Hair follicles pass through cyclic phases of activity and inactivity during which the inferior segment below the attachment of the arrector pilorum periodically appears and disappears. The Cycle starts with-
- Anagen (3-10yrs).
- Catagen (3days-3wks).
- Telogen (resting phase for 3mths).
Anagen: It is the growth phase. It lasts for 3-10yrs in scalp hair. Out of 1,00,000 hairs present on the scalp, about 80-95% are in this stage. It has thick, dark base with preserved inner and outer sheaths. The bulb is at an angle of 20 degree with shaft.
Catagen: It is the involution period. At the end of the anagen phase,the follicles go through involution or programmed cell death. About 1% hairs are in this stage and it lasts for about 2 weeks on human scalp. There is gradual thinning of hair with decrease of pigment at the base of hair shaft. At the cellular level the melanocytes stop producing melanin.
Telogen: It is the resting phase. The hair shaft matures into a club hair which is eventually shed. The hair shaft may dislodge on traction. About 10% of hair is in this stage and about 100 hairs are shed daily which is normal. This phase lasts for 1-3 months following which the follicles enter the anagen phase.