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Information about 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.

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. 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 a process known as the Anagen stage.

The average growth rate of 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.

Structure

Papilla

At the base of the follicle is a large 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.

Matrix

Around the papilla is the hair matrix, a collection of epithelial cells often interspersed with the pigment producing cells, melanocytes. 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. 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.

Hair Fiber

The hair fiber is composed of a cuticle that is continuous with the root sheath, an intermediate cortex, and an inner medulla.

Other Structures

Other structures associated with the hair follicle include arrector pili muscles, sebaceous glands and apocrine sweat glands.

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.

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

  • 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)
  • Ross, M. & Pawlina, W. (2006). Histology: A Text and Atlas, 5th Ed., Lippincott, Williams & Wilkins. 
 •  • [ e]
Integumentary system
structuresSkin • Sweat glands • Sebaceous glands • Hair (Hair follicle) • Nails • Scale
layersEpidermis (Stratum corneum, Stratum lucidum, Stratum granulosum, Stratum spinosum, Stratum germinativum/basale) • Dermis • Subcutis
Skin layers: epidermis, dermis, and subcutis, showing a hair follicle, sweat gland & sebaceous gland.]] In zootomy and dermatology, skin is the largest organ of the integumentary system made up of multiple layers of epithelial tissues that guard underlying muscles and organs.
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Hair is a filamentous outgrowth of protein, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other organisms, especially insects, show filamentous outgrowths, these are not considered "hair".
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The sebaceous glands are glands found in the skin of mammals.

Locations and morphology

A branched type of acinar gland, these glands exist in humans throughout the skin except in the palms of the hands and soles of the feet.
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The sebaceous glands are glands found in the skin of mammals.

Locations and morphology

A branched type of acinar gland, these glands exist in humans throughout the skin except in the palms of the hands and soles of the feet.
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gland is an organ in an animal's 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).
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The hands (med./lat.: manus, pl. manūs) are the two intricate, prehensile, multi-fingered body parts normally located at the end of each arm (medically: "terminating each anterior limb/appendage") of a human or other primate.
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The sole (foot) is the bottom of the foot.

Sole may also refer to:
  • Sole, in shoemaking, is the bottom supporting member of the shoe
  • Sole (fish), several species and groups of flatfishes:

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In physics, density is mass m per unit volume V—how heavy something is compared to its size. A small, heavy object, such as a rock or a lump of lead, is denser than a lighter object of the same size or a larger object of the same weight, such as pieces of
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muscle fiber, also spelled muscle fibre (see spelling differences), also technically known as a myocyte, is a single cell of a muscle. Muscle fibers contain many myofibrils, the contractile unit of muscles.
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Erectores pilorum (singular Erector pili) are tiny muscle fibers attached to each hair follicle, which contract to make the hairs stand on end, causing goose bumps. They exist in most mammals including humans.
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Goose bumps, also called goose pimples, goose flesh, chill bumps, chicken skin, or the medical term cutis anserina, are the bumps on a person's skin at the base of body hairs which involuntarily develop when a person is cold or experiences
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Stem cells are primal cells found in all multi-cellular organisms. They retain the ability to renew themselves through mitotic cell division and can differentiate into a diverse range of specialized cell types.
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Demodex

Species

Demodex brevis
Demodex bovis
Demodex canis
Demodex caprae
Demodex cati
Demodex equi
Demodex folliculorum
Demodex ovis

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A papilla (plural: papillae) can be:
  • A small projection, such as a nipple-like projection on the skin, at the base of a hair or the root of a feather; the base of a new tooth.
  • A pimple or blister
  • An interdental papilla is the part of gingiva located between teeth.

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Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, muscle, and nervous tissue.) It is largely a category of exclusion rather than one with a precise definition, but all or most tissues in this category are similarly:
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A hair matrix cell (also known as trichocyte or simply referred to as matrix cell) gives rise to non-hair non-keratinized IRSC (inner/internal root sheath cell) as well.

It is also commonly known as a Root Hair Cell.
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epithelium is a tissue composed of a layer of cells. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.
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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).

Melanogenesis

Through a process called melanogenesis
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Chemotherapy is the use of chemical substances to treat disease. In its modern-day use, it refers to cytotoxic drugs used to treat cancer or the combination of these drugs into a standardized treatment regimen.
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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).
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Alopecia
Classification & external resources

Bald redirects here; for other uses see Bald (disambiguation).
Baldness involves the state of lacking hair where it often grows, especially on the head.
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Hair is a filamentous outgrowth of protein, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other organisms, especially insects, show filamentous outgrowths, these are not considered "hair".
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Erectores pilorum (singular Erector pili) are tiny muscle fibers attached to each hair follicle, which contract to make the hairs stand on end, causing goose bumps. They exist in most mammals including humans.
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The sebaceous glands are glands found in the skin of mammals.

Locations and morphology

A branched type of acinar gland, these glands exist in humans throughout the skin except in the palms of the hands and soles of the feet.
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Apocrine is a term used to classify exocrine glands in the study of histology. Cells which are classified as apocrine bud their secretions off through the plasma membrane producing membrane bound vesicles in the lumen.
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In humans, there are four kinds of sudoriferous or sweat glands which differ greatly in both the composition of the sweat and its purpose.
  • eccrine glands - sweat (normal sweat used in temperature control)
  • apocrine glands - sweat,fats,proteins (slightly more viscous sweat)

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The eyebrow is an area of coarse skin hairs above the eye that follows the shape of the brow ridges.

Functions

The main function of the eyebrow is to prevent moisture, mostly salty sweat and rain, from flowing into the eye, an organ critical to sight.
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For the Egyptian hawk-god, see .


The month is a unit of time, used with calendars, which is approximately as extensive as some natural period related to the motion of the Moon.
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''For other uses of the word, see Scalp (disambiguation)
The scalp is the anatomical area bordered by the face anteriorly and the neck to the sides and posteriorly.
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