Observations of the ultrastructure of the myocardium of the common lobster (Homarus americanus), especially of the myofibrils

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Abstract
The ultrastructure of the myocardial cells of the common lobster, Homarus americanus, was examined with the electron microscope. The contractile elements of the cells show the usual A, I and Z band structures in longitudinally cut sections. Sarcomeres in slightly oblique section frequently exhibit distinct H zones at the midportions of the A bands. The fine filaments of the I band may be seen between the larger A band filaments in many areas of the sarcomeres, although in other areas there appears only to be filamentous “bridging” between the filaments of the A bands. Cross sections through the A band do not show a constant “array” of I and A filaments. The H zones, observed in these myocardial cells, appear to result from the separation of the finely filamentous materal between the A rods at the middle of A bands when the sarcomeres are stretched. Notation is also made of the appearance of intercalated disks and the thickening and folding of the basement membranes of the sarcolemma at points of cellular angulation. Mitochondria are numerous in these cells, and there is an extensive sarcotubular system.