Triads in Skeletal Muscle Fibers of 19-day Fetal Rats

The interfibrillar spaces of skeletal muscle fibers in adult vertebrates contain two distinct membranous systems, the T system and the sarcoplasmic reticulum. Most of the T system is oriented transversely between the fibrils. Occasionally, longitudinal connections between the transversely oriented T system are seen (14). The sarcoplasmic reticulum is, in general, oriented longitudinally between the fibrils. However, the terminal segments of the sarcoplasmic reticulum face the T system and show transverse orientation at this level. Two terminal segments of sarcoplasmic reticulum and a tubular element of the T system between these segments have been called a triad (13). A triad is found near each junction of the A and I bands in reptiles, birds, and mammals. Numerous investigators have noted that the space between the T system and the apposed sarcoplasmic reticulum is about 100 A and is remarkably constant. Revel (14) described rather regularly spaced, dense bridges traversing this space in bat muscle fibers. Similar dense bridges were reported (19) for rat muscle fibers, and evidence that these bridges are connections between the T system and sarcoplasmic reticulum has been presented (20). In a recent electron microscope study (1) on morphogenesis of rat skeletal muscle, sarcoplasmic reticulum was observed between the developing fibrils in 16.5-20-day rat fetuses; but triads were not reported in that study. In light microscope studies on skeletal muscle fibers from newborn mice and fetal guinea pigs, Veratti (17) observed an irregular reticulum in longitudinal sections. He found a regular reticulum, transversely oriented, in similar longitudinal sections of fibers from adult mice (17). The contrast in appearance of reticulum in light microscope studies when reticulum profiles in fibers from newborn and adult mice are compared led to the suggestion that development of transversely oriented triads might occur in rodents after birth. Electron microscope studies on skeletal muscle fibers from 19day rat fetuses have shown a striking sparsity of transversely oriented triads and a more numerous distribution of longitudinally oriented triads.