Figure 29

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.

well-defined regions of the cell membrane. These regions eventually become coated pits (Fig. 2.10a). The name "coated pit" is derived from their appearance in the electron microscope as an accumulation of electron-dense material that represents aggregation of clathrin molecules on the cytoplasmic surface of the plasma membrane. Cargo receptors recognize and bind to specific molecules that come in contact with the plasma membrane. Clathrin molecules then assemble into a basketlike cage that helps change the shape of the plasma membrane at that site into a vesicle-like invagination (Fig. 2.10b). Clathrin interacts with the cargo receptor via another coating protein, adaptin, which is instrumental in selecting appropriate cargo molecules for transport into the cells. Thus, selected cargo proteins and their receptors are pulled from the extracellular space into the lumen of a forming vesicle. The large (100-kDa) mechanoenzyme (GTPase, also called dy-namin) mediates the liberation of forming clathrin-coated vesicles from the plasma membrane during receptor-mediated endocytosis. The type of vesicle formed as a result of receptor-mediated endocytosis is referred to as a coated vesicle, and the process itself is known as clathrin-dependent endocytosis. Clathrin-coated vesicles are also involved in the movement of the cargo material from the plasma membrane to endosomes and from the Golgi apparatus to the plasma membrane.

• Phagocytosis [Cr., cell eating] is the ingestion of large particles such as cell debris, bacteria, and other foreign materials. In this process, large vesicles (larger than approximately 250 nm in diameter) called phagosomes are produced. 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. 2.11a). 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

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