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In adult skeletal muscle fibers of reptiles, birds, and mammals each sarcomere contains two transversely oriented triads. The triads are located near the two A-I junctions in the sarcomere. The central element of the triad is a tubule formed by invagination of the sarcolemma and the transverse tubules thus formed are called the T system (rE). The two lateral elements of the triad are terminal segments of the sarcoplasmic reticulum (SR). The terminal segments of SR are continuous with two networks of tubules, one traversing the M line level, the other the Z line level. Although a majority cf the tubules in these networks are longitudinally oriented, many of the tubules make transverse connections in the networks (6). The interfibrillar SR and T of adult mammlain muscle fibers have been described in many electron microscope studies as a continuum of sleeves with each fibril surrounded by one of the sleeves. In a recent report (1 l) concerned primarily with triads in muscle fibers of the 19 day fetal rat, an interfibrillar network of tubules was also described. This network of tubules, which extends uninterrupted in successive sarcomeres, is continuous with lateral elements cf triads. On the basis of this continuity the network was identified as SR (11). In further electron microscope studies on ske]etal muscle fibers of fetal and newborn rats, close contact between the network of tubules and the Z line has been observed. A review of the literature shows that several investigations have called attention to the close association of interfibrillar tubules and the Z line. In early electron microscope studies on breast muscle fibers of the adult fowl, Bennett and Porter (2) observed strands of SR extending from Z line to Z line across the interfibrillar space. Bennett and Porter (2) described the SR as sleeves around each fibril and viewed the strands of interfibrillar SR at the Z line level as structures attached to SR encircIing the Z line. In a recent study on developing muscle cells in embryonic and posthatched chicks, Allen and Pepe (1) found distinct Z lines in the 5 day embryo. The Z lines were occasionally associated with the interfibrillar tubular system, Tubules surrounding myofilaments at approximately 1.5 /1 intervals were found in the 4 day embryo. Although no distinct Z lines were seen in the 4 day embryo, material resembling the Z line was found in place of the tubule 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 (I). In a report on cultured breast muscle fibers from the chick embryo, Shimada, Fischman, and Moscona (9) showed elegant electron micrographs of the interfibrillar network of SR tubules at the Z line level of the sarcomere. However, the relationship of the Z line to the network of tubules was not considered in that report (9). At the same time, another report (5) on cultured breast muscle fibers from the chick embryo showed the interfibrillar network of tubules and called attention to the fact

ELECTRON MICROSCOPE STUDY OF THE SARCOPLASMIC RETICULUM AT THE Z LINE LEVEL IN SKELETAL MUSCLE FIBERS OF FETAL AND NEWBORN RATS