Effects of aging on the size, density, and number of rhesus monkey lateral geniculate neurons.

Visual abilities decline during aging, and many of these declines are due to neural changes in the retina or brain. We have begun studies of the monkey visual system to investigate the location and nature of these changes as well as to answer general questions about the effects of aging on neural structure and function. We began with the dorsal lateral geniculate nucleus (LGN) because it is the main structure through which visual information passes on the way to cortex and because the parallel parvicellular and magnocellular pathways are most easily identified and studied in the LGN. In the present experiment, we determined the sizes, densities, and numbers of LGN neurons in young-adult (5 to 12.5 years) and old (23 to 27.5 years) rhesus monkeys. The measures were corrected for tissue shrinkage, and stereological procedures were used that yield unbiased estimates. In young-adult monkeys, neurons densities were lower in the magnocellular layers (about 14,000/mm3) than in the parvicellular layers (23,000/mm3). Neuron density increased about 28% from anterior to posterior in both types of layers. There was an average of approximately 1,267,000 neurons in the parvicellular layers and 148,000 neurons in the magnocellular layers; however, there was substantial variability (1.9-fold) among five brains studied. Aging produced a statistically significant decrease in neuron density in both the magnocellular (29% average decrease) and parvicellular (41% average decrease) layers. However, there was no significant loss of neurons. Rather, the density decrease was due to a small (nonsignificant) decrease in the number of neurons combined with a small (nonsignificant) increase in LGN volume. The increase in LGN volume was due to a significant increase in neuron soma-size and proportional increase in the volume of glial cells, blood vessels, and neuropil. These results, together with those of other studies, suggest that the effects of aging on the primate visual pathway from retina through striate cortex are relatively subtle. It is possible that the major neural changes occur more centrally. Alternatively, individual differences in the effect of aging may require much larger samples or prior screening to observe consistent changes.