MIGRATION BEHAVIOR OF GRANULE CELL NEURONS IN CEREBELLAR CULTURES .2.AN ELECTRON-MICROSCOPIC STUDY

We examined the fine structure of migrating granule cell neurons in ce rebellar microexplant cultures. Radially migrating bipolar cells exten ded microspikes or small filopodia from their soma and processes and f requently made contact with neighboring cells. These microspikes conta ined microfilaments but no microtubules. At the later phase of the mig ration, in which they had symmetrical bipolar long processes, filopodi a extending from perikarial region of cells contained microtubules, su ggesting that they are precursors of the future thick perpendicular pr ocesses. When cell bodies changed orientation from radial to perpendic ular, microtubules that were nucleated from perinuclear centrioles fre quently extended into both thick radial and perpendicular processes fr om the perikarial region. Bundles of 10 nm intermediate filaments also appeared in these processes. During migration by the perpendicular co ntact guidance, many filopodia extending from both the thick leading p rocesses and thin trailing processes made close contacts with the radi al parallel neurite. These findings suggest that; 1) The direct contac t of the filopodia from both the growth cones and their processes of t he granule cells to the neurite bundle plays roles in both the paralle l and perpendicular contact guidances. 2) The spacial and temporal cha nges of cytoskeletons and the association of microtubules with perinuc lear centrioles are important for the formation of perpendicular proce sses and initiation of the perpendicular contact guidance.