Actin, mitochondria, and ER at axon branch. (A and B) Analysis of actin present in mature (A) and premature (B) branch from Fig. 2, colored according to filament length. (C) Distribution of actin lengths at mature and premature branches and axon shafts, shown as scatter plots with interquartile range (n = 325 mature, n = 414 premature, n = 148 shaft). (D and E) Distribution of actin orientations from A (D) and from B (E). (F) Mitochondria length in tomograms (n = 34 branch, n = 10 shaft). Length is defined as the longest longitudinal axis of a single mitochondrion. (G–I) Mitochondria analysis from live-cell imaging of neurons showing mitochondrial length (G), mitochondrial area as box-and-whiskers graphs (H), and mitochondrial density as bar graphs (I) with mean ± SD (n = 63 branch, n = 119 shaft, n = 104 dendrite; significance was tested using Mann–Whitney U test). (J) Slice from tomogram showing various types of ER (thin tubes and flat sheets). (K) Segmented tomogram of J; color code: gray, cell membrane; green, MTs; yellow, ER; white arrowhead shows ER in branch together with MTs. White squares depict areas in L and M. (L) ER–MT contact (black arrowhead). (M) Segmentation of ER tube wrapping around MT (white arrowhead). Scale bars: 100 nm (A, B, J, and K); 50 nm (L and M).