Cellular surfaces of amphibian atrial muscle.

The cells of the amphibian atrial myocardium contain many so-called micropinocytosis vesicles, irregularly distributed along the cell membrane. In freeze-etch replicas they appear as pits when the cell membrane is seen as from an external view point, or as irregular stumps, when viewed as from an internal or cytoplasmic aspect of the membrane. In occasional specimens in which the fracture has traversed a sagittal plane the vesicles exhibit the flaskshaped appearance noted in sectioned material. In sections from tissue fixed in aldehydes and post-fixed in an osmium-lanthanum solution, the vesicles contain lanthanum whether or not they appear to be attached to the surface membrane. Since this agent is an extracellular marker, administered after cell death, this observation is taken as an indication that all the vesicles communicate with the extracellular space, and that failure to visualize the connexion is merely a consequence of the plane of sectioning. Vesicles not connected to the membrane, and free of lanthanum could not be found, suggesting that if micropinocytosis does occur, abscission is an infrequent event, and/or the vesicles are very short-lived following separation from the membrane. The possibility that these organelles may act as an extension of the cell membrane and serve to enlarge its area has been suggested. This implies that calculations of cell membrane area based on the assumption of a smooth surface may be in error, though the magnitude of the error cannot be estimated from the present information.