III. Three-dimensional electron microscopy of mammalian cardiac sarcoplasmic reticulum at 80 kV.

The Golgi black reaction method was combined with stereoscopic techniques to obtain three-dimensional views of cardiac sarcoplasmic reticulum (SR) using a conventional electron microscope operating at 80 kV. We have previously described the SR in avian and mammalian skeletal muscles with similar techniques. It was necessary to modify these earlier techniques for cardiac muscle. Two regions of mammalian heart were explored: trabecular and papillary muscles. These muscles presented striking differences with regard to relative volume of mitochondria and myofibrils, but both muscles presented similar dispositions of the inner tubules of SR. The SR near myofibrils appeared heterogeneous and consisted of fenestrated collar, bulbous extensions at the Z line (corbular SR), and flat extended regions (cisternal SR). The SR near mitochondria, however, always formed a simple rete with occasional cisternal SR. Specific "staining" of the inner tubules of cardiac SR by the Golgi method offers new views of cardiac fibers that suggest a more extensively developed SR than previously acknowledged.