The sarcoplasmic reticulum and development of z lines in skeletal muscle fibers of fetal and postnatal rats

This is the authors' abstract. We don't add key points for this paper.

In electron microscope examinations of leg muscles from fetal and postnatal rats the development of SR tubules precedes the development of Z lines and the formation 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 μ thick) which are encircled by SR tubules separated from the Z lines by a space about 100 Å wide, and this space is traversed by electron‐opaque strands which appear to be 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.