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Postadolescent circuitry reorganization of associative layer IIIC pyramidal neurons in the human prefrontal cortex.

That human cognitive and emotional development continues through adolescence is a general fact accepted among developmental psychologists and in past decade supported by extensive data of functional brain development. These findings have been taken as indicators of protracted synaptic pruning or/and protracted dendritic growth. However, as per our knowledge, there are no available data of cortical circuitry development that can support this assumption. The aim of this study was to examine adolescent and post-adolescent dendritic growth and dendritic spine density on key elements of circuitry involved in processing of higher cognitive functions, the layer IIIC pyramidal neurons in dorsolateral prefrontal cortex (Brodmann’s area 9). We used available quantitative data of dendritic morphology and dendritic spine density obtained from 25 specimen of Zagreb neuroembryological collection. Data obtained per subject were used as a row data for statistical analysis between following groups: childhood (2-8 years), adolescence (9-18 years), early adulthood (19-30 years), adult (31-65 years) and aging (66 years+). Except intermediate segments, length and branching pattern of dendrites has not changed significantly. However, we found massive statistically significant decrease in dendritic spine density that occurred during adolescence, but also continued through early adulthood for all segment type analyzed. In parallel, the length of intermediate segments decreased for about 20%. Some differences in pattern of dendritic spine density between different segment types were seen. Dendritic spine density in adulthood was highest in the most distal segments of apical oblique dendrites, which showed also largest and most protracted decrease in spine density compared to proximal oblique and basal dendrites. Distal dendrites are known to be predominantly innervated by intracortical and associative cortico-cortical fibers. The maturation of layer IIIC pyramidal neurons might represent a biological basis for protracted cognitive development and, as such, may be highly influenced by education and social environment. Besides biomedical, educational and social implications, these results also point to highly protracted environmentally driven plasticity in associative cortex that influence circuitry development, suggesting mechanisms how and for how long the environment reshapes synaptic circuitries that builds biological basis of individuality.

prefrontal cortex; adolescence; development; working memory

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