The fine structure of obliquely striated body wall muscles in the earthworm, Lumbricus terrestris linn

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Abstract
The fine structure of obliquely striated muscle fibres from the body wall of the earthworm has been investigated. Certain details of the structure have been confirmed by cutting serial sections. The fibres contain both thick and thin myofilaments. The latter are attached to Z-material and the 2 types of myofilament are arranged in interdigitating arrays to give rise to A- and I-bands and an H-zone similar to those in cross-striated muscle. The A-bands contain both thick and thin myofilaments and the I-bands only thin myofilaments. The Z-material is rod-shaped and these Z-rods, oriented perpendicular to the sarcolemma, are arranged in numerous parallel rows which run obliquely along the length of the fibre A line drawn parallel to the longitudinal fibre axis through a Z-rod in one row passes through a Z-rod in the next row. A thin, sheet-like array of myofilaments lies between 2 such Z-rods, forming a single sarcomere containing an A-band and 2 I-bands. The flat surfaces of neighbouring sarcomeres are directly apposed to one another but, since the rows of Z-rods run diagonally along the length of the fibre, each sarcomere is displaced longitudinally with respect to the next, so that the A- and I-bands follow an oblique course, instead of a transverse course as in cross-striated muscle. Because of the regular stagger of the sarcomeres A- and I-bands are cut alternately in transverse sections. Also the sarcomeres are very narrow and are seen as bands lying perpendicular to the sarcolemma. In the A-band a variable number of thin myofilaments (up to 12) surrounds each thick one. Cross-links have been seen between the 2 types of filaments. In longitudinal sections an appearance similar to that seen in cross-striated muscle is obtained in one plane (perpendicular to the sarcolemma). In the plane at right angles to this (parallel to the sarcolemma) the A- and I-bands are at an acute angle to the longitudinal fibre axis. The thick myofilaments exhibit a banding of about 15 nm. There is a system of transversely oriented tubules and peripheral vesicles with dyad-like structures occurring at the juxtaposition between the penpheral vesicles and the sarcolemma. It is concluded that this system is sarcoplasmic reticulam, and it is compared with tubular systems in other muscles. Other cellular constituents are described, including a peripheral skeleton of fibrillar bundles.