American doctors have released details about another operation involving the transplant of pig organs into humans.

    A medical team from the University of Alabama at Birmingham said it had successfully transplanted pig kidneys into a brain-dead human. The operation took place last September, but was first reported January 20.

    Similar operations have taken place in recent months.

    In October, doctors at New York University temporarily attached a pig kidney to blood vessels outside the body of a brain-dead human. And earlier this month, doctors at University of Maryland School of Medicine in Baltimore transplanted a pig heart into a living human patient.

    In all of the operations, doctors used organs from genetically modified pigs provided by Virginia-based medical company Revivicor.

    The latest experiment in Alabama was performed on 57-year-old Jim Parsons, who was declared brain-dead after being injured in a motorcycle accident. His family donated his body to science.

    For a little more than three days -- until the man's body was removed from life support -- the two pig kidneys survived with no signs of immediate rejection, the medical team reported. The results were recently published in a study in the American Journal of Transplantation.

    Dr. Jayme Locke of the University of Alabama at Birmingham led the new study. She told The Associated Press the experiment marks the beginning of a planned series of pig kidney transplants.

    "The organ shortage is in fact an unmitigated crisis, and we've never had a real solution to it," Locke said.

    She added that an important finding of the latest operation helped answer a major question: Could the pig kidney blood vessels survive the force of human blood pressure? She said the operation proved that the answer was yes.

    One kidney was damaged during removal from the pig and did not work effectively, the team reported. But the other quickly started producing urine as a kidney is supposed to.

    Locke said no pig viruses were passed on to the human, and no pig cells were found in the man's bloodstream. She added that the latest experiment showed that brain-dead bodies can serve as much-needed human models to test out possible new treatments.

    In the donor pigs, scientists removed several genes linked to organ rejection. They also removed another gene in an effort to prevent too much growth of pig heart tissue.

    Dr. Robert Montgomery has led similar experiments at New York University's Langone Health in New York City. He told the AP that scientists still have a lot to learn about how long pig organs survive, and how best to genetically change them. "I think different organs will require different genetic modifications," he said.

    Organ donor organizations estimate there are about 110,000 Americans currently waiting for an organ transplant. And more than 6,000 patients die each year before getting an organ, reports organdonor.gov.

    I'm Bryan Lynn.