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頭髮生長週期

 

The Three Phases of The Hair Growth Cycle 

Hair is composed of a strong structural protein called Keratin. It is the same type of protein that makes up nails and the outer layer of skin. Below the surface of the skin is the hair root, which is enclosed within a hair follicle. At the base of the hair follicle is a small gland called the dermal papilla. The dermal papilla is fed by the bloodstream, which carries nourishment to produce new hair. Hair follicles grow in repeated cycles. One cycle can be broken down into three distinct phases. Each phase has specific characteristics that determine the length and strength of the hair. Random number of hair strands may undergo any of these three stages of hair shedding and hair growth.

1. Anagen (Growth) Phase

Normally, up to 90% of hair follicles are in the anagen phase at any given time where the fair follicles are in active growth. The cells in the dermal papilla divide to produce new hair fibers, and the follicle buries itself into the dermal layer of the skin to nourish the strand. During this phase, hair typically grows 1 cm every month and can continue growing for 2 to 6 years. Genetics, diet and stress levels determine the period of time the hair follicle stays in the anagen phase.

2. Catagen (Transitional) Phase

At the end of the growth period, hair follicles prepare themselves for a resting phase. This transition period is called the catagen stage. It allows the hair follicle to, in a sense, renew itself. During this stage, the hair follicle shrinks and the dermal papilla detaches from the follicle, cutting off the nourishing blood supply to the hair strand. Ultimately, the follicle reduces to 1/6 its original length, causing the hair root to be pushed upward towards the surface of the scalp. This stage usually lasts for about 1 to 2 weeks. 

3. Telogen (Resting or Shedding) Phase

This is the resting period of a hair follicle, which can last for two to four months. During the rest period the hair is still attached to the hair follicle, but it is not growing. When the body is subjected to severe stress, up to 70% of hair can enter the resting phase prematurely. The hair begins to fall, causing a noticeable loss of hair. This condition is called telogen effluvium. After the resting phase, older hair that has finished their life is shed and a new hair begins to grow, thus, starting a new hair growth cycle. Normally, 50 to 100 hairs are shed everyday from the scalp.

The length of time for these phases varies from person to person along with hair colour and follicle shape. Normally up to 90% of the hair follicles are in anagen (growing) phase, 1-2% in catagen (transitional) phase, while 10-14% are in telogen (resting or shedding) phase.