^ The endoplasmic reticulum (ER) is a continuous system of cell membranes, which are about 6nm thick. Dependent on cell specialization and activity, the membranes occur in different forms, such as stacks or tubules. The ER double membranes may be smooth or have granules attached to their outer surfaces. These granules are about 25 nm in diameter and have been identified as membrane-bound ribosomes. Therefore, two types of ER exist: the granular or rough form ( rER, rough ER) and the agranular or smooth form (sER, smooth ER).
Paired multiplanar stacks of lamellae are one characteristic forms of rER. The membranes are narrowly spaced and spread over large parts of the cell. The two associated membranes in this matrix are 40-70 nm apart. When cells assume a storage function, these membranes move away from each other and thus form cisternae, with a lumen that may be several hundred nanometers wide.
Elaborate systems of rER membranes are found predominantly in cells that biosynthesize proteins (see Figs. 19, 22-25). Proteins, which are synthesized on membranes of the rER, are mostly exported from the cell. They may be secreted from the cell (including hormones and digestive enzymes, etc.) or become part of intracellular vesicles (membrane proteins). The smooth endoplasmic reticulum (see Figs. 26-29) eluded light microscopy. The cisternae of the endoplasmic reticulum interconnect both with the perinuclear cisternae (see Fig. 11) and the extracellular space. This picture shows ergastoplasm (rER) from an exocrine pancreas cell, which produces digestive enzymes.
Electron microscopy; magnification: x 60 000
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