Feb. 8, 1999 NASHVILLE, Feb. 5 -- For years, scientists have believed that brain cells can't be born or newly generated following a stroke. But a new study in rodents finds that some brain cells are actually stimulated to regenerate following a stroke, a discovery that opens the door to treating memory disorders in stroke patients.
According to the study, this is the first evidence of a certain type of brain cell -- called a stem cell -- demonstrating plasticity and the ability to regenerate after a stroke.
Regeneration is the ability of the stem cells to divide and produce new neurons. Plasticity is the ability of the newborn neurons to make connections with existing neurons in the brain and integrate into their surrounding cellular environment. Though other studies have shown that new neurons are born under other conditions, this is the first study to show that new brain cells are born following stroke. The research was performed by inducing stroke in rodents.
"Data show that new neurons are born in the brains of adult monkeys and in the brains of adult humans," says Frank Sharp, M.D., of the department of neurology, University of California-San Francisco and one of the study's investigators. "It is not known whether there are new neurons born in the brains of humans following stroke. We certainly think there would be."
In research presented at the presented today at the American Heart Associations 24th International Conference on Stroke and Cerebral Circulation, scientists at the University of California-San Francisco discovered as much as a 12-fold increase in the birth of new cells in rodents after stroke in an area of the brain -- the hippocampus -- which controls memory.
"This region is important for new memories," says Sharp. "If you lose this region of the brain, you're unable to remember new sights, sounds and other experiences. You might remember your mother and father or something you learned in the eighth grade, but if you were taught a new way through town, you'd be unable to learn the new route."
In the future, say researchers, it may be possible that these stem cells can be stimulated to grow even more, perhaps helping to rewire the brain and help stroke survivors recover lost memory function.
Stem cells are important in the formative stages of brain development. Researchers say stem cells help form neurons, which mature and make the complex circuits that enable the brain to perform its many functions. Stem cells are present in the embryonic brain and remain in the brain throughout life, says Sharp. Most stem cells are located next to the ventricles in the brain and in the hippocampus.
Stem cells are present in the brain throughout life, and Sharp says there is evidence in rodents that the numbers of stem cells and newly formed neurons decreases as the brain ages.
For stroke survivors, recovering speech or motor skills typically requires many years of speech and physical therapy. Sharp cautions that more research is needed to determine whether or not sparking the growth of stem cells can be used as a companion treatment along with current types of therapy.
"The fact of the matter is, after stroke of all types, memory function is frequently impaired," says Sharp. "Though memory function often recovers to at least some extent, the mechanism by which this recovery occurs is not known. The birth of new neurons could very well be the mechanism which leads to memory recovery after ischemia."
Following the blockage of blood flow and decreased oxygen and glucose delivery to the brain -- a condition known as ischemia -- researchers found a 12-fold increase in the birth of new cells in the dentate gyrus of the hippocampus, a region of the temporal lobe that is crucial in laying down all long term memories. Sharp says that following the ischemia, half of the newborn cells become neurons and a quarter of the cells become glial cells.
Sharp says the research is the first to demonstrate that neural stem cells divide into neurons and astrocytes following ischemia. Though neurons are the information cells in the brain, gilal cells called astrocytes have important functions for maintaining the metabolic health of the neurons. Gilal cells have long been considered as only supporting cells, but recent research in the past few years has suggested that these cells are just as important in the transfer of information from the brain to other parts of the body.
The birth of new neurons and glial cells following a stroke could provide a new way to treat stroke survivors, according to Sharp. Recent studies have shown that new neurons are born in the human brain as well, and the hope for the future is that these cells to be stimulated to improve function even more.
"Our studies show that the newborn neurons do not occur because of the death of neurons in ischemic brain," says Sharp. "We are able to produce a degree of ischemia that stimulates neuronal birth without killing other neurons. Therefore, ischemia itself stimulates the new neurons. We believe that this represents a protective response that facilitates memory function that may be disturbed even with brief ischemia."
The study's lead author is Jialing Liu, Ph.D, also of the University of California-San Francisco and the San Francisco VA Hospital.
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