In the latest step towards transplantable bioengineered parts, researchers have built rat forelimb tissue – complete with working blood vessels and muscle fibers – in the lab.

After they transplanted the biolimb into a recipient rat, the blood vessels filled with circulating blood, and the muscles even flexed the rat’s wrists and the joints in its paws.

For people who have lost a limb, transplants could help to improve the quality of life. But this also means having to take immunosuppressant drugs so that their bodies don’t attack the donated parts.

That’s why a lot research has focused on using the patients’ own stem cells to regenerate their own replacement tissues, but what’s been missing so far is the scaffold (or matrix) to provide shape and support for growing cells as they become the complex tissues that make up a limb.

So, a team led by Massachusetts General Hospital’s Harald Ott tried stripping away cells from an existing rat forelimb and then repopulating the remaining matrix with progenitor cells. This decellularization technique has previously been used to build bioartificial organs like kidneys, livers, hearts, and lungs in animals, but engineering tissues for a bioartifical limb is a different kind of task.

“The composite nature of our limbs makes building a functional biological replacement particularly challenging,” Ott explains in a news release. “Limbs contain muscles, bone, cartilage, blood vessels, tendons, ligaments, and nerves – each of which has to be rebuilt and requires a specific supporting structure called the matrix.”