Miniature livers 'grown in lab'
Scientists have managed to produce a small-scale version of a human liver in the laboratory using stem cells.
The success increases hope that new transplant livers could be manufactured, although experts say that this is still many years away.
The team from Wake Forest University Baptist Medical Center presented its findings at a conference in Boston.
UK experts said it was an "exciting development" but it was not yet certain a fully-functioning liver was possible.
The demand for transplant livers far exceeds the number of available organs, and in recent years, research has focused on ways to use cell technology to support failing organs in the body, or even one day replace them.
Their building block is the stem cell, a "master cell" which can, in certain conditions, divide to produce different types of body tissue.
However, constructing a three-dimensional organ from stem cells is a difficult task.
The method used by the Wake Forest researchers, and other teams around the world, is to form new liver tissue on a scaffold made from the structure of an existing liver.
In this case, a detergent was used to strip away the cells from the liver, leaving only the collagen framework which supported them, and a network of tiny blood vessels.
The new stem cells - in this case, immature liver cells and endothelial cells, to produce a new lining for the blood vessels - were gradually introduced.
After a week in a "bioreactor", which nurtured the cells with a mixture of nutrients and oxygen, the scientists saw widespread cell growth within the structure, and even signs of some normal functions in the tiny organ.
Professor Shay Soker, who led the research, said: "We are excited about the possibilities this research represents, but must stress that we're at an early stage, and many technical hurdles must be overcome before it could benefit patients.
"Not only must we learn how to grow billions of liver cells at one time in order to engineer livers large enough for patients, we must determine whether these organs are safe to use."
UK researchers welcomed the findings, which are being presented to the American Association for the Study of Liver Diseases. Professor Mark Thursz, from Imperial College London, said the results were "encouraging".
"The report suggests that the authors have overcome one of the major hurdles in creating an artificial liver - to generate functioning human liver cells in a 'natural' liver structure.
"It is clear that the cells are growing well, but the next step is to show that they are functioning like normal human liver tissue."
Dr Mark Wright, from Southampton University added: "In an era of increasing liver disease and death with a chronic shortage of liver transplants this represents an exciting development in an important field of work.
"The researchers appear to have made the step of combining stem cell technology with bioengineering as a first step to producing artificial livers.
"Whilst 'off the shelf' new livers are clearly still a long way off, this work gives a glimmer of hope that this is no longer just the stuff of science fiction."