Hair growth cycle
Hairs grow from hair follicles in the skin, and new growth begins in the dermal papilla at the base of each hair follicle, which is nourished by blood capillaries. Normally hairs grow and fall out to be replaced by new hairs, following a hair growth cycle, with the following phases:
- Anagen or active growth phase, where the cells at the root of the hair are actively dividing and the hair grows in length by approximately 1cm in 28 days, over a 2-6 year period.
- Catagen or transition phase, which is the end of the growth phase and within the next 2-3 weeks, the fully formed hair becomes detached from the dermal papilla and its blood supply.
- Telogen or resting phase lasting 6-8 weeks, in which the old hair is shed from the hair follicle or remains until pushed out by a new hair that forms when the hair follicle enters anagen again and begins another cycle of hair growth.
What causes hair loss
Hair loss in men is due to an inherited condition called male pattern baldness or androgenic alopecia and this condition accounts for 99% of hair loss in men. This form of hair loss is caused by an inherited increased sensitivity to the male sex hormone dihydrotestosterone (DHT), which is an active form of testosterone. DHT is converted from testosterone in the hair follicles by the action of the enzyme 5-alpha reductase and men with androgenic alopecia usually have higher levels of this enzyme, which is found mainly in the scalp and prostate. Increased conversion of testosterone to DHT is also responsible for hair loss. Male pattern baldness is based on disruption of the normal hair growth cycle by DHT.
Loss of hair usually begins with a receding hairline followed by thinning of hair on the crown of the head and temples, which are areas of the scalp that have greatest sensitivity to DHT. Gradually the pattern of hair loss continues over the scalp leaving a rim of hair around the base of the scalp.
Baldness in women
Inherited baldness can also occur in women, and is known as female pattern baldness. It is seen in women with high levels of the male hormone testosterone, which is normally present in women but at much lower levels. The balding pattern is different in women, with a general thinning of the hair that gives an appearance of balding over the entire scalp.
Preventing hair loss
There is no cure for baldness but it is possible to prevent further balding in men with mild to moderate hair loss but not restore hair that has been lost for a long time. There are medications available that slow the progression of male pattern baldness and prevent further hair loss from all areas of the scalp, including the front.
Types of hair loss treatment
Several classes of hair loss treatment are available that work in different ways to prevent hair loss. These include:
- the 5α-reductase inhibitors finasteride and dutasteride, which are taken in tablet form and work by inhibiting the action of the enzyme type II 5α-reductase, which is found hair follicles and converts the male hormone testosterone to DHT.
- Tretinoin is a retinoid, which is an active metabolite of Vitamin A and has been found to stimulate skin cell growth including hair follicles, and this action promotes new hair growth. However, tretinoin does not prevent hair loss directly.
- Minoxidil and aminexil are both vasodilators that widen blood vessels and are used in topical applications for the scalp. They work by dilating small blood vessels in the skin, thereby increasing the blood supply to the hair follicles. Minoxidil is thought to work by stimulating cells that support hair follicle growth and trigger new hair growth in follicles that have not been inactive for long and have not yet become dormant.
- Saw palmetto is a dietary supplement of herbal extracts rich in fatty acids and phytosterols (plant sterols) that works as an anti-androgen in a way similar to the 5α-reductase inhibitors finasteride and dutasteride, blocking the convertion of testosterone to DHT.
- Ketoconazole is an antifungal that can be used as a shampoo and is thought to work as an anti-androgen to inhibit the production of testosterone in the skin; also it may inhibit the 5α-reductase enzyme thereby inhibiting the production of DHT.