Adult bone marrow stem cells fuse with existing cells to grow new tissues, rather than transforming into other types of cell, two new studies suggest. Researchers at Stanford University have shown that transplanted bone marrow cells fuse with specialised brain cells involved in controlling movement and balance. A second study, carried out by a team based at the University of California, San Francisco (UCSF) and Howard Hughes Medical Institute (HHMI), shows that transplanted bone marrow-derived cells fuse with cells in the heart, brain and liver.
Much excitement has surrounded findings that bone marrow stem cells might be capable of specialising not just into blood cells, but also into other tissues, including brain and heart. This research promised new treatments for a range of diseases, and also offered a possible alternative to 'master' stem cells obtained from early embryos. However, two studies published earlier this year cast doubt over these results - they showed that transplanted bone marrow cells in mice were simply fusing with existing liver cells, rather than turning into new liver cells. At the time, some scientists thought that cell fusion might be peculiar to the liver, but the two studies published last week show that the same thing may be happening in other tissues.
The UCSF/HHMI team transplanted mouse bone marrow cells into other mice, and found that cell fusion, rather than transformation, occurred when the cells migrated to the brain, heart and liver. They found no evidence of fusion in skeletal muscle, gut, kidney or lung, and no evidence that the transplanted cells had transformed into any new type of cell. 'The finding raises major questions about the plasticity of adult blood stem cells' said team member Sean J Morrison. The scientists warn that clinical trials involving transplanted bone marrow cells to repair damaged heart, brain and liver tissue should be postponed until more research has been conducted. But they also say that cell fusion should be explored further, as it might be one of the body's natural damage repair strategies. Their results were published in the advance online issue of Nature last week.
The scientists based at Stanford found that cell fusion was responsible for the appearance of new, specialised brain cells called Purkinje cells, after they transplanted bone marrow cells into mice. Team leader Helen Blau agreed that fusion might be a 'really important' biological mechanism. 'Fusion might be a sophisticated mechanism for rescuing complex damaged cells' she said, adding that the next step was to find out what biological signals could trigger fusion. 'If you know what those signals are, you could deliver the signal to damaged tissue and recruit the body's own bone marrow cells to treat disease' she said. The study will be published in the November issue of Nature Cell Biology.