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How nuclear fallout casts doubt on renewal of some adult braincells by 123wert sdfsf
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How nuclear fallout casts doubt on renewal of some adult braincells |
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The human body is a tireless gardener, growing new cells throughoutlife in many organs—in the skin, blood, bones and intestines.Until the 1980s most scientists thought that brain cells were theexception: the neurons you are born with are the neurons you havefor life. In the past three decades, however, researchers havediscovered hints that the human brain produces new neurons after birth in two places: the hippocampus —a region important for memory—and the walls offluid-filled cavities called ventricles, from which stem cells migrate to the olfactory bulb, a knob of brain tissue behind theeyes that processes smell. Studies have clearly demonstrated thatsuch migration happens in mice long after birth and that humaninfants generate new neurons. But the evidence that similarneurogenesis persists in the adult human brain is mixed and highlycontested.
A new study relying on a unique form of carbon dating suggests that neurons born during adulthood rarely if ever weavethemselves into the olfactory bulb's circuitry. In other words,people—unlike other mammals—do not replenish theirolfactory bulb neurons, which might be explained by how little mostof us rely on our sense of smell. Although the new research castsdoubt on the renewal of olfactory bulb neurons in the adult humanbrain, many neuroscientists are far from ready to end the debate. In preparation for the new study, Olaf Bergmann and Jonas Frisén of the Karolinska Institute in Stockholm and their colleaguesacquired 14 frozen olfactory bulbs from autopsies performed between2005 and 2011 at the institute's Department of Forensic Medicine.To determine whether the neurons were younger than the people theycame from—which would mean the cells were generated afterbirth—the researchers needed to isolate the cells' DNA.First, they dissolved the brain tissue into a kind of soup, whichthey spun at high speeds so that the dense cell bodies and nucleicontaining DNA sank to the bottom of the flasks. Using Y-shapedproteins called antibodies, which were hitched to fluorescentmarkers, the researchers tagged nuclei from both neurons and fromglia, non-neuronal brain cells.
After a laser-equipped cell-sortingmachine identified and separated the nuclei, the researchersisolated and purified the DNA within. Frisén and his colleagues analyzed the DNA with a uniqueform of carbon dating , the technique that paleontologists use to date the fossils andremains of ancient creatures. When the U.S. and Soviet Unionconducted nuclear bomb tests during the Cold War, the sparringnations doubled the amount of radioactive carbon 14 in Earth'satmosphere. After the Limited Nuclear Test Ban Treaty of 1963,atmospheric C 14 levels began to return to baseline, halving every11 years as plants and the oceans absorbed the radioactive isotopes (a differentprocess from radioactive decay, which halves the level of C 14 infossils every 5,700 years).
Scientists have documented this steadydecline by analyzing ice cores and tree rings, assigning uniqueatmospheric C 14 levels to each year since 1955. This record makes it possible to accurately date recently livingcells by looking at the amount of C 14 in their DNA. When one cellsplits into two—and duplicates its DNA in theprocess—it uses of some of the organism's current supply ofcarbon to make DNA's backbone. People get carbon from the animalsand plants they eat and plants absorb carbondioxide—including any C 14—from the air. Therefore,levels of C 14 in the DNA of preserved cells from a deceasedorganism should match the atmospheric levels of C 14 during themost recent cell division.
Scientists only have the opportunity tomake use of this unique form of carbon dating for a few moredecades, before C 14 levels drop to baseline. In their new study, Frisén and Bergmann found that the levelof C 14 in olfactory bulb neurons from all the autopsies almostexactly matched the levels of C 14 in the atmosphere when thosepeople were born. Their olfactory bulb neurons were as old as theywere—they had never been replaced. Levels of C 14 in glialcells, however, were lower than atmospheric levels of C 14 at thetime of the subjects' birth—these cells had divided after thepeople were born. The results appear in the May 24 issue of Neuron.
Frisén's findings do not change the fact that the humanbrain's ventricles are reservoirs of stem cells that can matureinto neurons. Likewise, the new results do not challenge the production of new neurons in tissue lining the nasal cavity . Frisén's latest study, however, adds significant weight toevidence suggesting that the garden of cells in the human olfactorybulb rarely plants new members during adulthood—a conclusionthat echoes several recent studies. Arturo Alvarez-Buylla of the University of California, San Francisco, has detailed howneurons migrate from the ventricles to the olfactory bulb in mice.When he looked for evidence of the same cellular journey in humanbrains, he found that the steady stream of new neurons producedbetween birth and 18 months of age dries up by early adulthood . Likewise, a study in China confirmed that ventricles harbor stemcells in the human brain, but found no new neurons in the olfactory bulb.
( Scientific American is part of Nature Publishing Group.) Contrasting these negative results, a 2007 paper in Science —of which Frisén was a co-author—concluded thatimmature cells continue to migrate to the olfactory bulb in theadult human brain and mature into neurons. The study relied onbrain tissue from people who were, for medical reasons, injectedwith a chemical called BrdU (bromodeoxyuridine), which closelyresembles thymidine—one of the building blocks of DNA. When acell divides, it incorporates BrdU into the newly duplicated DNA,which allows researchers to identify newborn cells. Pasko Rakic of Yale University, as well as other researchers, has q uestioned whether BrdU reliably tags newborn cells in adult tissues because the compound can trigger cell divisionand label dying cells, skewing the results. To compensate for theseshortcomings, some studies on neurogenesis have coupled BrdUlabeling with antibodies designed to tag proteins that onlyimmature neurons produce.
Frisén and his colleagues inventedtheir carbon dating technique in part to overcome thesefrustrations. On account of the contradictory evidence and technical challenges,many neuroscientists are not prepared to put the question ofneurogenesis in the human olfactory bulb to rest. In a commentary published in Neuron alongside Frisén's new study, Jeffrey Macklis of Harvard University argues that although the new study isrigorous, it does not definitively eliminate the possibility thatnew neurons join the adult human olfactory bulb after birth. AsMacklis points out, neurons that migrate from the ventricles to theolfactory bulb in mice, but fail to receive sufficient stimulationin the form of new smells, die. If the same thing happens in thehuman brain, Frisén's study would have missed it.
Perhapsnew neurons migrate through everyone's brains during adulthood, butonly remain in the olfactory bulb's circuitry if they findthemselves useful. Macklis wonders whether the 14 people in the newstudy aroused their noses enough to keep new neurons in theolfactory bulb alive. He suggests studying the brains of"chefs, sommeliers, perfumers, vintners,'foodies'"—connoisseurs of fragrance. Fred Gage of the Salk Institute for Biological Studies, who has workedextensively on neurogenesis in the human brain, thinks that the newfindings make sense from an evolutionary perspective.
" Animals on all fours, whose noses are right there on the track, have giantolfactory bulbs compared to humans," Gage says. "The newfindings might reflect the fact we are not as olfactorydependent.". I am a professional writer from Agriculture, which contains a great deal of information about wicker dining set , pvc sprinkler fittings, welcome to visit!
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