They are getting bigger and more complex. They might even allow scientists to replace brain tissue that’s been lost through injury or disease.īut Gage’s experiment, which was first announced at a conference last November, shows that the ethical discussion around organoids has yet to catch up with the fast-moving technology for creating them. Organoids, though also limited in their own ways, “allow us to capture aspects of human brain development that were previously inaccessible,” says Pasca. Animal brains are smaller than ours, and grow differently. Imaging technologies are still relatively crude. “There are many issues why psychiatric disorders have been hard to crack, but one is that we don’t have access to the functioning human brain,” says Sergiu Pasca, a neuroscientist at Stanford University. Another group, led by Hongjun Song at the University of Pennsylvania used organoids to confirm that the Zika virus can affect the fetal brain and cause microcephaly, and to pinpoint which cells it infects. Those organoids were also smaller than usual, and the team could work out why. For example, one group of researchers made organoids with a genetic mutation that’s linked to microcephaly-a condition where babies grow up with small brains. Scientists can use them to understand how brains develop, and how they differ in disorders. They do, however, capture some of the architectural features of parts of a brain, which is why they’re useful. They’re not mini-brains either, in the same way that a leaf is not a mini-plant and a doorknob is not a mini-building. They are emphatically not brains in jars. They don’t sense, learn, or make memories. On their own, their neurons don’t form networks like those in our heads. Even the biggest ones don’t have the full set of cells needed for a working brain. The biggest brain organoids are lentil-sized and contain 2 to 3 million cells a human brain is 20,000 times bigger and contains around 172 billion cells. To be clear, Gage’s mice weren’t running around with human brains, nor did they have a human mind trapped inside their heads. It was the first time such transplants had worked: Until now, organoids had only ever been grown in dishes. Once surgically implanted into rodent brains, the organoids continued growing, and their neurons formed connections with those of the surrounding brains. Gage’s team grew the blobs, known as brain organoids, from human stem cells. Last week, Rusty Gage and colleagues at the Salk Institute announced that they had successfully transplanted lab-grown blobs of human brain tissue into mice.
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