The Ultrastructure of Developing Sarcoplasmic Reticulum

Developmental changes in the sarcoplasmic reticulum and surface membranes of chicken skeletal muscles were investigated by electron microscopy. A dominant feature of freeze-etched sarcoplasmic reticulum membranes isolated from adult animals is the presence of spherical intramembranous particles of 75 A apparent diameter which are more numerous on the concave than on the convex cleaved faces. The sharp increase in the Ca2+ transport and Ca2+-sensitive ATPase activity of isolated sarcoplasmic reticulum membranes with development is accompanied by an increase in the number of 75 A particles. Corresponding changes were observed in intact muscles where the sarcoplasmic reticulum elements can be readily identified on the basis of their characteristic disposition between the myofibrils. The reasonably close correlation between Ca2+ transport activity and the intramembranous particle density during development supports earlier suggestions that the intramembranous particles may represent the Ca2+ transport system. Similar 75 A particles were also observed on the surface membrane of skeletal muscle but in lesser density than the corresponding structures in sarcoplasmic reticulum. Their number also increases with development.