Going bald? Blame stem cell inactivation, say researchers.

A new study, led by George Cotsarelis of University of Pennsylvania School of Medicine, has found that stem cells do play an unexpected role in explaining what happens in bald scalp, the 'Journal of Clinical Investigation' reported.

Using cell samples from men undergoing hair transplant, the researchers compared follicles from bald scalp and non-bald scalp, and 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.

The researchers surmised that 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.

"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," Cotsarelis said.

At this point, the researchers don't know why there is a breakdown in this conversion. "However, the fact that there are normal numbers of stem cells in bald scalp gives us hope for reactivating those stem cells," said Cotsarelis.

In 2007, the researchers found that hair follicles in adult mice regenerate by re-awakening genes once active only in developing embryos.

In the latest study, the team also found a progenitor cell population in mice that is analogous to the human cells; these cells were able to make hair follicles and grow hair when injected into an immunodeficient mice.

The researchers say their next steps will be to study the stem and progenitor populations in other types of hair loss, including female pattern hair loss.

The 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.

They will also focus on identifying factors that could be used topically to convert stem cells to progenitor cells to generate normal large hairs.

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.