Most bodily organs continually die and regrow a little at a time. It takes two years, for example, for all the cells of the liver to be replaced by fresh ones. Research from Karolinska Institutet in Sweden now shows that there is one important exception to this -- the nerve cells of the brain remain the same throughout a person's life.
Many scientists have suspected that the cerebral nerve cells lack this renewal ability. However, studies on apes and rodents have indicated that brain cells can reform in mammals after all -- including man. One problem with these findings has been the research methodology, as the most common way of measuring the age of nerve cells is extremely complicated, especially in man, and gives unreliable and controversial results.
Last year, stem cell researcher Jonas Frisén from Karolinska Institutet presented a way to settle the issue with help from unexpected quarters -- the nuclear tests of the cold war. He used a variation on the well-known carbon-14 method that is based on the fact that atmospheric concentrations of C14 peaked sharply during the cold war tests. By comparing levels of C14 in plant and animal cells with atmospheric levels, it is possible to pinpoint the exact time the cells were born.
Jonas Frisén has now teamed up with another Swedish research group from Göteborg University, led by Dr. Peter Eriksson, and scientists from Australia and America to measure levels of C14 in neocortical nerve cells from all the cerebral lobes of adult individual brains. They have been able to show that levels of C14 in the nerve cells from all areas of the cerebral cortex were just as high as the atmospheric levels at the time of the individual's birth. Put another way, no cell division had taken place in the neocortex during the person's life from infancy to adulthood.
The researchers stress that the method has certain limitations. For instance, newly formed nerve cells might remain undetected if they have made up less than one per cent of the total number of nerve cells a person has throughout his or her life. Another possibility, which has been indicated by research on apes, is that new cells can be born, but they are short-lived and die again quickly. The researchers are leaving this possibility open with the reservation that newly formed cells, if they have indeed lived, have not survived any longer than 4.2 months. Moreover, the study rule does not rule out the ability of the cerebral cortex to form new nerve cells after having suffered pathological conditions, such as stroke.
Other studies show that most other tissue types have a renewal cycle of varying lengths. Cells covering the inside of the stomach, for instance, live only a few days; the outer layer of skin is renewed every other week, while the skeleton is thought to be completely renewed once every ten years or so.
The research scientists are now using this new method to study new nerve cell formation in other parts of the brain in healthy and sick individuals.
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