Modulation of the cell cycle contributes to the parcellation of the primate visual cortex

An as-yet unresolved issue in developmental neurobiology is whether the discrete areas that form the mammalian cortex emerge from a uniform cortical plate or whether they are already specified in the germinal zone. A feature of the primate striate cortex is that the number of neurons per unit area is twice that of anywhere else in the cerebral cortex. Here we take advantage of this unique structural feature to investigate whether the extra striate cortical cells are due to increased neuron production during neurogenesis. We labelled precursors undergoing terminal cell division with 3H-thymidine and allowed them to migrate to the cortical plate. Cell counts revealed that their rate of production in the germinal zone of striate cortex is higher than in that given rise to extrastriate cortex. Also, we used 3H-thymidine pulse injections to investigate cell cycle dynamics and found that this phase of increased production of striate cortical cells is associated with changes in the parameters of the cell cycle. These results show that cortical area identity is at least partially determined at the level of the ventricular zone.