It may be possible to reverse this

It was thought hair follicles, once damaged, could never be replaced.

But a University of Pennsylvania, US, team, writing in the journal Nature, says hair growth can actually be encouraged using a single gene.

A UK expert said the study could prove more important in aiding development of better wound-healing techniques.

This paper provides convincing evidence that the skin has remarkable powers of regeneration, not just repair as previously known

Professor Des Tobin, University of Bradford

The human head comes equipped with 100,000 tiny hair follicles, from each of which grow a single hair.

These follicles are produced by the embryo in the first stages of pregnancy, and it was thought that no further replacement follicles could be produced during life.

The Pennsylvania team found that a particular gene important in wound healing, called wnt, appeared to play a role in the production of new hair follicles.

In its experiment, small sections of the outer skin layer, or epidermis, were removed from mice.

Just this act appeared to awaken stem cell activity in the area, the scientists said, which included the production of a number of hair follicles.

If the action of the wnt gene was blocked, no hair follicles were produced; but if it was boosted, then many more hair follicles were produced, with the skin layer eventually being indistinguishable from surrounding areas.

'Remarkable powers'

The researchers said that their findings "opened a window" for new treatments not only for baldness, but for other aspects of wound healing and regeneration.

British expert Professor Des Tobin, from the University of Bradford, said: "This paper provides convincing evidence that the skin has remarkable powers of regeneration, not just repair as previously known.

"It was long thought that hair follicle development, under physiological conditions, was limited to early developmental process in the embryo.

"Now this shows convincingly that under the conditions peculiar to the wound-healing environment, the highly complex hair follicle can be created anew from apparently unremarkable cells of the healing epidermis and its underlying dermis."

He added: "The implications of this observation are many fold, but principally perhaps for what it tells us about the reprogramming power of adult stem cells, and it applications in regenerative medicine and wound healing."