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Review

The Effects of Normal Aging on Nerve Fibers and Neuroglia in the Central Nervous System

In: Brain Aging: Models, Methods, and Mechanisms. Boca Raton (FL): CRC Press/Taylor & Francis; 2007. Chapter 5.
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Review

The Effects of Normal Aging on Nerve Fibers and Neuroglia in the Central Nervous System

Alan Peters.
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Excerpt

For the past several years, this laboratory has been involved in examining the effects of age on the brains of rhesus monkeys (Macaca mulatta). This species was chosen because these monkeys have a lifespan of about 35 years [1], so that one monkey year is equivalent to about one human year of life. Furthermore, although some senile plaques may be present in the brains of the older monkeys, there are no neurofibrillary tangles and the monkeys show no signs of developing Alzheimer’s disease as they become older. They do, however, exhibit cognitive decline with age, similar to the cognitive decline that occurs in normally aging humans, and the extent of the cognitive decline can be assessed in monkeys by psychological tests that are adapted from those used on humans (e.g., [2–4], see also Chapter 2). Taken together, these attributes make the rhesus monkey an excellent model in which to study the effects of normal aging on the brain. It is now generally accepted that there is no significant overall loss of neurons from the cerebral cortex of rhesus monkeys and other primates during normal aging (see [5–7]). Moreover, when one examines sections of cerebral cortex from old monkeys by either light or electron microscopy, there are few indications that neurons undergo morphological changes with age, beyond some accumulation of lipofuscin in their cell bodies and a loss of dendritic spines. However, Smith et al. [8] have recently asserted that when they compared the prefrontal cortex of young and aging monkeys, they found a 32% loss of neurons from area 8A of old monkeys. In contrast, they found that the numbers of neurons in the adjacent area 46 remained unchanged, as reported earlier by Peters et al. [9]. Smith et al. [8] suggest that their finding demonstrates that neuronal loss from the aging cerebral cortex may be localized. The cells that do show obvious alterations with age are the neuroglial cells. All three classical types of neuroglial cells in old monkeys show accumulations of material in their perikarya and, in addition, there are obvious changes in the morphology of the myelin sheaths and axons of some nerve fibers.

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