ORIGIN AND ROUTE OF TANGENTIALLY MIGRATING NEURONS IN THE DEVELOPING NEOCORTICAL INTERMEDIATE ZONE

Neuroblasts produced in the ventricular zone of the neocortex migrate radially and form the cortical plate, settling in an inside-out order. It is also well known that the tangential cell migration is not negli gible in the embryonic neocortex. To have a better understanding of th e tangential cell migration in the cortex, we disturbed the migration by making a cut in the neocortex, and we labeled the migrating cells w ith -dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Di l) in vivo and in vitro. We also determined the birth dates of the cel ls. Disturbance of tangential cell migration caused an accumulation an d disappearance of microtubule-associated protein 2 immunoreactive (MA P2-IR) cells on the ventral and dorsal side of the cut, respectively, which indicated that most of the MAP2-IR cells in the intermediate zon e (IZ) were migrating toward the dorsal cortex. The Dil injection stud y in vivo confirmed the tendency of the direction of cell migration an d suggested the origin of the cells to be in the lateral ganglionic em inence (LGE). Dil injection into the LGE in vitro confirmed that the L GE cells cross the corticostriatal boundary and enter the IZ of the ne ocortex. The migrating cells acquired multipolar shape in the IZ of th e dorsal cortex and seemed to reside there. A 5-bromo-deoxyuridine inc orporation study revealed that the migrating MAP2-IR cells in the IZ w ere early-generated neurons. We concluded that the majority of tangent ially migrating cells were generated in the LGE and identified as a di stinct population that was assumed not to have joined the cortical pla te.