Hair growth goes through several distinct stages:
- Anagen. The growth phase of hair. An unknown signal tells follicle stem cells to do their thing. Next, the permanent part of the follicle -- the dermal papilla -- gives the "go" signal to hair matrix cells. Those cells grow wildly and become pigmented, creating a new hair shaft. At any given time, 90% of hair cells are in this stage.
- Exogen. The new hair shaft pushes the old, dead hair shaft out of the skin. The old hair falls out.
- Anagen finished. The new hair extends beyond the surface of the skin and keeps growing. The hair shaft fully matures.
- Catagen. The lower two-thirds of the follicle shrivels up and is destroyed. The dermal papilla remains attached to the regressing follicle.
- Telogen. The withered follicle rests. It waits for a signal telling it to start all over again.
Losing hair is part of a normal cycle of growth and replacement. Hair follicles go through the growth and resting cycle in a nonsynchronized fashion. But sometimes things go wrong.
Common Hair Loss Problems: Androgenetic Alopecia
Most of us, when we think of hair loss, think about aging men. Nearly all men eventually get that receding M-shaped hairline and thinning hair on the top of the head, also known as male pattern baldness. It's called androgenetic alopecia, and it's caused by a by-product of testosterone called DHT.
Aging women have a similar problem. Their hair gets thin, although it's not clear that this is necessarily caused by sex hormones.
What is clear is that the same thing happens in aging men and women. Hair follicles get smaller. The anagen stage of hair growth gets shorter, and the resting (telogen) stage gets longer. The result: Thin, very short hairs -- and many follicles empty of hair shafts.
Why is this pattern of hair loss only in the front and on top? That's where hormone-sensitive follicles live. The follicles on the sides and back of the head aren't affected by DHT and usually stay healthy.