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    "original_url": "https:\/\/www.nytimes.com\/2026\/09\/16\/science\/human-mouse-brains-organoids.html?emc=edit_nn_20260917&nl=the-morning&segment_id=226633",
    "parsed_content": "These Mice Have Partly Human BrainsThe animals may help reveal the causes of brain disease in people and point the way to treatments. But researchers already are calling for new ethical guidelines.Neurons firing in the brain of a mouse given human cortex cells.These Mice Have Partly Human BrainsThe animals may help reveal the causes of brain disease in people and point the way to treatments. But researchers already are calling for new ethical guidelines.Neurons firing in the brain of a mouse given human cortex cells.Credit...Supported bySKIP ADVERTISEMENTListen \u00b7 8:49 min Share full articleBy Carl Zimmer and Carolyn Y. JohnsonSept. 16, 2026In order to learn more about brain disorders, scientists reported on Wednesday that they had created extraordinary mice whose brains are filled with millions of human neurons. The work marks a startling advance in replicating the complexities of the human brain outside the body.The research, described in the journal Nature, also poses some profound questions. \u201cWill there be some enhancement of function?\u201d asked Dr. Sergiu Pa\u0219ca, the Stanford University neuroscientist who led the work. \u201cWill they do better than other mice?\u201dDr. Pa\u0219ca and his colleagues have consulted regularly with ethicists about the implications of their research. And they\u2019ve kept careful watch on the mice, analyzing their behavior. So far, they behave mostly as if they had ordinary mouse brains.\u201cYou wouldn\u2019t be able to tell the difference,\u201d Dr. Pa\u0219ca said. Other experts not involved in the study agreed.\u201cIt may have a large proportion of its brain made up of human cells, but none of its behavior is made up of human behavior,\u201d said Madeline Lancaster, a developmental neurobiologist at the Medical Research Council Laboratory of Molecular Biology in the United Kingdom.Dr. Pa\u0219ca spent years creating these mice for one reason: It is bogglingly hard to study human brain diseases. Our brains contain about 80 billion neurons, joined by trillions of connections into intricate circuits.Some scientists suspect that when those networks go awry, they give rise to conditions such as schizophrenia and autism. \u201cNeuropsychiatric disorders are largely disorders of circuits,\u201d Dr. Pa\u0219ca said.Mice have complex brains, too, and they are useful experimental subjects. But the similarities to human brains are limited.Our brains are a thousand times bigger. They hold special types of neurons not found in most other animals, and they are organized into layers and other structures crucial to the human experience.In the 2010s, scientists created a new way to study the human brain by growing tiny replicas in the lab called organoids. Neuroscientists use them to observe the early stages of brain development \u2014 and to see how that development can veer off course.ImageDr. Sergiu Pa\u0219ca in his lab at Stanford.Credit...Mike Kai Chen for The New York TimesImageA vial of organoids under fluroescent light.Credit...Mike Kai Chen for The New York TimesDr. Pa\u0219ca and his colleagues have used brain organoids to study a rare form of profound autism called Timothy syndrome. Neurons from these patients behave differently than cells from people without the disease.More on Organ TransplantsU.S. Cuts Ties With Organ Donor Group: The federal government is ending its contract with a Kentucky nonprofit that it says pressured medical workers to remove organs from patients who were getting better.An Improved System: A crackdown on problems with fairness and safety is achieving results, including a big drop in the number of sick patients being passed over for transplants.Skipped Over in Line: The sickest patients are supposed to get priority for lifesaving transplants. But more and more, they are being skipped.Bungled Organ Retrievals:\u00a0Potential donors endured rushed or premature attempts\u00a0to remove their organs. Some were gasping, crying or showing other signs of life.Courting Overseas Patients:\u00a0Despite an organ shortage, some U.S. hospitals seek out international patients, who pay as much as $2 million for a transplant. In recent years, patients from abroad have typically received organs faster than U.S. patients.These clues led Dr. Pa\u0219ca and his colleagues to design a drug to restore the growth of brain cells in people with Timothy syndrome. The researchers have performed safety testing in animals and hope to start a clinical trial in the next few months.But organoids aren\u2019t perfect stand-ins for human brains. They are minuscule compared to the real thing, they don\u2019t get signals from the outside world, and they aren\u2019t nourished by blood vessels.So, for a decade, scientists have been trying to improve human brain organoids by transplanting them into rodents. Neuroscientist Fred Gage and his colleagues at the Salk Institute for Biological Studies achieved the feat for the first time in 2018.AdvertisementSKIP ADVERTISEMENTIn a follow-up study published in 2022, the researchers implanted human organoids in the visual center of adult mouse brains. Flashes of light caused both mouse and human neurons to respond, the scientists found.At Stanford, Dr. Pa\u0219ca and his colleagues inserted human brain organoids into young rats. The organoids grew until each took up about a third of one side of a rodent\u2019s brain, and the neurons grew longer. The rats were used to experiment with the drug for Timothy syndrome.Even these transplants had their limits. The scientists had to surgically remove a piece of the rat brain to make room for the organoid, and rat neurons developed connections more quickly than the human ones.Seven years ago, Dr. Pa\u0219ca and his colleagues decided to take a new approach. They started by genetically engineering mice with parts of their brains missing.The scientists altered the genes in mice that are essential for growing neurons in the brain\u2019s outer layers, known as the cortex. The cortex is active when mammals make decisions and carry out other complicated kinds of cognition.The cortex takes up a much bigger proportion of human brains than is the case in other species. It enables our sophisticated use of language and other forms of higher-order thinking.AdvertisementSKIP ADVERTISEMENT\u201cIt\u2019s the part of the brain that results in our humanness,\u201d said John Evans, a bioethicist at the University of California, San Diego, who was not involved in the study.The genetically altered mice had brains that were almost entirely missing the cortex. Fluid filled the space where half of their brains would normally have grown.Then Dr. Pa\u0219ca and his colleagues sought volunteers to provide skin cells that would be used to make brain organoids; the volunteers were told that these cells might end up in animals.The scientists went on to create organoids made of cells of the human cortex. Those organoids were placed into the empty spaces in the brains of the engineered mice. (The mice were engineered without an immune system so that they wouldn\u2019t reject human tissue.)The organoids thrived, feeding on the fluid in the skull and then connecting to the blood vessels in the mouse brain. From a few hundred thousand cells, the human neurons multiplied to as many as 4 million.VideoBy every measure the scientists have made, the mice with human neurons, like the one at left, behave pretty much as if they had an ordinary mouse brain, like the one at right.CreditCredit...\u201cThat\u2019s a big proof of concept, saying it\u2019s possible to create this kind of human-dominated cortex in a mouse,\u201d said Hongkui Zeng, the director of brain science at the Allen Institute in Seattle, who was not involved in the study.The human cells were still immature, Dr. Pa\u0219ca\u2019s team found, resembling the neurons inside the brain of a third-trimester fetus. But they still managed to connect with the mouse brain, becoming active as the mice went about their lives.The researchers filmed the movements of the mice, finding only subtle differences from ordinary mice. And in tests of memory and other cognitive tasks, the mice with human brain cells fared about as well.But the engineered mice were like humans in some crucial ways \u2014 ways that make them useful for studying brain disorders.Babies that suffer a loss of oxygen around birth can experience devastating damage to the cortex. But mouse pups exposed to low oxygen usually escape injury.Dr. Pa\u0219ca and his colleagues found that a lack of oxygen damaged the human neurons in the mice. Afterward, the animals had difficulty walking, mirroring the effects of oxygen deprivation in humans.AdvertisementSKIP ADVERTISEMENT\u201cThis is an ideal example of how you can study this devastating disorder,\u201d Dr. Pa\u0219ca said.Dr. Pa\u0219ca and his colleagues also discovered that some of the human brain cells in mice became quite large. These cells, the scientists discovered, are a special type, known as von Economo neurons.They only develop in large, social animals like whales, elephants and humans. When scientists look at the brains of people who suffer from frontotemporal dementia, they find that the von Economo neurons are among the first cells to die.Now that the scientists can make mice with von Economo neurons, they\u2019re starting to investigate their connection to the disease.\u201cWe\u2019re trying to see whether mutations that are associated with frontotemporal dementia are making the cells uniquely susceptible to disease,\u201d Dr. Pa\u0219ca said. \u201cAnd why? Why is this happening?\u201dThe new study is the latest evidence of how quickly research into brain organoids is moving \u2014 far faster than regulations intended to keep the research from crossing any red lines.In a report earlier this month, Dr. Pa\u0219ca and other researchers warned that oversight from governments and universities is evolving far too slowly to keep up with the science.If millions of human cortex cells can thrive in a mouse\u2019s brain, Dr. Zheng wondered, what might happen if scientists transplanted brain organoids into pigs or monkeys, with much more space for the human cells to grow?\u201cThere\u2019s really need of ethical oversight and regulation,\u201d she said.AdvertisementSKIP ADVERTISEMENTAdvertisementSKIP ADVERTISEMENT",
    "published_at": "2026-09-16T15:00:09+00:00",
    "author_name": "Carl Zimmer, Carolyn Y. Johnson",
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    "ai_headline": "These Mice Have Partly Human Brains",
    "ai_simplified_title": "Scientists Create Mice With Human Brain Cells to Study Disease",
    "ai_excerpt": "Scientists have created mice with millions of human neurons in their brains to study brain disorders. This research marks a significant advance in replicating human brain complexities outside the body, but also raises profound ethical questions.",
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