BOSTON – For the first time, scientists have rebuilt a complex human organ, the bladder, in seven young patients using live tissue grown in the lab – a breakthrough that could hold exciting promise for someday regenerating ailing hearts and other organs.

Only simpler tissues – skin, bone, and cartilage – have been lab-grown and transplanted in the past. This is the first time that a more intricate organ has been mostly replaced with tissue grown from the patient’s own cells.

“This suggests that tissue engineering may one day be a solution to the shortage of donor organs in this country for those needing transplants,” said Dr. Anthony Atala, the lead researcher. He said he believes the work provides a model for growing other tissues and organs.

The bladder transplants, performed on seven patients ages 4 to 19, were being reported online Tuesday in The Lancet medical journal. The research team at Children’s Hospital in Boston did the first procedure in 1999 but wanted to make sure it would work on others. The results weren’t announced while the doctors did the other surgeries and followed the progress of the last patient for almost two more years.

“It gives everyone in the field … the evidence and encouragement they’ve needed to say this can be done,” said Dr. Stephen Badylak, a University of Pittsburgh expert in tissue engineering.

Growing other organs will likely hold unforeseen challenges since organs are so specialized in their functions, scientists stress.

Even for people with bladder disease – and there are an estimated 35 million in the United States alone – Atala’s technique requires testing on more patients and for longer times, researchers say. Replacing an entire bladder would pose many more problems, including reconnecting urine tubes, blood supply, and nerve signaling, according to Dr. Steve Y. Chung, an Illinois urologist who wrote a commentary for The Lancet.

Still, he called the work “a tremendous, tremendous advance.”

For the children and teenagers in the study, the transplants reduced leaking from their bladders – a potentially big gain in quality of life. For 16-year-old Kaitlyne McNamara, the transplant has meant a new social life.

At the time of her surgery five years ago, her kidneys were close to failing as a result of her weak bladder. Now, they are working again, and she no longer wears a diaper. Instead, she was waiting for alternations on a dress for her junior prom.

“Now that I’ve had the transplant, my body actually does what I want it to do,” she said last week in Middletown, Conn. “Now I can go have fun and not worry about having an accident.”

Scientists, marveling at how animals like salamanders regenerate lost limbs, have long toyed with the possibilities of regrowing worn-out or injured human parts.