The fine structure of the contractile apparatus of the anterior byssus retractor muscle of Mytilus edulis

Abstract

By the use of improved conditions of fixation for electron microscopy, a very uniform preservation of muscle cells from fresh anterior byssus retractor muscle of Mytilus edulis was obtained. This same appearance of cells was also observed in 10–15% of cells in glycerinated muscles. From visual inspection and optical diffraction analysis of electron micrographs it is shown that the muscle cells have very regular arrangements of thin and thick filaments. The ratio between the numbers of thin and thick filaments seen in a cross-section of muscle from different preparations is very constant at about 17: 1. The thick filaments show a nearest-neighbor order with a separation between centers of filaments of about 700 A. Within small groups the thick filaments may be seen to be arranged with hexagonal order. The thin filament arrangement is consistent with X-ray diffraction data in showing, in the electron microscope, a so-called “actin lattice.” The thin filaments pack hexagonally in multi rows which are branched in two dimensions. On the basis of quantitative analysis of electron micrographs, a three-dimensional organization of the contractile elements is deduced and a contractile unit defined. The contractile unit proposed is a sarcomere with a length in the nonoverlap state of about 50 μm. The sarcomere consists of 3 to 4, 25-μm long, thick filaments and about 70, 11-μm long, thin filaments on each half of a dense body, the latter being equivalent to the Z-disk of striated muscle. From nuclei and cell counts in cross sections, the cell length was estimated as about 1.6 mm. The surfaces of the thick filaments show regular arrangements of projecting structures which follow the general symmetry of the Bear-Selby net and which are interpreted as myosin projections. On the basis of optical diffraction analysis it is shown that these projections are arranged on a two-stranded helix with a repeat of 720 A, pitch 2 × 720 A, and with 10 residues per turn.