George Costanza wore it with pride in the Seinfeld series, and Michael Jordan decided to just shave it all off. We all know about the "comb over" and the always adored toupee or hair piece. However, surprisingly little is known about its cause at the cellular level considering the stress it seems to cause amongst the male population.

Men go bald for many reasons, but for men 20 to 45 years old who start to lose scalp hair, the chances are 95% that one is experiencing male pattern baldness. Male pattern baldness follows a typical sequence or pattern, and air loss can start in different areas but is usually at the temples and/or on the crown of the head. Initial thinning of hair progresses over a number of years and may lead to total baldness but more typically loss of hair over the top surface of the head.

George Cotsarelis, MD, chair of the Department of Dermatology at the University Of Pennsylvania School Of Medicine in a new study, has found that stem cells play an unexpected role in explaining what happens in bald scalp. Balding may arise from a problem with stem-cell activation rather than the numbers of stem cells in follicles. In male pattern balding, hair follicles actually shrink; they don't disappear. The hairs are essentially microscopic on the bald part of the scalp compared to other spots.

Cotsarelis explains:

"We asked: 'Are stem cells depleted in bald scalp?' We were surprised to find the number of stem cells was the same in the bald part of the scalp compared with other places, but did find a difference in the abundance of a specific type of cell, thought to be a progenitor cell. This implies that there is a problem in the activation of stem cells converting to progenitor cells in bald scalp. However, the fact that there are normal numbers of stem cells in bald scalp gives us hope for reactivating those stem cells."

To date however, researchers don't know why there is a breakdown in this conversion. The research team decided to lift cell samples from men undergoing hair transplants and then compare follicles from bald scalp and non-bald scalp patients. They discovered that found that bald areas had the same number of stem cells as normal scalp in the same person. However, they did find that another, more mature cell type called a progenitor cell was markedly depleted in the follicles of bald scalp.

What are the next steps in researching this phenomenon? First, the stem and progenitor populations in other types of hair loss, including female pattern hair loss will need to be examines. This information may assist in developing cell-based treatments for male pattern balding by isolating stem cells and expanding them to add back to the scalp directly. Focus will also be on factors that could be used topically (such as Minoxidil) to convert stem cells to progenitor cells to generate normal large hairs.

First author Luis Garza, MD, PhD, a dermatologist and former postdoctoral fellow in the Cotsarelis lab, performed much of the work and is now an assistant professor of Dermatology at Johns Hopkins University.

The research was funded in part by the National Institute of Arthritis and Musculoskeletal and Skin Diseases; the Pennsylvania Department of Health; the Fannie Gray Hall Center for Human Appearance; and L'Oreal.

Written by Sy Kraft, B.A.