Regulation of growth cone morphology by nerve growth factor: a comparative study by scanning electron microscopy.

The object of this study was to document and analyze local regulation by nerve growth factor (NGF) of neuronal growth cone properties and to explore the possible diversity of this effect in various NGF-responsive preparations. In particular, scanning electron microscopy was used to characterize the morphology of neuronal growth cones in cultures of dissociated chick embryo dorsal root ganglia (DRG) under conditions of continuous NGF exposure, withdrawal of NGF for 5-6 hr, and restoration of NGF for various times. Comparison was made with similarly manipulated cultures of dissociated newborn rat sympathetic ganglia and neurite-bearing PC12 pheochromocytoma cells. The growth cones of most of the continuously NGF-treated DRG neurons (cultured on poly-L-lysine or collagen-coated glass coverslips) had relatively compact central flattened areas and numerous prominent filopodia. Withdrawal of NGF resulted in a marked spreading of the central growth cone area so that the average maximum width of this structure increased by about threefold as compared to nondeprived cultures. The mean number and lengths of filopodia were unaffected. Restoration of NGF brought about, over a time course of tens of minutes, a return of the original type of growth cone morphology. Rather different responses were observed for the sympathetic neuron and PC12 cultures. Here, surface ruffles, only rarely seen in the chick cultures, were a major feature of the growth cones, whereas filopodia, though present, were less prominent. Removal of NGF led to loss of ruffles and to rounding up of the growth cones; NGF readdition elicited a rapid (less than 30 sec) reinitiation of ruffling and a more gradual (over tens of minutes) respreading of growth cones. These findings illustrate not only that NGF can regulate growth cone properties, but also that there is a diversity as to how this is manifested. Possible mechanisms and biological roles for this regulation are discussed.