P2Y1 ͉ calcium transient ͉ embryonic neocortex ͉ SVZ F or almost four decades we have known that neurons destined for the cerebral cortex in all mammalian species, including the human, are generated in the primary and secondary proliferative layers situated near the embryonic cerebral ventricle, respectively called the ventricular (VZ) and subventricular zones (SVZ) (1, 2). It is also generally recognized that the SVZ is formed by a subclass of neuronal progenitors that lose their attachment to the ventricular surface and translocate their nucleus and surrounding somatic cytoplasm to the SVZ, where they continue to divide and produce neurons destined predominantly for the upper cortical layers (3-5). The use of in vitro imaging has revealed that the SVZ progenitors (recently renamed the intermediate neuronal progenitors or INPs, to emphasize their continuous proliferation), originate in the VZ by asymmetric mitotic division of the neuroepithelial cells (6). After translocation of their soma to the SVZ, INPs amplify production of cortical neurons via asymmetrical and symmetric modes of mitotic divisions (6-9). The relative large size of the SVZ in primates, particularly in humans, has inspired suggestions that cells in this zone contribute to the expansion and elaboration of the neocortex during evolution (2, 10, 11). Furthermore, genetic and environmental disturbances of neuronal production in the SVZ affect the development and function of the cerebral cortex (5,(12)(13)(14).Considering the developmental and clinical significance of the SVZ, surprisingly little is known about its origin and how INPs are distinguished from other proliferative neural precursors and postmitotic migrating neurons. Similar to the postmitotic neurons, INPs leave the VZ by extending their basal processes toward the cortical plate (CP) along radial glia (RG) shafts and translocating their nuclei within the growing leading processes (1, 6). However, they stop migration after reaching the level of the SVZ, where they re-enter the cell cycle. It has been found that genes, such as p27, Ngn2, FLNa, and MEKK4 are involved in the final mitotic division and onset of migration from the VZ via modulation of the actin cytoskeleton (15-17). Reelin (18) and the neurotransmitters GABA and glutamate and their receptors might also contribute to this process (19,20). However, the nature of the complex intercellular signaling involved in the migration and settlement of INPs in the SVZ remains unexplored.We started with the assumption that Ca 2ϩ , a ubiquitous secondary messenger, and ATP released via gap junctions and hemichannels in the VZ might be involved in the cell-cell communication and the initiation of INP migration. Our hypothesis was based on the previous findings that Ca 2ϩ signaling influences the mitosis of neuronal progenitors and their nuclear interkinetic translocation in the embryonic retina (21), and that [Ca 2ϩ ] fluctuations also affect cellular migration in cerebellar granular cells (22-24), cultured neuronal progenitors (25), and p...