Thick membrane extensions contain mitochondria, ER, and membranous compartments. (A) Partially segmented TEM image montage of thick membrane extensions (cyan) and thin filopodia (orange) running between ASP and myoblasts (red asterisks on disc diagrams). Some cells project thick membrane extensions that narrow into thin cytonemes (red arrow). The distinction between thick extensions and thin filopodia was made based on 10 serial sections of the region, not only the section shown. Thick membrane extensions contain mitochondria (purple), rough ER (red), and other membranous compartments of different sizes (green). Membrane extensions and filopodia that were in cross section were not segmented. (B) TEM image shown in A without segmentation. Insets show enlarged examples of mitochondria (purple border), rough ER (red border), and other membranous compartments (green and blue borders). (C and D) 3D projection of confocal image stack of ASP expressing CD8-mCherry and mito-GFP (C) and CD8-GFP and KDEL-RFP (D). (E–G) TEM images of a small membranous compartment inside a thick area of a membrane extension (E), a larger membranous compartment within a thick portion of a membrane extension and bifurcation into thin extensions (F), and a membranous compartment inside a varicosity of a cytoneme (G). Scale bars: 500 nm (A and B), 100 nm (B, inset), 20 µm (C and D), and 100 nm (E–G).