Examples of mitochondria fission, ER entering branch, thin ER tubes, and ER-bound ribosomes. (A) Mitochondria undergoing fission. White arrowheads depict ER pinching at mitochondria membrane or wrapping around the mitochondria. (B) ER tube present in axon branch together with MTs. Red arrowheads, ER; white arrowheads, MTs; black arrow, direction of new branch; white arrow, direction of main axon growth. (C–G) Analysis of ER diameter. In C, ER tube reaching extremely thin diameters; red arrowheads follow continuous ER tubes, and white arrowheads depict different ER. In D–F, ER (white arrowheads) wrapping around MTs (D), cross-section of ER tube (white arrowhead) wrapping around MTs (E), and diameter of thinnest ER tubes found in tomograms (median diameter 7.19 nm, n = 19; F). (G) Diameter of ER tube wrapping around MT (median diameter 12.21 nm, n = 29). (H) Slices of tomograms with ribosomes on the surface of ER. The area of ER is highlighted in yellow. Scale bars: 100 nm (A, B, H); 50 nm (C–E).