Hair Growth (Life) Cycle
Οn our scalp there are 55.000 – 60.000 follicular units on average, which give 100.000-150.000 hairs. Hair follicles are complete biological units that comprise nerves, blood vessels, sebaceous glands, collagen and the small muscle arrector pilli. Each hair follicle is a naturally occurring group of 1-4 hairs. Hair follicles are complete structures in which take place specialized operations for hair production.
The Hair growth Cycle is an asynchronous, repeated process and consists of 3 phases:
Ι. Anagen Phase
ΙΙ. Catagen Phase
ΙΙΙ. Telogen Phase
The greatest percentage of our hair, approximately 80%, is in the Anagen Phase, which lasts from 2 to 6 years. Hair follicles in the Anagen Phase are intensely active and lie at the bottom of epidermic fat. Hair in the Anagen Phase has enough quantity of pigment and intense colour; grows daily circa 0.035cm, preserving this rate of growth during the entire Anagen Phase.
Catagen Phase follows on the Anagen and lasts 2 weeks on average. During the Catagen Phase, hair follicles become inactive, while cell division stops. Hair follicles start shrinking, hair growth stops and hairs are being prepared to drop. Approximately 1 to 2% of hair lie in the Catagen Phase. It can be regarded as a transitional period which prepares hair to come into the Telogen Phase.
In some cases, Catagen Phase also includes Dystrophic Catagen Phase. During this phase hair break easily and fall, as a consequence of diseases, infections or medicine. Dystrophic Catagen, when it occurs, concerns approximately 1 – 2% of the hair, while the physiological rate is 0%.
Telogen Phase lasts approximately 100 days (3-4 months). Hair follicles become inoperative, hair growth is inhibited and hairs remain in hair follicles for a short period before falling out. Almost 20% of hair is in the Telogen Phase.
At the end of the Telogen Phase, the hair growth cycle starts again and hair enters the Anagen Phase. As hair follicles reform, new hairs start growing and emerging, boosting the old hair to drop. In hair loss cases, the afore-mentioned 3 phases of the hair growth cycle undergo alterations. More specifically, the percentage of hair in the Anagen Phase decreases while the percentage of hair in the Telogen Phase increases.
All the above are an average estimation of the hair growth cycle operation. Further genetic factors, external parameters and specific conditions may differentiate hair behavior from person to person or even in the same person.