Relationship of the sarcoplasmic reticulum to fibril and triadic junction development in skeletal muscle fibers of fetal monkeys and humans

Abstract

Examinations of stages of fibril development in muscle fibers of seven Rhesus monkey and six human fetuses reveal SR tubules encircling the Z lines at all stages of fibril development. The encircling SR tubules are continuous with the SR network of tubules which is found surrounding fibrils at all stages of development observed. The SR tubules encircling the Z lines show connections (electron-opaque strands) with the Z lines. The developing triadic junction shows a progressive increase in complexity of structures within the junction. First, membranes of T and SR become apposed with no visible structure between them- Second, tenuous connections are found traversing the space between apposed membranes. Third, well developed bridges are seen traversing the space. And finally, an intermediate density midway between the apposed membranes and parallel to them is found in favorable sections. Junctions between T tubule membranes were also observed and the structures in these junctions are somewhat similar to those found in junctions between T and SR membranes. The change in orientation of triads from predominantly longitudinal to predominantly transverse is complete in the 18-week monkey fetus and incomplete in the latest stage (28-week) of fetal development observed in humans.

Morphology of isolated triads.

“…The free strand observed in negative staining (Fig. 2 c), running parallel to the membrane, may be different from a medial line reported in thin section (28)(29)(30). The latter is located at the center of the junction, whereas that seen by negative staining displays a proximity closer to the t-tubule side of the junction (Fig.…”

A comparative study of charge movement in rat and frog skeletal muscle fibres.

“…Schneider & Chandler (1973) suggested that in frog muscle the movement of charge particles within the tubular membrane might be transmitted to the SR membrane by these feet, and that one charge particle was located at each foot. Feet are present in rat and other mammalian muscles (M. Cullen, S. Hollingworth & M. W. Marshall, unpublished observation;Luff & Atwood, 1971;Walker, Schrodt, Currier & Turner, 1975), and the difference in fibre type Qmax values revealed by this study may, therefore, be reflected in the distribution and structure of feet within the two muscle types. From the Qmax values in Table 3, and using a valence of 2, we estimate the density of charge particles to be 1800 and 600 tm-2 for e.d.l.…”

Development of the excitation-contraction coupling machinery and its relation to myofibrillogenesis in human iPSC-derived skeletal myocytes

“…According to a number of developmental studies, the spatial positioning of SR-TT junctions is a slow and final process that occurs about 2 weeks after birth in rodents [ 5 , 30 – 33 ]. In humans, change in orientation of the triads is still incomplete at 28 weeks of fetal development [ 34 ] . Interestingly, a similar timeframe is shared by muscle innervation which reaches its mature mono-synaptic form about 2 weeks after birth in rodents [ 35 , 36 ] and within the first year of life in humans [ 37 ].…”