Latest news with #neurons
Yahoo
03-07-2025
- Health
- Yahoo
Even old brains can make new neurons, study suggests
Your body is constantly generating new cells. In your digestive tract, the colon's lining turns over every five to seven days. Your red blood cells replace themselves every few weeks, skin cells about once a month. But certain organs are a big exception. Contrary to popular belief, we are not biological Theseus' ships, reconstructing ourselves entirely from fresh building blocks every seven years. Most of your neurons, the cells that fast-track information across the brain, spine, and sensory organs, have the same lifespan as you do. Until the late 20th century, the prevailing view in neuroscience was that, past childhood, humans stop making neurons, brain-wide. What we have in adolescence, is what we get, and all we can do is lose cells or reorganize them. However, the latest research adds to a mounting body of evidence finding that the timeline of neuron generation isn't so clear-cut. In at least two parts of the brain, a subset of neuroscientists believe that neurons may continue to form throughout life–the hippocampus and the ventral striatum. In the hippocampus, a critical brain region for learning and memory, new cells emerge in some people into late adulthood, according to a study published July 3 in the journal Science. The findings tip the scales in a still-active debate over how our brains continue to develop throughout life. A better understanding of adult neurogenesis (the formation of new neurons), and a firm answer to if and where it occurs, could help improve treatments for neurological diseases as well as normal aging. [ Related: Scientists mapped every neuron of an adult animal's brain for the first time. ] Past research in rodents, pigs, and monkeys has shown that neurogenesis does happen in these other mammals, throughout life. Birds do it too. Yet based on other noted structural and developmental differences in the human brain, some scientists argue that our species is unique and lacks the lifelong neurogenesis that might interrupt complex, streamlined brain function. (New neurons may sound great, but too much activity and connectivity can cause chaos.) And it's difficult to get a clear answer. It's a major technical challenge to pin down new brain cells in humans because you can't readily see through living skulls at the cellular level. Researchers largely have to rely on scarce brain tissue collected via surgery on those with medical conditions like epilepsy or tissue donated by the deceased. With this new study, 'I think they used really strong tools,' Mercedes Paredes, a neurologist and developmental neuroscientist at University of California, San Francisco, tells Popular Science. Paredes was not involved in the research, but has previously found contrasting results in her own lab. The new work, she says, is a 'good starting point' for applying novel methods to the brain and deciphering what types of cells are truly present. Other studies of potential adult neurogenesis have looked at protein and immune signatures in brain tissues to determine if, where, and how often new neurons are being formed. They've come to conflicting conclusions, with some (like Paredes' 2018 work) failing to identify the cells that other researchers report seeing. There've also been a few studies that rely on carbon dating to ascertain the age of neurons, which find young cells in adults. However, none of these methods have, so far, reliably pinpointed the stem cells or progenitors capable of yielding fresh neurons–leaving room for doubt. The new study seems to address this previously unresolved point. It's the 'missing link' of neurogenesis, Jonas Frisén, senior study author and a developmental biologist and stem cell researcher at Karolinska Institutet in Stockholm, tells Popular Science. To find that missing link, Frisén and his co-authors surveyed the scientific literature to compile a list of genes that are likely active in hippocampal neurogenesis. Then, they confirmed those gene markers (largely from animal studies) by comparing them to RNA sequences found in brain samples from six deceased, child and infant donors. Next, the team sequenced RNA from mitochondria in the brain tissue from 19 people between the ages of 13 and 78. They used three different machine learning algorithms to assess those sequences and identify likely intermediate, forming neural cells. The scientists validated their machine learning outputs by applying the same tools to datasets from mice and adult human cortex cells, and reported a false positive rate of just 0.37 all of their analyses, they identified 354 of cells out of hundreds of thousands across their 19 samples that appeared to be precursors to new neurons, including dozens of stem cells and neuroblasts from adults. The cells weren't distributed uniformly, only showing up in half the adolescent samples and five of the 14 adult samples. However, whether or not the cells were present didn't seem to wholly correlate with age or documented disease. One of the adults with the highest number of neurogenesis-related cells present was a 58-year-old with no known pathology, per the study.'We nailed down active neurogenesis in the adult human brain,' Marta Paterlini, a neuroscientist at Karolinska Institutet, tells Popular Science. She co-led the new work over eight years alongside her colleague Ionut Dumitru, who focused on the machine learning side. 'We are confident in our data,' Paterlini says. 'We would like to put an end to the controversy.' Still, not everyone in the field agrees that the cells identified in this study are indisputably emerging adult neurons. 'When I first heard about this study, I was excited. It's the sort of approach you would want to use to ID rare subtypes,' Shawn Sorrells, a neuroscientist at the University of Pittsburgh who co-authored the 2018 study with Paredes, tells Popular Science. But, after a closer look, he was 'disappointed by how few cells they found,' and in his view, there's an alternate potential explanation. 'The most likely conclusion is that the cells they are looking for are rare or nonexistent in most people. The other possibility is that the cells they claim are adult neural stem cells are associated with a disease process in these individuals or some other cell type altogether.' The human brain is full of cells that do divide and replicate throughout life called glial cells. These are the supportive and connective cells that enable neurons to do the job of conveying nerve signals. Glial cells are neurons' pit crew. It's possible that, in their efforts to identify neurons in progress, the study authors may have inadvertently included some glial stem cells in their analysis, Sorrells suggests. [ Related: New human brain atlas is the most detailed one we've seen yet. ] Both Sorrells and Paredes believe more research is needed to confirm the new study's conclusions. Direct, morphological comparisons between human and animal cells could boost the findings, says Paredes. Sorrells suggests we'll need more advanced brain imaging techniques to really resolve the issue. 'Following the same cells over time to see how they develop. That will be the best evidence for neurogenesis,' he explains, though notes that's currently not possible. And so, the science presses on. If, as the new research indicates, some healthy humans do continue to make neurons inside their hippocampuses for life, it could have major implications for psychiatric and neurodegenerative diseases like Alzheimers, where animal studies indicate a dearth of new cells plays a role. It could also aid in our ability to understand and maximize healthy aging and neuroplasticity, Frisén says. Perhaps down the line, adult neural stem cells could be used to help people recover after brain trauma, Paterlini suggests. 'The lab is working on regenerative medicine, so we will keep going on this.' And maybe all their effort will manage to change minds about how much human minds can change.
Yahoo
01-06-2025
- General
- Yahoo
Can adults grow new brain cells?
When you buy through links on our articles, Future and its syndication partners may earn a commission. The developing human brain gains billions of neurons while in the womb, and tacks on some more during childhood. For most of the 20th century, the conventional wisdom was that the brain cells grown before adulthood would be the only ones we would have for the rest of our lives. But over the past few decades, more and more research is challenging that belief. So is it actually possible for adults to grow neurons? While some experts believe there's strong evidence that we can gain brain cells after childhood, others are still skeptical of this notion. The process of creating new brain cells is called neurogenesis. Researchers first observed neurogenesis after birth in lab animals of various ages, including mice, rats and songbirds. In adult mice, they found new neurons growing in parts of the brain collectively called the subventricular zone, an area closely linked with sense of smell, as well as in the hippocampus, a structure that's central to memory. Researchers think that neurogenesis in these brain regions is important for plasticity, or the brain's ability to adapt and change over time. Plasticity underlies the ability to learn and form memories, for instance. In mice, it's clear that lifestyle factors such as living in a stimulating environment and exercising can promote the growth of new neurons. Conversely, in mouse models of diseases like Alzheimer's, neurogenesis is hampered. What's still up for debate is whether these findings extend beyond mice and other lab animals. "Most of our knowledge about adult neurogenesis came from studies in animal models," Hongjun Song, a professor of neuroscience at the University of Pennsylvania Perelman School of Medicine, told Live Science in an email. "Whether such knowledge can be directly translated to human studies is a challenge." That's because many of the studies that have established the existence of adult neurogenesis in animals used methods that aren't possible in human studies, such as injecting radioactive tracer molecules into the brain. These methods enable scientists to visualize if and where new neurons are growing, but the tracers themselves can be toxic. These methods also require dissecting the brain after the animal has been euthanized. "Unfortunately, there is no way to measure neurogenesis in the living person yet," Gerd Kempermann, a professor of genomics of regeneration at the Center for Regenerative Therapies in Dresden, Germany, told Live Science in an email. Related: Is there really a difference between male and female brains? Emerging science is revealing the answer. There are, however, some rare cases in which scientists have been able to apply similar methods to track neurogenesis in humans. For example, the radioactive tracer molecules used in animal neurogenesis studies are also sometimes used by doctors to track tumor growth in patients with brain cancer. While these radioactive tracer molecules are too toxic to give to healthy people, their benefits outweigh their risks in patients who already have cancer. Scientists behind a 1998 study published in the journal Nature Medicine used this approach and analyzed the brains of cancer patients after their deaths. They reported that, in addition to flagging cancer cells, the tracer molecules had marked new neurons in the hippocampus. This finding suggested that humans could grow new neurons well into adulthood, given the patients were 57 to 72 years old. Later, a 2013 study in the journal Cell used a form of radiocarbon dating to look for new neurons in humans. Radiocarbon dating usually determines the age of a sample by comparing the relative proportion of two different forms of carbon, or carbon isotopes, called carbon-14 and carbon-12. But to study neurogenesis in humans, scientists instead looked at carbon-14 concentrations inside the DNA of cells. Their approach took advantage of the fact that there was a spike in carbon-14 levels in the atmosphere caused by nuclear bomb tests in the 1950s and 1960s. People's bodies absorbed this carbon-14 via their diets, and it became incorporated into their DNA. The amount of carbon-14 in a given cell corresponds to the isotope's concentrations in the atmosphere at the time the cell formed, making it possible to roughly pinpoint the "birthday" of that cell — and determine whether it formed after its owner's birth. By analyzing postmortem brain tissue from people ages 19 to 92, this radiocarbon study identified newborn neurons in the adult hippocampus. But while compelling, the study's methods were so complex that the results have never been replicated. That said, there are also more indirect markers of neurogenesis, such as certain proteins that are only present in growing neurons. Using these methods, both Kempermann and other research groups have uncovered additional evidence of newborn neurons in the adult human brain. "There are many different markers that are more or less specific for adult neurogenesis," Kempermann said. "In tissue sections, one can study these markers under the microscope and look for patterns that are consistent with neuronal development." But some researchers aren't convinced by this evidence. Arturo Alvarez-Buylla, a professor of neurological surgery at the University of California, San Francisco, has spent his career studying the growth of new neurons. While he's observed new neurons being formed in children and adolescents, he's found little evidence to support the notion of neurogenesis in adults. Alvarez-Buylla believes there are a number of issues that may lead other researchers to find signs of neurogenesis in adult humans. For example, the chemical markers that some labs use to track new neurons may also show up in other cell types, such as glia, which are cells in the brain that support neurons' function in various ways. This may make it appear that new neurons are growing when they're actually not. He's also skeptical of the use of carbon-14 dating for this purpose, calling it "creative" but arguing that researchers can't confirm that the new cells are neurons, or if there could be other potential reasons for varying carbon-14 levels in cells. But Alvarez-Buylla isn't ruling out the possibility of human adult neurogenesis; he's only saying that — so far — the evidence hasn't convinced him. "I would say that is a rare phenomenon," said Alvarez-Buylla. "If it happens, it's very, very few cells." Kempermann, on the other hand, is a firm believer that people can grow new neurons well into adulthood. "The positive reports outnumber the critical papers by far, their take is much wider, and their quality is overall higher." The researchers said that understanding whether adult neurogenesis exists will continue to be a key question for the field of neuroscience. RELATED STORIES —Could we ever retrieve memories from a dead person's brain? —What happens in your brain while you sleep? —How much of your brain do you need to survive? "The question about whether adults can grow new neurons has tremendous implications for the plasticity of the adult brain," Song said. If new neurons can be grown and integrated into the brain, he explained, those mechanisms could form the basis for new therapies for a variety of conditions, including brain injuries and neurodegenerative disorders. Alvarez-Buylla said that even if he doesn't believe neurogenesis happens frequently in adults, it may nonetheless be possible to harness the mechanisms used by animals to grow new neurons for human therapies. "The whole idea that it can happen opens a huge door for repair," he said. "I hope that we can keep our plasticity open to things going either way."
