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THE SARCOPLASMIC RETICULUM

K. Porter

Published 1961 in The Journal of Biophysical and Biochemical Cytology

ABSTRACT

The contents of this volume, and most especially the modern morphological observations on the sarcoplasmic 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 Veratt i '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 ret iculum as the image of slender extensions of extracellular trophocytes having, supposedly, nutritive functions. Students of the Golgi component found compelling reasons to homologize the system with the Golgi apparatus. And finally, the very existence of the delicate lacework of silver-stained strands that Veratt i 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 " i t is astonishing that a struc-ture 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 reticulum 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 ret iculum 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 sarcoplasmic 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 this component of the sarcoplasm convinced the observers that they were seeing the electron microscope image of the cross-fiber ret iculum of Cajal (1890 (10)), Retzius (1890 (45)), Veratt i (1902 (57)), and Heidenhain (1911 (19))among others. These initial observations by electron microscopy introduced a period of recognition for the sarcoplasmic reticulum (or SR), and a number of investigators of muscle fine structure began to note the same or similar components in electron micrographs of striated muscle fi'om various sources. Ruska (1954 (50)) observed it in several skeletal muscles of the mouse and Weinstein ( 1954 (59)) in cardiac muscle cells of various vertebrates. Edwards and Ruska (1955 (15)) called attention to its presence in leg muscles of two different insects, but noted its apparent absence from flight muscles. Bennett (1955 (3)) provided some of the best illustrations up to that time of the system in mouse skeletal muscle, and Porter (1954, 1956 (38, 39)) reported it in micrographs of muscle fibers from the caudal myotomes of a larval amphibian. These early observations were soon supple-

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