Figure 4.

High resolution imaging of the lesion patch by electron tomography. (a) Overview of muscle tissue with a membrane repair patch between two muscle fibers (arrow). Scale bar, 10 µm. (b) Tomographic slice through the repair patch in panel (a) at 9600x magnification. The sarcolemma of the lesioned cell (asterisk) is already resealed (black arrowheads). Note that, the sarcomere organization appears still disturbed. A characteristic, rather thick and slightly electron dense coat, is lining the sarcolemma extracellularly over the lesioned region (double black arrow) while the lower right myofiber seems also affected by the laser. Scale bar, 500 nm. (c) The lesion site shows heterogeneous vesicle profiles with a grainy or a smooth aspect (black arrow in c). Scale bar, 200 nm. (d) Tomographic slice of a large repair patch at 9600x magnification with heterogeneous membranous profiles shown in detail in panels (e,f,g). Membranes appear loosely stacked in a multilamellar configuration (black arrowheads in e,f). Scale bar, d, 500 nm; (e), 200 nm; (f), 100 nm; (g), 100 nm. (g) Some vesicles of various sizes are also visible forming a row against the lesioned myofiber. These vesicles also show heterogeneity in their aspect (black arrows in g) as noticed in (c). (h) Image of a macrophage sitting on the repair patch. The plasma membrane of the macrophage shows a close apposition with the repair patch (black arrow in i). Scale bar, h, 10 µm; (i), 1 µm. The damaged cell shows a stronger contrast in comparison to the surrounding undamaged cells and appears shrunk. This presumably reflects that the sarcolemma damage triggered an apoptotic program in the myofiber as previously described for cells with large lesions (Middel et al., 2016).