Material and methods

Abstract

The ultrastructural organization of the motor end plate and various sarcoplasmic components of the skeletal muscle fiber of the mouse have been analyzed by means of three-dimensional reconstructions from serial sections. Motor end plate The motor end plates analyzed were of the classical type, “plaque terminale”. The end branchlets of the terminal axons are located in grooves of the muscle fiber surface. The plasma membrane makes invaginations, junctional folds, in the grooves. Dimensions of the junctional folds are shown in Table I (p. 26). The junctional folds have narrow and short openings on that part of the muscle fiber which faces the terminal axon branchlet. From these small openings the fold expands in one plane. The fold length in the interior of the muscle fiber often considerably exceeds the length of the entrances. The increase of the muscle fiber surface in the grooves by means of the junctional folds is about 4.5×. The whole surface area of the invagination is 42 times greater than the surface area of the entrances of the folds. The surface area of the entrances of the invaginations is about 10% of the total surface area of the grooves. The region of the junctional folds seems to correspond to that region where cholinesterase is apparently located. The mechanical effects of volume changes within the junctional folds in connection with contraction are discussed. The junctional folds and the space between the plasma membranes of the muscle and terminal axon branchlets (400–500wide) in the grooves are filled with a homogeneous basement membrane-like structure. In the sarcoplasm of the muscle fibers and in the axoplasm of the terminal axon branchlets, small vesicles or granules are found, called post- and presynaptic vesicles respectively. Most of the postsynaptic vesicles (average diameter 380 ) are localized at the bottom and edges of the junctional folds, with which they have close topographic relationship. The presynaptic vesicles (average diameter 450 ) are mostly scattered in the axoplasm but seem to be more numerous at regions where a particular kind of membranous component, X component, occurs. The X component consists of pairs of membrares, bounding a round or irregularly shaped space with Golgi apparatus The components interpreted as the Golgi apparatus of the muscle fiber are tightly packed vacuoles of different sizes surrounded by granules and vesicles with diameters from 200 to 500embedded in a homogeneous opaque material. The bounding membrane of the vacuoles and the granules and vesicles is about 50–60 . Occasionally, 4–9 tightly packed pairs of membranes (up to 1.5 μ long) are found. the thickness of the membranes and the space between them is about 50–60 . The space between the pairs of membranes is about 70 . The pairs of membranes fuse at their edges and enclose a narrow space. They do not vacuolize and are not observed to branch or anastomose. The different Golgi components do not seem to communicate with each other. Close topographic relationship are found between the Golgi components and mitochondria, cytoplasmic membranes, the sarcotubular system, and the nuclear membrane, but no direct continuity between the components of the Golgi apparatus and the other fiber components has been observed. The bounding membranes of the Golgi components do not exhibit attached small opaque particles.