Reelin is an essential glycoprotein for the establishment of the highly organized six-layered structure of neurons of the mammalian neocortex. Although the role of Reelin in the control of neuronal migration has been extensively studied at the molecular level, the mechanisms underlying Reelin-dependent neuronal layer organization are not yet fully understood. In this study, we directly showed that Reelin promotes adhesion among dissociated neocortical neurons in culture. The Reelin-mediated neuronal aggregation occurs in an N-cadherin-dependent manner, both in vivo and in vitro. Unexpectedly, however, in a rotation culture of dissociated neocortical cells that gradually reaggregated over time, we found that it was the neural progenitor cells [radial glial cells (RGCs)], rather than the neurons, that tended to form clusters in the presence of Reelin. Mathematical modeling suggested that this clustering of RGCs could be recapitulated if the Reelin-dependent promotion of neuronal adhesion were to occur only transiently. Thus, we directly measured the adhesive force between neurons and N-cadherin by atomic force microscopy, and found that Reelin indeed enhanced the adhesiveness of neurons to N-cadherin; this enhanced adhesiveness began to be observed at 30 min after Reelin stimulation, but declined by 3 h. These results suggest that Reelin transiently (and not persistently) promotes N-cadherin-mediated neuronal aggregation. When N-cadherin and stabilized β-catenin were overexpressed in the migrating neurons, the transfected neurons were abnormally distributed in the superficial region of the neocortex, suggesting that appropriate regulation of N-cadherin-mediated adhesion is important for correct positioning of the neurons during neocortical development.T he mammalian neocortex is highly organized into six neuronal layers. This laminar structure is responsible for the complex motor, sensory, and cognitive functions of the mammalian brain (1). Neuronal migration plays an important role in the establishment of this layered structure. Cortical neurons are generated within the ventricular zone (VZ) or subventricular zone (SVZ), and migrate along radial fibers toward the pial surface. Newly born excitatory neurons migrate radially into the cortical plate (CP) past the neurons born earlier, resulting in a birth date-dependent "inside-out" alignment of the neurons in the CP (2-4).Reelin is a glycoprotein that is secreted by the Cajal-Retzius cells in the marginal zone (MZ) of the cortex during neocortical development (5-7). This glycoprotein is essential for establishment of the aforementioned birth date-dependent layered structure of the neocortex, because the CP neurons show an almost inverted alignment in the neocortex of the Reelindeficient, reeler mice. In addition, neurons born at the same time tend to be distributed broadly and not to form clear layers in the reeler cortex (8, 9). Although extensive studies, including at the molecular level, have been conducted to determine the role of Reelin in neuronal migration i...