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A hair consists a hair bulb, a stem, an isthmus, an infundibulum and a shaft. The shaft is seen outside the skin while all the others within the skin. There are stem cells present in the bulge area of the hair (isthmus). These stem cells are also important for hair growth and regeneration. The stem cells are present at other sites as well such as root sheath. At, the bulb, these is dermal papilla which also regulates hair cycle.
When there is inflammation and injury in the lower part the hair can regenerate( alopecia areata). When there is injury in the isthmic area with destruction of stem cells, hair regeneration is not possible (cicatricial alopecia).
Hair growth occurs in cycles. Hair are in different phases of this cycle. The anagen (growing phase, catagen (transition phase), telogen (resting phase) and kenoogen ( shedding phase). At any given point, in scalp, 80-90% hair are in anagen phase, 1-2 % are in catagen phase and 10% are in telogen phase.
Considering that there are 1,00,000 hair on human scalp, 10% are in telogen phase. Hence, loss of upto 100 hair in a day is considered normal as these many hair are in the shedding phase.
The duration of all the phases especially growing phase depends on age,growth factors hormonal factors, nutrition, any disease state etc. Certain growth factors like IGF1 and fibroblast growth factor 5 potentiate hair growth.
There is evidence that injury to scalp also stimulates hair growth. Emotional stress and psychologic distress can cause hair loss.
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