NGF rapidly induces the formation of filopodia and the accumulation of β1 integrin at filopodial tips. Representative growth cones were fixed at various times after addition of 100 ng/ml NGF and double-stained for detection of F-actin and β1 integrin by fluorescence microscopy. Left panels show F-actin, andright panels show β1 integrin. A, B, The growth cone has few filopodia, and β1 integrin is distributed unremarkably before NGF is added. C, D, One minute after the addition of NGF, patches of F-actin and β1 integrin appear close to the leading edge of the growth cone. E, F, Filopodia have begun to extend by 3 min after the addition of NGF. β1 Integrin can be found in extending filopodia. G, H, By 10 min, filopodia have fully extended, and β1 integrin is concentrated at the tips. Scale bar in A, 10 μm. I, Quantitation of the number of filopodia (solid line) and accumulation of β1 integrin (broken line) at filopodial tips per growth cone after addition of NGF. The appearance of tip aggregates lags somewhat behind the formation of filopodia. Values are representative of three separate experiments. Sample number = 100. Error bars show SDs. Asterisks denote those values that are significantly different (p < 0.005) from control (0 min) values.