Introduction and background

It must be known to everyone that skin is an organ of integumentary system composed of layers of tissues that protect the organs and muscles underneath. Skin consists of hairs which can be seen above the surface of the skin as elongated keratinized structures. The hair has two parts; the hair follicle and the hair shaft. The resistant part above the epidermis of skin is the hair shaft. The hair follicle is the point from which the hair grows. Hair follicle has a stocking like appearance with several layers having different functions. The cells in the hair follicle divide every 23 to 72 hours, faster than any other cells in the body.


Structure of Hair Follicle/Root

Hair follicle is the deep down structure from which hair grows. The structure of hair follicles consists of following parts:

•Hair Bulb 

It is a bulbous structure at the terminus of skin. This is called hair bulb. Cells in hair bulb serve two functions. These cells produce the long, fine and cylindrically shaped hair fibers. There are some special cells, which produce the pigment that gives the hair its color. These cells are known as melanocytes and produce pigment melanin.
• Papilla
Moving down to the hair bulb there is a structure containing vessels known as papilla. Papilla performs the circulation function thus providing nourishment to the hair follicle. The dermal papilla is a structure very important to hair growth because it contains receptors for male hormones and androgens.

• Root Sheaths
Within the skin external and internal root sheaths cover the hair follicles. The external root sheath of a hair follicle is continuous along with the epidermis.

• Glands
There are some glands situated at around the hair follicle. The most important of these glands is the sebaceous gland, which secretes and produces natural oils lubricating hair known as sebum. Other glands include apocrine sweat glands. Sebum is poured to the skin through a skin pore.


• Other Structures
Other structures that are associated with the hair follicle are infundibulum, erector Pilli muscles, hair follicle receptors and a pore. Infundibulum is a cup that encases hair follicle. Erector Pilli muscles are bundles of muscle fibers near the hair follicle, responsible for the erection and dilation of the hair shaft with the skin or way from it. Hair follicle receptors sense the position of the hair. Pore is an oily opening of the hair to the skin.


Hair Follicle Growth Cycle
There is predetermined genetic way that each hair follicle follow to grow. Hair follicle grows in repeated cycles. Each follicle first gets into long dividing phase followed by a relatively smaller resting phase. During the rest period the hair is still attached to the hair follicle, but it is not growing. After the resting phase the hair is shed and a newer hair begins to grow, thus, starting a new hair growth cycle. Hair follicle growth cycle consists of the three distinct phases which are:
1.) Growth Phase (Anagen)
2.) Transitional Phase (Catagen)
3.) Resting Phase (Telogen)


Growth Phase (Anagen Phase)
The growing period of hair follicle is termed as anagen phase. This is the active growth phase of hair follicle. The Anagen phase or growth phase can vary from two to six years. Hair grows approximately 10cm per year and any individual hair is unlikely to grow more than one meter long. The amount of time the hair follicle stays in the anagen phase is genetically determined.


Transitional Phase (Catagen Phase)
At the end of anagen phase hair follicle enters into a short intermediate catagen phase. It signals the end of growth phase. Hair follicle grows in anagen phase and this is the time they prepare themselves to rest. This stage of the hair growth cycle usually lasts about 1 to 2 weeks or so. During the catagen phase the deeper portions of the hair follicles start to collapse and hair follicle shrinks to about 1/6 of the normal length. The lower part is destroyed and the dermal papilla breaks away to rest below.


Resting Phase (Telogen Phase)
This is the last phase of the cycle and is resting period of hair follicle. During this phase the hair does not grow but attached to the hair follicle passively. It is usually 3 to 4 months in length and at the end of this period older hairs that have finished their life will fall out and newer hairs will begin to grow. At the end of telogen phase the hair follicle reenters the anagen phase. The dermal papilla and the base of the follicle join together again and a new hair begins to form.
If the old hair has not already been drop the new hair rams the old one out and the growth cycle starts all over again.


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