Characterization of cortical organoids cultured with xanthan gum. Credit: Nature Biomedical Engineering (2025). DOI: 10.1038/s41551-025-01427-3

An interdisciplinary team working on balls of human neurons called organoids wanted to scale up their efforts and take on important new questions. The solution was all around them.

For close to a decade now, the Stanford Brain Organogenesis Program has spearheaded a revolutionary approach to studying the brain: Rather than probe intact brain tissues in humans and other animals, they grow three-dimensional brain-like tissues in the lab from stem cells, creating models called human neural organoids and assembloids.

Beginning in 2018 as a Big Ideas in Neuroscience project of Stanford's Wu Tsai Neurosciences Institute, the program has brought together neuroscientists, chemists, engineers, and others to tackle the neural circuits involved in pain, genes that drive neurodevelopmental disorders, new ways to study brain circuits, and more.

Still, one problem has dogged them: scale. If researchers could produce thousands of organoids at once with uniform size and shape, they could learn more about brain development and developmental disorders, and more efficiently test new drugs and gene therapies.

The trouble is, neural organoids have a habit of sticking to each other, making it hard to grow large batches of them with consistent size and shape.

Now, a team of neuroscientists and engineers led by Wu Tsai Neuro affiliates Sergiu Pasca, the Kenneth T. Norris, Jr. Professor of Psychiatry and Behavioral Sciences in the School of Medicine, together with Sarah Heilshorn, the Rickey/Nielsen Professor in the School of Engineering, has found a simple solution. As they reported in their study published June 27 in Nature Biomedical Engineering, all it took to keep organoids from sticking together was xanthan gum, a common food additive.

"We can easily make 10, 000 of them now, " said Pasca, the Bonnie Uytengsu and Family Director of the Stanford Brain Organogenesis Program. In keeping with the program's commitment to making their techniques widely available, they've already shared their approach so others can take advantage of it. "This, as with all of our methods, is open and freely accessible. There are already numerous labs that have implemented this technique."

So few you could name them

Things weren't always so easy. About a dozen years ago, Pasca had just developed a method for transforming stem cells into the three-dimensional tissues now known as regionalized neural organoids, and he could only manage to make a few of these early cultures.

"In the early days, I had eight or nine of them, and I named each of them after mythological creatures, " Pasca said.

But Pasca wanted to get a good handle on brain development—specifically, what happens during development that leads to disorders like autism or Timothy syndrome—and tackle other ideas, such as screening drugs for potential side effects on brain development. To do that, he said, "we needed to produce thousands of organoids, and they should all be the same."

He also realized he would need to involve a wide range of researchers. "I thought, "This is an emerging field and there are a lot of problems we're going to face, and the way we're going to face them and solve them is by implementing innovative technologies, '" Pasca said.

To move forward, Pasca teamed up with Wu Tsai Neuro affiliate Karl Deisseroth, a bioengineer and neuroscientist, and brought together an interdisciplinary team to launch the Stanford Brain Organogenesis Program with support from Wu Tsai Neuro's Big Ideas in Neuroscience grant.

The nonstick solution

The stickiness problem reared its head soon after. Organoids were fusing together, resulting in smaller numbers of organoids of different shapes and sizes.

"People in the lab would constantly say, 'I made a hundred organoids, but I ended up with 20, '" Pasca said.

That was both a blessing and a curse. On the one hand, it suggested that researchers could stick two different kinds of organoids together—say, a tiny cerebellum and spinal cord—to study the development of more complex brain structures. Indeed, these assembloids are now a key part of Pasca's and his colleagues' work.

On the other hand, the team still needed to be able to create large numbers of organoids so they could gather precise data on brain development, screen drugs for growth defects, or carry out any number of other projects at scale.

One possibility would be to grow each organoid in a separate dish, but doing so is often inefficient. Instead, the lab needed something to keep organoids apart while growing them in batches, so Pasca worked with Heilshorn, a Stanford Brain Organogenesis Program collaborator and materials engineer, to try out some options.

The team ultimately looked at 23 different materials with an eye toward making their methods accessible to others.

"We selected materials that were already considered biocompatible and that would be relatively economical and simple to use, so that our methods could be adopted easily by other scientists, " Heilshorn said.

To test each one, they first grew organoids in a nutrient-rich liquid for six days, then added one of the test materials. After another 25 days, the team simply counted how many organoids remained.

Even in small amounts, xanthan gum prevented organoids from fusing together, and it did so without any side effects on organoid development. That meant that researchers could keep the organoids separated without biasing their experimental results.

Scaling up at last

To demonstrate the potential of the technique, the team used it to address a real-world issue: Doctors often hesitate to prescribe potentially beneficial drugs to pregnant people and babies because they don't know whether those drugs might harm developing brains. (Although FDA-approved drugs go through extensive testing, ethical concerns mean they are generally not tested on pregnant people or babies.)

To show how organoids address that problem, co-lead author Genta Narazaki, a visiting researcher in Pasca's lab at the time the research was done, first grew 2, 400 organoids in batches. Then, Narazaki added one of 298 FDA–approved drugs to each batch to see if any of them might cause growth defects. Working closely with co-lead author Yuki Miura in the Pasca lab, Narazaki showed that several drugs, including one used to treat breast cancer, stunted the growth of the organoids, suggesting they could be harmful to brain development.

That experiment shows that researchers could uncover potential side effects—and do so very efficiently, Pasca said, "One single experimenter produced thousands of cortical organoids on their own and tested almost 300 drugs."

Pasca and his Stanford Brain Organogenesis Program colleagues are now hoping to use their technique to make progress on a number of neuropsychiatric disorders, such as autism, epilepsy, and schizophrenia. "Addressing those diseases is really important, but unless you scale up, there's no way to make a dent, " Pasca said. "That's the goal right now."

More information: Genta Narazaki et al, Scalable production of human cortical organoids using a biocompatible polymer, Nature Biomedical Engineering (2025). DOI: 10.1038/s41551-025-01427-3  Journal information: Nature Biomedical Engineering