A heart organoid with cell types glowing different colors

Mini hearts in a dish: A big win for cardiac research

Could we grow a human heart in a dish? It’s not that far-fetched. But before we go there, imagine looking at a blob the size of a sesame seed, rhythmically beating, and knowing it has the potential to unlock key mysteries surrounding the human heart. Scientists are now tapping into new stem cell-based technologies to grow highly complex 3D balls of heart cells functioning much like a full-size heart – but in a dish. Termed organoids, which literally translates to “organ-like,” these mini organs allow scientists to study the human heart in ways they never have before. 

“They look like organs, and work like organs,” said Aitor Aguirre, Ph.D., associate professor of Biomedical Engineering at Michigan State University. He paused, then added with a laugh, “Kind of.”

Aguirre’s an NHLBI-funded researcher who runs one of the few labs in the world that perfects this technology for the heart.  He explained that these organoids, which are isolated from the whole-body environment and literally grown from stem cells, can be turned into almost any organoid you like. Isolating them from other organ systems helps scientists understand how the organ develops and functions in both healthy and diseased states. Without other biological factors muddying the waters, like blood, hormones, or signals from other organs, researchers can home in on things like the genetic pathways that guide organ development, or structural changes that happen during disease. 

“Organoids give us direct access to the tissues for study,” said Aguirre, whose work focuses specifically on cardiovascular diseases. He gave the example of studying heart attacks in an animal model. “You can induce a heart attack in a mouse, but you can’t easily watch what is happening live because you have the whole animal surrounding the organ. These tiny organs in a dish are completely accessible.”

Another perk: because organoids are made directly from human stem cells, they give scientists a powerful tool to verify what they have learned in animal studies. Aguirre explained that much of what we’ve learned about human physiology over the decades comes from this kind of animal modelling research – not from studies on real humans. And while animals generally serve as good stand-ins, the differences between the species are still major enough to matter. 

A personal “aha!”

Aguirre launched into the world of organoids when he was a postdoctoral researcher working on stem cells. He stumbled upon a study from Yoshiki Sasai, a scientist from the Riken Center in Japan, who was growing optic cups – the precursor to eye retinas – in a dish.

“It never occurred to me to make a human part in a dish,” he said. When Aguirre started his own lab at Michigan State University in 2018, learning how to do this became his priority. Then, just weeks after beginning his experiment, he had what he called a “eureka moment”: he saw his first mini heart beating in its dish. 

Aguirre’s now-refined heart organoid is the first advanced human heart model with all the properties of a real human heart, including all the primary heart cell types organized into tiny atria and ventricles. And he has published a protocol that guides anyone in the world through the process of making them. 

A step-by-step guide to mini hearts

Growing mini hearts in a dish requires access to a sterile space, a few pieces of lab equipment, and pluripotent stem cells (PSCs), or cells primed to turn into any cell type in the body. PSCs typically come from human donors and are sold commercially.

Aguirre said his protocol builds on a key element of the stem cells: self-organization, or the ability that PSCs possess to build any part of the body if properly instructed. “This is what we are using in our favor to make heart organoids,” he said.

Step one: prep the PSCs for their future as heart cells. This involves allowing them to grow up in solutions known as media, which gives them all the nutrients they need to grow and thrive.  Over the next couple of days, the existing cells give rise to new cells until they’ve reached critical mass. Once that step is reached, divide them up in a special dish with 96 different wells, so there are just a few PSCs in each well.

Step two: Spin the dish at fast speeds, forcing the cells to ball together. After 24 hours, the cells in each of the 96 wells form blobs of not-yet-heart cells called embryoid bodies.

Step three is where the fun happens, because the PSCs in the embryoid bodies will be directed to turn into heart organoids. “We have to speak the cells’ language,” said Aguirre, by adding small molecules to the media that target specific genetic pathways in the cells. This should be done at different time points and in a specific order for about a week. Each day replace the media with new media containing different molecules that tells the cells which step to do next. By day six, the mini hearts in a dish begin to beat.

Big possibilities for curing disease

Aguirre said that a critical application of heart organoids is disease modeling. “Allowing us to understand a disease is more than half the effort to cure it. For most incurable diseases the critical element is our lack of understanding of the disease mechanism.”

One research arm in his lab specifically focuses on congenital heart defects. In this model the organoids are developed in media mimicking diabetes with higher-than-normal insulin and glucose levels. The idea is to study the effects of pregestational diabetes on the developing human heart as diabetes during pregnancy increases the risk of such defects tenfold. The team’s results showed that organoids grown in under diabetic conditions were larger and had irregular heartbeats and decreased oxygen consumption.

The findings underscored what Aguirre has come to firmly believe: that organoids, with their beginnings in human cells, will help pave a speedier path to precision medicine. For example, given the differences in our genetics, certain medications may work differently from one person to the next. Some drugs may be needed at a higher or lower dose depending on the person, while others may not be tolerated at all in an individual. Creating an organoid from a specific person’s stem cells would allow researchers to be able to test which medications might be safely used and at which dose for everyone. 

Looking even further ahead, the technology to make these mini hearts in a dish might become advanced enough to make full-sized hearts, this time from the cells of an actual patient. The heart would be completely identical to a real heart and could be used, say, as a fully compatible heart transplant. 

“Considering the progress we have made in just five years, I would think growing human hearts like grapes on a vine is possible within a lifetime,” said Aguirre. “Science-fiction for now, but not for long.”