Numbers of Meynert and layer IVB cells in area V1: A stereologic analysis in young and aged macaque monkeys

Visual impairments that are not related to optical changes are not, uncommo n during aging, and a number of psychophysical investigations have document ed deficits in motion detection as well as in spatiotemporal contrast sensi tivity in elderly people. However, little is known about the extent and nat ure of age-related changes in neural structure and how they may affect visu al function in aging. To address this question, the authors analyzed the ef fect of aging on two well-characterized neuronal populations in the primary visual cortex (area V1) of macaque monkeys. Four young adult (ages, 7-11 y ears) and four aged (ages, 26-32 years) rhesus monkeys were analyzed. The a nimals were perfused, and their brains were prepared for immunohistochemist ry with an antibody to neurofilament protein. Unbiased stereologic estimate s of the total numbers of neurofilament protein-containing layer IVB cells and Meynert cells were obtained by using the optical fractionator method fo r the calcarine cortex and the opercular cortex separately. Stereologic est imates of the volume of these parts of area V1 also were calculated by usin g the Cavalieri principle. A considerable degree of interindividual variabi lity in neuron numbers and cortical volume was observed among animals of bo th groups. However, there were no differences in either Meynert cell number s or layer IVB cell numbers between the aged group and the young group. It is noteworthy that the oldest animal in the sample had the lowest numbers o f Meynert cells, indicating that, despite the small size of the available s ample, it is possible that some animals have a certain degree of neuronal l oss in area V1 during aging. No change in the volume of area V1 was observe d as a function of aging. These data suggest that the deficits that occur d uring aging in the visual system are not due to the loss of highly specific neocortical neuronal populations, such as those analyzed in this study. Ra ther, it is possible that more subtle alterations in the neurochemical char acteristics or synaptic organization of the functional pathways subserving the different visual modalities are responsible for these deficits. J. Comp . Neurol. 420:113-126, 2000. (C) 2000 Wiley-Liss, Inc.