When he arrived at work the next morning, he was expecting all sorts of problems. “But Rocky is standing up in his cage, wanting breakfast and wagging his tail,” Dr. Badylak said. “I thought, well, this is pretty cool.”

Later experiments showed that over time the tube had lost the internal cells that are specific to intestines and gained cells specific to blood vessels. “It had morphed into a blood-vessel-like structure, which we thought was incredible,” he said. “Eventually we figured out that it was not the whole intestine but just the extracellular matrix that was responsible.”

Extracellular matrix from pigs, sheep and other animals has been used in the past decade as a reinforcing layer to help repair rotator cuff damage, hernias and other injuries. “Surgeons think of them as meshes that hold things together,” Dr. Badylak said. Most of them do not understand the matrix’s role in signaling and repair. “They don’t get it,” he said. “We didn’t either at first.”

The scaffolding is isolated by stripping out all of the living cells from a tissue or organ, leaving an intricate three-dimensional web of proteins and other compounds. Removing the cells eliminates the possibility that the material, of animal origin, will be rejected outright by the body when it is implanted. But the matrix does provoke a less intense immune response, Dr. Badylak said, which is necessary for it to work. “You actually need the immune system to recognize the material.”

“The body can say, ‘This is not me,’ but the signals that are there are actually telling me that I need to rebuild that tissue,” he added.

The matrix has to be in contact with healthy tissue, which is why scar tissue must be removed first. “If it’s put in the middle of a scar, it doesn’t remodel because it’s not exposed to the bloodstream and sources of cells,” Dr. Badylak said.