Amphibians such as the salamander can regenerate entire fore and hind limbs, while lizards can regrow their tails. Mammals had appeared to be quite incapable of this extraordinary act. If they are severely wounded they manage a crude repair of the damage with scar tissue.
But yesterday a scientist at a private medical research centre in Philadelphia revealed to the American Association for the Advancement of Science how, five years ago, she had stumbled across a strain of mouse which does have the ability to regenerate lost tissue. Her work is soon to be published in a medical journal.
Even when one centimetre - about a sixth - of its tail is sliced off at the tip, this type of mouse can regrow three-quarters of the missing portion of tail, with a normal looking covering of skin and hair.
Professor Ellen Heber-Katz, an immunologist at the Wistar Institute in Philadelphia, discovered the "healer" mouse when she was carrying out work on various mouse strains aimed at understanding what underlies multiple sclerosis. She was using a strain with a mutation which made it "autoimmune" - its infection- fighting immune system goes into unprovoked overdrive, eventually killing the mouse prematurely.
In most research work on miceeach animal has to be marked individually so that the researchers can identify it. Usually this is done by making a pattern of small holes through their thin ears, which are then permanent.
But in the mice Dr Heber-Katz was using these holes closed up and disappeared. The researchers, thinking they had made a mistake, re-pierced the mice ears. The holes closed again, with a full replacement of the layers of skin - the epidermis and dermis - along with cartilage, fatty tissue, sweat glands and small blood vessels. The regrown, regenerated ears looked normal with no scar tissue.
This regeneration seems very similar to what happens in amphibians. At the site of the wound a bulge of rapidly dividing immature cells form with the potential to become different tissues - rather like a very early embryo. Furthermore, in the flesh next to the wound a thick layer of protein, the extracellular matrix, which normally separates different types of tissue is seen to break down during the rebuilding process.
Since making the discovery, the research team has found there are several genetic differences linked to its ability to regenerate body parts. There is probably a network of genes shared between mammals and amphibians which carry the instructions for limb regeneration, but in mammals they have been permanently switched off in the course of evolution.