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endocytosis (ĕnˌdōsītōˈsəs), in biology, process by which substances are taken into the cell. When the cell membrane comes into contact with a suitable food, a portion of the cell cytoplasm surges forward to meet and surround the material and a depression forms within the cell wall. The depression deepens and the movement of the cytoplasm continues until the food is completely engulfed in a pocket called a vessicle. The vessicle then drifts further into the body of the cell where it meets and fuses with a lysosome, a vessicle normally found in the cell that contains digestive enzymes known as acid hydrolases. The food is then broken down into molecules and ions that are suitable for the cell's use. There are two types of endocytosis: pinocytosis, the engulfing and digestion of dissolved substances, and phagocytosis, the engulfing and digestion of microscopically visible particles. Phagocytosis is the process by which many protozoans obtain most of their food supply. It is also the process through which specialized cells in animals eliminate foreign matter, such as infecting microorganisms, as part of the body's defense system (see blood; immunity). The various phagocytic cells in higher animals are derived from relatively unspecialized cells called stem cells that are either fixed within a network of supporting (reticular) cells and fibers of the spleen, thymus, and bone marrow, or that wander freely throughout body tissues. Many phagocytic cells respond chemically to substances produced by foreign bodies or by degenerating tissue by moving toward the substances, a mechanism known as chemotaxis. When a particle of the proper charge or chemical composition adheres to the cell surface, the cell cytoplasm moves so that it finally surrounds the particle and traps it within a cytoplasmic vacuole. Various enzymes are then secreted into the vacuole to digest the foreign substance. In higher animals each phagocyte can ingest about 5 to 25 invading bacterial cells. Phagocytosis often precedes production of antibodies by the body, but some species of bacteria cannot be phagocytized unless specific antibody is already present. Although phagocytosis is an effective response to infection, some organisms, such as the bacteria causing brucellosis and tuberculosis, can survive for years within the descendant cells of the phagocytes that ingested them. The process of phagocytosis was first described in the late 19th cent. by the Russian zoologist Élie Metchnikoff.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the engulfing by the cell surface of fluid and substances that the fluid contains. Pinocytosis is one of the principal mechanisms by which high-molecular-weight compounds enter the cell, particularly proteins and carbohydrate-protein complexes. The phenomenon of pinocytosis was discovered by the American scientist W. Lewis in 1931. During pinocytosis short, slender outgrowths on the plasma membrane surround a drop of fluid. Then the region of the plasma membrane that contains these outgrowths invaginates and pinches off to form a bubble inside the cell. The formation of pinocytotic bubbles, whose diameters do not exceed 2 microns (μ), has been traced by phase-contrast microscopy and microcinematography. With the aid of the electron microscope, bubbles with diameters that range from 0.07 to 0.1 μ can be discerned. Pinocytotic bubbles are capable of freely moving within the cell or of merging with each other or with other intracellular membranous structures. The most active pinocytosis is observed in amoebas, in epithelial cells of the intestines and renal tubules, and in the endothelium of blood vessels and growing oocytes. The level of pinocytotic activity depends on the physiological state of the cell and on the environment. Active inducers of pinocytosis are γ-globulin, gelatin, and certain salts.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(cell and molecular biology)
Deprecated term formerly used to describe the process of uptake or internalization of particles, macromolecules, and fluid droplets by living cells; the process is now termed endocytosis.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.