Modified growth cones in mouse neuroblastoma cultures; an electron microscopic study

Growth cones of differentiated neuroblastoma cells in monolayer cultures were studied by electron microscopy. Morphological differentiation of the growth cone formation was induced by sodium bromide. Upon prolonged application of 10(-4) to 10(-5) M sodium bromide to the cultures, a peculiar or modified formation of the growth cone occurred. Growth cones lengthened gradually. The ultrastructure of the growth cone in contrast to the control was typified by a round to oval structure, midway being electron-dense and carrying laterally denser cytoplasmic protrusions. Bundles of microtubules, aggregates of many dense-cored vesicles, 70-150 nm in diameter, a few less electron-dense, as well as some agranular vesicles were present. Comparing the findings with previous ultrastructural accounts of growth cones of cultured ganglion cells or neuroblastoma cells, differences outnumbered similarities. The organization of the microtubule bundles and the abundance of dense-cored vesicles, sometimes extending distally was remarkable. The presence of an electron-dense substance, of unknown origin, extending laterally with the cytoplasmic protrusions has not been describe as yet.