Stem cell researchers in Hong Kong and the United States are trying to grow spare parts for the human heart that may be ready for tests on people within five years, they said on Thursday.

Scientists have already made basic heart muscle from stem cells, but the Hong Kong-led team wants to refine it so it can replace any part damaged in heart attacks, and to recreate the natural pacemaker, where the heartbeat originates.

When you get a heart attack, there is a small time window for a cure when the damage is still small. You can cure with a patch, a small tissue, so you won't progress to late stage heart failure, said team leader Ronald Li, director of the University of Hong Kong's Stem Cell & Regenerative Medicine Consortium.

We have the muscle strip now, but we want it to mimic what we see in the native heart better, (and) that requires engineering, Li told Reuters in an interview.

An organ or section of tissue grown from a person's stem cells can, in general, be surgically implanted only in that same person.

There are many different types of heart cells. If cells that are responsible for electricity aren't going right, you get arrhythmias or heart rhythm disturbances. There are heart muscle cells that do mechanical heart pumping that work all the time.

The team will use approved human embryonic stem cell lines to build these human heart muscle strips, as well as the native pacemaker for people with arrhythmia, or irregular heart beat.

The team plans first to transplant these muscle strips and pacemakers into pigs, and, if successful, to move to human clinical trials where they will transplant parts of the heart that are grown using the patients' own stem cells in about five years.

The question is whether we can put it in the heart to integrate with the recipient organ. Even if it becomes integrated, will it last? Li said.

He added that the team chose to use pigs because porcine hearts were anatomically and functionally more similar to human hearts.

I am hoping that at the end of the five years, we will have a number of blueprints for designing different prototypes that can be tested, he said.

Stem cells are the body's source of all cells and tissues. They can generate all the cell types of the organ from which they originate. Because of their ability to generate different types of cells, to multiply and self-renew, scientists hope to harness stem cells to treat a variety of diseases and disorders, including cancer, diabetes and injuries.

As well as the Hong Kong experts, the team will include scientists from the Harvard Stem Cell Institute, National Institutes of Health (NIH) and the Mount Sinai School of Medicine in the United States.

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