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Quantitative analysis of basal dendritic tree of layer IIIC pyramidal neurons in different areas of adult human frontal cortex

Human cognition is the most complex function of the cerebral cortex. The recent studies in primates have disclosed that in the basis of cognitive processing are large, long projecting layer IIIc pyramidal neurons. These results lead to the conclusion that dendritic trees in highly ordered associative cortical areas became more complex. However, comparative data of morphological complexness among pyramidal neurons of distinct cortical areas in the adult human brain are still very limited. Therefore, the aim of the present study was to examine if there are any significant differences in morphology in the largest layer IIIc pyramidal neurons among four Brodmann cortical areas (BA) of different order: the primary motor cortex (BA4), the dorso-lateral prefrontal region (BA9) ; Broca’ s motor speech area (BA45) of the left (L) and right (R) hemisphere. Post-mortal brain tissue from three adults without signs and history of neurological and psychiatric diseases was prepared using rapid Golgi and Golgi Cox staining methods. From each analyzed area, we have selected the five largest layer IIIc pyramids, per each Golgi method and per subject, making a total of 120 analysed neurons. The basal dendritic tree of each analyzed neuron was three-dimensionally reconstructed, using Neurolucida 3.0 software. The numeric data obtained were statistically analyzed to evaluate possible regional differences. Furthermore, adjacent tissue sections were processed immuno-cytochemically with an antibody to non-phosphorilated epitops of neurofilaments (SMI32), which in associative areas superiorly labels large layer IIIc projecting pyramidal neurons. The statistical analysis did not show any significant differences among distinct cortical areas. In all areas, regardless of the Golgi method used, a single pyramidal neuron had 5 basal dendrites on average, and the basal dendritic tree was approximately 2700µ ; m long, whilst the total dendritic spine number was around 1000 (mean spine density 0.4 per µ ; m length). However, a qualitative analysis of the SMI32 stained section showed the most numerous and most intensive staining of the neurons in associative BA9 ; a medium intensive staining in BA45 (with slightly higher staining in dominant BA45L, then in BA45R), while neurons in primary motor BA4 were only sparsely stained. Surprisingly, in contrast to present literature data, we did not found differences in dendritic tree morphology of individual, largest layer IIIc neurons, between different areas of the frontal lobe. However, the SMI32 qualitative analysis supports the view that the cortico-cortical network (the thickness of cortically projecting layers II and III, which correlates with the absolute number of cortico-critically projecting neurons, as well as the density of the most composite layer IIIc pyramidal neurons) is increased in highly ordered associative areas. We suggest that, at some point, progression of the complexity of cortico-cortical network is established principally due to the increase of the total number of cortico-cortically projecting neurons, as well as to the increasing percentage (ratio) of the most complex layer IIIc pyramidal neurons among them.

associative cortex; higher cognitive functions; human

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