This microscope image shows neurons (yellow) attached to the silk-based scaffold (blue).
Image credit: Tufts University
The research team, including senior author David Kaplan, PhD, a Stern Family professor and chair of biomedical engineering at Tufts School of Engineering, says the model paves the way for new studies into brain function, injury and disease, and treatment.
They recently published their findings in The Proceedings of the National Academy of Sciences (PNAS).
To study the function of brain neurons, researchers currently grow them in petri dishes. But the complicated structure of brain tissue - which is made up of segregated areas of grey and white matter - cannot be duplicated with these 2D neurons.
Grey matter mainly consists of neuron cell bodies, and white matter consists of bundles of nerve fibers, or axons. These axons are responsible for transmitting signals between neurons.
When the brain is subject to damage or disease, the grey and white matter are affected in different ways, meaning there is a need for brain tissue models that allow each of these areas to be studied separately.
"There are few good options for studying the physiology of the living brain, yet this is perhaps one of the biggest areas of unmet clinical need when you consider the need for new options to understand and treat a wide range of neurological disorders associated with the brain," says Kaplan.
Scientists have recently tried creating functional brain tissue by growing neurons in 3D collagen gel-only environments but without success. Such models have died quickly and have failed to produce strong enough tissue-level function.
But the Tufts team has found a way to create functional 3D brain-like tissue that not only incorporates segregated gray and white matter regions, but that can also live for more than 9 weeks.
How was the 3D brain-like tissue created?
Firstly, Kaplan and colleagues combined two biomaterials: a silk protein and a collagen-based gel. The silk protein acted as a spongy scaffold to which neurons attached, while the gel encouraged nerve fiber growth.
This diagram shows the scaffold donut and the different areas of grey and white matter.
Image credit: National Institute of Biomedical Imaging and Bioengineering
The researchers then cut the spongy scaffold into the shape of a donut and colonized it with rat neurons, before filling the middle of the donut with the collagen-based gel, which infiltrated the whole scaffold.
The team found that the neurons created functional networks around the scaffold outlets in only a few days, and nerve fibers passed through the gel in the middle of the donut to connect with neurons on the other side. This created separate grey and white matter regions.
The researchers then conducted a series of experiments on the 3D brain-like tissue in order to test the health and function of its neurons, and compare them with neurons grown using the existing 2D method or in a gel-only environment.
Kaplan and colleagues found higher expression of genes involved in neuron growth and function in the 3D brain-like tissue.
The neurons grown in the 3D-like brain tissue demonstrated stable metabolic activity for almost 5 weeks, while such activity in neurons grown in a gel-only environment began to fade within 24 hours. Furthermore, electrical activity and responsiveness similar to that found in the intact brain was seen in the 3D brain-like tissue neurons.
Commenting on this creation, Rosemarie Hunziker, PhD, program director of tissue engineering at the National Institute of Biomedical Imaging and Bioengineering, which funded the study, says:
"This work is an exceptional feat. It combines a deep understanding of brain physiology with a large and growing suite of bioengineering tools to create an environment that is both necessary and sufficient to mimic brain function."
Model could improve studies of brain function, injury and disease
As the 3D brain-like tissue appeared functional, the team wanted to see whether their model could be useful for studying traumatic brain injury (TBI).
They simulated a TBI by dropping weights onto the model from different heights. They found that the chemical and electrical activity in the neurons of the tissue changed following TBI, which the researchers say is similar to observations reported in animal studies of TBI.
According to Kaplan, this finding shows that the 3D brain-like tissue model could provide a more effective way of studying brain injury.
"With the system we have, you can essentially track the tissue response to traumatic brain injury in real time," he explains. "Most importantly, you can also start to track repair and what happens over longer periods of time."
But the advantages of this model do not stop there. Kaplan notes that the brain-like tissue survived for more than 2 months, meaning it could allow researchers to gain a better insight into an array of brain disorders:
"The fact that we can maintain this tissue for months in the lab means we can start to look at neurological diseases in ways that you can't otherwise because you need long timeframes to study some of the key brain diseases."
"Good models enable solid hypotheses that can be thoroughly tested. The hope is that use of this model could lead to an acceleration of therapies for brain dysfunction as well as offer a better way to study normal brain physiology," adds Hunziker.
The researchers say they now plan to tweak the model to make it even more similar to the brain. They have already found that they can adjust the donut scaffold to incorporate six rings, each of which can be colonized with different neurons. This, the team says, would simulate the six layers of the human brain cortex.
Last year, Medical News Today reported on a study published in the journal Nature, revealing how scientists successfully grew "mini-brains" from stem cells.