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 , 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.  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. . Smith et al.  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.
Copyright © 2007, Taylor & Francis Group, LLC.
The effects of aging on area 46 of the frontal cortex of the rhesus monkey.Cereb Cortex. 1994 Nov-Dec;4(6):621-35. doi: 10.1093/cercor/4.6.621. Cereb Cortex. 1994. PMID: 7703688
Aging and the myelinated fibers in prefrontal cortex and corpus callosum of the monkey.J Comp Neurol. 2002 Jan 14;442(3):277-91. doi: 10.1002/cne.10099. J Comp Neurol. 2002. PMID: 11774342
Disrupted myelin and axon loss in the anterior commissure of the aged rhesus monkey.J Comp Neurol. 2003 Nov 3;466(1):14-30. doi: 10.1002/cne.10859. J Comp Neurol. 2003. PMID: 14515238
Structural changes in the normally aging cerebral cortex of primates.Prog Brain Res. 2002;136:455-65. doi: 10.1016/s0079-6123(02)36038-2. Prog Brain Res. 2002. PMID: 12143402 Review.
A review of the structural alterations in the cerebral hemispheres of the aging rhesus monkey.Neurobiol Aging. 2012 Oct;33(10):2357-72. doi: 10.1016/j.neurobiolaging.2011.11.015. Epub 2011 Dec 21. Neurobiol Aging. 2012. PMID: 22192242 Free PMC article. Review.