TORONTO — A Canadian-led international research team has determined how to grow early-stage human heart cells from embryonic stem cells, a step that could one day allow scientists to create new tissue for repairing a person’s damaged heart.

“Probably something a little more in the near future would be that this gives us an opportunity to efficiently generate human heart cells that can be used for drug screening and drug toxicology,” said principal investigator Gordon Keller.

In other words, scientists would be able to create these early-stage heart cells – known as progenitor cells – then allow them to grow into different types of cardiac cells that could be used for drug testing right in the lab.

“A lot of people have been talking about this, but I think when you see the … relative ease we have in producing such cells, we can now start interacting with biotech companies and pharmaceutical companies,” said Keller, director of Toronto’s McEwen Centre for Regenerative Medicine at the University Health Network.

Keller stressed that this is not the first time heart cells have been grown from embryonic stem cells. What the Toronto-led lab has done is figure out the “recipe” for making embryonic stem cells give rise to distinct types of immature progenitor heart cells.

This recipe, detailed in the latest issue of the journal Nature, is believed to mimic the process that directs embryonic stem cells at life’s earliest stages to develop heart progenitor cells.

These progenitors are able to make three major cell types found in the human heart – cardiomyocytes (the ones that beat), endothelial cells and vascular smooth muscle cells that will form cardiac blood vessels.

“By isolating these cells, these progenitors, first and then allowing them to mature, we can get populations highly enriched for cells that are beating or cells that are not beating – in other words, some of the vascular cells,” Keller said.

The process involves placing human embryonic stem cells – from existing stem cell lines scientists have long had access to – in Petri dishes, adding growth factors and other substances and then culturing them in special incubators.

After five or six days, the stem cells have given rise to heart progenitor cells. Under the microscope, some of these cells can be seen to have spontaneously begun pulsing – just as certain cardiac cells work together to cause the human heart to beat as a whole.

“You can view it as the first step in heart cell development,” Keller said.

But when it comes to the idea of injecting such cells directly into a person’s heart in the hope they would grow healthy tissue to repair damage from a heart attack, Keller believes that is “a long way off.”

“That’s why we talk about tissue engineering. The notion of us taking cells that we grow in a dish and putting them straight into a heart is probably naive,” he said.

“We will have to interact with our colleagues in tissue engineering to make structures that more approximate the three-dimensional aspects of our organogenesis (production of organs).”

Still, the researchers are already planning to transplant heart progenitor cells into animals with heart damage – likely pigs or sheep – to see if they will at least mature into the three cells types they’ve been able to create in culture dishes.

Keller said U.S. colleagues included in the Nature study have injected them into laboratory mice, and they “made all three heart cells but at a very low frequency.”

In the future, he believes heart progenitors could be grown from stem cells derived from individuals. (Japanese and U.S. scientists recently reported finding a way of culturing “pluripotent” stem cells, those which give rise to all 220 cell types in the body, from people’s skin).

“In principal, one could do this from any individual,” he said.

“Our hope is that we can turn any of these pluripotent stem cells into these cardiovascular progenitors and ultimately into cardiac cells,” Keller said.

Ideally, the researchers would like to grow large numbers of progenitors and freeze them, so they could be used as needed.

“And then we don’t always have to go back to the stem cell. We already have the cell that’s poised and ready to make the heart cells.”