Miniature Human Brains Grow for Months When Implanted in Mice Skulls

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The mice behaved just like others of their kind, as far as scientists could tell, and they also looked the same—except for the human mini brain that had been implanted into each rodent’s own cortex, made visible by a little clear cover replacing part of their skull.

The report on Monday by scientists at the Salk Institute is the first publication describing the successful implant of human cerebral organoids into the brains of another species, with the host brain supplying the lentil-sized mini cerebrums with enough blood and nutrients to keep them alive and developing for months. It won’t be the last, as scientists use the approach to understand human brain development and test whether the tiny entities might one day serve as cortical repair kits, replacing regions of the brain that have been injured or failed to develop normally.

It’s “an important technical advance,” said neuroscientist Michal Stachowiak of the State University of New York, Buffalo, who created human cerebral organoids to study schizophrenia, and “an important initial step toward using organoids in regenerative medicine.”

When the Salk researchers briefly described their experiments last November at the annual meeting of the Society for Neuroscience, bioethicists raised questions about what implants of human brain organoids would do to mice’s intelligence, consciousness, and even their identity as mice. The published paper, in Nature Biotechnology, fills in details about how successfully the human organoid integrated into the mouse brain and addresses one of those concerns: At least in the tests the scientists ran, the mice with human brain organoids seemed no different, and no smarter, than standard lab mice.

Since the first human brain organoids were created from stem cells in 2013, scientists have gotten them to form structures like those in the brains of fetuses, to sprout dozens of different kinds of brain cells, and to develop abnormalities like those causing neurological diseases such as Timothy syndrome. Researchers hope the organoids will be better than lab animals or cells growing in culture at revealing how the human brain develops, both normally and when things go awry, and identify potential therapeutic or genome-editing targets.

The basic recipe takes human stem cells, makes them differentiate into brain cells, and lets them grow into entities a few millimeters across that mimic the structure, cell populations, and even the electrical activity of the full-blown version.

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