Ultrastructure of dyads in muscle fibers of Ascaris lumbricoides.

The dyads of Ascaris body muscle cells consist of flattened intracellular cisternae applied to the sarcolemma at the cell surface and along the length of T-tubules. In specimens prepared by conventional methods (glutaraldehyde fixation, osmium tetroxide postfixation, double staining of sections with uranyl acetate and lead hydroxide), both the sarcolemma and the limiting membrane of the cisterna exhibit unit membrane structure and the space between them is occupied by a layer of peg-shaped densities which is referred to as the subsarcolemmal lamina. The lumen of the cisterna contains a serrated layer of dense material referred to as the intracisternal lamina. In specimens fixed in glutaraldehyde, dehydrated, and then postfixed in phosphotungstic acid, with no exposure to osmium tetroxide or heavy metal stains, the membranous components of the dyads appear only as negative images, but the subsarcolemmal and intracisternal laminae still appear dense. Except for the lack of density in membranes and in glycogen deposits, the picture produced by the latter method is very much like that of tissue prepared by conventional methods.