Structural organization of the normal and anoxic heart of Scyllium stellare

The general and ultrastructural organization of the heart of the elasmobranch, Scyllium stellare, was studied in normal and in anoxic animals. The rich coronary supply was revealed three-dimensionally by the use of corrosion casts, showing a thebesian system of coronary arterioles and capillaries in the thin, outer compact layer as well as in the predominant, inner spongy layer of trabeculae. Only the sinus venosus received a neuronal input of large bundles of granule-containing axons terminating at fenestrated regions of the endocardium and suggesting a neurohormonal function. A simple, tubular sarcoplasmic reticulum with flattened junctional cisternae was present in myocardial cells of 1-5 microns diameter, which contained one or two bundles of myofibrils. The latter were closely apposed to the inner aspect of the plasmalemma. Mitochondria were located centrally in the cells, which were joined by unfolded desmosomes involving Z-band material. Long periods of anoxia were tolerated without loss of heart function, but at the expense of cytoplasmic glycogen. Lipid granules were abundant in all layers and chambers, notably in animals prepared in the summer. The lipid granules displayed a marked increased in electron density when the heart was incubated in a buffered oxalate solution prior to fixation. A glycogen-sparing effect of the lipids during anoxia was observed.