Studies on membrane specializations in tentacular retractor muscle of the gastropod, Limax sp

Retractor muscle cells of the optic tentacle of Limax sp. occur as a network beneath the epithelium. The cells are spindle-shaped, irregularly cross-striated, and they contain a large number of subsarcolemmal caveolae. Freeze-fracture images of the sarcoplasmic reticulum, caveolae and sarcolemma demonstrate distinct particulate organizations. Membranes of the sarcoplasmic reticulum contain typical 7-9 nm PF-face particles. The caveolae membranes contain linear, sometimes rhombic arrays of 12-15 nm EF-face particles. An extensive area of the sarcolemmal surface is occupied by caveolar invaginations. Other areas of the sarcolemma contain linear arrays of 7-9 nm PF-face particles and a few rhombic ordered, 7-9 nm PF-face particles. The results of the study are discussed relative to previous studies on particulate arrays in muscle membranes. It is concluded that these highly specialized sarcolemmal and caveolar particulate organizations may, in some way, reflect the large surface area changes which occur in these muscle cells.