Its original owner is no longer alive, but this heart may soon beat again. Harald Ott and researchers at Massachusetts General Hospital are engineering the organ to be used for a transplant. They stripped the heart down to a scaffold of structural proteins so that they can repopulate it with new stem cells that are compatible with a transplant patient's immune system. If it works, the technique would boost the supply of transplantable hearts, in part by allowing human cells to be grown on cardiac scaffolds sourced from pigs and other animals.

A similar engineering process has worked with simpler, hollow organs such as bladders, but the heart is a work in progress. The key challenge is choreographing the growth of intricate vascular networks and specialized cells that must act as one to produce not just a single heartbeat but another lifetime's worth.