A functioning strip of heart muscle has been created from mouse embryonic stem cells, thanks to the identification of a new type of cardiac stem cell. The research has not yet been repeated with human cells, but it lays a blueprint for how to generate heart muscle that could be used to repair damage from heart attacks and to test new drugs. The scientists, from Harvard University, are now working on isolating similar cardiac cells from lines of human stem cells.

Stem-cell therapy for heart disease has so far focused on trying to repair heart-attack damage with injections of patient-derived stem cells from bone marrow, but studies have yielded mixed results. Rather than using undifferentiated cells, “the push now is to try to obtain cardiac myocytes [heart muscle cells] from people and use them as patches that would be placed over damaged tissue in someone who has had a heart attack,” says Benoit Bruneau, a researcher at theGladstone Institute of Cardiovascular Disease, in San Francisco. “They made engineered cardiac tissue from embryonic stem cells. From a bioengineering point of view, that’s significant.”

Embryonic stem cells, which are capable of forming any type of tissue in the body, can spontaneously form clumps of beating heart cells when grown in a dish. But it has been difficult to isolate large numbers of these cells from the mix of tissue types that can develop from embryonic stem cells. A heart patch would require a huge number of these cells, perhaps billions, says Christine Mummery, a biologist at the Leiden University Medical Center, in the Netherlands.

The Harvard team, led by Kenneth Chien, director of the Massachusetts General Hospital Cardiovascular Research Center, in Boston, has made progress toward this goal, previously developing a method of isolating a master cardiac stem cell from embryonic stem cells and fetal tissue–one capable of producing all the cell types that make up the heart. In the most recent study, published today in the journalScience, Chien’s team developed a way to isolate particularly desirable progeny of this master stem cell, cells that produce only ventricular muscle cells, the type damaged in heart attacks. “If you want to create a cardiac patch, you want cells that will behave–that would line up nicely like they do in the heart,” says Bruneau.

Scientists genetically engineered mice to express two markers of different colors–one that marked the master cardiac stem cell, and the other that turns on when the cells start making muscle. They then isolated the 0.5 percent of cells in the developing mouse embryo that expressed both markers. In addition to making only ventricular muscle cells, these cells also have the ability to continue to reproduce, enabling the production of large volumes of cells. “This ability to divide and make muscle is something that normal heart cells do not have,” says Chien.

Using a technology previously developed by Chien’s collaborator, Kevin “Kit” Parker, a bioengineer at Harvard, researchers then grew the cells on a thin polymer film that had been patterned with molecules typically found outside of cells, such as collagen. “The cells recognize the geometric cues on the film and reorganize themselves to spontaneously form a piece of cardiac tissue,” says Parker. The cells can contract, and they express the same genes as those expressed by normal heart muscle. (See a video of the muscle cells contracting.) “We can use them to test new drugs, as well as the safety of different drugs, chemicals, and nanomaterials,” says Parker. “We can also graft them onto the heart and restore contractility of that injured region of the heart.”

The researchers still have several steps to surmount before they can test how well the patches will repair the heart. They must find ways to isolate human versions of these cells. To make patches that are therapeutically useful, they must create three-dimensional versions of the two-dimensional patches of muscle cells. That will require the addition of blood vessels to feed the muscle. “We are working on additional technology to template in a vascular system within the cardiac tissue,” says Parker. “Once we’re comfortable with that, we will take it into animals.”

Ultimately, scientists would like to generate these heart-muscle-producing cells from induced pluripotent stem cells, a type of adult stem cell that can be made from a patient’s skin cell. This would allow physicians to take a skin biopsy from a heart-attack patient, generate a heart patch that is genetically matched to the patient, and implant it over the damaged heart tissue.