Structures linking the myonemes, endoplasmic reticulum, and surface membranes in the contractile ciliate Vorticella

An electron microscope investigation of the interface between the myonemes of Vorticella convallaria and their associated endoplasmic reticulum (ER) has revealed structures of a complex morphology linking these two organelles. These structures are named "linkage complexes". Each complex contains a spindle-shaped midpiece which lies in a groove of the ER membrane. Microfilaments splay out from the tips of the midpiece and may come in contact with the inner alveolar sac membrane. Three to six raillike structures lie on each side of the midpiece and parallel it. The ER membrane appears to pass through the sides of the rails. In the lumen of the ER these rails are associated with a meshwork of filaments. A cradle of five rods lies within the groove under the midpiece. The ER membrane also passes through these rods which contact the same meshwork. In the scopular region and in the stalk the microfilaments from the midpiece form a bundle which passes into the lumen of modified basal bodies. These basal bodies are connected to the alveolar sac which, in the stalk, passes as a flattened tube along its length. The parts of the dissociated linkage complex are scattered throughout the spasmoneme of the stalk along membranes of the intraspasmonemal tubules. Thus, both stalk and body contractile bundles have linkage complexes that link their associated membrane systems to the microfibrils and, in turn, connect this membrane-microfibrillar interface to the pellicular membranes. The arrangement of the linkage complex suggests an involvement in the control of the transport of calcium ions between ER and microfibrils, and possibly the transfer of a message from the surface membranes to the sites of calcium release to trigger myonemal contraction.