FIGURE 2.9 • 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 fnally, 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 (arrows) within the cytoplasm of endothelial cells of a blood vessel. 60,000.
therefore may be referred to as clathrin-independent endocytosis.
 Phagocytosis [Gr., cell eating ] is the ingestion of large particles such as cell debris, bacteria, and other foreign materials. In this nonselective process, plasma membrane sends out pseudopodia to engulf phagocytosed particles into large vesicles (larger than approximately 250 nm in diameter) called phagosomes. Phagocytosis is performed mainly by a specialized group of cells belonging to the mononuclear phagocytotic system (MPS). Phagocytosis is generally a receptor-mediated process in which receptors on the cell surface recognize non–antigen-binding domains (Fc fragments) of antibodies coating the surface of an invading microorganism or cell (Fig. 2.10a). Phagocytosis is also triggered by recognition of pathogen-associated molecular patterns (PAMPs) that are commonly expressed on pathogen surfaces by Toll-like receptors (page 278). This recognition of PAMPs leads to activation of nuclear factor kappa B (NF-B) transcription factor, which regulates genes that control cell responses in phagocytosis. However, nonbiologic materials such as inhaled carbon particles, inorganic dusts, and asbestos fibers, as well as biologic debris from inﬂammation, wound healing, and dead cells, are sequestered by cells of the MPS without involvement of Fc receptors (Fig. 2.10b). This process does not require clathrin for phagosome formation. However, because of initial pseudopodial extensions of plasma membrane that contribute to the formation of phagosome, the actin cytoskeleton must be rearranged in a process that requires depolymerization and repolymerization of the actin filaments. Thus, phagocytosis is referred to as clathrin-independent but actin-dependent endocytosis.
 Receptor-mediated endocytosis allows entry of specific molecules into the cell. In this mechanism, receptors for specific molecules, called cargo receptors, accumulate in well-defined regions of the cell membrane. These regions, which are represented by the lipid rafts in the plasma membrane, eventually become coated pits (Fig. 2.11a). The name coated pit is derived from these