7.2.3: Follicle
- The root of each hair is contained in a tube-like structure, called a follicle, which is situated approximately a quarter of an inch (6 millimeters) into the skin.
- The hair follicle is a complex mini-organ. Along with the sebaceous gland and the arrector pili muscle, the hair follicle is part of the ‘pilo-sebaceous unit’. This also includes the outer root sheath and the inner root sheath.
- Although it is part of the epidermis (outer layer of skin), the follicle extends through the dermis (middle layer of skin) into the subcutaneous tissue (inner layer of skin). The hair grows from the bottom of the follicle and is connected to a tissue structure called the papilla, which is nourished by a system of blood vessels. The cells of the papilla reproduce and move up the hair follicle, causing the hair to grow.
- The hair follicle can be divided into 3 regions: the lower segment (bulb and suprabulb), the middle segment (isthmus), and the upper segment (infundibulum). The lower segment extends from the base of the follicle to the insertion of the erector pili muscle. The middle segment is a short section that extends from the insertion of the erector pili muscle to the entrance of the sebaceous gland duct. The upper segment extends from the entrance of the sebaceous gland duct to the follicular opening at the scalp.
- The Outer Root Sheath (ORS) is a continuation of the epidermis. The "bulge" region in the ORS is the site at which the arrector pili muscle is attached.
- The arrector pili muscle is a smooth muscle which makes hair stand erect and produces goose bumps in the skin.
- The bulge region is thought to be the storage area for hair follicle stem cells.
- Also extending from the ORS is the sebaceous gland which contains cells that produce oils (lipids). The sebaceous gland has a duct with an opening into the tube where the hair fiber sits.
- Sebum is an oily solid that is expelled from the hair follicle. Sometimes overproduction of sebum can build up in the hair follicle and harden into comedones (blackheads). Sebum is a nutrient rich material and an ideal habitat for bacteria resulting in inflammation and acne. Sebum is also responsible for the oily ‘T-Zone’ on the forehead and around the sides of the nose, as well as for certain dandruff conditions.
- At puberty there is an increase in oil production from our skin as the sebaceous glands become larger and increase their activity.
- The ORS is thinnest at the level of the bulb and thickest in the middle portion of the hair follicle. The ORS cells contain clear vacuolated cytoplasm because of the presence of large amounts of glycogen. The ORS does not keratinize below the level of the isthmus (in contrast to the IRS). However, the ORS keratinizes at the level of the isthmus where the IRS disintegrates.
Follicle (Inner Root Sheath)
- The inner root sheath (IRS) is closely positioned to the hair shaft, and, because the sheath contains no pigment, it can easily be distinguished from the hair shaft. The IRS coats and supports the hair shaft up to the level of the isthmus, at which time it breaks down and exfoliates.
- The inner root sheath (IRS) is produced by matrix cells positioned at the periphery of a hair follicle. The cells that make up the IRS gradually become differentiated and keratinized as they are pushed away from the bulb region (DP). As keratinization occurs, the cells die and form the IRS surrounding and protecting the development of the hair fiber.
- The inner root sheath (IRS) can be divided into three layers: the cuticle (Cl), Huxley layer (Hu), and Henle layer (He). These 3 layers are distinct just above the dermal papilla. However, they keratinize relatively low in the hair follicle and are indistinguishable at higher levels where they function as a single unit covering the hair shaft.
- Adjacent to the hair fiber is a single cell thick IRS cuticle layer (Cl) that closely interlocks with the hair fiber cuticle layer (Ch). The next IRS layer is called the Huxley layer (Hu) which consists of up to four cell layers. The IRS Henle layer (He) runs adjacent to the ORS layer.
