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.

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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. 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 follicle receptors sense the position of the hairs.

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. Growth cycles are controlled by a chemical signal like epidermal growth factor.

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.

Categen 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

References