WASHINGTON (Reuters) - Mexican salamanders who can re-grow amputated legs are not pulling off quite as big a biological trick as scientists had first thought, which may help doctors trying to regenerate human limbs.

A salamander swims in an aquarium at Mexico City's National Autonomous University of Mexico (UNAM ) laboratory June 10, 2009. REUTERS/Felipe Leon

The little buds that eventually produce a brand-new leg have not completely reverted to an embryo-like stage, the researchers reported in the journal Nature.

Instead, they seem to form a new leg from cells that partly remember how to make bone, muscle, or nerve tissue, Elly Tanaka of the Center for Regenerative Therapies in Dresden, Germany, and colleagues reported.

However, how the little animals called axolotls or water monsters do this is still a mystery.

“How this is achieved in the salamander and why it does not occur in mammals remains an important question,” the researchers wrote.

“It gives you more hope for being able to someday regenerate individual tissues in people,” Malcolm Maden of the University of Florida, who worked on the study, said in a statement.

“If you can understand how they regenerate, then you ought to be able to understand why mammals don’t regenerate.”

All animals can regenerate to some degree. A human fingertip can sometimes grow back and cuts often heal with minimal scarring.

In salamanders, the blood vessels contract quickly and limit bleeding when a limb is cut. Skin cells quickly cover the wound and form what is called a blastema.

Researchers thought these cells must be pluripotent stem cells -- the body’s master cells, which retain the ability to start growing into any kind of cell in the body.

These cells are found in days-old embryos, but quickly differentiate -- becoming the various cell types from muscle to nerve to skin or blood.

Tanaka’s team found the blastema is not full of embryonic-like stem cells, but something slightly more mature.

“It is likely that cells at the amputation plane are indeed undergoing reprogramming events that allow them to re-enter embryonic programs of tissue formation, even if they do not revert back to the earliest pluripotent state,” they wrote.

But each cell in the blastema does not have the power to grow bone, muscle and nerve. Instead, there appear to be small groups of predestined cells, the researchers found. And at least some of the new tissue seems to come from ordinary skin.

It may be possible to replicate this some day in other animals and eventually people, researchers said.

“Nonetheless, the adult salamander tissue uniquely generates these blastema cells to reconstitute an entire limb after amputation, a property that other systems do not have,” the researchers noted.