Here's a bold, encouraging step forward in medicine: Surgeons attached a kidney grown in a genetically altered pig to a human patient and found that the organ works normally. This dramatic breakthrough eventually could yield a vast new supply of organs for severely ill patients – though that's likely years away because medical and regulatory hurdles remain.

Testing experimental animal organs in humans raises ethical questions, so New York University hospital doctors took an unusual step in September. With permission from a brain-dead patient's family, they attached the pig's kidney to the woman being kept alive on a ventilator -- the first operation of its kind. The fact that the organ functioned outside the body is a strong sign that it will work in the body, said Dr. Robert Montgomery, the lead surgeon. "There didn't seem to be any kind of incompatibility between the pig kidney and the human that would make it not work,” he added. "There wasn't immediate rejection."

Researchers tracked the results for just 54 hours, leaving many questions about long-term results. Still, it's seen as a very encouraging advance. A steady supply of organs from pigs — potentially including hearts, lungs and livers — would offer a lifeline to the more than 100,000 Americans now on transplant waiting lists, including the 90,240 who need a kidney. An even larger number of Americans with kidney failure — more than a half-million — depend on grueling dialysis treatments to survive. "It's truly mind-boggling to think of how many transplants we might be able to offer," says Dr. Amy Friedman, a New York specialist. Pig heart valves are routinely transplanted into humans, and some patients with diabetes have received pig pancreas cells. Pig skin has been used as temporary grafts for burn patients.

NYU surgeon says: "It was better than I think we even expected. It just looked like any transplant I've ever done from a living donor. A lot of kidneys from deceased people don’t work right away, and take days or weeks to start. This worked immediately." – Dr. Robert Montgomery, who performed the procedure

Outside expert says: "This is a huge breakthrough. It’s a big, big deal." -- Dr. Dorry Segev, a professor of transplant surgery at Johns Hopkins School of Medicine

Critics say: "Pigs aren't spare parts and should never be used as such just because humans are too self-centered to donate their bodies to patients desperate for organ transplants." -- People for the Ethical Treatment of Animals (PETA), a national group in Norfolk, Va.