Figure 230

Photomicrograph of plasma cells. This photomicrograph of a plastic-embedded specimen showing the lamina propria of the small intestine is stained with toluidine blue. The plasma cells, where appropriately oriented, exhibit a clear area in the cytoplasm near the nucleus. These negatively stained regions (arrows) represent extensive accumulation of membranous cisternae that belong to the Golgi apparatus. The surrounding cytoplasm is deeply metachromaticaliy stained because of the presence of ribosomes associated with the extensive rER. x 1,200.

a series of stacked, flattened, membrane-limited sacs or cisternae and tubular extensions embedded in a network of microtubules near the microtubule-organizing center (MTOC) (see page 53). Small vesicles involved in vesicular transport are seen in association with the cisternae. The Golgi apparatus is polarized both morphologically and functionally. The flattened cisternae located closest to the rER represent the forming face, or cis-Golgi network (CGN); the cisternae located away from the rER represent the maturing face, or trans-Golgi network (TGN) (Figs. 2.31 and 2.32). The cisternae located between the TGN and CGN are commonly referred as the medial Golgi.

The Golgi apparatus functions in the posttranslational modification, sorting, and packaging of proteins

Small vesicles called transport vesicles carry newly synthesized proteins (both secretory and membrane) from the rER to the CGN. From there, they travel within the transport vesicles from one cisterna to the next. The vesicles bud from one cisterna and fuse with the adjacent cisternae. As proteins and lipids travel through the Golgi stacks, they undergo a series of posttranslational modifications that involve remodeling of N-linked oligosaccharides previously added in the rER.

In general, glycoproteins and glycolipids have their oligosaccharides trimmed and translocated. Glycosylation of proteins and lipids uses several carbohydrate-processing enzymes that add, remove, and modify sugar moieties of oligosaccharide chains. M-6-P is added to those proteins destined to travel to late endosomes and lysosomes (page 28). In addition, glycoproteins are phosphorylated and sulfated. The proteolytic cleavage of certain proteins is also initiated within the cisternae (Fig. 2.33).

Four major pathways of protein secretion from the Golgi apparatus disperse proteins to various cell destinations

As noted, proteins exit the Golgi apparatus from the TGN. This network and the associated tubulovesicular array serve as the sorting station for shuttling vesicles that deliver proteins to various locations (Fig. 2.34). These locations are

• Basolateral plasma membrane. Proteins targeted to the basolateral domain have a specific sorting signal attached to them by the TGN. This constitutive pathway uses clathrin-coated vesicles with an epithelium-specific adaptor protein; the transported membrane proteins are continuously incorporated into the basolateral cell surface.

• Apical plasma membrane. Many extracellular and membrane proteins are delivered to this site. This pathway represents another example of constitutive secretion. Secretory proteins also use clathrin-coated vesicles for their transport to the apical surface, which is mediated by specific carbohydrate-lecithin interactions.

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