Minuscule blobs of human brain tissue have come a long way in the four years since scientists in Vienna discovered how to create them from stem cells.
The most advanced of these human brain organoids — no bigger than a lentil and, until now, existing only in test tubes — pulse with the kind of electrical activity that animates actual brains. They give birth to new neurons, much like full-blown brains. And they develop the six layers of the human cortex, the region responsible for thought, speech, judgment, and other advanced cognitive functions.
A human brain organoid cannot reach anything close to the size of even a child’s brain in the tiny confines of a mouse’s (which is one one-thousandth the volume of a person’s brain), almost certainly limiting how complex it can become. “These little suckers are not going to say ‘Hi,’” said legal scholar and bioethicist Hank Greely of Stanford University. But the rapid advances, he said, are raising the question “of whether you are creating something human-ish that you have to take seriously in terms of according it dignity and respect — and figuring out what that even means.”
Those issues emerged 17 years ago, when a Stanford colleague of Greely’s proposed implanting human stem cells into mouse brains to see what would happen when the former turned into neurons; the experiment has not yet happened. But the ethical consensus was that it might be OK if the fraction of human neurons in the mouse brain was so small that the implant had no chance of giving the mouse human consciousness or enhanced intelligence.
That could well be only a matter of time. Blood flow would make arrays of brain organoids more likely to survive, grow, and develop. “People are talking about connecting three or four,” said Stanford’s Greely. “But what if you could connect 1,000? That would be getting close to the number of cells in a mouse brain,” raising the distant prospect of a human brain organoid with cognitive and even emotional capacities, all while sitting in a lab dish.
“At some future point,” Greely said, “it could be that what you’ve built is entitled to some kind of respect.” Next year is the 200th anniversary of the publication of Mary Shelley’s “Frankenstein,” he noted: “I think that story is relevant to what we’re talking about.”Read More