The hair cycle
Hair grows in groups of 1 to 4 hairs called hair follicles hair follicles. We are born with all the hair follicles we will ever have in the scalp(approximately 100,000). Hair follicles replenish themselves by going through three phases, the Anagen phase Catagen phase and Telogen phase, known as the hair cycle.
This is the reason why hair grows to a particular length and thickness. The duration of the Anagen phase, between 2 to 6 years, determines the length of the hair and the hair bulb determines the hair shaft diameter. The growth rate is approximately 1cm per month.
Diagram showing a cross-section of the skin and hair follicles.
In the Anagen phase in a normal scalp, most follicles are growing (90-95%). Anagen is the active growth phase of hair follicles. The cells in the root of the hair are dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2-7 years. The amount of time the hair follicle stays in the Anagen phase is genetically determined and is shorter for men with male pattern baldness.
At the end of the Anagen phase an unknown signal causes the follicle to go into the Catagen phase. The Catagen phase is a short transition stage that occurs at the end of the Anagen phase. It signals the end of the active growth of a hair. This phase lasts for about 2-3 weeks while a club hair is formed. Note in the image that the bulb is retracting.
The hair follicle is completely at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root. At any given time, 10 – 20% of all hairs are in the Telogen phase. This phase lasts for about 100 days for hairs on the scalp. At the end of this resting phase the old hair is pushed out by the actively growing new hair shafts, approximately 100 – 150 hairs per day. Note in the image that the hair is being extruded and replaced by fibrous tissue.