MODULATION OF THE CELL-CYCLE CONTRIBUTES TO THE PARCELLATION OF THE PRIMATE VISUAL-CORTEX

AN as-vet unresolved issue in developmental neurobiology is whether th e discrete areas that form the mammalian cortex emerge from a uniform cortical plate or whether they are already specified in the germinal z one1,2. 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 c ortex3. Here we take advantage of this unique structural feature to in vestigate whether the extra striate cortical cells are due to increase d neuron production during neurogenesis. We labelled precursors underg oing terminal cell division with H-3-thymidine and allowed them to mig rate to the cortical plate. Cell counts revealed that their rate of pr oduction in the germinal zone of striate cortex is higher than in that giving rise to extrastriate cortex. Also, we used H-3-thymidine pulse injections to investigate cell cycle dynamics and found that this pha se of increased production of striate cortical cells is associated wit h 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.