermis that encloses a small section of the dermis at its base. The human hair is formed by divisions of cells at the base of the follicle. As the cells are pushed upward from the follicleās base, they become keratinized (hardened) and undergo pigmentation.
Hair is continually shed and renewed by the operation of alternating cycles of growth, rest, fallout, and renewed growth. The average life of different varieties of hair varies from about four months for downy hairs to three to five years for long scalp hairs. Each human follicle follows this cycle independently of others, so the total amount of hair remains constant; some animalsā hair follicles have synchronous cycles, causing periodic shedding, or molts.