Comparative Ultrastructure of Cell Membrane Specializations in Vertebrate Cardiac Muscles

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Abstract
The ultrastructure of ventricular cardiac muscles from representatives of all the vertebrate phyla [mouse, mole, sparrow, lizard, frog, rainbow trout, lamprey] is examined with EM using thin sections and freeze-fracture replicas. Mammalian heart muscle fiber width is usually over 10 .mu.m, and T-tubules and sarcoplasmic reticulum (SR) are well developed. Non-mammalian cardiac muscles in general show small fiber diameter of less than 5 .mu.m, lack T-tubules, and reveal a wide variety in the degree of SR development. The rainbow trout, lamprey and sparrow, reveal well-developed SR and numerous subsurface cisternae (SC) in proximity to Z line level. However, lizard and frog ventricles show poor development of SR and infrequent SC at A band level. Nexus (gap junction) can be identified in all the vertebrate hearts examined by freeze-fracture technique. However, it is much smaller and more sparsely distributed in nonmammalien hearts than in mammals. These morphological differences may imply different electrophysiological properties correlated with the small fiber diameter of the lower vertebrate cardiac muscles.