Three-dimensional structure of the sarcoplasmic reticulum of skeletal myofibers in chicken and frog.

The three-dimensional structure of the sarcoplasmic reticulum (SR) of the twitch and slow tonic fibers in the skeletal muscles of the chicken and frog was examined using a modified Golgi method combined with stereoscopy employing high-voltage electron microscopy. In both types of fibers the SR network was classified into two categories, intermyofibrillar SR and subsarcolemmal SR. The intermyofibrillar SR in twitch fibers was basically similar and consisting of three or four typical structural regions. The configuration of the intermyofibrillar SR in twitch fibers was divided into two main groups which represented the SR in the fast twitch and the slow twitch fibers. The form of intermyofibrillar SR in slow tonic fibers is entirely different from that in twitch fibers in chicken, whereas it was rather similar in frogs. Regarding their function, the most striking feature of the SR structure was the paucity of the junctional SR in slow tonic fibers. The structure of subsarcolemmal SR in slow tonic fibers of the chicken and frogs was almost the same as in twitch fibers, forming polygonal mesh at the center of A-band and a loose tubular network including distended cisternae.