The hearts have been grown using only stem cells, for the very first time, the New Scientist reports. As such, it mimics the processes that happen when humans hearts' grow for the first time — except it happens in a lab, at the prompting of researchers.

The new hearts were created using stems cells that were made by reversing human skill cells, so that they turned back to something like an embryo. Once that was done, the scientists encouraged the cells to grow into the right formation, changing their shape and then eventually forming first into the cells that help hearts beat, then into those that connect the heart up and after that into tiny ventricles.

The techniques could eventually be used to create a full-sized heart, scientists suggest to the New Scientist. "Our model is the first step towards building a heart relying on self-organisation of cells, without any external three-dimensional supporting materials," says Zhen Ma, from the University of California at Berkeley, told the magazine.