Learning how to grow new limbs.

Growing new limbs to replace lost ones sounds like something created by Hollywood special effects wizards for science fiction movies, In the real world, humans can't grow new limbs, but some animals can. Fishermen who sell stone crabs' claws know that they can return the creatures to the ocean to grow new claws for another harvest.

Crustaceans - such as crabs, crayfish, and lobsters-are capable of creating a new limb completely from scratch. While scientists have long been intrigued by this uncanny ability little is known about the process. Working with fiddler crabs, University of Oklahoma zoologists Penny Hopkins and David Durica are trying to determine how, when a crustacean loses one of its appendages, it can grow a new one that is fully functioning and identical to the lost limb.

Crabs lose their legs quite frequently, Hopkins explains. When a bird decides to dine on one of the small crunchy animals, it grabs it by one of its 10 limbs and flies away. The crab can free itself by constricting a special set of muscles that causes the captive leg to fall off. It then can grow a new one in about two weeks - even the showy large claw that male fiddler crabs wave about to attract a mate. "These animals grow new nerve, new muscle, new everything!"

In many ways, this sort of regeneration resembles embryonic development. The cells that migrate into the site of the lost limb must have the ability to give rise to different kinds of cells. Each of these migrant cells is full of little packets of material that explode and cause a scab to form, the beginning of the regeneration process.

"Every cell in an organism has the same genetic information," Durica explains. "We want to know how that information gets programmed to develop into a particular structure and how that program gets carried out."

The scientists have determined that they can interfere with the laying down of the muscle protein in a regenerating crab leg by manipulating levels of the relevant steroid hormone (ecdysteroid). Hormones can go into every cell in an animal's body, but they only have an...