Figure 231

Electron micrograph of the Golgi apparatus. This electron micrograph shows the extensive Golgi apparatus in an islet cell of the pancreas. The flattened membrane sacs of the Golgi apparatus are arranged in layers. The c/s-Golgi network (CGN) is represented by the flattened vesicles on the outer convex surface, whereas the flattened vesicles of the inner convex region constitute the frans-Golgi network (TGN). Budding off the TGN are several vesicles (1). These vesicles are released (2) and eventually become secretory vesicles (3). x55,000.

• Endosomes or lysosomes. Most of the proteins bearing M-6-P markers are destined for these sites (see page 28).

• Apical cytoplasm. Proteins that were aggregated or crystallized in the TGN because of changes in pH and Ca2+

concentration are stored in large secretory vesicles. These vesicles eventually fuse with the plasma membrane to release the secretory product by exocytosis. This type of secretion is characteristic of secretory cells found in exocrine glands.

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