Hair provides many important functions for the human body. There are thousands of hairs growing out of the human body at any given time. These hairs are in a constant cycle of growth and loss revolving around a structure of the skin called the hair follicle.
Structure
The hair follicle is located in the outer layer of skin called the epidermis. Through the hair follicle, a single shaft of hair can find access to the outside environment. Made up of a tube of cells, there is a single hair follicle for every hair on your head.
Function
The hair follicle has many important functions in regards to hair growth and health. First, it connects via the papilla to surrounding capillaries, which provide the nutrients needed for hair to grow. The follicle also holds the bulb of the hair, which is where cell division creates the actual hair shaft. The hair follicle also helps support the many other structures that mediate hair growth such as the sebaceous glands, apocrine glands (in areas such as the groin and armpits) and the erector pili muscle.
Anagen
One of the most important aspects of the hair follicle is the role it plays in each of the three stages of hair growth. The first of these stages is the anagen phase. During the anogen phase, the hair follicle provides the needed support for cells to multiply and grow into hair. As the hair moves up the follicle, it is covered in keratin, a substance that gives hair its structure. The follicle holds the hair until it enters the catagen stage.
Catagen
In the categen stage, the hair follicle shrinks, which prevents the hair from continuing to grow. It does this by blocking the hair-forming cells' access to the capillaries that supply the necessary nutrients for hair growth. It also cuts the hair off from the cells that can produce new hair. This lasts for approximately two weeks, before the follicle enters the telogen stage.
Telogen
During the telogen phase, the hair follicle is at rest. Because the current hair growth no longer has access to the nutrients it needs to grow, it often falls out of the follicle during this stage. The telogen stage can last for up to 100 days. Once the resting phase is over, the follicle creates a new connection to the capillary blood supply, and starts the growth of a new hair. If for some reason the old hair does not fall out during the telogen stage, it will be pushed out by the new hair as it grows up the hair follicle.
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