What are mini brains? - Madeline Lancaster

599,509 views ・ 2018-01-16

TED-Ed


Please double-click on the English subtitles below to play the video.

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This pencil-eraser-sized mass of cells is something called a brain organoid.
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It’s a collection of lab-grown neurons and other brain tissue
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that scientists can use to learn about full-grown human brains.
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And it can be grown from a sample of your skin cells.
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Why would we need such a thing?
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Neuroscientists face a challenge:
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shielded by our thick skulls and swaddled in layers of protective tissue,
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the human brain is extremely difficult to observe in action.
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For centuries, scientists have tried to understand them using autopsies,
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animal models,
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and, in recent years, imaging techniques.
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We’ve learned a lot through all these methods,
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but they have limitations.
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Conditions like Alzheimer’s and schizophrenia,
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and the effect on the human brain of diseases like Zika,
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continue to hide beyond our view, and our understanding.
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Enter brain organoids, which function like human brains
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but aren’t part of an organism.
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Each one comes from an undifferentiated stem cell,
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which is a cell that can develop into any tissue in the body,
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from bone to brain.
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Scientists can make undifferentiated stem cells from skin cells.
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That means they can take a skin sample from a person with a particular condition
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and generate brain organoids from that person.
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The hardest part of growing a brain organoid,
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which stumped scientists for years,
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was finding the perfect combination of sugars, proteins, vitamins, and minerals
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that would induce the stem cell to develop a neural identity.
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That was only discovered recently, in 2013.
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The rest of the process is surprisingly easy.
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A neural stem cell essentially grows itself,
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similar to how a seed grows into a plant,
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all it needs are the brain’s equivalents of soil, water, and sunlight.
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A special gel to simulate embryonic tissue,
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a warm incubator set at body temperature,
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and a bit of motion to mimic blood flow.
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The stem cell grows into a very small version
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of an early-developing human brain,
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complete with neurons that can connect to one another
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and make simplified neural networks.
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As mini brains grow, they follow all the steps of fetal brain development.
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By observing this process, we can learn how our neurons develop,
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as well as how we end up with so many more of them in our cortex,
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the part responsible for higher cognition like logic and reasoning,
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than other species.
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Being able to grow brains in the lab, even tiny ones,
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raises ethical questions, like:
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Can they think for themselves, or develop consciousness?
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And the answer is no, for several reasons.
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A brain organoid has the same tissue types as a full-sized brain,
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but isn’t organized the same way.
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The organoid is similar to an airplane
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that’s been taken apart and reassembled at random;
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you could still study the wings, the engine, and other parts,
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but the plane could never fly.
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Similarly, a brain organoid allows us to study different types of brain tissue,
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but can’t think.
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And even if mini brains were organized like a real brain,
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they still wouldn’t be able to reason or develop consciousness.
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A big part of what makes our brains so smart is their size,
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and mini brains have only about 100,000 neurons
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compared to the 86 billion in a full-sized brain.
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Scientists aren’t likely to grow larger brain organoids anytime soon.
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Without blood vessels to feed them,
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their size is limited to one centimeter at most.
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Finally, mini brains aren’t able to interact with the outside world.
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We learn by interacting with our environments: receiving inputs
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through our eyes, ears, and other sensory organs, and reacting in turn.
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The complex neural networks that underlie conscious thoughts and actions
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develop from this feedback loop.
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Without it, the organoids can never form a functional network.
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There’s nothing quite like the actual human brain,
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but mini brains are an unprecedented tool
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for studying everything from development to disease.
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With luck, these humble organoids can help us discover
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what makes the human brain unique,
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and maybe bring us closer to answering the age-old question:
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what makes us human?
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