For Some Worms, It's Heads or Tails Yoshihiko Umesono Two species of flatworm with differing abilities to regenerate body parts. Dugesia japonica, left, can regrow a head from a tail frament, while Phagocata kawakatsui can not. Losing your head is a bigger problem for some species than others. Among flatworms, there are those that can fully regenerate a severed head or tail, and those that can regrow only a tail. The question is, Why the difference? The answer, courtesy of three separate studies published last week in Nature, seems to be a single signaling pathway known as WNT/beta-catenin. When left to function normally, the intercellular pathway in some flatworms enables tail regrowth but prevents formation of a head. But by inhibiting the pathway, scientists were able to induce head regeneration in parts of animals that didn't previously have that ability. "This pathway is required for making tails in the animal," said an author of one of the studies, Phillip A. Newmark, at the University of Illinois at Urbana-Champaign. When you eliminate the ability of this pathway to function, you don't make a tail, and instead you make a new head. Why some animals can regrow lost body parts and others can't is one of the most intriguing questions in biology, Dr. Newmark said. And while human limb (or head) regeneration is not in the foreseeable future, studying the process in simple animals may yield benefits for larger ones. "The more we study this," he said, "the more themes will hopefully emerge that will help us figure this out for more complex organisms like ourselves."