Once a muscle cell, always a muscle cell--or so scientists used to think. But they were wrong. Stem cells found in muscle can take up residence in bone marrow and give rise to blood cells, according to research described in the current Proceedings of the National Academy of Sciences. The work is another surprising example of the versatility of adult stem cells, which has forced biologists to rethink their ideas about how cells develop.
Based on the observations of early embryologists, scientists had assumed that cells travel an ever-narrowing path during development. The cells in an early embryo were most versatile, they thought, but in adult tissue, even stem cells were restricted to becoming one of the few cell types present in that tissue. But recent studies have challenged that simplistic view: Stem cells from rodent brains can transform into blood cells, for instance, and cells from bone marrow can become muscle.
Inspired by those finds, stem cell biologist Margaret Goodell of the Baylor College of Medicine in Houston wondered if the muscle stem cells she was studying "had the potential to do much more" than simply blossom into muscle cells. To test this possibility, Goodell and her colleagues grew muscle cells from adult mice in culture dishes for 5 days. They then injected an aliquot of the cells--a mixture of muscle cells and their stem cells--into mice that had received enough radiation to completely destroy their bone marrow. The researchers also injected these mice with a large dose of bone marrow cells from a different mouse strain. Six weeks later, the animals had blood cells derived from both sources, suggesting that the muscle stem cells were pressed into service to form blood cells. The muscle cells were prolific, accounting on average for more than half of the blood cells in each animal.
The researchers aren't sure exactly which cells in the muscle are so versatile, although they suspect it is the so-called muscle satellite cells, stem cells that are involved in repairing damaged muscle. These cells may be primitive enough, Goodell says, to take on a new identity once they are exposed to developmental signals in the blood stream. It's also possible, notes stem cell biologist Ihor Lemischka of Princeton University, that the 5 days the partially differentiated muscle stem cells spent in culture somehow reprogrammed them to accept new fates. Either way, scientists have harbored "many preconceptions about what stem cells are" and what they're capable of, he says. "Maybe it's time to rethink that."