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ADULT HAIR FOLLICLES AND CYCLING

Under normal circumstances hair growth in each hair follicle occurs in a cycle. There are three main phases of the hair growth cycle; anagen, catagen and telogen with anagen further subdivided into proanagen, mesanagen, and metanagen. Anagen is the active growth phase when hair fiber is produced. Proanagen marks initiation of growth with RNA and DNA synthesis in a follicle which then quickly progresses through mesanagen to metanagen and maximum hair follicle length and girth. In this mature anagen state of proliferation and differentiation, the hair follicle consists of a total of eight concentric cell layers and melanogenesis occurs within pigmented hair follicles.

The hair fiber is the core part of any growing anagen hair follicle. Epidermal derived cells close to the DP remain as undifferentiated cells. These cells focus on multiplying and proliferating to produce yet more 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. Cells in the center of the hair follicle, called cortex cells, are destined to become the hair fiber itself. 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 Diagram of Hair incorporated into the cortex. The cortex cells become keratinized and harden. As they keratinize it becomes impossible for the cells to function properly and they die. The keratinized cells are then pushed up and away from the hair bulb region as new cells come in below. The cortex cells are now part of the dead keratinized fiber.

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, cuticle cells also keratinize and die as they move towards the skin surface.

Around the hair cortex and cuticle are several layers of cells called sheaths. These sheaths protect the hair fiber as it grows in the skin. The inner root sheath (IRS) lies adjacent to the hair cuticle. 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 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 immune system responds to the antigens in the modified sebum and the bacteria resulting in inflammation and acne.

Around the IRS lies the outer root sheath (ORS). This layer is an extension of the epidermis and contains a bump of cells called the "bulge". 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. 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 bulge region is believed by some dermatologists to be the source of these cells although 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. While the rest of a hair follicle cycles through activity and rest, the sebaceous gland remains active.

Anagen is followed by catagen, a period of controlled regression of the hair follicle. On entering catagen the dermal papilla condenses as the cells become inactive. With a lack of dermal papilla cell stimulation, the hair fiber and root sheaths stop growing. Ultimately the hair follicle enters telogen, when the follicle is in a so-called resting state. In telogen the dermal papilla can become isolated in the dermis as the remaining hair follicle regresses and sits high up in the dermis. Although a telogen hair follicle is resting, it may still have a hair fiber in it. When hair follicles enter telogen and stop making hair they dump their last cells on to the end of the fiber. This lump of cells acts as an anchor to hold the hair fiber in the tube of the hair follicle. The hair is now called a "club" hair as the lump of cells on the end give it a club appearance. As a hair follicle enters the next anagen cycle and starts producing a new hair fiber, the new fiber will push out the old club hair. Also, club hairs can easily be pulled out by combing, brushing, or other daily hair care.

Anagen is the longest phase with up to 90% of follicles on a normal human scalp in this active hair growth state at any given time and correspondingly telogen hair follicles comprise up to 10% on the scalp. The average rate of hair fibre growth is around 0.35mm a day but this rate varies depending on the site of the hair follicle and the age and sex of the individual. The length of the anagen growth phase for scalp hair is usually 6-10 years while telogen lasts just 30-90 days and catagen is best estimated at 14-21 days. Hair follicles continue to cycle through growth and rest throughout adult life. The rate of cycling may gradually slow down and in old age the time period of the telogen resting stage may increase. This may effectively lead to thin looking scalp hair as a greater percentage of hair follicles are in a resting stage.