A Golgi study of the sixth layer of the cerebral cortex. III. Neuronal changes during normal and abnormal cortical folding.

During normal development in the cat Layer VI cortical neurons, including pyramidal cells, fusiform neurons, inverted pyramidal cells and bipolar neurons located in the gyral regions retain the vertical orientation characteristic of earlier developmental stages. However, Layer VI pyramidal neurons and the subtypes in the sulcal zones develop long horizontal basilar dendrites which seem to form dendritic bundles with those of neighbouring pyramidal cells; in addition, fusiform neurons and bipolar cells are tangentially orientated and horizontal pyramidal neurons appear in place of inverted pyramidal cells in the gyral and intermediate regions. In human cortical malformations, neurons of the external cellular layer (Layers V and VI) in lissencephaly are vertically arranged, but Layer VI neurons in polymicrogyria are tangentially orientated when located in microsulci. These features suggest that neuronal changes occur in Layer VI neurons as a consequence of cortical folding and that cortical folding determines the cellular shape of Layer VI neurons in normal as well as in abnormal development of the cerebral cortex.