Cytology and neuron-glial apposition of migrating cerebellar granule cells in vitro

In developing mammalian brain, many neurons migrate to their final position by moving in direct apposition to radially oriented glial cells. Glial-guided migration can be visualized in microcultures of mouse cerebellar cells by the combined use of cellular antigen markers and high resolution time-lapse video microscopy (Hatten et al., 1984; Edmondson and Hatten, 1987). Such studies have demonstrated the behavior of migrating cells and revealed a motile leading process on the migrating neuron that resembles an axonal growth cone and grows along extended glial fibers. To study the fine structural details of the migrating neuron and its neuron-glial apposition, we identified and monitored neurons in microcultures with video microscopy and examined the cytology and cellular contacts of the same cells with transmission electron microscopy. The cytology of the soma and leading process of migrating cells closely matches that described for granule cells in intact brain (Rakic, 1971). Newly observed structures include the presence of longitudinally oriented microtubules extending from a basal body in the soma into the leading process, and microfilament-rich filopodia arising from the soma and leading process. The most striking feature of actively migrating neurons is a specialized junction between the neuronal cell soma and apposing glial fibers. At this junction, here termed "interstitial density," the extracellular space is dilated to 20 nm and filamentous material in the intracellular cleft either spans the cleft or runs parallel to the cell membranes. Some interstitial fibrils are contiguous with, or are transmembranous extensions of, submembranous cytoskeletal elements that attach to microtubules. Interstitial junctions were not found between neurons that did not translocate in the observation period before fixation. Instead, stationary cells formed desmosomes (puncta and macula adhaerentia) at appositions with glial processes.