Researchers Grow Human Heart Muscle

Many organs in the human body regenerate cells after they have been damaged, but the heart is not one of them. If heart muscle is damaged from a heart attack, the damaged or dead cells do not regenerate and are replaced with scar tissue. Because the scar tissue cannot transmit electric impulses and contract, the heart often fails, resulting in death. More than 12 million people worldwide die annually from heart failure.

Now, researchers at Duke University have created human heart muscle in the laboratory, and successfully grown it large enough to provide a patch that contracts and transmits electrical signals.

The work fills a void in heart research. Today, existing therapies attempt to reduce symptoms from the damage already done, says Ilia Shadrin, a biomedical engineering doctoral student who leads the research. His work was recently published in Nature Communications.

“No one has been able to solve the problem of scarring,” he says. “There is nothing that can bring back the muscle that was lost.”

Shadrin moved the bar by using human pluripotent stem cells and growing them outside of the body. These types of stem cells have the ability to transform into any tissue, eliminating the controversy over using human fetal stem cells in research. The trick to culturing the cells lies in slowly rocking them in a nutrient-rich bath.

Image: Duke University

Most researchers working with heart cells today focus on drug development and disease modeling studies instead of creating functional heart tissue for human therapy, says Nenad Pursac, a Duke bioengineering professor and advisor to Shadrin.  

“This is a really big achievement, scaling up this technology,” Pursac says. “The difficulty is scaling up tissue size but not losing the function [of smaller scale cells.] If a large patch does not have functionality, it can generate lethal cardiac arrhythmia.”

Pursac points out that it takes about 10 years for humans to develop full functioning heart cells. Trying to mimic that behavior in a lab over a much shorter timeframe is extremely difficult.

“The whole trick is to get to the highest level of function in the shortest possible time,” he says.

Shadrin accomplished the goal partly by trial and error, building on previous knowledge. Finding the proper combination of growth factors, cells, support structures, nutrients and culture conditions took the team years of work, beginning in 2013. The researchers were able to grow a variety of heart cells from the stem cells, including cardiomyocytes, fibroblasts, endothelial, and smooth muscle cells. Cardiomyocytes are the cells responsible for contraction; fibroblasts provide structural framework; and endothelial cells form blood vessels.

Growing cardiomyocytes from a single cell to 3D tissue with functionality in just five weeks was perhaps the biggest challenge.. “We really had to think about how nutrients are delivered to tissue,” Shandrin says.

The successful combination used about 85% heart cells and 15% of a variety of others, embedded in hydrogel as a service matrix for the tissue. But it was the dynamic rocking motion that made the difference in growing them. “The splashing forces make for more dynamic action,” he says. “It is simple, but has great results.” The team obtained three-to-five times better results using the rocking cultures compared to static samples.

The researchers built on earlier patches of one square centimeter and four square centimeters, growing a functioning patch scaled up to 16 square centimeters (about one-quarter the size of an iPad Air screen) and eight cells thick.

“Scaling up, you want to make sure not to lose the functionality of tissue,” Shandrin says, adding that tests show the tissue is fully functional, with properties that mimic an adult human heart.“We did this on the 1-by-1-centimeter and worked up to the 4-by-4-centimeter tissue. And we were able to measure the functionality, both electrical and mechanical.”

“Ilia devised a number of recipes and tricks to generate a highly functional heart patch,” Bursac says. “But simple is best, otherwise there will be more obstacles to regulatory work. He used the simplest method.”

Concurrently, other researchers are working on injecting stem cells derived from blood, the heart or bone marrow directly into damaged heart tissue as a means of regeneration. But results, while promising, are mixed and not fully understood. The risk for generating arrhythmia is very large, Bursac says.

The Duke team believes a heart patch could remain viable for a long time, provide strength for contractions and a path for electrical stimulation. The heart patches also secrete enzymes that could help adjacent tissue that has not died.

Already, they have successfully tested the patch on mice hearts, proving functionality, vascular growth and survival. The next step is to test them on larger animals, primarily pigs.  But to do that, the team must grow a patch that is larger, thicker and stronger, in order to put out more force.

The team now is working with the University of Alabama at Birmingham and the University of Wisconsin under an $8.6-million, six-year National Institutes of Health grant to take the next step.

“We want maximum use of the patch,” Bursac says. “When you put a patch on top of the heart, ideally it should integrate with the host heart. So far, no one has been successful in the electrical coupling of the host. We want to provide coupling.”

Read more about Tissue Engineering on AABME.org.