A heart won't beat once it's been ripped out of the chest Indiana Jones style. But disembodied heart cells, when nurtured in a petri dish, will beat on their own.
Stem cells have been transformed into adult cells of almost every kind, so it's not surprising to learn that stem cells have been isolated and nurtured into the three types of cell that make up the heart.
Scientists plopped down stem cells in a petri dish and fed them on nutrients that encouraged growth of the smooth muscle cells that make up the blood vessels of the heart, the endothelial cells that line the heart's vessels, and the cardiac muscle cells that contract the heart and push blood through it.
Once all the cells develop, though, they "try" to form a heart on their own. They don't have the resources needed to develop the actual organ, but they can (and do) start sending out a pulse that causes the cardiac cells to contract.
This — along with the ability to make stem cells with a person's undifferentiated blood cells — may be an incredible step for science. The disembodied heart cells respond to drugs the way interior heart cells would. This allows researchers to test drugs without putting someone's intact ticker at risk. For example, if a person's heart can't take chemo, the petri dish will let doctors know. Right now the test takes about a year and costs $15,000, but someday we could sample medicinal treatments externally.