Telltale blue. This muscle's blue nuclei could have come only from bone marrow cells.

New Muscle Bred in the Bone

Molecular and developmental biologists have discovered a potentially useful source of replacement muscle for people suffering from muscular dystrophy. As reported in today's issue of Science, experiments in mice show that bone marrow cells can migrate through the blood and transform into new muscle fibers in damaged muscle. The results overturn current dogma that only nearby cells can repair muscle tissue.

Researchers knew that stromal cells--the support cells of the marrow--could be coaxed into forming muscle cells in lab dishes, but all the existing evidence suggested that this did not occur in living organisms. A team led by Fulvio Mavilio of the San Raffaele-Telethon Institute for Gene Therapy in Milan suspected, however, that those experiments may not have been sensitive enough to track the fate of transplanted bone marrow. So Mavilio's group turned to a strain of transgenic mice carrying a so-called marker gene that, when activated, causes cell nuclei to turn blue--but only in muscle cells.

Mavilio's colleague, Giuliana Ferrari, transplanted bone marrow from the transgenic mice into other mice whose own bone marrow had been destroyed by irradiation. After several weeks, she injected a toxin into the forelimbs of these mice to damage the muscle there. Two weeks later, Ferrari found that the damaged muscle areas in nine mice not only showed signs of recovery but also had numerous blue nuclei throughout--indicating that these new cells had started out as transplanted bone marrow cells. "These [bone marrow] cells are transplantable, and can circulate and reach the muscle," Mavilio says.

The finding presents "a whole avenue of potential therapies that didn't exist before," says Terry Partridge, a cell biologist at the Medical Research Council Clinical Research Centre at Imperial College School of Medicine in London. Researchers might one day be able to rig bone marrow cells to carry a good copy of the gene that causes muscular dystrophy. That could provide decaying muscles with a plentiful source of new--and normal--muscle cells, but Partridge cautions that such therapies are "certainly not around the corner."