Female muscle-derived stem cells make more skeletal muscle cells (red) than do their male counterparts.
The authors were led into the realm of muscle and sex when they realized that they were relying heavily on cell lines derived from female mice to study muscle regeneration. Most of the male lines, by contrast, were stuck away in the freezer. They supposed that the female ones were more popular because they were working better.
The group now shows that female muscle stem cells are indeed better at regenerating diseased muscle in a mouse model. While the male cells proliferated briefly and abruptly differentiated into muscle, the female cells held out. They proliferated more slowly at first, but cycled longer before becoming muscle. By multiplying first, the stem cells can eventually create more fibers.
The female cells also seem better equipped to deal with the hypoxic conditions and reactive oxygen species found in injured and diseased muscle, as they up-regulated more stress-related and antiapoptotic genes. Artificially turning on antiapoptotic Bcl-2 in male stem cells was not enough to improve their regeneration, however.
Hormones may also have some influence, as females were better recipients as well as donors. But estrogen treatments did not prod stem cells of either sex to make muscle more efficiently.
The authors still need to determine the ultimate cause of the female cells' superior regenerating ability. Perhaps a Y chromosome gene encourages quick differentiation. Whatever the cause, biologists should make sure to note cell sex when describing their experimental methods with stem cells of any sort. 
