Hair Life Cycle
The human hair follicle is a complex cutaneous appendage in which more than 20 cell populations are actively involved in the process of hair fiber formation. Hair growth is the consequence of proliferative activity of the hair follicle cells, which exhibit one of the highest mitosis rates in the human organism.
The hair growth cycle consist of three stages:
Anagen (Period 2-6 years)
A new hair bulb develops during the Anagen phase, and hair formation starts. The duration is genetically predisposed. On average 85% of the human scalp hair is in this phase. The follicle is divided into an upper and a lower part; the permanent upper portion is located above the bulge, and the transient part is below the bulge. The bulge is a bump on the upper part of the follicle’s outer root sheath, just below the entrance of the sebaceous glands. After the hair follicle has grown for a period of time, it discards the transient portion and enters into catagen.
Catagen (period 1-3 weeks)
When the follicle is nearing the end of its growth period and prepares to enter catagen, the matrix cells stop differentiating and become a column of cells. Melanocytes stop functioning, so that the last segment of each hair formed is white. The club is then formed in the keratogenous zone, and the mitosis stops in the matrix cells. The transient part of the follicle, which shrinks to one-third of its original size. In the meantime, the inner root sheath metamorphoses into an anchoring substance that secures the hair club into the epithelial capsule. The follicle is surrounded by a row of macrophages that clean up the debris from the follicle.
Telogen (period 2-3 months)
The percentage of scalp hair in the telogen stage is about 15%. The quiescent follicle represents what remains of the original follicle (the permanent part) after catagen. During telogen, the follicles remain quiescent from area to area gets longer with aging. When the end of telogen varies from area to area and gets longer with aging. When the end of the telogen stage is reached, the dead hair is discarded, and the formation of a new hair starts, which enters the Anagen stage.