Hair follicle

Hair follicle
Hair follicle
A photograph of hair on a human arm emerging from follicles
Details
System	Integumentary system
Artery	Supratrochlear, supraorbital, superficial temporal, occipital
Vein	Superficial temporal, posterior auricular, occipital
Nerve	Supratrochlear, supraorbital, greater occipital, lesser occipital
Lymph	Occipital, mastoid
Identifiers
Latin	Folliculus pili
MeSH	D018859
TA98	A16.0.00.023
TA2	7064
TH	H3.12.00.3.01034
FMA	70660
Anatomical terminology [edit on Wikidata]

The hair follicle is an

The function of hair in humans has long been a subject of interest and continues to be an important topic in society, developmental biology and medicine. Of all mammals, humans have the longest growth phase of scalp hair compared to hair growth on other parts of the body.[1] For centuries, humans have ascribed esthetics to scalp hair styling and dressing and it is often used to communicate social or cultural norms in societies. In addition to its role in defining human appearance, scalp hair also provides protection from UV sun rays and is an insulator against extremes of hot and cold temperatures.^[1] Differences in the shape of the scalp hair follicle determine the observed ethnic differences in scalp hair appearance, length and texture.

There are many human diseases in which abnormalities in hair appearance, texture or growth are early signs of local disease of the hair follicle or systemic illness. Well known diseases of the hair follicle include alopecia^[2] or hair loss, hirsutism or excess hair growth and lupus erythematosus.^[3][2]

Structure

The position and distribution of hair follicles varies over the body. For example, the skin of the palms and soles does not have hair follicles whereas skin of the scalp, forearms, legs and genitalia has abundant hair follicles.

• The papilla is a large structure at the base of the hair follicle.[4] The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non-existent.^{[contradictory]}
• Around the papilla is the hair matrix.
• A root sheath composed of an external and internal root sheath. The external root sheath appears empty with cuboid cells when stained with H&E stain. The internal root sheath is composed of three layers, Henle's layer, Huxley's layer, and an internal cuticle that is continuous with the outermost layer of the hair fiber.
• The bulge is located in the outer root sheath at the insertion point of the arrector pili muscle. It houses several types of stem cells, which supply the entire hair follicle with new cells, and take part in healing the epidermis after a wound.^[5][6] Stem cells express the marker LGR5+ in vivo.^[7]

Other structures associated with the hair follicle include the cup in which the follicle grows known as the infundibulum,

Attached to the follicle is a tiny bundle of

There are ethnic differences in several different hair characteristics. The differences in appearance and texture of hair are due to many factors: the position of the hair bulb relative to the hair follicle, size and shape of the dermal papilla, and the curvature of the hair follicle.[1] The scalp hair follicle in Caucasians is elliptical in shape and, therefore, produces straight or wavy hair, whereas the scalp hair follicle of people of African descent is more curvy, resulting in the growth of tightly curled hair.^[1]

Development

In utero, the epithelium and underlying mesenchyme interact to form hair follicles.^[11][12]

Aging

A key aspect of hair loss with age is the aging of the hair follicle. Ordinarily, hair follicle renewal is maintained by the stem cells associated with each follicle. Aging of the hair follicle appears to be primed by a sustained cellular response to the DNA damage that accumulates in renewing stem cells during aging.^[13] This damage response involves the proteolysis of type XVII collagen by neutrophil elastase in response to the DNA damage in the hair follicle stem cells. Proteolysis of collagen leads to elimination of the damaged cells and then to terminal hair follicle miniaturization.

Hair growth

Hair grows in cycles of various phases:^[14] anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase (names derived using the Greek prefixes ana-, kata-, and telos- meaning up, down, and end respectively). 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 or several hairs that might arise from a single follicle exits. Normally up to 85% 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 hair have a much shorter length limit compared to hair on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor. DLX3 is a crucial regulator of hair follicle differentiation and cycling.^[15][16]

Anagen phase

Anagen is the active growth phase of hair follicles^[17] during which the root of the hair is dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. A hair pulled out in this phase will typically have the root sheath attached to it which appears as a clear gel coating the first few mm of the hair from its base; this may be misidentified as the follicle, the root or the sebaceous gland by non-health care professionals. Scalp hair stays in this active phase of growth for 2–7 years; this period 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.^[18] It signals the end of the active growth of a hair. This phase lasts for about 2–3 weeks while the hair converts to a club hair. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. A bulb of keratin attaches to the bottom tip of the hair and keeps it in place while a new hair begins to grow below it. A hair pulled out in this phase will have the bulb of keratin attached to it which appears as a small white ball on the end of the hair. This process cuts the hair off from its blood supply and from the cells that produce new hair. When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase.

Telogen phase

The telogen phase is the resting phase of the hair follicle, about three months.^[19] When the body is subjected to extreme stress, as much as 70 percent of hair can prematurely enter the telogen phase and begin to fall, causing a noticeable loss of hair. This condition is called telogen effluvium.^[20] The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair.^[11] Fifty to one-hundred club hairs are shed daily from a normal scalp.^[11]

Timeline

• Scalp: The time these phases last varies from person to person. Different hair color and follicle shape affects the timings of these phases.
  • Anagen phase, 2–8 years (occasionally much longer)
  • Catagen phase, 2–3 weeks
  • Telogen phase, around 3 months
• Eyebrows:
  • Anagen phase, 4–7 months
  • Catagen phase, 3–4 weeks
  • Telogen phase, about 9 months

Clinical significance

Disease

This section needs expansion. You can help by adding to it. (January 2018)

There are many human diseases in which abnormalities in hair appearance, texture or growth are early signs of local disease of the hair follicle or systemic illness. Well known diseases of the hair follicle include