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Growing a heart is much harder than, say, growing a windpipe, because the heart is so big and has several types of cells, including those that beat, those that form blood vessels, and those that help conduct electrical signals. For a long time, scientists didn't know how to make all the cells grow in the right place and in the right order.
The problem had been cracked by Dr. Taylor. She said that when human stem cells were put into a heart scaffold in 2010, they seemed to know just where to go. "They organized themselves in a way I didn't believe," said Dr. Taylor, who now works at the Texas Heart Institute but makes regular visits to Madrid to help with the experiments. "It's amazing that the [scaffold] can be as instructional as it is. Maybe we don't need to micromanage every aspect of this." Science Fiction Comes Alive as Researchers Grow Organs in Lab - WSJ.com.