Figure 1. Mitochondria in close apposition to sarcolemma.

A. Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) image of ventricular myocytes. Notice the presence of mitochondria between z-disks (red box). Scale bar 2 μm. B. FIB-SEM single plane of complete 3D-volumetric image of cell membrane and subjacent mitochondria. Notice mitochondria under the crest. Enlarged images in bottom frames: sequential z-sections showing proximity of membranes and points where membranes come in close apposition (red arrows). C. Segmentation analysis of subsarcolemmal mitochondria connecting to mitochondrial reticulum. Top left: Mitochondria in bottom images of panel B, highlighted in blue. Top right: Segmentation of layers below shows subsarcolemmal mitochondria coupling to first chain of interfibrillar mitochondria. Dotted circles identify points of proximity. Lower panel in left shows subsarcolemmal mitochondria (purple) that bifurcates to reach two interfibrillar chains. Bottom right: bifurcating mitochondrial from a different angle. Through these connections, mitochondria form a complete reticulum, as documented by Glancy et al.^37,38