After two months in culture, organoids that mimic the human cortex were transplanted into young rat brains. The organoids were placed in the same location in each rat’s brain, which made monitoring their development easier.
Rat cells quickly migrated to human tissue. Rat endothelial cells infiltrate human brain implants and populate blood vessels. Blood vessels supply human cells with nutrients and signaling substances and carry away waste metabolites. Resident immune cells from rat brain were also incorporated into human grafts.
“They moved right away,” Paska said.
Transplanted human organoids survived, thrived, and grew. It was probably one-fifth of an inch in size when implanted, but after six months it had grown to occupy a full one-third of the transplanted rat brain hemisphere.
Individual neurons of human organoids in rat brains were at least six times larger than neurons of organoids (generated simultaneously by the same method) that remained in the dish without being transplanted. exhibited a much more sophisticated branching pattern.
Neurons from the organoids are placed in the rat brain and form working connections with intrinsic circuits. His one of the rat brain structures to which human neurons formed direct connections was the thalamus. The thalamus is a region deep in the brain that relays multiple sensory inputs to the cortex.
“This connection may have provided the signaling necessary for optimal maturation and integration of human neurons,” Pasca said.
Ultimate control experiment
To assess the ability of transplanted organoids to flag the molecular basis of neuropsychiatric disorders in humans, Paska and his colleagues identified Timothy’s syndrome, a rare genetic condition strongly associated with autism and epilepsy. I looked. Scientists transplanted organoids generated from skin cells from a patient with Timothy’s syndrome into one side of a rat’s brain. Corresponding sites on the contralateral side were transplanted with organoids generated from healthy control individuals.
After 5-6 months, researchers observed a marked difference in the electrical activity of the two sides. Neurons in Timothy’s syndrome were also much smaller and lacked sprouting branches, brush-like extensions called dendrites that act as antennas for input from nearby neurons.
“We learned a lot about Timothy Syndrome by studying organoids in dishes,” Pasca said. “But only with transplantation could we see differences associated with these neuronal activities.”
A similar comparison of transplanting organoids from people with and without conditions such as schizophrenia and autism on the opposite side of the brain of the same rat reveals differences in size, complexity, function, and connectivity. Pasca said it could be revealed.
Researchers at the Stanford University School of Medicine took care to place organoids in areas of the rat brain that process sensory information from the animal’s whiskers. Long hairs protruding from its nose help it detect nearby objects and surfaces and avoid collisions when moving quickly or in unfamiliar locations.
“Without removing tissue from the human brain, we are now able to study not only healthy brain development, but brain disorders that are understood to be developmentally rooted in greater detail than ever before.”
Scientists have clearly shown that human neurons can be activated by sensory input in rats. Perturbing the whiskers of adult rats with puffs of air electrically activated human neurons in the rat brains that responded in synchronism to each puff of air.
In cognitive tests about 200 days after transplantation, rats were less frightened than control rats, retained similar memory abilities, and did not experience seizures.
“Overall, we didn’t see any improvement or deficit,” Paska said.
In another experiment, scientists implanted human organoids whose constituent neurons were modified to be activated by specific frequencies of blue laser light. About three months later, the researchers inserted a very thin fiber optic cable. This allows laser light to be sent to human organoids in rat brains from a distance. These rats were then placed in a glass box with a small water spout. The researchers employed a form of Pavlovian conditioning by delivering random bursts of blue light to activate human organoid tissue in rat brains, but only after a blue light pulse did rats We made water available. By the end of the 15-day training period, pulsing the organoids with blue light was enough to make the rats start running towards the spout.
Rats learned to associate blue light stimulation of transplanted human neurons with water reward, demonstrating that transplanted human tissue can be integrated and engaged in reward-seeking circuitry in rats. Mammals direct behavior toward activities associated with previous pleasurable outcomes.
“This is the most advanced human brain circuit ever constructed from human skin cells, and shows that transplanted human neurons can influence behavior in animals.” Pasca said. “Our platform hopes to provide the first behavioral readout of human cells and accelerate our understanding of complex psychiatric conditions.”
This study was funded by the National Institutes of Health (Grants R01MH115012, K99DA050662, and S10RR026917-01), Stanford Wutsai Neuroscience Institute, Stanford Brain Organogenesis Program, Bio-X, Kwan Funds, Senkut Funds, New York Stem Cell. it was done. Foundation, Chan-Zuckerberg Initiative, Coates Foundation, Ludwig Family Foundation, Alfred E. Mann Foundation.