AN INVESTIGATION OF CROSS STRIATIONS AND MYOSIN FILAMENTS IN MUSCLE

¹ Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts

1. Electron micrographs were made of myofibrils isolated from frog and other skeletal muscles fixed in formalin. The structure with respect to the location and disposition of the principal cross striations is in good agreement with that previously determined from histological studies.

2. The myofibrils are composed of bundles of myosin filaments ranging in width from about 50 to 250 Å and extending continuously and in relatively straight lines through the isotropic and anisotropic bands in both the extended and contracted states. The anisotropic bands also contain material of high electron scattering power and affinity for phosphotungstic acid. The distribution of this "A substance" changes with contraction in characteristic fashion. The evidence indicates that the myosin filaments are the contractile units.

3. While the myosin filaments have an indefinite length in the intact fibril, they are fragmented extensively during extraction in weakly alkaline salt solutions (method of Greenstein and Edsall). Filaments from such extracts have fairly uniform widths (50 to 250 Å) but highly variable lengths, in general below 15,000 Å. Filaments from rabbit, frog, lobster, scallop and clam muscles are similar in appearance; widths are fairly uniform but lengths vary significantly from one form to another.

4. The relation of these findings to physical chemical data previously obtained by others on similar myosin extracts is discussed.

5. In the one smooth muscle examined (clam adductor) no striations comparable to those of skeletal muscle fibrils were found. However, a regular structure with a period of about 1,100 Å was observed.

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