In Nissl stained sections of both monkey and human cortices it can be seen that aging neurons acquire some lipofuscin in their cell bodies, but in this type of preparation there are no other obvious alterations of neurons. However, when Golgi preparations, neuronal tracers, or intracellular filling are employed, a different picture emerges.
Some of the most comprehensive studies of the effects of age on neurons have been carried out by injecting tracers into area 46 and then examining the labeled cortico-cortical projection neurons in the temporal cortex [e.g., (Page et al., 2002
; Duan et al., 2003
)]. These studies have shown that when young and old monkeys are compared, there are only minor qualitative differences in the dendritic lengths and complexity of dendritic trees of the pyramidal neurons, but there is extensive loss of dendritic spines from all portions of the dendritic tree. Overall, there is a loss of some 25% of dendritic spines on apical and basal dendrites (Duan et al., 2003
). A similar observation was made in a later study that examined the effects of normal aging on the structural properties of layer 2/3 pyramidal neurons in area 46 from rhesus monkeys (Kabaso et al., 2007
). In addition to reduced branch lengths in the apical dendrites and increased numbers of segments in the basal dendrites, the aged neurons had reduced numbers of dendritic spines, amounting to 50%, with fewer thin and more stubby spines being evident (Kabaso et al., 2007
). Since thin and small spines are important for learning, the shift from thin to stubby spines may contribute to the learning deficits that occur during aging. In a recent study Kabaso et al. (2009)
have shown that not all pyramidal neurons in the cortex are affected in the same way by age. Thus Kabaso et al. (2009)
find that although all pyramidal cells lose spines, the dendrites of the long projecting neurons from superior temporal cortex to area 46 become shorter and less branched, while the lengths of dendrites of locally projecting pyramidal cells in area 46 are largely unaltered.
These recent studies on monkey cortex largely confirm earlier work on the effects of age on neurons in monkey prefrontal cortex (Cupp and Uemura, 1980
). Examination of Golgi impregnated neurons revealed that with age entire branches are lost from the apical dendrites of pyramidal neurons, accompanied by a loss of about 25% of dendritic spines from all branches of the dendritic trees. By contrast, another study of Golgi impregnated neurons from prefrontal area 10 and visual area 18 of young and old human cerebral cortices concluded that with age there is only a slight loss in the overall extent of the dendritic trees of pyramidal neurons but a dramatic loss of about 50% of dendritic spines (Jacobs et al., 1997
The loss of spines on area 46 pyramidal neurons during normal aging in the monkey is paralleled by a loss of synapses. Electron microscopic analyses of the numerical density of both asymmetric (excitatory) and symmetric (inhibitory) synapses in the neuropil of layer 2/3 and layer 5 of behaviorally tested rhesus monkeys reveal that with age there is no change in the length of the synaptic junctions, or in the percentage distribution of synapses relative to postsynaptic spines and dendritic shafts (Peters et al., 2008a
). However, in layer 2/3 there is a loss of about 30% of synapses with age and both asymmetric and symmetric synapses are lost at the same rate. Layer 5 is different, since only about 20% of synapses are lost with age, and this is almost entirely due to a loss of asymmetric (excitatory) synapses. As far as we are aware, the only other study of the effects of aging on synapses in monkey prefrontal cortex is that of Uemura (Uemura, 1980
), who used electron microscopy to examine superior frontal cortex, or area 9, which is situated above area 46 and has related functions. The overall loss of synapses calculated in this study (Uemura, 1980
) is similar to that of Peters et al. (Peters et al., 2008a
), who also found loss of synapses from the upper and middle thirds of the depth of the cortex to be greater (25%) than from the lower third (16%).
A piecewise analysis of the data brought to light the interesting fact that most of the synapse loss occurs after 20 years of age. When the synapse loss data are correlated with the cognitive status of the monkeys, it emerges that for layer 2/3 there is a strong correlation between cognitive impairment and the numerical density of asymmetric synapses and a weaker correlation between symmetric synapse loss and cognitive impairment. In contrast, for layer 5 there is no correlation between synapse loss and cognitive impairment.
The greatest loss of synapses (30–60%) is found in layer 1, which becomes significantly thinner during aging (Peters et al., 1998b
). The loss of synapses in layer 1 is accompanied by a reduction in the frequency of profiles of dendrites and dendritic spines. In layer 1 there is a significant correlation between the numerical density of synapses and the cognitive decline exhibited by aged monkeys. Interestingly, although there is a similar thinning of layer 1 in area 17 of the monkey cortex, accompanied by a loss of dendritic branches, dendritic spines and synapses, these alterations do not correlate with behavioral changes in memory function. The probable basis for this difference is that while area 46 is implicated in cognition, the same is unlikely to be true of area 17 (Peters et al., 2001a
The extensive loss of synapses from layer 1 of area 46 with age is very interesting in light of the fact that there is a massive input of thalamic afferents from the M (matrix) neurons in thalamic nuclei to layer 1 of cerebral cortex in several species [reviewed in (Jones, 2007
)]. Moreover, inputs from several thalamic nuclei in rats converge on layer 1 throughout the cerebral cortex (Rubio-Garrido et al., 2009
). These findings are consistent with the strong projections from most thalamic nuclei to layer 1 in monkeys, which run in parallel to the classically described thalamic projection system to the middle layers of the cortex [reviewed in (Jones, 1998
; Jones, 2007
)]. Recent studies show that in rhesus monkeys there is a significant projection from the ventral anterior thalamic nucleus to the entire prefrontal cortex, including area 46 (Zikopoulos and Barbas, 2007b
). In addition to projections to the middle layers, significant projections from the ventral anterior thalamic nucleus reach layer 1 of area 46 and the adjacent area 9 (Zikopoulos and Barbas, 2007b
). The large majority of these projections originate from calbindin positive ‘matrix’ thalamic neurons. It is highly probable that there is a similar input to layer 1 of area 46 by other thalamic nuclei in the monkey. The reduction in the number of dendritic branches and synapses from the apical tufts would interfere with the feedback interactions between area 46 and thalamic nuclei, which appear to be crucial for associative learning and attention.
The results of electron microscopic analyses of aging and synapse numbers in human frontal cortex are much more confusing. For example, Scheff et al. (Scheff et al., 2001
) examined layers 3 and 5 in area 9 from the brains of cognitively normal humans and concluded that during the first eight decades of life there is no significant loss of synapses. These findings support a similar conclusion reached earlier by another group, who examined synapses in layer 3 of frontal cortex (Huttenlocher, 1979
). In contrast, another study reported a significant loss of synapses with age from human frontal, but not temporal cortex, in brains from “apparently intellectually normal people”. Interestingly, when these brains were compared with ones from patients diagnosed as suffering from Alzheimer type dementia, the numbers of synapses in the two groups were similar (Gibson, 1983
). The results of determining synapse numbers in the frontal cortices of human brains using antibodies, such as ones against synaptophysin, to label axon terminals are also conflicting. For example, several studies found that individuals older than 60 years showed an average of 20% decrease in the numbers of synapses in the frontal cortex compared to young individuals (Masliah et al., 1993
). By contrast, a fourth study found no loss of synapses (Zhan et al., 1993
Since there is a loss of dendritic spines from pyramidal neurons in non-human primate frontal cortex with aging, it follows that there has to be a loss of synapses involving these spines. Why disagreement has arisen about whether there is a loss of synapses from human frontal cortex with normal aging is not clear, but various authors have attributed the discrepant findings to factors such as the inadvertent inclusion of patients with early stages of Alzheimer’s disease among the normal subjects, morphological changes brought about by long postmortem intervals, and inappropriate methods used to make accurate synaptic counts.