Hair follicle
A hair follicle is part of the skin that grows hair by packing old cells together. Inside the follicle the sebaceous gland is found, a tiny oil-producing gland found everywhere except on the palms and soles of the feet. The thicker density of hair, the more sebaceous glands are found. At the end of the hair , tiny blood vessels form the root, around the root there is a white structure called a bulb, which is visible on plucked healthy hairs.
Hair grows in cycles of various phases. Anagen(hair growth), catagen(involuting)and telogen(resting). Normally up to 90% of the hair follicles are in anagen phase while, 10-14% are in telogen and 1-2% in catagen. The cycles length varies on different parts of the body. For eyebrows the cycle is completed in around 4 month while it takes the scalp 3-4 years to finish, this is reason hair on eyebrows have a fixed lenght, while hair on the head seems to have no lenght limit. Growth cycles are controlled by chemical signals like epidermal growth factor.
Hair growth cycle times
- Scalp:
- anagen phase, 2-3 years (occasionally much longer)
- catagen phase, 2-3 weeks
- telogen phase, around 3 months
- Eyebrows etc:
- anagen phase, 4-7 months
- catagen phase, 3-4 weeks
- telogen phase, about 9 months
|
