Hair grows in continuous cycles that are partially controlled by a number of steroid hormones produced in the hair follicle and other organs of the body. Steroid hormones are a group of biologically active compounds that act as signalling molecules influencing the hair growth cycle.
Hair follicles undergo cyclical rounds between growth (anagen), transition to rest (catagen), rest (telogen) and shedding (exogen).
The longest phase in the cycle is anagen phase, this phase can last up to 7 years. In anagen phase keratin cells are constantly multiplying, the increased number of keratin cells cause keratin cells to move up the hair shaft and emerge from the scalp. Hair will continue to grow as long as keratin cells in the base of the hair follicle continue to multiply.
The catagen phase is a short transition period which is a type of regression from growth. The keratin cells in the base of the hair follicle receive a signal to stop active growth, the hair fibre detaches from its blood supply and from the hair follicle stem cells in the epithelial column that promote new growth. A ‘club hair’ is formed at the base of the hair fibre which features a bulb of keratin protein at the root tip of the hair strand. The keratin bulb keeps the hair follicle embedded within the dermis until the hair sheds.
The telogen phase is the resting phase where hair the club hair sits in the dermis. This phase usually lasts around 3 months but the length of time a hair can remain in telogen is dependant on the thickness of the dermis and the size of the keratin bulb anchoring the resting hair fibre in the scalp.
The exogen phase is the point where the hair fibre completely detaches and is shed from the scalp. This phase should occur when a new active follicle in early anagen phase is growing below the resting hair fibre.
Normally up to 10% of hairs are in resting phase and 90% of hairs are in growth phase. When the percentage of hairs in telogen phase increase, there will be an increased rate of shedding. Thinning tends to become noticeable when the number of hairs in anagen phase falls below 60%.
A number of hormones affect hair growth, the include: prolactin, melanocyte-stimulating hormone and thyroid hormones. Four hormones that play a particularly important role in maintaining the balance between resting hairs and growing hairs are androgens, progesterone, estrogens and cortisol.
The hair growth cycle. Each hair follicle goes through an independent cycle of growth and rest.