Figure 1.

Filopodia initiation in neuronal cells. (A–C) Phase-contrast time-lapse sequences of rat hippocampal neuron (A), differentiated B35 neuroblastoma cell (B), and Xenopus neuron (C). Large panels at left and right show the beginning and the end of the sequence, respectively. Montages in the middle show individual time frames for the boxed regions in the corresponding large panels (B, all; A and C, top) or for other time-lapse sequences (A and C, bottom). Arrows and arrowheads mark individual filopodia formed during the sequence. Filopodia initiation occurs from lamellipodia-like regions and is usually preceded by formation of a small bulge at the leading edge of a growth cone (A–C, top), or at the tip of a secondary neurite (C, bottom), or within a lateral protrusion (A and B, bottom). (D and E) Dynamics of YFP-actin in a growth cone (D) and a lateral protrusion (E) of B35 cells. A corresponding phase contrast sequence is shown for D. Cone-shaped condensation of actin fluorescence precedes filopodia formation (arrows and arrowheads). Time in minutes:seconds. Bars, 5 μm.