Examples of mitochondrial networks at the micron and nanometer scales. (a) Mitochondrial networks can vary from separated structures to interconnected networks. Indian Muntjac deer skin fibroblast (left) and BPAE (bovine pulmonary artery endothelial) cell (right), both expressing a pEYFP-mitochondrial plasmid vector to label mitochondria (yellow-orange). The thin yellow line is an approximate outline of the cell. White arrows point to small boxes indicating either a tubule (left) or tubule branch (right) that are illustrated in (b). (b) Diagram of the organization of the mitochondrial membranes (ultrastructure) at the nanoscale for (i) a tubule and (ii) a tubule branch. Abbreviations in (i) include the outer and inner mitochondrial membrane (OMM and IMM, respectively) and the intermembrane space (IMS). (c) The mitochondrial network in a budding yeast cell. Thin yellow line indicates the outline of the mother (larger, bottom) and bud (smaller, top) compartments. Image is a maximum intensity projection of a three-dimensional z-stack. The size of the cell is approximately the same size as the nuclei in the mammalian cells shown in (a). Images in (a) are reproduced and slightly altered courtesy of and with permission from Michael Davidson (Florida State University) and are featured on the Nikon MicroscopyU website .