Stereological analysis of glial cell subtypes in the primary visual cortex across the life span of rhesus monkeys
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The normal aging process is accompanied by mild declines in visual function in both humans and non-human primates, independent of ocular deficiencies, indicating an involvement of structures in the central visual pathway. Studies examining loss of cortical neurons as a potential explanation have concluded that neuron numbers are largely preserved with age both in visual cortex and as well as other cortices. In contrast to the stability of neuron numbers with age, a significant increase in the total number of glia was found in the infragranular layers of the primary visual cortex, in rhesus monkeys (Giannaris and Rosene, 2012). Unpublished data indicates that increase in glial density is correlated with decreased visual function assessed by the behavioral performance metric. In order to understand the basis of glial increase with age in the rhesus monkey, we used immunohistochemistry to parcellate the total number of glia in primary visual cortex into three subtypes: microglia with Iba1, astrocytes with GFAP and oligodendrocytes with Olig2. These were then quantified using unbiased stereology in a subset of 12 animals whose ages ranged from young to old (6 male and 6 female), from the original study of 26 monkeys. Adding together all three subtypes in the current subset of animals showed a modest but non-significant trend toward the increase observed in the larger sample of 26 animals. In this study, examining the three subtypes showed no significant increase and the total number of glial cells was found to be unchanged with age. A definitive answer to how the different subtypes contribute to the overall increase in glia will require analyzing the remainder of the full data set.