Pinocytosis. a. Pinocytosis involves the dynamic formation of small vesicles at the cell surface. First, substances to be pinocytosed (e.g., small soluble proteins, colloidal tracers) make contact with the extracellular surface of the plasma membrane; next, the surface becomes indented; and finally, the invaginated portion of the membrane pinches off from the membrane to become a pinocytotic vesicle within the cell. b. This electron micrograph shows numerous smooth-surfaced pinocytotic vesicles within the cytoplasm of endothelial cells of a blood vessel. x60,000.

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