The article by Steinman et al. focuses on the properties and dynamics of endocytic vacuoles, emphasizing the rapid movement of vesicles and the recycling of plasma membrane (PM) components. Key points include: 1. **Rapid Vesicle Movement**: Endocytosed membrane and contents move between cellular compartments within seconds to minutes. 2. **Large amounts of PM are interiorized**: This requires significant flow of PM. 3. **Recycling of PM**: In many cases, internalized PM must return to the cell surface intact. 4. **Sorting during recycling**: Contents and membrane components can be sorted, allowing solutes to accumulate while the membrane recycles. The authors highlight examples from cultured mouse macrophages, noting that rapid membrane flow and recycling can explain observed phenomena. Two striking examples are: - Fibroblasts interiorize 50% of their surface area and 5-10% of their cell volume per hour during pinocytic activity, yet cell dimensions remain constant. - During receptor-mediated endocytosis, the amount of ligand delivered to cells per hour can be much higher than the binding capacity of the PM, suggesting repeated recycling of PM receptors. The article also discusses the vacuolar system, which includes the cell surface, phagocytic and pinocytic vacuoles, digestive granules (lyosomes), and the Golgi apparatus. Key points include: - **Phagocytic Vacuoles**: Form by phagocytosis and contain particles and fluid. Phagocytic vacuoles can be isolated and studied, and their membranes are similar to PM. - **Fluid Phase Pinocytic Vesicles**: Contain extracellular fluid and dissolved solutes. These vesicles are spherical, lack associated coats or filaments, and can be visualized using tracers like HRP. - **Adhesive Pinocytic Vesicles**: Bind to specific ligands and form larger vacuoles (endosomes) that can fuse with lyosomes. Coated vesicles, which mediate adsorptive pinocytosis, are enriched in clathrin and ligand-binding sites. The article concludes with a discussion of the pathways of endocytosis, including phagocytosis, fluid phase pinocytosis, and transcellular transport. It emphasizes the rapid and directed nature of membrane fusion and the importance of membrane recycling in maintaining PM area and function.The article by Steinman et al. focuses on the properties and dynamics of endocytic vacuoles, emphasizing the rapid movement of vesicles and the recycling of plasma membrane (PM) components. Key points include: 1. **Rapid Vesicle Movement**: Endocytosed membrane and contents move between cellular compartments within seconds to minutes. 2. **Large amounts of PM are interiorized**: This requires significant flow of PM. 3. **Recycling of PM**: In many cases, internalized PM must return to the cell surface intact. 4. **Sorting during recycling**: Contents and membrane components can be sorted, allowing solutes to accumulate while the membrane recycles. The authors highlight examples from cultured mouse macrophages, noting that rapid membrane flow and recycling can explain observed phenomena. Two striking examples are: - Fibroblasts interiorize 50% of their surface area and 5-10% of their cell volume per hour during pinocytic activity, yet cell dimensions remain constant. - During receptor-mediated endocytosis, the amount of ligand delivered to cells per hour can be much higher than the binding capacity of the PM, suggesting repeated recycling of PM receptors. The article also discusses the vacuolar system, which includes the cell surface, phagocytic and pinocytic vacuoles, digestive granules (lyosomes), and the Golgi apparatus. Key points include: - **Phagocytic Vacuoles**: Form by phagocytosis and contain particles and fluid. Phagocytic vacuoles can be isolated and studied, and their membranes are similar to PM. - **Fluid Phase Pinocytic Vesicles**: Contain extracellular fluid and dissolved solutes. These vesicles are spherical, lack associated coats or filaments, and can be visualized using tracers like HRP. - **Adhesive Pinocytic Vesicles**: Bind to specific ligands and form larger vacuoles (endosomes) that can fuse with lyosomes. Coated vesicles, which mediate adsorptive pinocytosis, are enriched in clathrin and ligand-binding sites. The article concludes with a discussion of the pathways of endocytosis, including phagocytosis, fluid phase pinocytosis, and transcellular transport. It emphasizes the rapid and directed nature of membrane fusion and the importance of membrane recycling in maintaining PM area and function.