Cell‐to‐cell Interactions during Development of the Neocortex, with Special Reference to Neuronal Migration *

Neuronal migration plays a key role in forming a characteristic laminar cytoarchitecture of the neocortex. In this review, the localization of cell‐surface molecules in developing murine neocortex including that of the reeler mutant were described in association with their functional significance. L1 antigen appeared solely on neurons which were placed in the intermediate zone, the cortical plate and the marginal zone. In the cortical plate the surface of neuronal cell body was positively stained until embryonic day 17. Later on, the cell body became negative whereas cell processes in the intermediate and the marginal zones remained positive. Migrating neurons in the intermediate zone were L1 positive. N‐CAM, on the contrary, were detectable on both matrix cells and neurons. The L2/HNK‐1 epitope was present on endfeet of matrix cells, in addition to on the neuronal surface. In vitro perturbation assays using embryonic neocortical explants showed that the antibody for L1 could modify the rate of neuronal migration to some degree. These results suggest the functional roles of the L1 molecule in neuronal migration. However, L1 was present in the reeler mouse that is supposed to have a genetic defect in neuronal migration resulting in a disorganized cytoarchitecture. The scanning electron microscope (SEM) fractographic studies revealed a defective formation of the bundles of matrix cell processes in the developing reeler neocortex. Our observations indicate deranged cellular interactions among matrix cells in the reeler, although no molecular abnormalities have so far been demonstrable. The significance of cell‐to‐cell interactions in neocortical histogenesis was discussed.