Age and hemisphere effects on dendritic structure.

The dendritic structures of 187 small supragranular pyramidal neurons of the posterior superior temporal gyrus were studied with rapid Golgi impregnations in postmortem samples from 10 men aged 21-71 years. The number of primary basilar dendritic branches, the total number of basilar dendritic endings, the total basilar dendritic length, the total number of visible basilar dendritic spines and the cell soma sizes were all positively inter-correlated and all features were correlated to age (r = -0.77, -0.88, -0.82, -0.72, -0.86, respectively; all P < 0.05). These neuronal measures all correlated with brain weight (r = 0.79*, 0.65*, 0.51, 0.45, 0.55, respectively; *denotes P < 0.05). A first principle component derived from the inter-correlations of the neuronal features plus brain weight correlated almost perfectly with age (r = -0.93). The neuronal features differed between the right and left hemispheres (Wilks' Lambda = 0.91, P < 0.01). Post hoc tests showed that the dendritic trees of the right hemisphere were longer (P = 0.002), more branched (P = 0.008) and possessed more dendritic spines (P = 0.0009; Sheffe's tests). In conclusion, there are hemispheric differences in the dendritic structure of the small pyramidal neurons of presumptive human speech cortex and its right hemisphere analogue. Generalized neuronal atrophy is highly correlated with both brain weight and age, and is a candidate process to explain the decline in cognition with age.