The linearity of neurite elongation depends on the dimension of culture substrates and actin filaments. (A and B) Phase-contrast images of hippocampal explants. Neurites from an explant turned rightward on a 2D substrate of PDL/laminin (A) but grew practically straight in a 3D substrate of collagen gels (B). B is a composite of four photomicrographs. (C) DIC and phalloidin fluorescent images of growth cones in the absence (no treatment) or presence of cytochalasin D. Treatment with 10 ng/ml cytochalasin D inhibited the formation of filopodia. (D–G) Hippocampal explants on PDL/laminin substrates in the absence (D) or presence of cytochalasin D at concentrations of 1 ng/ml (E), 10 ng/ml (F), or 100 ng/ml (G). (H) The y axis represents the curvature of neurites on PDL/laminin substrates, with positive values indicating rightward turning. Numbers in parentheses indicate the total number of neurites examined. Data represent mean ± SEM. ***, P < 0.001 versus no treatment (Bonferroni’s multiple comparison test). Bars: (A, B, and D–G) 500 µm; (C) 10 µm.