Cytoplasmic morphology weaver (wv) mouse cerebellar neurons at the culture substratum

The purpose of this study was to determine the structural basis for the hypermotility and impaired growth cone elongation of the homozygous weaver (wv/wv) mouse cerebellar granule cell neurons in culture. Two-day cultures of dissociated week-old normal (+/+) and wv/wv cerebellum were processed for electron microscopy of intact cells and cytoskeleton. Serial sections parallel to and starting from the substrate were examined. Fine-caliber neurites of normal granule cells are packed with parallel arrays of microtubules at all levels. Microfilament-packed microspikes are present at substrate level emanating from a cortical microfilament lattice at the terminus of neurites of varying length. Homozygous weaver granule cells at substrate level have lateral cytoplasmic extensions along the neurite. Microtubules that curve throughout the neurite are separated by cytoplasm. The lateral extensions and growth cone cytoplasmic projections contain microfilaments and occasionally microtubules. Microfilament-packed microspikes are not observed. Immunofluorescent detection of actin confirms the ultrastructural picture. A hallmark of the wv/wv cytopathology is the presence of large numbers of coated vesicles throughout the neurite shaft at the cell-substratum interface. These are rare at similar locations in +/+ neurites. We hypothesize that reduced tension in the growth cone and neurite owing to the presence of lateral extensions and absence of stable microspikes are responsible for the impaired elongation and hypermotility of mutant neurons.