Organization of Pyramidal Cell Apical Dendrites and Composition of Dendritic Clusters in the Mouse: Emphasis on Primary Motor Cortex

It has been proposed that neurons in sensory cortices are organized into modules that centre on clusters of apical dendrites belonging to layer V pyramidal neurons. In the present study, sections reacted for microtubule‐associated protein (MAPP) were examined in order to determine the three‐dimensional inter‐relationships of pyramidal cell dendrites in mouse primary motor cortex (MsI) cortex. Results indicate that pyramidal cell dendrites in MsI cortex can be interpreted to be arranged in a modular fashion, and that these modules are organized similarly to those in the sensory areas of the cortex. Also included in the present study are experiments designed to determine if the clusters of apical dendrites, around which the modules are centred, are composed of dendrites belonging to one or to more than one type of projection cell. Callosal neurons in MsI cortex were labelled by the retrograde transport of horseradish peroxidase deposited onto severed callosal fibres in the contralateral hemisphere. Examination of tangential thin sections through layer IV of MsI cortex shows clusters of apical dendrites in which every dendrite is labelled with horseradish peroxidase. Adjacent clusters are composed of unlabelled dendrites, suggesting that the apical dendrites of callosal neurons aggregate to form clusters that are composed exclusively of dendrites belonging to this type of projection cell. These findings suggest a hitherto unsuspected degree of specificity in the cellular composition of cortical modules.