3-DIMENSIONAL ELECTRON-MICROSCOPY OF THE SARCOPLASMIC-RETICULUM AND T-SYSTEM IN EMBRYONIC CHICK SKELETAL-MUSCLE CELLS-INVITRO

The formation of the sarcoplasmic reticulum (SR) and the transverse tubular system (T-system) in embryonic chick skeletal muscle cells in vitro was studied by either the critical point drying-physical rupturing or physical rupturing-freeze drying together with rotary shadowing. In these cells, two membranous systems were observed. One was composed of flattened sacs which were either isolated or were connected to each other with slender processes to form mostly longitudinally oriented strands. Initially, these sacs had small granules at their surface and were found mainly under the sarcolemma. Later, they became smooth at their surface, extending throughout the cytoplasm to form irregular and dense networks. At later phases, the networks tended to be disposed at right angle to nascent myofibrils, exhibiting a characteristic honeycomb appearance. From the similarities in thin section images, they were identified as developing SR. The other membranous system were tubules with many enlargements. They were frequently associated with coated vesicles which appeared to take part in the formation, elongation, and anastomosing of developing tubules. These tubules could be impregnated with a tannic acid-glutaraldehyde-potassium ferrocyanide complex and, thus, were identified as T-tubules. © 1990 Springer-Verlag.