Hair follicle

A hair follicle is a 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 the density of the hair, the more the number of sebaceous glands that 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 (piloerection) and a pore incased with skin oil. 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 healthy hair follicles on the scalp is half an inch per month.

Certain species of Demodex mites live in the hair follicles of mammals (including those of humans) where they feed on sebum.

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. 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 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.

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

Bulge
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.

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.

Morphogenesis
In utero, the epithelium and underlying mesenchyma interact to form hair follicles.