More than 105,000 people are on the U.S. waiting list for an organ transplant. Thousands will die before it’s their turn. Thousands more never even get put on the list, considered too much of a long shot.

“The number of organs we have available are never going to be able to meet the demand,” said Dr. Amit Tevar, a transplant surgeon at the University of Pittsburgh Medical Center. “This is our frustration.”

That’s why scientists are looking to animals as another source of organs. A Maryland man lived two months after receiving the world’s first heart transplant from a pig last January — an animal genetically modified so its organs didn’t trigger an immediate attack from the human immune system. The FDA is considering whether to allow additional xenotransplantation experiments using kidneys or hearts from gene-edited pigs.

If the Food and Drug Administration agrees, the initial experiment will be outside a patient’s body. Researchers would place a pig-turned-humanlike liver next to a hospital bed to temporarily filter the blood of someone whose own liver suddenly failed. And if that novel “liver assist” works, it would be a critical step toward eventually attempting a bioengineered organ transplant — probably a kidney.

More complex is getting human cells to take over.

“We can’t take billions of cells and push them into the organ at once,” Ross said. When slowly infused, “the cells crawl around and when they see the right environment, they stick.”