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A quantitative analysis of membrane particles of freeze-fractured sarcoma-180 cells
The freeze-fracture technique was used to analyze intercalations within the hydrophobic interior of the plasma membrane and the nuclear envelope of Sarcoma-180 ascites tumor cells. These membranes are extremely deformable and reflect basic, intrinsic alterations 1vi thin the membrane matrix. A quantitative analysis was made of membrane fracture faces in terms of particle densities, particle size frequency distributions, and mean density ratios for different membrane loci. These data were interpreted in terms of topological changes of these membranes, especially the extension and retraction of surface and nuclear lobation. The data suggest that a) particles are differentially bound to one leaflet versus the other, with a particle partition coefficient (kp) difference of 0. 66 between villous and non-villous membrane regions, and that b) membrane leaflets have different fluidity characteristics with respect to particle movements (displacement) and that c) these factors account for deformability modes within membranes which are reflected in topological changes of membranous structures as a whole. Other factors such as extrinsic proteins, microtubules and microfilaments may play a role in topological changes, depending upon cell type. In Sarcoma-180 cells, however, such extra-membranous components have not been observed in thin sections, suggesting that phase changes within the membrane per se may initiate cellular topological changes.