3H-thymidine autoradiographic analysis of telencephalic histogenesis in the chick embryo: II. Dynamics of neuronal migration, displacement, and aggregation.

The movements of cells from their sites of origin in the proliferative neuroepithelium to their final positions in the chick telencephalon were traced by autoradiographic analysis of 3H-thymidine-labeled brains. A series of chick embryos were labeled on successive days of development between days 5 and 9 and fixed for autoradiography between days 6 and 10. Isochrone maps visualizing neuronal positions on each day of development between days 6 and 10 provided direct information concerning cell migrations, displacements, and aggregations leading to compartmentalization of the telencephalic wall and the generation of the "outside-in" pattern of histogenesis characteristic of the avian telencephalon. The topological divisions of the telencephalic wall appear to result from two factors: (1) the specification of neuronal precursors within the neuroepithelium and (2) the intrinsic associative and migratory properties of postmitotic neuronal populations expressed within the mantle layers. There was no evidence that glial cell barriers mediated the initial compartmentalization of neuronal populations.