Boston — The once-fanciful dream of regrowing the heart and other failing organs has suddenly edged closer to reality: The first complex organ, the bladder, has been rebuilt in seven patients from living tissue cultivated in the lab.

"It's really science fiction at its best," marvels Tracy McNamara whose daughter got one of the first transplants and is living a more normal life because of it.

The breakthrough was reported online Tuesday in The Lancet medical journal where doctors explained how they mostly replaced diseased bladders from seven youngsters with tissue grown from the patients' own cells.

Though simpler tissues like skin and bone have been regrown and transplanted, this is the first time it has been accomplished with a more complex organ.

"It's startling and exciting," declared Dr. Eli A. Friedman, a transplant surgeon at SUNY-Downstate Medical Center in New York who was briefed on the findings.

"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.

Growing other organs will likely hold unforeseen challenges, however, 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 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, of Middletown, Conn., 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 alterations on a low-cut champagne-colored dress for her junior prom.

"Now that I've had the transplant, my body actually does what I want it to do," she said in an interview last week at her uncle's house in Haddam, Conn. "Now I can go have fun and not worry about having an accident."