Electron micrograph of a nerve cell body. This image shows profiles of rER as well as numerous free ribosomes located between the membranes of the rER. Collectively, the free ribosomes and membrane-attached ribosomes are responsible for the characteristic cytoplasmic basophilia (Nissl bodies) observed in the light microscope in the perinuclear cytoplasm of neurons. x45,000.
contain RNA; it is the phosphate groups of the RNA of the ribosomes, not the membranous component of the endoplasmic reticulum, that accounts for basophilic staining of the cytoplasm.
Smooth-Surfaced Endoplasmic Reticulum sER consists of short anastomosing tubules that are not associated with ribosomes
Cells with large amounts of smooth endoplasmic reticulum (sER) may exhibit distinct cytoplasmic eosinophilic
(acidophilia) when viewed in the light microscope. It is biochemically similar to the rER but lacks the ribosome-docking proteins. sER tends to be tubular rather than sheet-like, and it may be separate from the rER or an extension of it. The sER is abundant in cells that function in lipid metabolism, and it proliferates in hepatocytes when animals are challenged with lipophilic drugs. The sER is well developed in cells that synthesize and secrete steroids, such as adrenocortical cells and testicular Leydig (interstitial) cells (Fig. 2.29). In skeletal and cardiac muscle, the sER is also called the sarcoplasmic reticulum. It sequesters Ca2+ that is essential for the contractile process and is closely apposed to the plasma membrane invaginations that conduct the contractile impulses to the interior of the cell.
sER is the principal organelle involved in detoxification and conjugation of noxious substances
The sER is particularly well developed in the liver and contains a variety of detoxifying enzymes related to cytochrome P450 that are anchored directly into sER plasma membranes. The degree to which the liver is involved in detoxification at any given time may be estimated by the amount of sER present in liver cells. The sER is also involved in
• Lipid and steroid metabolism
• Glycogen metabolism
• Membrane formation and recycling
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