If we knew exactly how stem cells, progenitor cells and other developing cells can produce regeneration without generating new tissue themselves, then we wouldn't need the cells. An example of that truism in action is provided by ScienceDaily: "a protein called connexin43, expressed by the transplanted embryonic heart cells, improved electrical connections to other heart cells. The researchers showed that the improved connections helped activate the transplanted cells deep within the damaged section of the heart tissue. The technique reversed the risk of developing ventricular arrhythmias after a heart attack ... In the past, scientists have transplanted a variety of cell types into failing hearts with modest improvement of function, although transplanting skeletal muscle cells made things worse and led to more arrhythmias. Surprisingly, when [researchers] transplanted embryonic cardiac cells, the hearts' electrical stability and function returned to normal. ... we were able to see how cells used in therapy are working with other cells in a complex organ within a living animal, establishing the mechanism of the therapeutic effect ... [researchers] engineered skeletal muscle to express connexin43 and achieved the same restorative results as they did with the embryonic heart cells."