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