Phagocytosis and fluid-phase pinocytosis

The generation, flow, directionality and fusion of phagocytic and fluid-phase pinocytic vesicles in cultured macrophages and fibroblasts are reviewed. Specific plasma membrane (PM) receptors, receptor mobility, contractile cytoplasmic elements and lipid composition of the PM serve to regulate the flow of large phagosomes into the perinuclear zone. Fluid-phase vesicles are constitutively generated and carry large quantities of PM, fluid and solutes into the cytoplasm. Quantitative information is cited on the rates of vesicular generation, fusion with other members of the vacuolar system, fluid and solute uptake, and digestion and solute release. The nature and composition of fluid-phase vesicles, phagocytic vacuoles and PM are compared. Once interiorized, PM and its component polypeptides rapidly cycle back to the cell surface. The flow rates of both the centrifugal and the centripetal compartments as well as the fate of a minor degradation pool are illustrated and compared to the turnover of individual membrane polypeptides. Implications of membrane flow for cell shape, motility and new PM insertion are discussed.