Golgi Apparatus(redirected from Trans Golgi network)
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An organelle, named after the Italian histologist Camillo Golgi, found in all eukaryotic cells but absent from prokaryotes such as bacteria. It consists of flattened membrane-bounded compartments known as cisternae. In most cells, the Golgi cisternae are organized into stacks. Different cell types contain from one to several thousand Golgi stacks. The Golgi apparatus sorts newly synthesized proteins for delivery to various destinations, and modifies the oligosaccharide chains found on glycoproteins and glycolipids. See Cell organization
The Golgi apparatus acts at an intermediate stage in the secretory pathway. A subset of the proteins synthesized by the cell are inserted into the endoplasmic reticulum. Most such proteins are then delivered to the Golgi apparatus by means of coat protein II (COPII) transport vesicles, which form at endoplasmic reticulum exit sites. Newly synthesized proteins traverse the Golgi stack until they reach the trans-most Golgi compartment, which is termed the trans-Golgi network to connote its extensive tubulation. The trans-Golgi network sorts the proteins into several types of vesicles. Clathrin-coated vesicles carry certain proteins to lysosomes. Other proteins are packaged into secretory vesicles for immediate delivery to the cell surface. Still other proteins are packaged into secretory granules, which undergo regulated secretion in response to specific signals. This sorting function of the Golgi apparatus allows the various organelles to grow while maintaining their distinct identities. See Cell membranes, Endoplasmic reticulum, Lysosome
The best understood of the processing reactions carried out by the Golgi apparatus is the remodeling of oligosaccharides (chains of six-carbon sugars) that are attached to glycoproteins. During insertion of a newly synthesized protein into the endoplasmic reticulum, one or more copies of a 14-sugar oligosaccharide may be attached to the amino acid asparagine at specific locations in the polypeptide chain. As the protein passes through the Golgi stack, the asparagine-linked oligosaccharides are modified to generate a diverse range of structures. Additional oligosaccharides may become linked to the amino acids serine and threonine. Although the particular oligosaccharide modifications are quite different in animal, plant, and fungal cells, the Golgi apparatus always functions as a “carbohydrate factory.” See Oligosaccharide
The Golgi apparatus also carries out other processing events, including the addition of sulfate groups to the amino acid tyrosine in some proteins, the cleavage of protein precursors to yield mature hormones and neurotransmitters, and the synthesis of certain membrane lipids such as sphingomyelin and glycosphingolipids. See Lipid, Protein
an intracellular reticular structure; the organelle associated with the synthesis of various inclusion bodies. Named for the Italian histologist C. Golgi.
The Golgi apparatus has a highly variable form; its appearance under the microscope varies from that of a complex netlike structure (a compact form occurring in vertebrates) to that of individual sinuous or rodlike bodies (a dispersed form, called a dictyosome, occurring in plants and invertebrates). Investigations with the electron microscope have shown that the Golgi apparatus comprises three elements: a system of (five to eight) flat cisternae (γ-membranes, 70–80 angstroms thick), vesicles (300–500 angstroms in diameter), and large vacuoles (0.2–0.3 microns in diameter). The Golgi apparatus is located near the nucleus or around the centrosome. On fixed material, it is revealed by impregnation with silver salts or osmic acid. The observation of living cells has confirmed the existence of the Golgi apparatus. The organelle contains lipids, lipoproteins, and phospholipids, but is relatively low in enzymes. The function of the Golgi apparatus is involved with the production of the various formed elements of the cell’s vital activities, such as secretory granules and vitelline lamellae, collagen, lipid, and glycogen inclusions, and melanosomol granules. Deposits of vital dyes, ascorbic acid, iron salts, and colloid silver are observed in the Golgi region. The activity of the Golgi apparatus is also implicated in the production of primary lysosomes. the acrosomes of spermatozoa, and the lamellae of plant cells. The role of the various components of the Golgi apparatus in these processes differs; the vesicles and cisternae are regarded as the functional parts of the Golgi apparatus, while the vacuoles only serve in the transport of substances. The dynamics of the incorporation of tagged amino acids confirm the fact that the Golgi apparatus is the final link in an intracellular “conveyor” that produces a protein secretion. The proteins synthesized on the ribosomes enter the Golgi apparatus through the cisternae of the endoplasmic reticulum, where they are concentrated and packed into granules. In the synthesis of protein products, the Golgi apparatus— the “packing plant”—forms the secretory granules. In the production of carbohydrates (mucopolysaccharides, glycoproteins), the intracellular “conveyor” in effect switches over to another product and the Golgi apparatus participates in the synthesis of polysaccharides. In mucus, bone, and cartilage cells, tagged sugar molecules are first incorporated into the Golgi apparatus and appear in other cell components only later. Sulfation, which is associated with the synthesis of acid mucopolysaccharides, also takes place in the Golgi apparatus.
REFERENCESRukovodstvo po tsitologii, vol. 1. Moscow-Leningrad, 1965. Pages 185–200.
Alov. I. A., A. I. Braude. and M. E. Aspiz. Osnovy funklsionainoi morfologii kletki. Moscow, 1966. Pages 82–113.
I. A. ALOV