Starting from induced pluripotent stem cells (iPSCs), Japanese researchers created human liver buds complete with blood vessels that performed normal metabolic activities when implanted in mice.
The achievement represents the first functional, three-dimensional, vascularized organ grown from stem cells, according to the researchers, led by Takanori Takebe, MD, and Hideki Taniguchi, MD, of Yokohama City University in Japan, who reported their results online in Nature.
But Takebe, speaking at a telephone press briefing, said much more work remained to be done before their approach could even be tested in humans. He estimated that it could be 10 years before a human clinical trial takes place.
He and his colleagues have not yet shown that the liver buds they developed, which were only a few millimeters in diameter, could grow big enough to accommodate human metabolic needs, and numerous safety issues remain to be addressed.
Takebe said that it would probably be necessary to implant "tens of thousands of liver buds" to generate a functioning liver in a single human adult. Doing so would require an automated process that, he said, could take 5 to 7 years to design and test.
Previous efforts to develop self-organized, functional organs from stem cells had all failed, the researchers indicated in the Nature report. The major hurdles have been the transition from two-dimensional growth into the third dimension, and the development of vasculature to support a 3-D structure and further growth.
However, those efforts typically involved culturing the initial stem cells -- whether human embryonic stem cells or iPSCs -- in isolation, by feeding them cocktails of growth factors designed to induce differentiation into specific mature cell types.
Takebe and colleagues said they overcame these obstacles by first inducing human somatic cell-derived iPSCs to develop into hepatic endoderm cells -- a type of progenitor cell destined to mature into normal liver cells -- and then co-culturing them with human umbilical vein endothelial cells and human mesenchymal stem cells.
This approach, they believed, would "recapitulate" the process by which real human livers develop during the early fetal stage, which "depends on the exquisite orchestration of signals between endodermal epithelial, mesenchymal, and endothelial progenitors before blood perfusion."
After observing that liver buds formed successfully in culture and developed a primitive vasculature, Takebe and colleagues then transplanted them into mice, initially by cutting a window in their skulls and placing the liver buds on their brains. At this location, the buds could engraft and connect to the mouse vasculature while allowing easy observation.
Indeed, the researchers reported, the liver buds' vessels made connections with the animals' cranial vessels, at which point the buds further matured into tissues strongly resembling the adult human liver. Takebe and colleagues confirmed that these mini-organs carried out metabolic functions expected in the liver. They expressed normal liver proteins such as albumin and alpha-1-antitrypsin, and metabolized drugs including ketoprofen and debrisoquine.
With these successes under their belts, Takebe and colleagues then implanted lab-grown liver buds into an abdominal site in mice. Again, the researchers confirmed that the transplanted buds engrafted and survived.
Moreover, the transplants were metabolically active enough that they improved survival in mice with drug-induced liver failure, compared with mice receiving sham surgery and with mice implanted with unorganized adult human hepatocytes.
Takebe and colleagues called the effort "proof of concept that organ bud transplantation offers an alternative approach to the generation of a three-dimensional, vascularized organ."
The study was funded by Japanese government grants and the Yokohama Foundation for Advanced Medical Science.
Study authors declared they had no relevant financial interests.
- Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania and Dorothy Caputo, MA, BSN, RN, Nurse Planner