GIANT SMOOTH MUSCLE FIBERS OF THE CTENOPHORE MNEMIOPSIS LEYDII: ULTRASTRUCTURAL STUDY OF IN SITU AND ISOLATED CELLS

¹ Histologie et Biologie Tissulaire, Université Claude Bernard, 43 Boulevard du 11 Novembre, 69622—Villeurbanne, France

The lobate ctenophore Mnemiopsis leydii possesses giant smooth muscle fibers grouped in two sagittal bundles. Functional isolated cells were obtained by an enzymatic digestion of mesoglea and epithelia.

Each bundle is made of 30 to 50 multinucleated cylindrical cells which may reach 35 µm in diameter and 4 cm in length. The nuclei and non-contractile organelles (mitochondria, golgi, rough endoplasmic reticulum) are contained in a discontinuous axial core, surrounded by a thick sheath of myofilaments. Thin (actin) filaments, 5.9 nm in diameter, form irregular rosettes around the thick (myosin) filaments, 16.1 nm in diameter. An actin:myosin filament ratio of 7:2 and a myosin density of 249 filaments per µm² were found in cross-sections of relaxed in situ cells. No dense bodies nor attachment plates were observed. From the coiled shape of contracted single cells and from the rearrangement of organelles in such coiled cells, we propose that myofilaments are organized in thin long myofibrils attached to the cell membrane at both ends, and that the attachment sites follow two (sets of) enantiomorphic helices. The sarcoplasmic reticulum is a longitudinally oriented 3-dimensional network of narrow tubules among the myofilaments. Its relative volume, estimated from cross sections, amounts to 0.9% of the contractile cytoplasm. No peripheral couplings have been observed, nor any tubular or vesicular invagination of the sarcolemma.