Scientists at Edinburgh University have for the first time been able to build a complete and functional organ in a living animal using cells created in a laboratory.
The breakthrough has been described as an important first step in developing a substitute for organ transplants which could eventually overcome problems with donor supplies and the use of immunosuppressive therapy.
Professor Clare Blackburn, from the Medical Research Council Centre for Regenerative Medicine at Edinburgh University, who led the team of scientists, said: "The ability to grow replacement organs from cells in the lab is one of the 'holy grails' in regenerative medicine.
"But the size and complexity of lab-grown organs has so far been limited.
"By directly reprogramming cells we've managed to produce an artificial cell type that, when transplanted, can form a fully organised and functional organ."
The technique, which produces a thymus, a vital immune system "nerve centre", has so far only been tested on mice.
It may be another 10 years before the treatment is proven to be effective and safe enough for human patients.
However, Professor Blackburn said: "This is an important first step towards the goal of generating a clinically useful artificial thymus in the lab."
The research is published in the journal Nature Cell Biology.
It reports that, while fragments of organs, including hearts, livers and even brains, have been grown from stem cells, no-one before has succeeded in producing a fully intact organ from cells created outside the body.
Dr Rob Buckle, head of regenerative medicine at the Medical Research Council, described the research as exciting. However, he warned more work is needed before organ transplants can be eradicated.
He said: "Growing 'replacement parts' for damaged tissue could remove the need to transplant whole organs from one person to another, which has many drawbacks - not least a critical lack of donors.
"This research is an exciting early step towards that goal, and a convincing demonstration of the potential power of direct reprogramming technology, by which once cell type is converted to another.
"However, much more work will be needed before this process can be reproduced in the lab environment, and in a safe and tightly controlled way suitable for use in humans."
Dr Paolo de Coppi, consultant paediatric surgeon at Great Ormond Street Hospital and head of Stem Cells and Regenerative Medicine at the Institute of Child Health, London, added that there could also be concerns with problems such as tumour growth on the cells. He said: "Research such as this demonstrates that organ engineering could, in the future, be a substitute for transplantation, overcoming problems such as organ donor shortages and by-passing the need for immunosuppressive therapy.
"It remains to be seen whether, in the long term, cells generated using direct reprogramming will be able to maintain their specialised form and avoid problems such as tumour formation."
The process of building the organs begins by taking connective tissue cells and "tweaking" their DNA to increase production of a certain protein.
After four weeks, the cells produce well-formed organs with the same structure as a healthy thymus.