Muscle organelles found

Muscle cells contain many mitochondria which are often present as reticulum-like structures extending longitudinally in the fiber near the sarcolemma, rather than as discrete ellipsoidal organelles found in many other cells. These provide much of the high-energy phosphate needed to power contraction and to operate the Ca + pumps that control the cytosolic calcium concentration. Different types of skeletal muscle fibers differ considerably in the extent and organization of both their SR and mitochondria. 

As further tissues were examined it became evident that the details of mitochondrial morphology were very variable. While most cells had rod-or sausage-shaped organelles, some were spherical. Other cells had mitochondria with spiral cristae or with massive crystalline inclusions. In confirmation of earlier suggestions from classical microscopists the position of mitochondria in cells was also seen to be linked with the site in the cell where energy was required. In skeletal muscle the mitochondria were adjacent to the myofibrils in the renal tubules they were close to the inner (non-luminal) surface of the cell which was then found to be the location of the Na/K-ATPase involved in active... 

The principal cytoskeletal proteins in non-muscle cells are actin, tubulin, and the components of intermediate filaments. Actin can exist either as monomers ( G-actin ) or polymerized into 70 A diameter double filament ( F-actin ). Polymerized actin usually is localized at the margins of the cells, linked by other proteins to the cell membrane. In contrast, tubulin forms hollow filaments, approximately 250 A in diameter, that are distributed within a cell in association, generally, with cell organelles. Stabilized microtubule structures are found in the flagella and cilia of eucaryotic cells however, in other instances - examples being the mitotic apparatus and the cytoskeletal elements arising in directed cell locomotion - the microtubules are temporal entities. Intermediate filaments, which are composed of keratin-like proteins, are approximately 100 A thick and form stable structural elements that impart rigidity, for example, to nerve axons and epithelial cells. 

There are several self-assembling macromolecules that are of interest to us in this text. They include (1) collagen, the primary structural material found in the extracellular matrix (2) actin, a component of the cell cytoskeleton that is involved in cell locomotion and in formation of the thin filaments of muscle (3) microtubules, which are involved in cell mitosis, movement, and organelle movement and finally (4) fibrinogen, which forms fibrin networks that minimize bleeding from cut vessels. Self-assembly is important in these systems because the function of these macromolecules can be modified via processes that increase the molecular axial ratio and hence decrease the solubility. 

Later the same approach was applied to study heart muscle mitochondria that represent mainly spherical bodies. It was found that these organelles form electrically conductive intermitochondrial contacts. As a result, heart mitochondria can be united to clusters composed of tens spherical organelles (we coined them Streptio mitochondriale). Both mitochondrial filaments and clusters were assumed to be used by the cell to transmit ApH+ inside the cell [4-6]. 

Lipids occurring in plant and animal materials consist of structural lipids, which build the cell membranes, and depot fats. The cell and organelle membranes of animal organisms are made of phospholipids and non-esterified cholesterol, whereas in plants they consist of phospholipids and glycolipids. The latter are also found in the central nervous systems of some animals. If a muscle tissue like that in lean fish contains only 0.3% w/w of lipids, they consist almost entirely (90%) of phospholipids. Galactoglycerols and phospholipids serve as important factors in nutrient and antioxidant delivery systems (Herslof, 2000). 

Because phospholipids are typically the other main lipid class found in fish flesh, the leaner the fish and the higher the proportion that phospholipids contribute to total lipids. For this reason, phospholipids comprise almost 90% of total lipids of lean fish such as cod, with TAG contributing as little as about 1%. Due to anatomical and physiological reasons, the amount of structural lipids (phospholipids) varies between 0.3 and 0.5 per 100 g w/w of fish muscle and does not usually exceed 1% w/w. This is most likely the minimum level of phospholipids essential for the cell and organelle membranes, of which they are a major component. Phospholipids are the major lipid class in most Australian fish, and in mollusks and crustaceans, all of which are typically lean (Table 12.3). In contrast to finfish, which tend to store lipid as TAG, an increase in the lipid content of shellfish is usually due to an accumulation of polar lipids (Nichols et al., 1998). This is similar to the case in Antarctic krill Euphausia superba D.), the phospholipids of which serve as storage lipids along with TAG. 

In untreated diabetes, the peripheral muscles are, like the liver, depleted of glycogen. In contrast, glycogen has been found in the hearts of diabetics. However, on the basis of histochemical and chemical analyses, researchers concluded that there was no correlation between the amount of glycogen deposited in the heart and the severity of the diabetes. The presence of glycogen in the liver nuclei has been contested by Byrd and Fischer. These authors believe that the vacuolization observed in that cellular organelle results from fluid imbalance rather than from the accumulation of glycogen. 

When C-labeled lipoic acid is injected in the rat intraperitoneally or administered per os, the label is found to a large extent in the urine, associated to small amounts of lipoic acid and to partly identified cataboli-tes (7 ). Part of the radioactivity has been found associated with tissues, mainly liver and muscle. Lipoic acid is found to be metabolized inside the mitochondria, and must therefore be taken up by these organelles (8 ). Ad-... 

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