Histology of Muscle Tissue PDF

Histology of muscle tissue  OBJECTIVES a. Ultrastructure of Skeletal and smooth muscles. b. Ultrastructure of cardiac muscle.  Muscle tissue,is composed of cells that optimize the universal cell property of contractility. Three types of muscle tissue ca...

Histology of muscle tissue  OBJECTIVES a. Ultrastructure of Skeletal and smooth muscles. b. Ultrastructure of cardiac muscle.  Muscle tissue,is composed of cells that optimize the universal cell property of contractility. Three types of muscle tissue can be distinguished on the basis of morphologic and functional characteristics: 1. Skeletal muscle 2. Cardiac muscle 3. Smooth muscle The cytoplasm of muscle cells is often called sarcoplasm. the smooth ER is the sarcoplasmic reticulum.  the muscle cell membrane and its external lamina are the sarcolemma.  Skeletal Muscle Skeletal (or striated) muscle consists of muscle fibers,  which are long  cylindrical multinucleated cells  diameters of 10-100 μm,with cross-striations. Their contraction is quick, forceful, and usually under voluntary control. Elongated nuclei are found peripherally just under the sarcolemma, a characteristic nuclear location unique to skeletal muscle fibers/cells. A small population of reserve progenitor cells called muscle satellite cells remains adjacent to most fibers of differentiated skeletal muscle.  Organization of a skeletal muscle Thin layers of connective tissue surround and organize the contractile fibers in all three types of muscle, and these layers are seen particularly well in skeletal muscle. The concentric organization given by these supportive layers resembles that in large peripheral nerves: 1. The epimysium, an external sheath of dense irregular connective tissue, surrounds the entire muscle. Septa of this tissue extend inward, carrying the larger nerves, blood vessels, and lymphatics of the muscle. 2. The perimysium is a thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a fascicle. Each fascicle of muscle fibers makes up a functional unit in which the fibers work together. Nerves, blood vessels, and lymphatics penetrate the perimysium to supply each fascicle. 3. Within fascicles a very thin, delicate layer of reticular fibers and scattered fibroblasts, the endomysium, surrounds the external lamina of individual muscle fibers. In addition to nerve fibers, capillaries form a rich network in the endomysium bringing O2 to the muscle fibers.  Collagens in these connective tissue layers of muscle serve to transmit the mechanical forces generated by the contracting muscle cells/fibers; individual muscle fibers seldom extend from one end of a muscle to the other. All three layers plus the dense irregular connective tissue of the deep fascia, which overlies the epimysium, are continuous with the tough connective tissue of a tendon at myotendinous junctions which join the muscle to bone, skin, or another muscle.  Organization within muscle fibers Longitudinally sectioned skeletal muscle fibers show striations of alternating light and dark bands. The sarcoplasm is highly organized containing primarily long cylindrical filament bundles called myofibrils that run parallel to the long axis of the fiber. The dark bands on the myofibrils are called A bands; the light bands are called I bands.  Mitochondria and sarcoplasmic reticulum are found between the myofibrils. Myofibrils consist of an end-to-end repetitive arrangement of sarcomeres.the lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit a characteristic pattern of transverse striations. The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin myofilaments, composed of myosin and F-actin, respectively, organized within each myofibril in a symmetric pattern containing thousands of each filament type.  The thick myosin filaments are occupy the A band at the middle region of the sarcomere. Myosin is a large complex with two identical heavy chains and two pairs of light chains.  Myosin heavy chains are thin, rodlike motor proteins twisted together as myosin tails. Globular projections containing the four myosin light chains form a head at one end of each heavy chain. The myosin heads bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity).  Several hundred myosin molecules are arranged within each thick filament with overlapping rodlike portions and the globular heads directed toward either end. The thin, helical actin filaments are each 1.0-μm long and 8- nm wide and run between the thick filaments. Each G-actin monomer contains a binding site for myosin. The thin filaments have two tightly associated regulatory proteins. Tropomyosin. Troponin  I bands consist of the portions of the thin filaments which do not overlap the thick filaments in the A bands, which is why I bands stain more lightly than A bands. Actin filaments are anchored perpendicularly on the Z disc by the actin-binding protein α-actinin and exhibit opposite polarity on each side of this disc.  An important accessory protein in I bands is titin, the largest protein in the body, with scaffolding and elastic properties, which supports the thick myofilaments and connects them to the Z disc. Another large accessory protein,nebulin, binds each thin myofilament laterally, helps anchor them to α-actinin, and specifies the length of the actin polymers during myogenesis.  The A bands contain both the thick filaments and the overlapping portions of thin filaments. Close observation of the A band shows the presence of a lighter zone in its center, the H zone, corresponding to a region with only the rodlike portions of the myosin molecule and no thin filaments.  Bisecting the H zone is the M line, containing a myosin- binding protein myomesin that holds the thick filaments in place, and creatine kinase.  Despite the many proteins present in sarcomeres, myosin and actin together represent over half of the total protein in striated muscle. The overlapping arrangement of thin and thick filaments within sarcomeres produces in TEM cross sections hexagonal patterns of structures which were important in determining the functions of the filaments and other proteins in the myofibril.  Sarcoplasmic reticulum & transverse tubule system In skeletal muscle fibers the membranous smooth ER, called here sarcoplasmic reticulum, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils. the sarcolemma has tubular infoldings called transverse or T-tubules. These long fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm and encircle each myofibril near the aligned A- and I-band boundaries of sarcomeres.  Adjacent to each T-tubule are expanded terminal cisternae of sarcoplasmic reticulum. In longitudinal TEM sections, this complex of a T-tubule with two terminal cisternae is called a triad. The triad complex allows depolarization of the sarcolemma in a T-tubule to affect the sarcoplasmic reticulum and trigger release of Ca2+ ions into cytoplasm around the thick and thin filaments, which initiates contraction of sarcomeres.