Though they can regrow part of the liver, humans and other mammals are generally thought to be incapable of true regeneration - growing a new organ or limb when it has been lost entirely.

But today, University of Pennsylvania dermatologists report they have indeed performed this feat of biological renewal, regrowing a "mini-organ" that is a sore subject for millions of older men: hair.

The researchers, whose findings appear in tomorrow's issue of the journal Nature, said they cut out small pieces of skin in mice, thereby triggering a genetic pathway that normally goes dormant after embryonic development. New hairs grew on bare patches where follicles had been completely removed, and the process worked especially well when researchers artificially boosted levels of a special signaling protein.

So far, the research has not been done in people, but already the wheels of commerce are churning: a startup company has licensed a patent related to the research, and one of the authors has a stake in the venture. The findings are of interest beyond the multibillion-dollar hair-loss industry, shedding light on how doctors might do a better job of healing people with burns and other wounds.

Bruce Morgan, an associate professor of dermatology at Harvard Medical School who was not involved with the research, called the research by Penn's George Cotsarelis "striking."

"He reprogrammed the cells to regenerate those organs from scratch," Morgan said of the hair regrowth.

The results appear to confirm a finding reported by a another Penn dermatologist more than 50 years ago. Albert Kligman, a professor emeritus who is better known for inventing the wrinkle cream Retin-A, reported that tiny new hairs grew in wounded skin. But his research was not well received, he recalled this week.

"People made fun of that paper, because they said . . . the epidermis of an adult is incapable of producing new follicles," Kligman said. "Sometimes happily you do something, and it sinks out of sight, and then it's rediscovered 50 years later."