Hair follicles are elements of the skin responsible for hair production. Most of the human skin is covered with follicles, the only areas without hair are the palms, the bottom of the feet and the lips.
Every hair follicle is paired with one or more sebaceous glands. Their role is to produce an oily compound called sebum, which keeps both the hair and the skin surface well lubricated. Sweat glands also play a role in the lubrication of hair, in areas like the armpits or the groin. Stronger hair, like the one on the scalp, need more lube and the number of sebaceous glands is increased. Every follicle also has a small muscle, named the arrector pili, which can contract the hair in an old mechanism of our body. The continuous production of hair is also provided using stem cells located at the base.
The papilla is the bottom area, or the base, of the hair follicle. It is fed by a small blood vessel and consists of connective material. Around it there is the so-called hair matrix, a mix of melanocytes and epithelial cells, both with a key role in hair growth. While the growth itself is caused by the fast division of the epithelial cells, melanocytes are in charge of the color, since they produce the required pigments.
Around the hair follicle there is the root sheath, a structure needed for its protection. It is divided in two parts: the internal and external root sheath. Both of these have divisions of their own. The internal one consists of three parts: the internal cuticle at the core (which bonds with the hair fiber itself), the Hukley's layer in the middle and Henle's layer on the outside. The layers of the fiber are the cuticle, the cortex and the medulla.
Every person has a different rate of hair growth but the natural process has four steps. The long growth phase is the anagen, its duration is variable and has a direct impact on the length of the hair. It can last up to 7 years on the scalp hair but just a few months in other locations, such as the eyebrows. A much shorter phase follows, the catagen, which only takes a few weeks.
The next step is the telogen, also known as the rest phase. By this time, the hair is already dead but might still remain in position. However, many of them usually fall off by themselves. It is another short phase, with a maximum duration of three weeks on the scalp. The last part of the cycle is the exogen, when the old hair is discarded so that the natural process can restart and a new hair starts to grow.
The main role of the hair follicles is to produce a hair fiber. Every hair consists of a visible and an invisible part, the visible one is called the shaft. It has an important role in the overall human biology and it plays an even greater psychological role.
Every hair fiber has three layers, these are named the medulla, the cortex and the cuticle. The largest part is located in the middle, also known as the cortex, which gives the hair its color due to the content of melanin. The outside part, or the cuticle, is extremely thin and only consists of one row of cells. Finally, the core of the hair is named the medulla.
Besides its biological role, human hair is extremely important in society for its aspect. A change in hair style can alter the look of one person completely and even make him unrecognizable, it can also signal a change in behaviour. However, the hair has a number of important roles, as a protection for the skin against strong sunlight, insect attacks and even physical harm.
The hair fiber is also part of the sensitive system. When external forces move the hair, the follicle reports the information to the central nervous system. The hair of some animals can function like a very accurate set of sensors. The most important role of the hair used to be thermal protection against any sudden change in temperature, like animal fur.
The hair follicle is directly linked with the hair shaft and its main purpose is producing it. The follicle can be divided into several parts, all of them with a separate task related to hair growth. All of the characteristics of the shaft depend on the follicle, for example a curved follicle will generate a curly fiber, while a straight follicle produces a straight one.
The second very important role of the follicle is to act as an anchor for the shaft. This is mainly accomplished by a sticky substance produced at the base of the hair fiber, through the trichilemmal keratinization of the root during the telogen phase. This area of keratin moors the shaft in place. Chemical analysis has revealed that the cells around the anchor point of the hair shaft have a high content of keratin K14 and desmosomes. Such enzymes seem to play a major role in anchoring the hair, since experiments on rodents have revealed that the ones lacking them had very loose fur.
The glands that produce sebum are located on the outer root sheath of the hair follicle. The same area also synthetizes all of the compounds used to repair the hair and recover from injuries. These include cytokines, adhesion compounds, keratin and other growth elements. Some of these cells in the upper root sheath have specialized roles. For example, the Langerhans cells repair the damage caused by ultraviolet radiation, while epithelial cells are tasked with recovery after ulcerated wounds.
The base of the follicle has a shape similar to a dome and is named the dermal papilla. Its role is to support the basal lamina, which is under a lot of stress because of the constant cell division that takes place inside. The specific shape is designed to host more matrix cells, resulting in greater structural strength.
The hair follicle also represents the first sensory organ of the body, providing early warning. Even small movements of the shaft are detected by the nervous terminations located in the follicle, who send a warning to the central system. The special neural cells in the follicle are named Langerhan's cells and Merkel cells. Langerhan's cells seem to play a more important role, since they are also responsible of activating the immune response.
Hair follicles are special skin structures that change the area around them. The research on the hair and the one on the skin are closely related and their biology is often studied together.