Starting Point
The heart in front of Taylor has been in the bioreactor for 10 days. How well would it beat on its own, when only one of its ventricles has been repopulated with cells? Kren switches off the current, and the heart’s motion decreases noticeably. Still, a rhythmic twitch is apparent in the left ventricle, indicating that cells there are beating in sync, and that the heart has started to gain some function.

The group has maintained rat hearts in the bioreactor for up to 40 days. In results published in Nature Medicine, they reported that after eight days, the hearts were able to generate roughly 2 percent as much force as an adult rat’s organ. The researchers hope to improve on this number, in part by adding cells to the scaffold more effectively.

But the next goal is truly daunting: to successfully transplant the hearts into animals. To date, the scientists have implanted hearts in the abdomens of rats for up to seven days. “We needed to show that we could sew them in and that they wouldn’t leak,” says ­Taylor. Now, however, the scientists must try to make the re-created organs take the place of the animals’ own hearts.

When it comes to making substitute hearts for people, the ­hurdles are even higher. “Where do we start?” asks Taylor. One of the greatest challenges is finding a suitable source of cells that will repopulate the heart scaffolding. Adult heart muscle cells do not divide readily. And cardiac­-derived progenitor cells, which are akin to cardiac stem cells, are not plentiful, although Taylor says her group has had some success isolating them and growing them in the laboratory. A human heart contains trillions of cells, she says. “We don’t believe we’re going to put every cell back. We believe we’re going to put some cells back and let them divide and figure out where they need to be themselves.”

If it works–and it will probably be years before that is known–the approach could transform the field of organ transplantation. “One of the best things about this process is that we’re harnessing natural processes to make things happen as they do in life,” says Kren. “We don’t have to do the heavy lifting. Nature will do it for us.”

Amanda Schaffer is a science and medical columnist for Slate and a frequent contributor to the New York Times.