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Encyclopedia > Hair follicle

A hair follicle is part of the skin that grows hair by packing old cells together. Attached to the follicle is a sebaceous gland, a tiny sebum-producing gland found everywhere except on the palms, lips and soles of the feet. The thicker density of hair, the more sebaceous glands are found. Image File history File links HairFollicle. ... In zootomy and dermatology, skin is an organ of the integumentary system made up of multiple layers of epithelial tissues that guard underlying muscles and organs. ... Young Girl Fixing her Hair, by Sophie Gengembre Anderson Hair is a filamentous outgrowth of dead cells from the skin, found mainly in mammals. ... Cells in culture, stained for keratin (red) and DNA (green). ... Schematic view of a hair follicle with sebaceous gland. ... The sebaceous glands are glands found in the skin of mammals. ... A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... // The hands (med. ... Look up Sole in Wiktionary, the free dictionary. ... Density (symbol: ρ - Greek: rho) is a measure of mass per volume. ...


At the base of the follicle is a small structure that is called the papilla. The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non-existent. Around the papilla is the hair matrix, a collection of epithelial cells often interspersed with melanocytes. Cell division in the hair matrix is responsible for the cells that will form the major structures of the hair fibre and the inner root sheath. The hair matrix epithelium is one of the fastest growing cell populations in the human body, which is why some forms of chemotherapy that kill dividing cells or radiotherapy may lead to temporary hair loss, by their action on this rapidly dividing cell population. The papilla is usually ovoid or pear shaped with the matrix wrapped completely around it except for a short stalk-like connection to the surrounding connective tissue that provides access for the capillary. A papilla (plural: papillae) can be: A small projection, such as a nipplelike projection on the skin, at the base of a hair or the root of a feather; the base of a new tooth. ... In the human body there are four types of tissue: (1) Epithelial, (2) Connective, (3) Muscle, and (4) Nervous Tissue. ... In zootomy, epithelium is a tissue composed of a layer of cells. ... Melanocytes are cells located in the bottom layer of the skins epidermis. ... Chemotherapy is the use of chemical substances to treat disease. ... Radiation therapy (or radiotherapy) is the medical use of ionizing radiation as part of cancer treatment to control malignant cells (not to be confused with radiology, the use of radiation in medical imaging and diagnosis). ...


Also attached to the follicle is a tiny bundle of muscle fiber called the arrector pili that is responsible for causing the follicle and hair to become more perpendicular to the surface of the skin, and causing the follicle to protrude slightly above the surrounding skin. This process results in goose bumps (or goose flesh). Stem cells are located at the junction of the arrector and the follicle, and are principally responsible for the ongoing hair production during the Anagen stage. Global view of a neuromuscular junction: 1. ... A hair follicle, showing its Arrector pili muscle attached on the right. ... Goose bumps on a human Goose bumps (AE), also called goose pimples, goose flesh (BE), chicken skin (Hawaiian Pidgin), or cutis anserina, are the bumps on a persons skin at the base of body hairs which involuntarily develop when a person is cold or experiences strong emotions like fear. ... Mouse embryonic stem cells. ...


The average growth rate of hair follicles on the scalp is .04 cm per day. This numerical value can vary depending on smoking habits and intervals between masturbation.


Certain species of Demodex mites live in the hair follicles of mammals (including those of humans) where they feed on sebum. Species Demodex brevis Demodex bovis Demodex canis Demodex caprae Demodex cati Demodex equi Demodex folliculorum Demodex ovis Demodex phyloides The demodex mites form a genus of tiny parasitic mites which live in or near hair follicles of mammals. ...


Hair growth phases

Hair grows in cycles of various phases: anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase. Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one of several hairs that might arise from a single follicle exits. Normally up to 90% of the hair follicles are in anagen phase while, 10–14% are in telogen and 1–2% in catagen. The cycle's length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hairs have a fixed length, while hairs on the head seem to have no length limit. Growth cycles are controlled by a chemical signal like epidermal growth factor. The eyebrow is a bony ridge above the eye that protects the eye and bears a tuft of facial hair in most mammals. ... Look up Month in Wiktionary, the free dictionary. ... The scalp is the anatomical area bordered by the face anteriorly and the neck to the sides and posteriorly. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... Epidermal Growth Factor or EGF is a 6045 Da protein with 53 amino acid residues and three intramolecular disulfide bonds. ...


Hair growth cycle times

  • Scalp: The time these phases last varies from person to person. Different hair colour and follicle shape effects the timings of these phases.
    • anagen phase, 2–3 years (occasionally much longer)
    • catagen phase, 2–3 weeks
    • telogen phase, around 3 months
  • Eyebrows etc:
    • anagen phase, 4–7 months
    • catagen phase, 3–4 weeks
    • telogen phase, about 9 months

References

  • K. S. Stenn and R. Paus (2001). "Controls of Hair Follicle Cycling". Physiological Reviews 81 (1): 449–494. PMID 11152763. (comprehensive topic review, successor to landmark review of 1954 by HB Chase)
Integumentary system - edit
Skin | Sweat glands | Sebaceous glands | Hair | Nails
Skin
Epidermis (Stratum corneumStratum lucidum, Stratum granulosum, Stratum spinosumStratum germinativum/basale)
Dermis | Subcutis
Nervous system - Sensory system - Somatosensory system - edit
Spinal pathway: Somatosensory information
Medial lemniscusTouch (Pressure & Vibration) | Proprioception
Spinothalamic tractPain | Temperature
Receptors
Touch: Pacinian corpuscles | Meissner's corpuscles | Merkel's discs | Ruffini endings | Free nerve endings | Hair follicle receptors
Proprioception: Golgi organ | Muscle spindle (Intrafusal muscle fiber)
Pain: Nociceptors    Temperature: Thermoreceptors

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