Unlike other mammals that shed or grow hair according to the season, in humans hair growth and loss are random and not strictly but somewhat seasonal. At any given time, in humans a random number of hairs will be in various stages of growth and shedding.
The three phases of hair growth cycle are as follows: anagen, telogen, and catagen. Anagen is the active growth phase during which the cells in the matrix are dividing rapidly, adding to the hair shaft. During this phase, the hair grows approximately 1–2 cm (i.e., 1/2–1 inch) per month. The catagen stage is a short transition phase that occurs at the end of the anagen phase during which the hair follicle receives a signal to stop growing. Shortly after the hair growth stops, the dermal papilla contracts and releases the hair shaft from the follicle allowing hair to shed. The telogen phase describes the resting time for the hair follicle. During this phase, a secondary hair germ forms from a band of epithelial cells that move upward, often seen during graft dissection as a short, smaller version of the hair bulb.
Each hair follicle goes through 10–20 cycles in a lifetime, while each cycle lasts a different time: anagen lasts 3–10 years, catagen 2–3 weeks, and telogen 3–4 months. This may explain why some hairs live longer than others, as the hairs on the back of the head may be programmed to survive for twenty 10-year cycles and live for 200 years. In the human scalp, the cycles are not synchronized. Therefore, at any one time, an average of 10% of the hair is in the telogen phase (with the thought that it can range from 4% to 24%), only 1–2% is in catagen and the rest (85–90%) of the hair remain in the anagen phase. About 25–100 hairs are shed normally each day.
Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth. However, people with very long hair have a long active phase of growth. The hairs on the arms, legs, eyelashes, and eyebrows have a very short, active growth phase (1–7 months) explaining why they are so much shorter than scalp hair.
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