The team at the University of Pennsylvania say though damaged hair follicles were thought to be irreplaceable, hair growth can be encouraged using a single gene.

The human head has as many as 100,000 tiny hair follicles, from each of which grows a single hair and as the follicles are produced in the embryo stage of human development it was thought that no further replacement follicles could be produced during life.

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

In an experiment using mice, small sections of the outer skin layer, or epidermis, were removed from the creatures and the procedure appeared to awaken stem cell activity in the area, which included the production of a number of hair follicles.

The scientists found that 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, and the skin layer eventually became indistinguishable from surrounding areas.

Dr. George Cotsarelis, a dermatology professor at the University of Pennsylvania School of Medicine in Philadelphia who led the study, says the findings could lead to remedies for male-pattern baldness and other types of hair-loss.

Experts say the study provides convincing evidence that the skin has remarkable powers of regeneration, and shows that under certain conditions hair follicles can be created.

They say the study demonstrates that mammals possess greater regenerative abilities than commonly believed and the findings could prove more important in the development of better wound healing techniques.

The study is published in the journal Nature.

The authors note that few studies have focused on pregnancies of Rh negative mothers who received RhIg during pregnancy, probably because the thimerosal is diluted before reaching the fetus and has been assumed to be innocuous. Nevertheless, there is a concern that even very small doses delivered when the brain is especially sensitive can be toxic. Numerous internet sites and one research study assert that RhIg causes autism and that a high percentage of mothers of children with autism are Rh negative, neither of which was shown to be true in the current study. In addition, a recent study hypothesized that Rh incompatibility itself could disrupt fetal neurodevelopment thus playing a role in autism, but the current study found no increase in the proportion of Rh incompatibility in mothers of autistic children. In response to the claim that only certain groups of children are at risk, the authors also analyzed specific ASD subgroups and found that none had significant increases in either Rh negativity or thimerosal exposure during pregnancy.

"This study adds to the evidence that there is no causal association between thimerosal and childhood autism," the authors state. They point out that even though RhIg and childhood vaccines are now thimersol free in this country, it is important to analyze questions of safety since thimerosal continues to be used in many places around the world to preserve vaccines, which makes them affordable. They conclude, "We hope this report of no association between autism, Rh negativity and thimerosal exposure during pregnancy will offset some of the decreased compliance with immunization recommendations which is known to increase morbidity and mortality from childhood infectious diseases."

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