Endocytic activity correlates with axon outgrowth and requires Rho GTPases. (A) Summary of axon length measurements from spinal neuron cultures grown on poly-d-lysine (PDL) or PDL with fibronectin. (B) Summary of fluid-phase endocytosis levels measured in spinal neuron growth cones cultured on the indicated substrates. Fluorescent dextran was applied for 10 minutes and the amount of internalization was measured by fluorescence microscopy. The mean fluorescence intensity of each growth cone was normalized to the mean control (PDL). (C) Summary of growth rate measurements during the one-hour growth assays with or without pre-treatment with Toxin B (20 ng/ml). (D) Summary of fluid-phase endocytosis levels measured as in (B) under the indicated conditions. All data (A-D) are the mean ± standard error of the mean and the number of axons measured per condition is indicated in each bar. *P < 0.05, **P < 0.001, t-test or Mann-Whitney U-test (see Methods).