Pathology of Muscle

NORMAL SKELETAL MUSCLE: C muscle cell = myocyte = muscle fiber = myofiber. C embryologically skeletal muscles are derived from paraxial mesoderm: P muscles of the back, body wall & limbs are derived from the myotome (ie part of mesodermal somites) & muscles of the head are from cranial paraxial mesoderm that migrates into the branchial arches. P the mesoderm differentiates into myoblasts ÿ align and fuse together to make longer, multinucleated tubes called myotubes ÿ myotubes synthesize myofilament proteins (begin to see cross striations on LM) ÿ continued differentiation to form functional myofilaments and nuclei displaced against the sarcolemma. C the organization of muscle from gross to microscopic is as follows: i) a muscle seen grossly is surrounded by dense fibrous connective tissue which consists of external deep fascia that blends internally with the epimysium. ii) thin septa from the epimysium, extend into the muscle and divide it into fascicles (groups) of myofibers; this is called the perimysium. iii) individual myofibers are surrounded by endomysium which consists of a external basal lamina produced by the myofibers and some extracellular matrix from fibroblasts. iv) a major role of this connective tissue arrangement is to transfer the mechanical force of the contracting muscle cells, because individual myofibers rarely extend the entire length of the muscle. C skeletal myofibers are non-branching, elongate, cylindrical, multi-nucleated cells (100-300 nuclei/cell) which can range from 1 mm to ~10 cm’s in length and 10-100 :m in diameter. C the multiple nuclei of the myofibers are arranged at the periphery of the cell beneath the sarcolemma. C the sarcoplasm (myofiber cytoplasm) contains abundant myofibrils (~ 60% of volume; which are long cylindrical filamentous bundles, 0.5 to 1 um in diameter, which lie parallel to the long axis of the cell), sarcoplasmic (endoplasmic) reticulum, glycogen +/fat droplets, mitochondria and lysosomes. C the sarcoplasmic reticulum has a special configuration, which along with the T-tubules (invaginations of the sarcolemma) allow for rapid simultaneous release of Ca throughout the myofiber, for uniform contraction. 2+