Axon terminals on Betz cell somata of area 4 in rhesus monkey throughout adulthood

Previous work in our laboratory demonstrated an age-related decline in the size of Betz cell somata in cortical area 4 of the adult rhesus monkey brain. The present study was conducted to determine whether changes might also occur in the axon terminals upon these cortical cells. Tissue from area 4 was collected from seven rhesus monkeys and prepared for electron microscopy. The ages of the monkeys ranged from 5 to 35 years, covering the entire adult life span of this species. A total of 140 Betz cell profiles (20 per monkey) were examined. Measurements of these profiles confirmed our earlier finding of a decline in the perimeters of Betz cell somata with advancing age. The 1,540 axon terminals upon these cells, however, remained unchanged in size and length of membrane apposition, as well as in their number of mitochondria throughout the adult life (greater than or equal to 5 years) of the rhesus monkey. In addition, the total number of axon terminals on Betz cells did not change with age. Because the axosomatic terminals showed no age-associated changes, the material was used to calculate parametric characteristics of Betz cells and associated terminals. Betz cell somata of the rhesus monkey were estimated to have a mean membrane surface area of 5,700 microns2. Axosomatic terminals on Betz cell somata had a mean appositional area of about 3.33 microns2 and covered about 15% of the somal surface. Thus, on average, each Betz cell appeared to receive approximately 260 axosomatic terminals. There were also some conspicuous age-associated changes in the motor cortex that were not quantified. These included an accumulation of lipofuscin and the presence of a novel inclusion body in the somata of Betz cells. Age-related occurrences in the neuropil included the degeneration of axons and their myelin, membrane-bound holes, and neuritic (senile) plaques.