By squirting cells from a 3D printer, researchers have created tissue that looks—and acts—like a chunk of brain. In recent years, scientists have learned how to load up 3D printers with cells and other scaffolding ingredients to create living tissues, but making realistic brainlike constructs has been a challenge. Now, one team has shown that, by modifying its printing techniques, it can print and combine multiple subtypes of cells that better mimic signaling in the human brain.
"It's remarkable that [the researchers] can replicate" how brain cells work, says Riccardo Levato, a regenerative medicine researcher at Utrecht University who was not involved with the study. "It's the first demonstration that, with some simple organization [of cells], you can start getting some interesting functional [responses]."
The new technology, described last week in Cell Stem Cell, could offer advantages over existing techniques that neuroscientists use to create 3D brain tissues in the lab. One common approach involves using stem cells to grow miniature brainlike blobs called organoids. But researchers can't control the types of cells or their precise location in these constructs. Each organoid "is unique," making it difficult to reproduce research results, says neuroscientist Su-Chun Zhang of the University of Wisconsin–Madison, an author of the new study. With the right kind of 3D printing, however, "you can control where different cell types are placed," says developmental biologist Francis Szele of the University of Oxford.
Past studies have used 3D printers to construct brain tissues that allowed researchers to study how the cells matured and made connections, and even integrate printed tissue into mouse brains. But those constructs had limited functionality. And efforts that produced more functional printed tissue used rat cells, not human cells.
In the new study, Zhang's team printed separate horizontal lines of human neural and glial progenitor cells, which can develop into multiple brain cell types. They also tinkered with the composition of the "ink"—called hydrogel—which functions as a glue between the cells. Their novel hydrogel recipe lent support to the tissue, but wasn't so stiff that it prevented the cells from moving or forming connections, as they do in a real brain. The resulting 3D structures mimicked developing brains, with the cells forging connections with cells in their own band, as well as extending links toward other bands. That activity allowed researchers to observe how the progenitor cells mature and link up.
Researchers used a printer and different types of human brain cells to create a thin sheet of tissue that functioned similarly to a developing brain.YAN ET AL., 2024, CELL STEM CELL 31, 260–274
The team then created different constructs by printing a variety of cells with specific ratios of each. One construct, for example, combined inhibitory and excitatory neurons that communicate using different types of signaling molecules, called neurotransmitters. The researchers then added astrocytes, an important type of supporting cell. In many cases, neurons produced electrical signals and astrocytes performed their job soaking up the neurotransmitter glutamate, suggesting they were making functional connections, similar to those in the brain. When researchers combined two cell types seen in the brain's outer cortex and deeper striatum, they found the cortical cells extended projections toward the striatal cells, but not the other way around, just as seen in the human brain. This told researchers that the constructs could replicate how the brain is organized.
"It's really cool that despite [the simplicity of the constructs], they got proper connectivity," Szele says. And Levato says that although previous studies involving 3D printing of brain tissue had observed some functionality, they involved constructs that were "nowhere near the type of quality of tissue that they get here."
Some experts, however, note that the printed tissues are still relatively thin—about 50 microns, or the diameter of a human hair—so they don't fully mimic the 3D complexity of a real brain. "The disadvantage is that they can only print one layer and stack them together, so it's more like 2.5D," says Oxford bioengineer and chemical biologist Linna Zhou, who was not involved in the study.
Still, researchers say the technology could improve studies of brain development and diseases. For example, when Zhang and his group printed cells that had a mutation associated with a neurodegenerative condition called Alexander disease, they saw that the cells formed fewer connections, echoing observations in humans. Printing techniques like this one could "make bioengineered tissue more compatible with disease modeling," Zhou says. Eventually, researchers envision printing tissue that is fit for transplanting into patients who have lost brain tissue because of stroke, neurodegeneration, or a traumatic brain injury.