While isolating cells from mouse skeletal muscle, the authors noticed that a few floating cells in their culture were beating. These cells (which they named Spoc cells) eventually adhered and developed into normal looking and functioning cardiac myocytes. Spocs are the first heart precursors shown to beat and develop in vitro without long-term culture. They may finally provide a convenient set of cultured beating cardiac cells from the various transgenic mice used by cardiac researchers.
Spocs also know their place in vivo. The group injected the freshly isolated cells into the bloodstream of a mouse with a recent heart attack injury. Within days, the precursor cells were visible in the injured region of the heart (fusion was ruled out). By 3 months, the cells had striations associated with mature cardiomyocytes.
Why skeletal muscle holds these cells, and why these cells do not normally repair heart injuries, remains unknown. Heart muscle that is too abundant or too thick can be deadly. “So maybe,” Epstein says, “we need the active suppression of stem cells in the heart.” But that does not explain why nature keeps them around elsewhere. “It's definitely a mystery,” says Epstein.