ORIENTED PATTERN OF MEMBRANE-ASSOCIATED VESICLES IN FIBROBLASTS

The topography of membrane-associated (pinocytic or surface) vesicles was investigated with freeze-fracturing of cultured human (MRC-5) fibroblasts. The vesicles had a tendency to form longitudinal rows in the direction of the long axis of the cell or its main cytoplasmic processes. Parallel rows often formed irregular areas leaving smooth membrane surfaces between them. Cells in both confluent and nonconfluent cultures showed a similar tendency of the vesicles to be arranged in longitudinal rows. This arrangement was partially disturbed by 10-5 and 10-6 M colchicine and was completely abolished by Cytochalasin B in concentrations of 5, 8 and 10 .mu.g/ml. The linear orientation of the vesicles is thought to be induced by close association of oriented microfilament bundles with the plasma membrane. The patchy distribution of the vesicles on the membrane indicates that the membrane is heterogeneous in structure (dynamic mosaicism of the membrane).