Clustering of dendrites in the cerebral cortex begins in the embryonic cortical plate

In this study, we have asked what organizational features can be detected in the embryonic rat cerebral cortex. We have found evidence using light microscopy, and scanning and transmission electron microscopy, that soon after the first cortical neurons join the cortical plate, their apical dendrites form clusters. These clusters are characterized by the close apposition of dendritic membranes, and by intermediate-type junctions. Clustering occurs within 24 h of cortical plate formation, at a time when the cortical plate is about six cell diameters thick, and before the cortex has any afferent input or the cortical plate any synapses. Thus the clustering of apical dendrites in the cortical plate appears to be a very early organizational feature in the development of the cerebral cortex.