This is just awesome the first steps in using stem cells for something other than hyperbole and annoying the Christians. Getting stem cells to construct an organ outside the specialised set up of an embryo is tricky and constructing one from scratch very complex. So these guys and girls took a living heart out of a convenient rat stripped it off all it's living cells so only a skeleton of non living connective tissues were left and then used stem cells which changed into the heart muscle cells to rebuild the heart. They then zapped it with some current and the hearts started beating and kept beating even after stimulation was removed some for as long as 40 days.
The ultimate goal with this is to take a human donor heart strip it of cells to get the structure then they could regrow it using stem cells from the recipient and you would have a human heart with 100% biological compatibility with it's new body no problems with rejection and the patient would not have to take immuno suppressants for the rest of their days.
Early work but undeniably cool stuff.