Surface features of striated muscle: I. guinea-pig cardiac muscle

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Abstract
A general survey of guinea-pig myocardium was undertaken using the freeze-etch technique. Replicas of myocardial cell membranes were obtained. These showed an ordered array of pits or stumps situated at Z levels. The pits are interpreted as the apertures of the transverse tubules (T-tubules) seen from outside the cell, and the stumps as the remnants of the T-tubules remaining attached to the cell membrane after the cell contents have been removed. Pinocytotic vesicles were also present. T-tubules, mitochondria and myofilaments could be seen in replicas of the interior of myocardial cells. Capillary endothelial cells were seen from various aspects; pinocytotic vesicles were their most prominent feature. The appearances of the cell membranes in the present study suggest that the fracture plane tends to pass along either the outer or the inner surface of the membrane rather than to split the membrane.