A researcher at Massachusetts General Hospital has successfully grown the world's first biolimb, The Washington Post reports. It's a rat's forearm and hand, and it looks a little smaller than an adult human's thumb. Researchers are hopeful this could be the first step toward growing fully functional limbs in labs.

"We're focusing on the forearm and hand to use it as a model system and proof of principle," Harald Ott, the researcher who grew the limb, told New Scientist. "But the techniques would apply equally to legs, arms, and other extremities."

Ott, a thoracic surgeon, used a process of decellularization and recellularization to grow the limb. First, he stripped living cells from the limbs of dead rats to expose their "scaffolds," or inert protein collagen. The rat's forearm "scaffold" included the collagen structures that make up blood vessels, tendons, muscles, and bone, New Scientist reports. Then, for the recellularization process, Ott seeded those collagen structures with cells from the recipient. The seeded scaffolding was then nursed in a bioreactor and given nutrients, oxygen, and electrical stimulation, which allowed blood vessels and muscles to regenerate. After the limb was in the bioreactor for about two to three weeks, Ott coated the limb, which doesn't have bones or cartilage, with skin grafts.

Once the process is complete, none of the original donor's soft tissue remains. Because the limb is grown using only the recipient's cells, a biolimb would likely negate the need for immunosuppression, which prevents the body's rejection of an artificial limb.

Although the biolimb is an encouraging first step towards growing fully functional human limbs, scientists note there is still a long way to go. "It's a very exciting development, but the challenge will be to create a functioning limb," Daniel Weiss, a researcher at the University of Vermont College of Medicine, told New Scientist.

The rat limb can respond to stimuli and circulate blood, but it's not clear yet if the muscle can function on its own. Using electrical pulses to activate the muscles, Ott found that the rat's hand could clench and unclench. He also attached the limb to anesthetized living rats and found it was able to circulate blood, but he didn't test for muscle movement. The limb still needs to be seeded with bone and cartilage to see if those cells can regenerate, and the researchers need to prove that a nervous system can develop around the biolimb.

Decellularization, or decel, has been used to grow hearts and lungs, and simpler structures like windpipes have been grown and used in human transplants. But a hand has many more cell types and nerves than something like a windpipe, making it much more difficult to grow in a lab.

Biolimb engineering could eventually create human limbs that look and function just like any other, but Ott estimates it will be at least a decade before the first human biolimbs are ready for testing.