Development human brain Gains billions of neurons in the uterus and packed more during childhood. For most of the 20th century, conventional wisdom was that the cells of the brain cultivated before adulthood would be the only ones that we would have for the rest of our lives. But in recent decades, more and more research disputes this belief.
So, is it really possible for adults to cultivate neurons? While some experts believe that there are solid evidence that we can earn from brain cells after childhood, others are still skeptical about this notion.
The process of creating new brain cells is called Neurogenesis. Researchers first observed neurogenesis after birth in laboratory animals of various ages, including mice, rats And Singers birds. In adult mice, they have found new neurons growing in certain parts of the brain subventricular zonean area closely linked to smell, as well as in the hippocampusA structure at the heart of memory.
Researchers think that neurogenesis in these brain regions is important for plasticity, or the capacity of the brain to adapt and change over time. Plasticity underpins the ability to learn and train memories, for example.
In mice, it is clear that lifestyle factors such as living in a Stimulating environment And exercise can promote the growth of new neurons. Conversely, in models of disease mice like Alzheimer’sNeurogenesis is hampered. What is still to be discussed is whether these results extend beyond the mouse and other laboratory animals.
Neurogenesis in human adults?
“Most of our knowledge of adult neurogenesis comes from studies on animal models”, ” Hongjun songProfessor of neuroscience at the University of Pennsylvania Perelman School of Medicine, told Live Science in an email. “The question of whether such knowledge can be directly translated into human studies is a challenge.”
Indeed, many studies that have established the existence of adult neurogenesis in animals have used methods that are not possible in human studies, such as the injection of molecules of radioactive tracers into the brain. These methods allow scientists to visualize if and where new neurons develop, but the tracers themselves can be toxic. These methods also need to dissect the brain after the animal’s euthanasia.
“Unfortunately, there is no way to measure neurogenesis in the living person,” Gerd KempermannProfessor of genomic of regeneration at the Center for Regenerative Therapies of Dresden, Germany, told Live Science in an e-mail.
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However, there are rare cases in which scientists have been able to apply similar methods to follow neurogenesis in humans. For example, radioactive tracer molecules used in animal neurogenesis studies are also sometimes used by doctors to follow tumor growth in patients with brain cancer. Although these radioactive tracer molecules are too toxic to give healthy people, their advantages prevail over 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 brain of cancer patients after their death. They reported that in addition to pointing out the cancer cells, the tracer molecules had marked new neurons in the hippocampus. This observation suggests that humans could develop new neurons in adulthood, since patients were 57 to 72 years old.
Later, a 2013 study in the journal Cell used a form of Radiocarbon dating To seek new neurons in humans. Radiocarbon dating generally determines the age of a sample by comparing the relative proportion of two different forms of carbon or carbon isotopes, called carbon-4 and carbon-12. But to study neurogenesis in humans, scientists have rather examined the concentrations of carbon-14 within cell DNA.
Their approach took advantage of the fact that there was a peak in carbon 14 levels In the atmosphere caused by nuclear bombs tests in the 1950s and 1960s. The body of people absorbed this carbon-14 via their diet and it became incorporated into their DNA. The quantity of carbon -14 in a given cell corresponds to the concentrations of the isotope in the atmosphere when the cell has formed, which makes it possible to roughly determine the anniversary of this cell – and to determine if it formed after the birth of its owner.
By analyzing the post-mortem brain fabric of 19 to 92, this radiocarbon study has identified newborn neurons in the adult hippocampus. But although unconventional, the study methods were so complex that the results were never reproduced.
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 others research groups have discovered additional evidence of newborns in the adult human brain.
“There are many different markers who are more or less specific to adult neurogenesis,” said Kempermann. “In fabric cuts, we can study these markers under a microscope and search for models consistent with neural development.”
Not everyone is convinced
But some researchers are not convinced by these evidence. Arturo Alvarez BuyllaProfessor of neurological surgery at the University of California in San Francisco, spent his career studying the growth of new neurons. While he observed New neurons are formed in children and adolescentsHe found little evidence in support of the concept of neurogenesis in adults.
Alvarez-Buylla believes that there are a number of problems that can lead other researchers to find signs of neurogenesis in adult humans. For example, the chemical markers that some laboratories use to follow new neurons can also appear in other types of cellsLike glie, which are brain cells that support the function of neurons in various ways. This may reveal that new neurons develop when they are not.
It is also skeptical about the use of carbon 14 dating for this purpose, the caller “creative” but arguing that researchers cannot confirm that new cells are neurons, or if there could be other potential reasons for different carbon-14 levels in cells.
But Alvarez-Buylla does not raise the possibility of human adult neurogenesis; He only says that – so far – the evidence has not convinced him. “I would say it is a rare phenomenon,” said Alvarez Buylla. “If it happens, it’s very few cells.”
Kempermann, on the other hand, is a firm believer that people can develop new neurons in adulthood. “There are far more positive reports than critical articles, their point of view is much wider and their quality is generally higher.”
The researchers said that understanding the opportunity for adult neurogenesis will continue to be a key question for the field of neuroscience.
“The question of whether adults can cultivate new neurons have enormous implications for the plasticity of the adult brain,” said Song. If new neurons can be cultivated and integrated into the brain, he explained, these mechanisms could form the basis of new therapies for a variety of conditions, including brain damage and neurodegenerative disorders.
Alvarez-Buylla said that even if it does not believe that neurogenesis occurs frequently in adults, it could nevertheless be possible to exploit the mechanisms used by animals to develop new neurons for human therapies.
“The idea that it can happen opens up a huge door for repair,” he said. “I hope we can keep our plasticity open to the things that take place in both directions.”
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