Fig 1. In 2008, bioengineers at the University of Minnesota stripped rat hearts of cells using detergent — you can see the results of three trials here. This process left untouched the blood vessels, collagen, and various proteins that compose the heart’s physical structure.
Fig 2. The ghost heart is flushed with red dye to show that major and minor blood vessels were left intact.
Fig 3. A researcher injects the ghost heart with heart cells from newborn mice.
Fig 4. Researchers adjusted the environmental conditions to simulate natural conditions, meaning they provided oxygenate fluids, pressure, and an electrical stimulus. You can see the bioreactor schematic here.
GIF source here. Research paper here. Less technical writeup here. More videos from the lab itself here (Supplementary Movie 1 in particular is pretty awesome).