An electron microscopic study on the extraocular muscles of a lamprey, Lampetra japonica

This paper reports an EM study on the observations of the extraocular muscles of the lamprey, Lampetra japonica. Individual muscles consisted of numerous subunits, each being surrounded by a common external lamina and containing two, S- and F-types, of muscle fibers. A subunit contained an average of 8.25 muscle fibers consisting of 5.58 S-type fibers and 2.67 F-type fibers. In a rectus anterior muscle, the total number of the S-type fiber was 852, the average cross-sectional area being 144 micrometer 2. The total number of the F-type fiber was 397, the average cross-sectional area being 300 micrometer 2. The S-type fiber was characterized by the arrangement of myofibrils as a single layer in the cortical sarcoplasm as well as by the development of peripheral couplings. The F-type fiber was characterized by the presence of central myofibrils in the juxtanuclear sarcoplasm in addition to peripheral myofibrils and by the development of the T-system and triads at the level of the Z line. M lines appeared distinct in the F-type fibers and less distinct in the S-type fibers. Muscle fibers within individual subunits were separated from each other by a space about 20 nm, but no membrane specializations such as gap junction or desmosome have been determined to exist between adjacent muscle fibers.