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Dr.Marie-Ève Tremblay of Laval University in Québec in Canada recently announced the discovery of "mysteriously shrunken" brain cells that seem to be associated with Alzheimer's disease.
Not sure whether the cells cause Alzheimer's disease or a consequence of the condition, Dr. Tremblay says that the dead or dying cells are microglia, a type of brain cell that is normally tasked with keeping the brain free of infection. Microglia also cut unwanted connections between functioning brain cells. Dr. Tremblay said that the cells had not been noticed before because they do not absorb the stains most often used to identify cells on slides to be placed under the microscope.
Brain Connection Killers in Lab Animals
The aberrant and destructive brain cells congregate and synapses, the tiny gaps between neurons over which information passes. As the microglia gather at synapses, it seems to degrade, before the neurons themselves die.
The numbers of microglia, at least in laboratory mice, increase as the brain ages, and they also seem to increase during stress. The brains of mice that have a condition analogous to Alzheimer's disease in humans have 10 times as many microglia as are found in the brains of normal mice.
Also Found in Humans
These cell-sized connection killers have also been found in human brains. Although they are not as prominent in human brains as they seem to be in mouse brains, Dr. Tremblay noted that the brain of a person who died of Alzheimer's at the age of 45 had twice as many microglia as would be expected in a healthy brain. Tremblay and other researchers believe that these especially destructive "dark" microglia may power the destructive processes that cause dementia.
Why Would Microglia Smother Brain Connections?
One of the questions addressed by researchers is why microglia, which normally prune superfluous connections between neurons, would start cutting connections needed for normal function of the brain. Researchers think that one possibility may be erroneous signaling, which causes the microglia to accelerate their usual cleanup schedule.
Another possibility is that the destruction of brain tissue by the dark microglia may be a response to physical stress. It is well know that inflammation from outside the brain accelerates chronic brain conditions such as Alzheimer's and Parkinson's diseases. Conditions that destroy joints or kidney tissue or skin generate inflammatory cytokines that can also damage the brain. Generally poor health makes dementia worse.
But yet another possibility is that the stress that is the problem is a stress in the microglia themselves. As these tiny clean up cells become more destructive, they also take on a darker coloration under the microscope. Experiments have found that microglia darken as they are deprived of oxygen (something that would happen if hardening of the arteries reduced circulation to the brain) and as they respond to assault by free radicals of oxygen, which are more active when blood sugar levels are high and during infection, injury, or chemotherapy. Dark microglia have been found to have defective DNA and damaged proteins.