It's the first time a major organ - in this case, a bladder - has been grown from a patient's own cells. It's something scientists have been trying to do for many years. Before this breakthrough, only simpler tissues like bone, skin and cartilage had been grown from stem cells in laboratories. It offers hope for people awaiting transplants that other organs can be grown and transplanted in the same way. It's still a long way before this becomes commonplace, but it may mark the beginning of the end of the long wait to find a suitable organ donor.

What difference does it make whether the organs are grown in a laboratory or taken from another person?

Growing organs from a patient's own cells means their immune system won't reject the new tissue. Organ recipients take a life-long course of anti-rejection drugs because their immune systems recognise non-self tissue and try to get rid of it.

Who grew the bladder?

Well, to be clear, it wasn't an entire bladder, just a large part of one that was grafted onto the patients' own failing bladders. The Institute for Regenerative Medicine at Wake Forest University School of Medicine carried out the work, and did the first transplant in 1999.

1999? Why did it take so long to release details?

The team wanted to carry out other procedures to make sure it wasn't a one-off. They also wanted to track the patients' progress for a few years.

How are the patients doing?

The seven patients at Boston Children's Hospital had poor bladder function because of spina bifida, a congenital birth defect that causes incomplete closure of the spine and an abnormally formed bladder. After being monitored for several years, none have shown signs of immune rejection and the bladders are functioning normally.

Now they've proven they can do this, could this be the start of spare-parts surgery?

The medical community agrees that this is all in the early stages and you won't be able to drop into your local doctor next month and order a spare bladder. Besides, the bladder is a relatively simple organ, at least compared to the heart or liver. The editor of The Lancet, Dr Richard Horton, said: 'The bladder is a pretty simple structure and if it was going to work with any organ, it was the bladder. When you talk about the heart and the kidneys, then it becomes much more complex.'

How advanced is the field of organ transplantation?

The first organ transplant, of a kidney, was in 1933, but the kidney never functioned. The first successful kidney transplant was more than 20 years later, from one twin to another, in Boston. The first successful liver transplant was carried out in 1967; the world's first heart transplant was done in South Africa the same year, but the patient died 18 days later. Advances in medicine with the advent of tissue typing and better anti-rejection drugs mean thousands of successful transplants of organs, veins and even ankles have been carried out over the decades.