Hair follicle

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]

In utero, the epithelium and underlying mesenchyme interact to form hair follicles.[10][11]

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.[12] 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.

Anagen is the active growth phase of hair follicles[16] 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.

The catagen phase is a short transition stage that occurs at the end of the anagen phase.[17] 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.

The telogen phase is the resting phase of the hair follicle, about three months.[18] 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.[19] The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair.[10] Fifty to one-hundred club hairs are shed daily from a normal scalp.[10]

• 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

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 or hair loss, hirsutism or excess hair growth, and lupus erythematosus.[2] Therefore, understanding the function of the normal hair follicle is fundamental to diagnosing and treating many dermatologic and systemic diseases with hair abnormalities.[2] Studies of Witka et al 2020 has shown the role of microbiome in the biology, immunology and diseases of scalp hair follicle. Studies further shown that change in hair follicle microbiome result into scalp disease like; Seborrheic dermatitis of the scalp and dandruff, Folliculitis decalvans, Androgenetic alopecia, Scalp psoriasis and Alopecia areata. [20]

• 
  male alopecia
• 
  Alopecia scale
• 
  Lupus erythematosus
• 
  Hidradenitis suppurativa

Hair follicles form the basis of the two primary methods of hair transplantation in hair restoration, Follicular Unit Transplantation (FUT) and follicular unit extraction (FUE). In each of these methods, naturally occurring groupings of one to four hairs, called follicular units, are extracted from the hair restoration patient and then surgically implanted in the balding area of the patient's scalp, known as the recipient area. These follicles are extracted from donor areas of the scalp, or other parts of the body, which are typically resistant to the miniaturization effects of the hormone DHT. It is this miniaturization of the hair shaft that is the primary predictive indicator of androgenetic alopecia,[21] commonly referred to as male pattern baldness or male hair loss. When these DHT-resistant follicles are transplanted to the recipient area, they continue to grow hair in the normal hair cycle, thus providing the hair restoration patient with permanent, naturally-growing hair. More than 60% of men and 10% of women suffer from hair loss.[22]

While hair transplantation dates back to the 1950s,[23] and plucked human hair follicle cell culture in vitro to the early 1980s,[24] it was not until 1995 when hair transplantation using individual follicular units was introduced into medical literature.[25]