Hair growth and its regulation

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Hair growth regulation

All mature follicles go through a pattern of cycling. Hair follicle cycling is characterized by three main stages - anagen (growth), catagen (regression) and telogen (rest). Subsequently the hair shaft is shed and the process is called exogen. The cycle varies with species and also with different types of follicles. As for example, animals like sheep (except merino sheep) go through a seasonal replacement of hairs twice a year whereas a mouse experiences it 6 or more times a year. However, this regeneration and replacement process may not always apparent to the naked eye. As old hair is shed it normally gets replaced leading to a seamless cycle. The only way to really look at hair cycling is through experiments that involve looking at the hair follicles themselves. A number of experimentations in mice, have led to better understanding of the factors that regulate the hair follicle cycling, but there is no one factor that can be said to be having ultimate control over hair growth.

Human hair follicles apparently go through the cycle of growth and rest which are independent of each other. There definitely is a biological mechanism that promotes anagen hair growth and the involution of the hair follicle to a resting stage. For many years the scientists are aware of the clues to the basics of this mechanism. Any form of skin damage will force telogen hair follicles in rodents into anagen growth in and around the site of injury. Any skin damage leads to temporary hair growth. With the healing of the injury hair follicles go back to normal. There is similar evidence that skin injury in humans will also promote growth and induce anagen in hair follicles nearby if hair follicles are in telogen. Cuts, scrapes, heat/chemical burns including sun burn, limited local necrosis, and some forms of skin irritation may work for hair growth stimulation. As long as the hair follicles are not significantly damaged, then mild skin injury seems to promote hair growth. This activation of growth is explained by the increase in cytokines (signaling chemicals) due to the skin injury and damage.

It is impossible to list all the factors that affect hair growth. Various pages on this site list different groups of factors that modify hair follicle activity. Among the products naturally produced by the body, the major hair growth affecting factors are hormones and cytokines. Hair follicles have receptors for androgens and estrogens which affect hair follicle activity. Among other hormones, those derived from the thyroid and the pituitary glands have significant effects on hair growth. There are few other naturally produced chemicals called cytokines that resemble the functions of hormone in their action on cells.

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