The Sarcoplasmic Reticulum

The contents of this volume, and most especially the modern morphological observations on the sar-coplasmic reticulum, place in strong relief the neglect that was accorded this component of the sarcoplasm in the 30 to 40 years preceding its recent rediscovery. From the time of publication of Veratti's paper, as D. S. Smith points out in his historical survey (54), various related or unrelated cytological discoveries served to submerge the earlier observations on the sarcoplasmic reticulum and finally to divert attention almost completely from them. The trophospongium hypothesis of Holmgren, which attracted considerable attention in the first decade of the century, interpreted the sarcoplasmic reticulum as the image of slender extensions of extracellular trophocytes having, supposedly , nutritive functions. Students of the Golgi component found compelling reasons to homolo-gize the system with the Golgi apparatus. And finally, the very existence of the delicate lacework of silver-stained strands that Veratti had depicted in his figures came to be doubted by almost everyone. Reviews on muscle, appearing during the 1940's and early 1950's, neglect the structure entirely. One can readily agree with Bennett (6) in his comment that "it is astonishing that a structure once described as accurately and as beautifully as the reticulum was by Veratti (1902) should have so quickly become almcst lost to man's knowledge". It was from this position of neglect that "man's knowledge" of the sarcoplasmic retic-ulum was rescued by electron microscopy. As attested by this volume, the study of the system is now back to a level of interest fittingly proportioned to Veratti's early work. The first paper of recent times which can be said to have identified the sarcoplasmic reticulum in electron micrographs was published in 1953 (7). The techniques for electron microscopy were then crude and the evidence somewhat short of convincing , but the report adequately made the point that within the sarcoplasm between the myofibrils and beneath the sarcolemma, there is a reticular structure, distributed in repeating patterns related to the sarcomeres of the myofibrils. The unit of structure was observed to be tubular or vesicular and reminiscent of the endoplasmic reticulum (or ER) then being recognized as a common component of other cell types (33, 37, 40). Connections, apparently comprised of elements of the sarcoplas-mic reticulum, were observed to run from the marginal fibrils of the fiber to the sarcolemma at the level of the Z line. These and other features of …