AN EARY TALE

A live mouse with a human ear growing out of its back? Surely it’s a freak or a fake, something out of a carnival sideshow or supermarket tabloid? No, the startling creature that showed up in newspapers and on television last week is quite real and actually serves a scientific purpose. It is the latest and most dramatic demonstration of progress in tissue engineering, a new line of research aimed at replacing body parts lost to disease, accident or, as is often the case with a missing ear, a schoolyard fight.

Today doctors correct such defects by reconstructing missing organs with synthetic materials or transplanting tissue from other humans or animals, a procedure that carries the risk that the body will reject the implant. The idea behind tissue engineering is to trick the body into regenerating its missing parts. The mouse with the extra ear was created by scientists at the University of Massachusetts and M.I.T. to prove that the basic technology can work. In practice, humans would grow their own tissue, without the help of mice.

To make the mouse’s third ear, scientists fashioned a precision mold out of porous, biodegradable polymer, seeded it with human cartilage cells, then tucked the structure under the skin of a mouse bred without an immune system (to prevent rejection). Nourished by mouse blood, the cartilage cells multiplied, taking the shape of the dissolving polymer scaffold and creating a perfectly formed human ear.