After transplantation the buds hooked up to the mice's blood supply, matured and were ultimately able to break down drugs given to the mice - a sign that they were working.
Experts were quick to applaud a development which, if confirmed, would be an immense step forward in regenerative medicine. While other body parts have already been grown and used clinically (see BioNews 485), solid organs like the liver are the most difficult to generate from stem cells. At the same time, there is a great need for therapies for such complex organ systems which do not rely entirely on organ donation.
While scientific commentators were unanimous in their praise for the paper, most were cautious in anticipating any immediate benefits for patients.
'Although the promise of an off-shelf-liver seems much closer than one could hope even a year ago', Dr Dusko Ilic, reader in stem cell science at King's College London said, 'the paper is only a proof-of-concept. There is much unknown and it will take years before it could be applied in regenerative medicine'.
The team behind the research, led by scientists at Yokohama City University, were trying to mimic the conditions of early liver growth in the embryo in their experiments. For this, they used human induced pluripotent stem (iPS) cells, originally generated from donated adult tissue, and coaxed them into becoming liver cells.
These were then mixed with two other kinds of cells, including from human umbilical cords. Professor Takanori Takebe told BBC News that he was 'completely gobsmacked' to see the cells 'self-organising to form a three-dimensional liver bud - this is a rudimentary human liver'.
The buds developed their own blood vessels and on being injected under the skin of lab mice connected to the mice's blood supply. Once transplanted, the buds grew from their embryonic state into what appeared to be adult liver tissue.
To check whether the liver buds worked, the scientists gave two groups of transplant mice two different drugs. Blood tests showed breakdown products, or metabolites, that could only be made by human liver and not mouse liver.
In a further experiment, groups of normal and transplant mice were given injections of diphtheria toxin in their tails. As expected, all the normal, 'control' mice suffered liver failure and died within 10 days. Several of the transplant mice, however, survived for over 40 days.
The scientists say that they would like to test liver bud injections in liver failure patients. In a change from previously envisaged therapies, the liver buds would not replace the diseased liver but sit nearby in the abdomen and support liver function.
But Professor Chris Mason, chair of regenerative medicine bioprocessing at University College London, who was not involved in the study, suggested that the liver buds' first application may be not in a clinical setting but in drug testing.
The kinds of cells currently used in metabolism and toxicity testing of new drugs, Professor Mason said, 'are only available in very limited quantities, insufficient for routine early stage research'.
'However, from Takebe’s data, mice transplanted with human iPS cell-liver buds might help developers predict drug metabolite profiles for patients and thus enable early stage detection of unwanted side effects rather than later during clinical trials, or worse, after the drug enters routine clinical practice'.
The study was published in the journal Nature.