Granule cell behavior on laminin in cerebellar microexplant cultures

In order to study roles of the extracellular matrix (ECM) in the cerebellar granule cell migration, cerebellar microexplants of neonatal to postnatal 11-day-old mice were cultured on 3 kinds of substrata, poly-L-lysine (PL), PL/fibronectin and PL/laminin. A prominent outgrowth of small granule cells, which did not uptake GABA, was observed only on the PL/laminin substratum. The granule cells showed the following sequence of events: (1) Many polygonal undifferentiated cells migrated out from the microexplants. These blast cells differentiated into small bipolar neurons with long fine neurites which extended radially from the explants. (2) These cells then changed their orientation perpendicular to their radial neurites, by protruding a short process from the cell body at right angles. (3) Finally, cell bodies of these granule cells adhered to each other to form cell aggregates. Quantitative labelings by bromodeoxyuridine revealed that there were less mitotic cells in explants from the later postnatal cerebellar compared to the earlier postnatal ones. Anti-MAP2 immunoreactivity was localized in short perpendicular processes of the aggregated granule cells. Thus, this unique cell behavior exhibited on the PL/laminin substratum provides the first defined experimental system for studying the granule cell differentiation in vitro.