Answer:
The living cells at the base of the hair show active mitotic growth. They eventually form a compact column, which extends toward the surface of the skin. A zone of keratinization forms directly above the actively dividing cells. The living cells become dehydrated, eventually die, and are converted into a mass of keratin. The average rate of growth of scalp hair is 1 cm per month.
The life cycle is divided into three phases:
Anagen- actively growing period. About 90% of the hairs on the scalp are in the anagen phase, which lasts about 3 years. Catagen- When a hair approaches catagen, the base of the hair becomes keratinized to form a club, and the melanocytes change and stop producing melanin. A constricted epidermal strand remains, joining the dermal papilla with the hair club, which moves toward the surface, completing catagen. The follicular bulb is destroyed. Catagen lasts about 1 to 2 weeks.
Telogen- The dermal papilla becomes released from its epidermal investment. A small nipple of undifferentiated tissue known as the secondary germ is left. Telogen lasts about 3 to 4 months. During this period, the follicle is inactive. Approximately 10% of the hairs are in telogen.