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hair follicle structure

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Follicular structure

The total number of hair follicles for an adult human is estimated at 5 million with 1 million on the head of which 100,000 alone cover the scalp. In humans, the only external regions of skin devoid of hair follicles are the palms of the hands and soles of the feet. The basic hair follicle structure remains essentially the same throughout the range of mammalian species with modifications for specialized functions. The hair follicle can be recognized as a separate entity within the skin with formation and maintenance based on interaction between dermal and epidermal components.

Under the influence of the DP, epidermal cell differentiation during anagen produces a keratinized hair fiber and associated products. The source epidermal cells, called matrix cells, that lie in the immediate vicinity of the dermal papilla are a living, actively proliferating group of cells which differentiate and become keratinized to form the hair cortex (Co) and surrounding hair cuticle (Hc) of the hair shaft at the center of which is situated the medulla (M). Cells Around the hair shaft comprise the inner root sheath (IRS) which can be divided into three layers the cuticle (Cu), Huxley layer (Hu) and Henle layer (He) based on structure, patterns of keratinization and incorporation of a product called trichohyalin. The IRS breaks down at the level of the sebaceous gland to leave only the hair cortex and surrounding cuticle to protrude above the epidermis.
Diagram showing the main differentiated layers in a mature anagen hair follicle. The hair follicle penetrates the dermal layer of the skin composed of fibroblast cells and collagen connective tissue interspersed with blood vessels, sweat glands and sensory nerves. The bulb region sits in the subcutaneous (adipose fat) tissue layer.

Annotated close up view of layers of a hair follicle jpg
Image showing the layers in a hair follicle from the outside (left) to the center (right).

Dermal papilla

It is the dermal papilla (DP) which directs and dictates the embryonic generation of a hair follicle and it also retains this instructive ability throughout the life of the hair follicle. The DP presents as a healthy "pear" shape in normal hair follicles. As the name suggests, derived from the dermis mesenchyme the DP consists of a highly active group of cells shown to be capable of inducing follicle development from the epidermis and production of hair fiber (Oliver 1966a, Oliver 1966b, Oliver 1967).

The DP consists of a small group of fibroblast cells derived from the mesoderm. The cells are held close to the base of the epidermal derived cells that produce the hair fiber and root sheaths but there is a thin layer, called the basement membrane (or basement lamina, or glassy membrane) that separates the DP cells from the hair fiber/sheath cells. In other words, the basement membrane provides a physical dividing line between cells descendant from embryonic ectoderm (epidermis) and embryonic mesoderm (dermis). This physical barrier has a role to play in our immunological protection. Holding the DP cells in place is a capsule that surrounds the DP cells in a cup and extends up the sides of the hair follicle to the epidermis. The whole follicle structure sits on a pad of fibrous tissue called the Arao-Perkins body. Nerve fibers and blood vessels penetrate through small gaps in the base of the hair capsule and invade into the DP area.

The bigger the DP, the more cells it has, then the thicker the hair fiber that the hair follicle produces. The DP cells are very active with lots of cytoplasm when the hair follicle is producing a hair fiber although the DP cells do not multiply and proliferate unlike the hair producing cells above the DP. When a hair follicle is not producing a fiber the DP cells lose much of their cytoplasm and become inactive.

Oliver et al revealed that the removal of the DP stops hair growth but that the lower third of the dermal sheath is capable of supplying new cells for regeneration of a new DP by infiltrating and transforming at the site of the original DP with subsequent hair follicle regrowth (Oliver 1966a, Oliver 1966b, Oliver 1966c, Jahoda 1992). With removal of more than the lower third of a hair follicle, reformation of a DP is unable to occur and the hair follicle is effectively permanently destroyed. The DP cells retain their embryonic functional abilities and are able to induce new hair fiber growth in mature, adult skin when implanted into previously deactivated hair follicles and in close association with ORS epidermal cells (Oliver 1967, Horne 1986).

DP cells can also interact with adult epidermis to induce the development of new hair follicles (Jahoda 1990). In the established hair follicle the DP cells act in conjunction with epidermal cells via mechanisms similar to those in embryogenesis to permit hair follicle cycling through hair production and resting phases. DP cells are almost unique in maintaining their embryogenic regenerative properties in adults making them potentially attractive for investigation with a view to gaining an insight on organ/limb regeneration and similar studies.

Annotated close up view of the dermal papilla jpg
Image showing the bulb region of a hair follicle.

Hair fiber

The hair fiber is the core part of any hair follicle. Epidermal derived cells close to the DP remain undifferentiated cells, called matrix cells, that focus on multiplying and proliferating to produce more cells. Those cells made in the center of the hair follicle are destined to become part of the hair fiber and are called cortex (cortical) cells. As the cells multiply the constant stream of production pushes the cells upwards towards the skin surface. As they move up the hair follicle they begin to differentiate into particular cell types. The cortex cells change from a round into a flattened appearance. They are squeezed together into layers (lamella). If the hair follicle contains melanocyte cells then melanin pigment is incorporated into the cortex cells. These cortex cells become keratinized and harden. As they do so it becomes impossible for the cells to function properly and the cells die. The keratinized cells are then pushed away from the hair bulb region and upwards as new cells come in behind. The cortex cells are now part of the dead keratinized fiber.

Some large hair follicles have a central strand of cells that are loosely organized and not packed together. This tube in the very center of the hair fiber is called the medulla.

Around the outside of hair fiber we see a cuticle. The cuticle is made up of more keratinized cells but they arrange themselves in a slightly different way to cortex cells. As the cuticle cells are produced, they lay over the cortex cells and flatten into an overlapping roof tile fashion. Cuticle cells become progressively flatter as they get older. As with cortex cells, when they keratinize the cell can no longer function properly and dies.

Root sheaths

The Outer Root Sheath (ORS) is distinct from other epidermal components of the hair follicle being continuous with the epidermis. The "bulge" region in the ORS is the site at which the arrector pili muscle is attached. The arrector pili muscle is connected to the epidermis at the other end. This is the muscle that makes hair stand erect and produces goose bumps in your skin when you are cold. The contraction of the muscle pulls on both the hair to make it erect and pulls on the skin making a bumpy surface.

The bulge region is believed to be the storage area for hair follicle stem cells. Hair follicles go through a cycle of growth and rest (below). With each renewed attempt to produce hair fiber, the hair follicle must obtain a source of cells to form the matrix cell population that make hair fibers. The source of these cells is believed by some dermatologists to be the bulge region. Other dermatologists suggest that stem cells are not present in the bulge region at all and that new matrix cells are obtained from the root sheath.

Also extending from the ORS is the sebaceous gland. It consists of a few cells focused on production of oils (lipids). These cells are large with their cytoplasm filled with vacuoles containing lipid. The cells are often divided into several lobes of the sebaceous gland connected together by a sebaceous duct. The duct has a single opening into the tube where the hair fiber sits.

The ORS surrounds the hair fiber and inner root sheath until deep into the dermis. Just above the bulb region containing the dermal papilla the ORS tapers and ends so the ORS does not entirely cover the hair fiber and inner root sheath. The ORS consists of several layers of cells that can be identified with unique ultrastructural properties.

The inner root sheath (IRS) is produced by matrix cells sitting above the hair follicle. While those matrix cells in the center of a hair follicle proliferate and produce the hair fiber and cuticle, the matrix cells towards the periphery of a hair follicle proliferate and produce the IRS. As with cells making up the hair fiber, the cells destined to be IRS gradually become differentiated and keratinized as they are pushed away from the bulb region. As keratinization occurs, the cells die and form the IRS surrounding and protecting the development of the hair fiber. The IRS can be subdivided into several layers. Adjacent to the hair fiber we see a single cell thick IRS cuticle layer that closely interdigitates with the hair fiber cuticle layer. The next IRS layer is called the Huxley layer that may consist of up to four cell layers. Outside of this there a single cell layer called the IRS Henle layer. The Henle layer runs adjacent to the ORS layer.

The products of the sebaceous gland are believed to help break down the IRS. The IRS does not extend out of the hair follicle. Only the hair fiber itself protrudes above the skin surface. The IRS disintegrates at the level of the sebaceous duct opening. The debris from the IRS break down mixes with the sebaceous gland oils and the result is sebum. Sebum is an oily solid that is expelled from the hair follicle and normally scraped or washed away in our general skin care habits. Sometimes overproduction of sebum can build up in the hair follicle and harden into plugs of material called comedones (blackheads). Sebum is a nutrient rich material and an ideal habitat for bacteria. Colonies of bacteria, particularly one called Propionibacterium acnes may proliferate in hair follicles using sebum as a nutrient supply. The bacteria feeding on the sebum changes its composition that makes it irritating. The immune system responds to the antigens in the modified sebum and the bacteria resulting in inflammation and acne.

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