Studies on the Endoplasmic Reticulum

Abstract

Previous papers in this series (1, 2) as well as numerous other reports on cell fine structure have provided evidence for the existence of a finely divided vacuolar system in the cytoplasm of all animal cells thus far examined except mature erythrocytes. The reticular character of the system and its exclusion from the thin cortical layer (ectoplasm) of the cytoplasm, as first observed in cultured cells (3,4), account for the name "endoplasmic reticulum." More recently, in electron microscope studies of thin sections, the term has been applied to small tubular or vesicular elements of the cytoplasm that are limited by a single continuous membrane and have a homogeneous, structurefree content. In the majority of cells, such elements make up a system that is continuous and reticular in form. By its presence the cytoplasm is divided into two phases: (a) a constantly continuous phase, the cytoplasmic matrix, containing such resolvable components as ribonucleoprotein (RNP) particles and fibrous elements of several kinds, and (b) the inner phase of the membranelimited reticulum, in general also continuous, but certainly discontinuous at times, and showing usually no resolvable elements. In some cells the system is abundantly represented; in others sparsely so. Its form, volume, and distribution tend to be characteristic for all cells of a particular type so that it becomes a feature of cellular differentiation. The system has also been observed to show local differentiations (2, 6) and to develop special associations with other components (e.g., the RNP particles (5) and the nucleus (6)). Since it is reasonable to look to them for clues to the functions of the system, these latter associations seem especially significant. Some attention is therefore being given, in this and subsequent studies, to their occurrence, form, and behavior.There are various indications to be found in published accounts of the structure of striated muscle that an equivalent of the endoplasmic reticulum is present in this as in Other types of cells, and that it may show a pattern of distribution relative to the myofibrils. For example, Bennett and Porter (7), in a report on the fine structure of striated muscle of the domestic fowl, directed attention to an interfibrillar component--in some instances vesicular--localized opposite the Z or N bands of the myofibrils. Similar elements were later depicted 269 J, BIOPHYSXC, ~tr~D Bloc~tz~, Cx-roL., 1957, Vol. 3, No. 2 on May 10, 2018 jcb.rupress.org Downloaded from http://doi.org/10. 1083/jcb.3.2.269 Published Online: 25 March, 1957 270 STUDIES ON T/-~ ENDOPLASMIC RETICULUM. HI in the sarcoplasm of insect muscle (8) and more clearly in muscle of Amblystoma larvae (9, 10) and the gracilis muscle of the mouse (11).These miscellaneous and preliminary observations have failed, however, to give more than a suggestion of the pattern the system adopts with respect to the myofibrils. In this regard, in fact, these recent observations are perhaps not so useful as older, somewhat neglected light microscop...

On the Structural Continuities of the Transverse Tubular System of Rabbit and Human Myocardial Cells

“…The relative infrequency of triads and dyads and of the en face views of the longitudinal network compared with skeletal muscle indicates that this component of the sarcoplasmic reticulum is less well developed in cardiac than in skeletal muscle (2). Fig.…”

“…In rat cardiac muscle, the sarcoplasmic reticulum was less highly developed than in skeletal muscle, the intermediary element of the triad was large relative to the lateral profiles, and occasionally the intermediary element was associated with only one lateral profile to form a "dyad." We assume that the transverse tubular system in rabbit and human myocardium corresponds to the series of vesicles at the Z line level in rat cardiac muscle (2). In most of our material, however, the transverse tubular profiles have the structure of sarcolemmal invaginations.…”

“…A simple mechanism might be provided by physical continuity of the lumen of this transverse tubular system with the extracellular space, a relationship not yet demonstrated in vertebrate skeletal muscle (2,6,7).…”

“…After Bennett and Porter's first electron microscopic observations of the sarcoplasmic reticulum in the breast muscle of the domestic fowl (I), Porter and Palade presented detailed descriptions of this subcellular structure in different types of striated muscle (2). These findings provided a structural basis for consideration of the mechanisms of excitation-contraction coupling in muscle.…”

Endoplasmic reticulum: a dynamic patchwork of specialized subregions.

“…On the one hand, ER-SR membrane continuities, evident during development, are difficult to detect in the mature fibers (Porter and Palade, 1957); on the other hand, the two systems differ markedly in their composition. In particular, the complex protein pattern of the ER contrasts with the simplicity of the SR, where Ca" ATPase and CSQ account for the bulk of the membrane and luminal protein, respectively (see Campbell, 1986).…”