A stereological study of the numbers of neurons and glia in the primary visual cortex across the lifespan of male and female rhesus monkeys.

Mild age-related declines in visual function occur in humans and monkeys, independent of ocular pathology, suggesting involvement of central visual pathways (Spear [1993] Vision Res 33:2589-2609). Although many factors might account for this decline, a loss of neurons in primary visual cortex (V1) could be a contributing factor. Previous studies of neuron numbers in V1 reported stability across age, but were limited in the ages and genders studied and sampled only limited parts of V1 or limited cell types, allowing for the possibility of a subtle loss of neurons. We pursued this question in 26 behaviorally tested adult male and female rhesus monkeys ranging from 7.4 to 31.0 years of age by using design-based stereology to estimate numbers of NeuN-labeled neurons and thionin-stained glia within three laminar zones, supragranular (layers II-IVB), granular (IVC), and infragranular (V-VI), across the entirety of V1. There were no significant differences between males and females on any measures, except for total brain weight (P = 0.0038). There was an average of 416,000,000 neurons in V1, but no effect of age on this total or numbers within any laminar zone. Similarly, there was an average of 184,000,000 glia in V1 (44% of the number of neurons), but no effect of age on this total. However, there was a significant age-related increase in numbers of glia in the infragranular zone, perhaps reflecting a glial response to pathology in myelinated projection fibers. This study provides further evidence that in normal aging neurons are not lost and hence cannot account for age-related dysfunction.