Skeletal muscle structure development

In striated muscles, SR is well developed to surround the myofibrils and is divided into two parts, the terminal cisternae (TC) and longitudinal tubules (LT). TC forms triad (skeletal muscle) or dyad (heart) structure with transverse tubules. The ryanodine receptor is located only in the TC, whereas the Ca2+ pump/SERCA is densely packed in both TC and LT. 

Figure 1. Muscle development. A skeletal muscle fiber is formed by the fusion of many single cells (myoblasts) into a multinucleated myotube. Myotubes then develop into the muscle fiber (see text). Sarcomeres form in longitudinal structures called myofibrils. The repeating structure of the sarcomere contains interdigitating thick and thin filaments.

A variety of other calcium transport systems are associated with Ca21-activated ATPases. The extraembryonic structure, the chorioallantoic membrane, of the chick embryo is responsible for the translocation of over 120 mg of eggshell calcium into (he embryo during development. The enzyme responsible for this is a (Ca2+, Mg2+)-ATPase with Km values for Ca2+ of 30 p,mol dm-3 and 0.3 mmol dm-3, and a molecular weight of 170 000. The enzyme can be crossiinked and co-isolated with a calcium-binding protein.158 Transport of Ca2+ is also associated with (Ca2+, Mg2+)-ATPases in neutrophil plasma membranes,159 transverse tubule membranes from rabbit skeletal muscle,160 rabbit myocardial membrane,161 endoplasmic reticulum,162 sar-colemma,163 brain microsomes,164 the Golgi apparatus165 and rat liver plasma membranes.166... 

The receptor for CNTF has recently been cloned (Davis et al., 1991) and appears to be exclusively expressed in the nervous system and in skeletal muscle. In contrast to other known receptors, the receptor for CNTF is anchored to the plasma membrane by a glycosyl-phospha-tidylinositol linkage (Davis et al., 1991). Its primary structure is most similar to the IL-6 receptor. Furthermore, both CNTF and the structurally related leukemia-inhibitory factor (see above) use the IL-6 signal transducer gpl30 (Ip et al., 1992). These observations raise the possibility that the receptors for CNTF, leukemia-inhibitory factor and hematopoietic cytokines are able to interact with each other and to activate related signaling pathways in diverse cell types (Bazan, 1991 Davis and Yancopoulos, 1993). It has been shown by Lillien et al. (1990) that some biological functions of CNTF, e.g. induction of type-2 astrocyte development, require cooperation with as yet unknown ECM-associated molecule(s). 

The neurotoxicity of acrylamide in humans is well known from occupational and accidental exposures [51], For instance, Calleman et al. [52] reported peripheral neuropathy symptoms to highly exposed workers in China. It is characterized by skeletal muscle weakness, numbness of hands and feet, and ataxia. Acrylamide has been shown to be toxic to both the central and the peripheral nervous system [53], although the nerve terminal is now considered to be the primary site of acrylamide action [54,55]. Acrylamide induces nerve terminal degeneration and has effects on the cerebral cortex, thalamus, and hippocampus [56]. In double-blind studies of factory workers, no neurotoxicity was found in workers exposed to less than 3.0 mg/kg/ day as determined by biomonitoring [55]. A very recent study demonstrates structural and ultra structural evidence of neurotoxic effects of fried potato chips on rat postnatal development. 

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