A lab at the University of Minnesota has done something cool: they’ve grown a functioning heart from stem cells. The problem with building complex organs in a lab is that their normal construction required an elaborate context in the developing embryo, something that is impossible to replicate, short of just growing the whole embryo. The Doris Taylor lab did something very clever: they took an adult rat or rabbit heart and stripped it of its cells, leaving behind a scaffold of nonliving connective tissue. Then they recellularized it with stem cells, and they differentiated appropriately to make a new, beating heart.
They’ve got a long way to go yet — the resynthesized hearts only beat with 2% of the strength of the normal adult heart — but it’s a good start.
You can watch a video describing the work. Warning: it does show one dead rat and a guy with a knife, and there are pulsing pink blobs of hearts in glass chambers, so it may not be for everyone.