Neuron loss is another potential morphological correlate of cognitive decline in normal and pathological aging in humans. Stereological studies have revealed regionally specific neuron loss in the hilus and subiculum of the hippocampus (West, 1993), the islands of the entorhinal cortex (Simic et al., 2005), and mild loss in the neocortex including the frontal, temporal, parietal and occipital cortices (Pakkenberg et al., 1997) in humans with age. In Alzheimer's disease the extent of neuron loss is even greater, occurring in the same regions as normal aging, hilus and subiculum, but extending into additional areas including area CA1 (West et al., 1994). Age-related neuron loss in the canine brain occurs in the cingulate gyrus, superior colliculus, and claustrum (Ball et al., 1983; Morys et al., 1994). Calbindin-positive GABAergic neurons in the prefrontal cortex of the canine brain are also lost with age (Pugliese et al., 2004). Our own stereo-logical studies in the canine hippocampus reveal a reduction in the number of hilar neurons in aged dogs (Siwak et al., 2005a).
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