Adult stem cells from bone marrow may be fusing with other types of body tissue, rather than replacing them, two new US studies suggest. The research, published in Nature, shows that in mice, liver damage can be repaired using transplanted bone marrow stem cells. But it seems that the stem cells are fusing with existing damaged cells, rather than turning into new liver cells, as was previously believed. There is now serious concern over whether these earlier studies 'reflected reality', says lead researcher Markus Grompe, of Oregon Health and Sciences University in Portland. However, another recent study of cheek cells derived from transplanted bone marrow cells shows no evidence of fusion.
Previous studies suggested that bone marrow stem cells, which normally make different types of blood cell, can also turn into liver, muscle and pancreas cells. These findings promise new treatments for a range of diseases, and offer an alternative to the use of 'master' stem cells obtained from early embryos. But Grompe says he's not convinced that bone marrow stem cells give rise to anything other than blood. His team found that damaged mouse livers were repaired following bone marrow transplants, but that the new liver cells contained two or three times the normal amount of DNA. This finding indicates that the donor bone marrow cells had fused with the recipient liver cells. In a separate study, a team from the University of Washington, Seattle, reported similar results. Grompe thinks that bone marrow cells can 'reprogramme' themselves as liver cells, and maybe other types of cell, by fusing with them.
But Eva Mezey, lead author of another stem cell study published recently, thinks that cell fusion could be peculiar to the liver. Her team, based at the National Institute of Neurological Disorders and Stroke in Maryland, US, studied cheek cells from female leukaemia patients who had received bone marrow transplants from men. They found that between two and 12 per cent of the women's cheek cells originated from the male transplanted cells, and that only 2 out of 9700 cells studied showed evidence of fusion.