A hair follicle is part of the skin that grows hair by packing old cells together. The skin is the outer covering of living tissue of an animal (or plant Hair is a keratinised protein filament that grows through the epidermis from follicles deep within the Dermis. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called 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 sebaceous glands are Glands found in the Skin of Mammals Locations and morphology A branched type of Acinar gland, these The sebaceous glands are Glands found in the Skin of Mammals Locations and morphology A branched type of Acinar gland, these A gland is an organ in an animal's body that synthesizes a substance for release such as Hormones or Breast milk, often into the Bloodstream The hands ( med / lat: manus pl manūs are the two intricate prehensile multi- Fingered body parts normally located at the end of each arm of a The thicker density of hair, the more sebaceous glands are found. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different
Also attached to the follicle is a tiny bundle of muscle fiber called the arrector pili that is responsible for causing the follicle lissis to become more perpendicular to the surface of the skin, and causing the follicle to protrude slightly above the surrounding skin (piloerection). Skeletal muscle is a type of Striated muscle, which usually attaches to tendons Arrectores pilorum (singular Arrector pili) are tiny Muscle fibers attached to each Hair follicle, which contract to make the This process results in goose bumps (or goose flesh). Goose bumps, also called goose flesh, chill bumps, chicken skin, people bumps, or the medical term cutis anserina, Stem cells are located at the junction of the arrector and the follicle, and are principally responsible for the ongoing hair production during a process known as the Anagen stage. Stem cells are cells found in most if not all multi-cellular Organisms.
The average growth rate of healthy hair follicles on the scalp is . 04 cm per day.
Certain species of Demodex mites live in the hair follicles of mammals (including those of humans) where they feed on sebum. Demodex is a Genus of tiny parasitic Mites that live in or near Hair follicles of Mammals About 65 species of Demodex
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Structure
Papilla
At the base of the follicle is a large structure that is called the papilla. [1] The papilla is made up mainly of connective tissue and a capillary loop. Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, Muscle, and Nervous tissue) Cell division in the papilla is either rare or non-existent.
Matrix
Around the papilla is the hair matrix, a collection of epithelial cells often interspersed with the pigment producing cells, melanocytes. Trichocytes are the specialized Epithelial cells from which the highly mechanically resilient tissues Hair and nail are formed In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body Melanocytes are cells located in the bottom layer (the Stratum basale) of the skin's epidermis and in the middle layer of the eye (the Uvea) Cell division in the hair matrix is responsible for the cells that will form the major structures of the hair fiber 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. Chemotherapy, in its most general sense refers to treatment of disease by chemicals that kill cells specifically those of micro-organisms or Cancer. Radiation therapy (or radiotherapy) is the medical use of Ionizing radiation as part of Cancer treatment to control Malignant Baldness involves the state of lacking hair where it often grows especially on the head 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.
Root Sheath
The root sheath is composed of an external root sheath (Henle's layer), a middle layer (Huxley's layer), and an internal cuticle that is continuous with the outermost layer of the hair fiber. Henle's layer is the third layer of the inner root sheath of the Hair, consisting of a single layer of Cubical cells with clear flattened nuclei The second layer of the inner root sheath of the Hair consists of one or two layers of horny flattened nucleated cells known as Huxley's layer.
Hair Fiber
The hair fiber is composed of a cuticle that is continuous with the root sheath, an intermediate cortex, and an inner medulla. Hair is a keratinised protein filament that grows through the epidermis from follicles deep within the Dermis.
Other Structures
Other structures associated with the hair follicle include arrector pili muscles, sebaceous glands and apocrine sweat glands. Arrectores pilorum (singular Arrector pili) are tiny Muscle fibers attached to each Hair follicle, which contract to make the The sebaceous glands are Glands found in the Skin of Mammals Locations and morphology A branched type of Acinar gland, these The skin contains two different groups of sweat glands: Apocrine sweat glands and Merocrine sweat glands. Hair follicle receptors sense the position of the hairs. A mechanoreceptor is a Sensory receptor that responds to mechanical pressure or distortion
Hair growth phases
Hair grows in cycles of various phases:[2] 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. The eyebrow is an area of thick delicate hairs above the Eye that follows the shape of the brow ridges. The month is a unit of Time, used with Calendars which is approximately as long as some natural period related to the motion of the Moon; The scalp is the anatomical area bordered by the Face anteriorly and the Neck to the sides and posteriorly A year (from Old English gēr) is the time between two recurrences of an event related to the Orbit of the Earth around the Sun Growth cycles are controlled by a chemical signal like epidermal growth factor. Epidermal growth factor or EGF is a Growth factor that plays an important role in the regulation of Cell growth, Proliferation, and
Anagen Phase
Anagen is the active growth phase of hair follicles. [3] The cells in the root of the hair are dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2-6 years. The amount of time the hair follicle stays in the anagen phase is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
Catagen Phase
The catagen phase is a short transition stage that occurs at the end of the anagen phase. [4] It signals the end of the active growth of a hair. This phase lasts for about 2-3 weeks while a club hair is formed.
Telogen Phase
The telogen phase is the resting phase of the hair follicle. [5] At any given time, 10%-15% of all hairs are in the telogen phase. This phase lasts for about 100 days for hairs on the scalp and much longer for hairs on the eyebrow, eyelash, arm and leg.
During this phase the hair follicle is completely at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root.
About 25-100 telogen hairs are shed normally each day.
Hair growth cycle times
- Scalp: The time these phases last varies from person to person. Different hair colour and follicle shape affects 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
Additional images
 Cross-section of all skin layers. |
References
- ^ Pawlina, Wojciech; Ross, Michael W. ; Kaye, Gordon I. (2003). Histology: a text and atlas: with cell and molecular biology. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-683-30242-6.
- ^ 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)
- ^ Brannon, MD, Heather. Anagen Phase. The New York Times. Retrieved on 2007-11-14. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1533 - Conquistadors from Spain under the leadership of Francisco Pizarro arrive in Cajamarca, Inca
- ^ Brannon, MD, Heather. Categen Phase. The New York Times. Retrieved on 2007-11-14. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1533 - Conquistadors from Spain under the leadership of Francisco Pizarro arrive in Cajamarca, Inca
- ^ Brannon, MD, Heather. Telogen Phase. The New York Times. Retrieved on 2007-11-14. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1533 - Conquistadors from Spain under the leadership of Francisco Pizarro arrive in Cajamarca, Inca
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