Unlike living tissue in the body, organoids lack a blood vessel system that delivers oxygen and nutrients to every cell. Beyond about 3 millimeters in diameter, an organoid can no longer sustain itself by absorbing resources directly from its environment.
“When you grow organoids to a certain size they start to die inside because they can’t get oxygen and nutrients to the center,” said Oscar Abilez, MD, PhD, a senior scientist in the Division of Pediatric Cardiac Surgery.
But for a study published June 5 in Science, Abilez and a team of Stanford Medicine researchers grew heart and liver organoids replete with tiny blood vessels, potentially allowing them to overcome the current size limit.
The ability to grow vascularized organoids overcomes a major bottleneck in the field, said Abilez, who is a co-lead author of the study. The integrated blood vessels could allow the organoids to not only grow larger, but also to reach a more mature state, making them more useful as biological models.
Huaxiao (Adam) Yang, a former instructor at the Stanford Cardiovascular Institute and now an assistant professor in biomedical engineering at the University of North Texas, co-led the study.
They could also be the next step in regenerative therapies, said Joseph Wu, MD, PhD, the study’s senior author. Wu is a professor of medicine and of radiology, the director of the Stanford Cardiovascular Institute and the Simon H. Stertzer, MD, Professor.
In a separate clinical study led by Wu, Stanford Medicine researchers are injecting lab-grown heart muscle cells, called cardiomyocytes, into patients with heart dysfunction. “But actual heart tissue contains more than cardiomyocytes,” Wu said. “There are endothelial cells that line blood vessels, smooth muscle cells that surround blood vessels, pericytes that connect blood vessels, fibroblasts and other cells.”
In the future, perhaps vascularized cardiac organoids grown from a patient’s own stem cells could be surgically implanted to replace lost or damaged tissue.
“The thought is that if organoids have a vascular system, they could connect with the host vasculature, and that’ll give them a better chance to survive,” Abilez said.
Recipe testing
Scientists grow organoids from pluripotent stem cells by bathing the cells in various chemicals — growth factors and other small molecules — to induce their transformation into different cell types.
But attempts to grow vascularized cardiac organoids have produced inconsistent levels of the cell types needed to form blood vessels. Other researchers have tried an engineering approach, separately growing endothelial cells, or even 3D bioprinting vascularized networks, then combining them with a cardiac organoid. But none have achieved organoids with realistic blood vessel systems.
“They don’t really make branched vessels with passageways,” Abilez said.
In the recently published study, the team set out to optimize a chemical recipe to grow heart organoids that could reliably generate nearly all the cell types in the human heart, including cells that form a robust network of blood vessels.
The researchers reviewed the established methods for creating three key types of cells: cardiomyocytes, endothelial cells and smooth muscle cells. They combined these methods into 34 different recipes, or growing conditions — specifying which growth factors, how much and when to add them — for creating cardiac organoids containing all three cell types.
They also modified stem cells to fluoresce in different colors when they transformed into the three cell types.
When they tested the 34 recipes on stem cells and allowed them to grow for about two weeks, one in particular — condition 32 — was the clear winner. It produced the most colorful cardiac organoid.
“It was pretty obvious,” Abilez said. “We picked the one that gave us the most amount of those three fluorescent colors, which correspond to the most cardiomyocytes, endothelial cells and smooth muscles cells.”
Under 3D microscopy, the doughnut-shaped organoids were organized with cardiomyocytes and smooth muscle cells on the inside, along with an outer layer of endothelial cells that formed unmistakable blood vessels. These tiny branching, tubular vessels resemble the capillaries in the heart, which are 10 to 100 microns, about a hair’s width, in diameter.
When the researchers analyzed the other cells in the organoids using single-cell RNA sequencing, they were surprised to find nearly all the other cell types of the heart. Each organoid contained 15 to 17 different cell types, comparable to a six-week-old embryonic heart, which has 16 cell types. An adult heart has 21 cell types.
“It had all these other cell types that are found in the heart,” Abilez said. “That was unexpected in a positive way.”
Model of development
The winning recipe seems to approximate the conditions found in early stages of embryonic development, when different cell types emerge and blood vessels begin to form.
That suggests the organoids could be valuable as models of the earliest stages of human development, a period that is difficult to study.
“There’s this black box of development in early pregnancy when it’s not possible, ethically, to test drugs,” Abilez said.
As proof of concept, the researchers tested fentanyl, a potent and often misused opioid, on the vascularized cardiac organoids. They found that organoids exposed to fentanyl generated more blood vessels.
“We don’t know how that might manifest in a newborn yet, but it’s a difference,” Abilez said.
Other organs
The researchers also showed that their vascularization strategy could be adapted to create other organoids. By combining established methods to differentiate the key cell types in the liver, they created liver organoids with robust networks of blood vessels.
In future studies, the researchers will allow the vascularized organoids to develop longer to see how large and mature they become. They also plan to further optimize their vascularization recipes to generate even more cell types in the organoids, like immune cells and blood cells, to more closely resemble the makeup of an adult heart and better model adult diseases, Wu said.
“I’d love to be able to do this in all the different organoid types,” Abilez said. “After all, almost every organ in our body has a blood vessel system.”
Researchers from the University of North Texas, Rosebud Biosciences, Bullseye Biotechnologies and Greenstone Biosciences contributed to the work.
The study was supported by funding from the National Institutes of Health (grants K01HL130608, L30HL138771, R15HD108720, R56HL174856, T32GM136501, K08HL119251, R01HL150693, R01HL141371, R01HL146690, R01HL145676, P01HL141084, R01HL150414, R01HL139679 and K99HL166693), the American Heart Association, Tobacco-Related Disease Research Program, California Institute for Regenerative Medicine, Stanford Maternal and Child Health Research Institute Transdisciplinary Initiatives Program, Stanford Cardiovascular Institute, and Stanford Bio-X Program.
# # #
About Stanford Medicine
Stanford Medicine is an integrated academic health system comprising the Stanford School of Medicine and adult and pediatric health care delivery systems. Together, they harness the full potential of biomedicine through collaborative research, education and clinical care for patients. For more information, please visit med.stanford.edu.
END