I11 electron microscope examinations of leg muscles from fetal and postnatal rats the development of SR tubules precedes the development of Z lines and the fosrmation of fibrils, thus showing that a function of SR in Z line development is possible. Early stages of fibril formation reveal very small Z lines (0.06 p thick) which are encircled by SR tubules separated from the Z lines by a space about 100 A wide, and this space is traversed by electron-opaque strands which appear to lbe connections between SR tubules and Z lines. The encircling SR tubules are observed from the beginning to the completion of Z line development. The small Z lines contain dense lines transversely oriented between thin filament tips that resemble transversely oriented dense lines found in fully developed Z lines. It is concluded that Z line enlargement involves growth in circumference of encircling SR tubules as the tubules incorporate additional layers of thin filament tips. It is suggested that the substance represented by electronopaque strands connecting SR tubules and Z lines might provide precursors for the substance represented by transversely oriented dense lines between thin filament tips.In electron microscope studies on breast muscle fibers of the adult fowl, Bennett and Porter ('53) found sarcoplasmic reticulum (SR) extending across the interfibrillar space from Z line to Z line of adjacent fibrils. The interfibrillar SR at the Z line level was described ,as a structure continuous with SR encircling the fibril at the Z line. Allen and Pepe ('65) observed distinct Z lines in the five-day chick embryo and the Z lines were occasionally associated with the interfibrillar tubules of SR. Although no distinct Z lines were seen in the four-day chick embryo, material resembling the Z line was found in place of the tubules in several cases. In fibers from breast muscle of the posthatched chick, small tubular connections were seen between interfibrillar vesicles and the tubules surrounding the fibrils at the Z line level. Bergman ('62) observed vesicles, interpreted as SR, attached to Z lines in studies on myogenesis in fetal rats.Electron micfoscope examination of cross-sections from ox cardiac muscle fibers revealed SR tubules which appear to en-ANAT. REC., 169: 661-678.circle each fibril at each Z line level (Simpson, '65) and studies on serial thin sections of ferret cardiac fibers have shown that the encircling SR tubules are continuous with a network of SR tubules that extends uninterrupted in the interfibrillar space of successive sarcomeres (Simpson and Rayns, '68). In a comparative study of ventricular and Purkinje fibers from hearts of guinea pig, rabbit, cat, dog, goat, and sheep, Sommer and Johnson ('68) found a component of the SR that is closely associated with the Z line. In fetal and newborn skeletal muscle fibers (Walker et al., '68) and in adult dog cardiac fibers (Edge and Walker, '70) the Z line and the SR tubule encircling it are separated by a space about 100 A wide and this space is traversed by electro...
Electron micrographs of dog papillary muscle fibers show the sarcoplasmic reticulum (SR) as a n extensive three-dimensional network of diverging and converging tubules that is continuous throughout each sarcomere and i n successive sarcomeres. The network surrounds each myofibril and is continuous across the fiber.
The space between the sarcoplasmic reticulum (SR) apposed to the transverse ( T ) tubules and the sarcolemma in dog papillary muscle fibers is traversed by structures of varying densities. The distance between apposed membranes is about 70 A at the level of densities and about 100 A at the level of less dense areas that alternate with the densities. The apposed SR shows a marked increase in electrondense material when comparison is made with the SR not apposed. The electron-dense material is located in a dense layer parallel to and about midway between the membranes of the apposed SR and in numerous projections from the inside surface of the membranes. With appropriate staining procedures the central dense layer and the projections are as thick as membranes and they frequently show paired densities separated by a distance about equal to the distance between the outer dense layers of trilaminar membranes. The paired densities in the central dense layer and in the projections indicate that the electron-dense material within apposed SR has structure. The projections are frequently in register with scallops or serrations in the SR membranes and they also appear to contribute to a waviness in the central dense layer. The senations and waviness are interpreted as evidence that the projections exert a holding force because they project &om the SR membranes and connect with the central dense layer.Veratti's ('02) elegant observations on reticular structures in skeletal muscles of invertebrates and vertebrates have been clarified by electron microscope examinations showing that the sarcoplasmic reticulum (SR) encircled each fibril and formed a close association with Z lines in fowl breast muscle (Bennett and Porter, '53) and that tubules of the SR formed a network in rat sartorius and cardiac muscle fibers (Porter and Palade, '57). Segments of the SR tubules were found in apposition with what appeared to be vesicles. The apposition was seen near the A-I junction in sartorius muscle and near the Z line in cardiac muscle. Serial sections of mouse leg muscle indicated that the structures between terminal segments of the network were also tubules transversely oriented across the fiber (hdersson-Cedergren, '59). A space about 100 A wide was observed between apposed membranes of transverse tubules (T tubules) and terminal segments of the network of tubules. Continuity of the T tubules with the sarco-AM. J. ANAT., 128: 33-44. lemma has been demonstrated near the Z line in mammalian cardiac fibers (Lindner, '57; Simpson and Oertelis, '61), in fish and tadpole tail muscle fibers (Franzini-Armstrong and Porter, '64) and near the A-I junction in rat gastrocnemius muscle fibers (Walker and Schrodt, '65a). Tubules of the sarcoplasmic reticulum are continuous with SR apposed to the sarcolemma of rabbit cardiac muscle fibers (Johnson and Sommer, '67) and skeletal muscle fibers of fetal and newborn rats (Walker et al., '69). The network of SR tubules extends without interruption throughout successive sarcomeres in mammalian car...
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