Origin
Even before we are born, the human head contains specialized cells intended for hair growth. The number of hairs, the color and the texture depend heavily on genes. Hair growth begins as cells collect in a compartment called the matrix, surrounding a sac called the papilla---which intakes nutrients and amino acids from the blood stream for use in hair development. As excess cells push upward from the matrix they die, creating a string-like substance called keratin, which is held together by disulphide compounds.
Expansion
The hair shaft is what scientists call the hair we actually see on our head. It grows from a follicle, which is made of a ball of cells and a cavity---the tiny holes on our head. Melanocytes provide hair with its color just as they provide skin with its color. Combined with keratinocytes, the hair shaft emerges from the skin as part of the skin's complete structure, meaning hair grows as the epidermis (skin) grows.
Growth Cycle
Hair goes through three stages of development: anagen (growth), catagen (regression) and telogen (rest). During the anagen phase, which lasts two to six years and accounts for 85 percent of human hair, hair continues to grow as normal from the follicle at a rate of six inches per year. In the catagen phase, the hair follicle shrinks to 1/6 of its normal size and detaches from the papilla, no longer receiving nourishment. Two weeks later, hair enters into the telogen phase. Ten percent of hair, or about 10,000 hairs, reside in this phase for a period of five to six weeks, during which time the follicle prepares for new activity and roughly 100 hairs are shed daily. From there, the cycle repeats itself, becoming shorter in duration with age.
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