Electron Microscopy, 5

remarkable similarities between various striated muscles. This is not surprising, for, as a rule, structural differentiations once having proven their effectiveness are usually preserved through phylogeny and ontogeny; already established structure-function complexes, instead of being replaced by a de novo alternative solution to an emerging physiological problem, tend to persist in a modulated form: Natura nonfacit saltum. Ultrastructural organization in cardiac muscle is relevant to both cell survival per se and to the purposeful discharge of unique functions given each cell, individually and severally, as they are gathered into aggregates. This chapter is devoted mainly to emphasizing those ultrastructural features that express the unique functions of intact cardiac myocytes (i.e., excitability and contractility). This review is brief and introductory. We urge the reader to fill in details by consulting the extant literature (e.g., refs. 26,100,104), especially the chapter on the ultrastructure of cardiac muscle in the Handbook of Physiology (137). The first part of the present chapter is devoted to a discussion of a number of technical matters. Our aim is to provide a working acquaintanceship with some of the methods used in electron microscopy to give the reader a feel for the m9des of morphological inquiry that are possible today, as .well as to caution against their obvious limitations. Throughout; we have employed terminology that is useful in defining structures traceable through various species and different kinds of striated muscle (137). The ultrastructural branch of anatomy is an essential source for comprehending function in biological systems: It attempts to describe morphology at a level approaching molecular organization. Early electron microscopy of the heart immediately asserted its lasting impact on matters functional by proving that cardiac muscle was composed of individual and separate cells instead of being an anatomical syncytium (cf. Figs. 1-10), even as the electrical behavior of cardiac muscle clearly proscribes such an anatomy (177). This apparent contradiction, in turn, was resolved by electron microscopy again when communicating junctions were discovered that bridge the extracellular space (ECS) and so allow for electrical current to flow directly from one cell to the next. The comparative branch of cardiac anatomy, especially, has often provided heuristic insights by suggesting or eliminating potential links between structure and function. Indeed, a large body of information, as well as many methods of inquiry into the ultrastructure of cardiac muscle, is derivative of knowledge gained from studying other striated muscles, skeletal muscle above all (104,137). Whereas skeletal muscle cells lend themselves readily to physiological studies because they are large and can be isolated with relative ease, recently the study of single cardiac myocytes, too, has come of age (36,95,123). On the whole, comparative anatomy has disclosed