Variations in the appearance of membrane particles after various pretreatments.

In freeze-fracturing human erythrocytes beside the intramembraneous particles fibrillar structures can be observed. They are in close relation to the intramembraneous particles and occur frequently under special conditions. 1. In erythrocytes incubated with MgCl2-solutions (25-100 mM) striking fibrillar structures are mostly located on the E-fracture face or extend from the E-face to the cytoplasmic surface aspect of the membrane. Fibrillar structures located on the P-face or extending from the P-face to the extracellular surface aspect do not often occur. This asymmetrical distribution indicates that elements pulled out from the cytoplasmic membrane half play a special role in the fibril formation. Intermediate forms between particles and fibrillar structures can occur. 2. By use of the microtome technique for freeze-fracturing in a frozen 60 vol.-% glycerol solution at temperatures upward from -100 degrees C melting can occur at the knife edge during cutting. In connection with the formation of a melting zone in isolated erythrocyte membranes incubated in 60 vol.-% aqueous glycerol a somewhat modified (unfinished) membrane splitting is possible. Lying side by side both the membrane fracture halves remain connected by the unsplit part of the membrane. In the region where they are connected fibrils between the two fracture faces occur indicating a deformation process during the particle formation. The results give further evidence that plastic deformation during freeze-fracturing takes place in the formation of the intramembraneous particles in native erythrocyte membranes.