Histology SSN 10/2006 Muscle (Skeletal, Smooth, Cardiac) Ting Zhou: email@example.com Muscles are responsible for movement of the body and for change in the size and shape of internal organs. Almost all muscle tissues are of mesodermal origin. Skeletal Muscles I. Location: a. Muscles of axial and appendageal skeleton, visceral skeletal muscles II. Organization a. Muscle fibers i. Run parallel and straight ii. Muscle tissue has many fasciculi iii. A fasciculus is a bundle of muscle fibers (muscle cells) 1. muscle fibers are really a syncytium of small myoblasts that fused to form a large multinucleated cell iv. A myofiber is comprised of myofibrils 1. in cross-section, the myofibrils are arranged in polygonal structure v. Myofibrils are bundles of myofilaments vi. Thick filaments are myosins, and thin filaments are actins. b. Connective Tissue i. Endomysium: - loose connective tissue that surrounds each muscle fiber - reticular fibers - smallest capillaries and neuronal branches ii. Perimysium: - connective tissue + fat + blood vessels and nerves - group muscle fibers into fascicles iii. Epimysium: - dense connective tissue that surrounds muscle groups - large blood vessels and nerves penetrate this layer to reach muscle fibers http://training.seer.cancer.gov/module_anatomy/images/illu_muscle_structure.jpg III. Characteristics a. Shape: i. Single, large, elongated cell b. Nucleus: i. Polynucleated, peripherally displaced c. T Tubule System: i. Present in a triad with two adjacent terminal cisternae of the sarcoplasmic reticulum d. Contractile Apparatus i. Arranged in longitudinal arrays, giving rising to the striated look ii. Myofilaments 1. myosin and actin iii. Contractile unit 1. sarcomere – the portion of a myofibril between two adjacent Z lines. iv. Striation – alternate dark and light bands under microscope 1. A band: stained dark, are the myosin filaments 2. I band: stand light, are the actin filaments not overlapping myosin 3. H band: myosin not overlapping actin 4. Z disk: disk in the center of I band from which actin anchors and extends 5. M line: myomesin and C protein, function to anchor the myosin 2+ IV. Ca regulation of contraction a. At the level of the thin filaments – by binding to troponin and changing the configuration of the thin filaments to expose myosin binding sites V. Histology Slides: a. Muscle Tendon Junction i. Tendon: 1. lightly stained in H&E 2. homogeneous 3. nuclei are sparse and squished in between sheets of collagen ii. Muscle: 1. striated!!! 2. more basophilic due to the abundance of rER 3. euchromatous nuclei (more darkly stained) b. Upper esophagus i. Skeletal muscle can only be found in the upper and middle portion of the esophagus ii. When you see both skeletal muscle and smooth muscle together in one slide, think esophagus! c. Tongue i. When you see skeletal muscle appearing with glandular tissue, think tongue! Cardiac Muscle I. Location: a. Heart, pulmonary veins, superior and inferior vena cava II. Characteristics: a. Shape: i. Short, narrow cells that exhibit branching b. Nucleus: i. Mononucleated, centrally placed c. T Tubule System: i. Present in a diad with small terminal cisternae of the sarcoplasmic reticulum, pairing is much less extensive than skeletal muscle d. Contractile Apparatus i. Similar to skeletal muscle ii. Intercalated Disks 1. High specialized attachment sites between adjacent cardiac cells 2. Fascia adherens (adhering junctions) a. The major constituent of the transverse component of the intercalated disk, and is responsible for its staining in routing H&E preparations 3. Maculae adherents (desmosomes) a. Bind the individual muscle cells to one another, help prevent the cells from pulling apart under the strain of regular repetitive contractions, serves to reinforce the adhering junctions 4. Gap junctions a. Constitute the major structural element of the lateral component of the intercalated disk, provides ionic continuity between adjacent cardiac muscle cells 2+ VI. Ca regulation of contraction a. At the level of the thin filaments b. Also exhibit intrinsic rhythm – does not need ACh or norepinephrine for contraction Smooth Muscle I. Location a. Blood vessels, ducts, GI system, respiratory system, urogenital system, etc II. Characteristics: a. Shape: i. Short, elongate fusiform cells b. Nucleus: i. Mononucleated, centrally placed c. T Tubule System: i. None! Smooth muscle cells have a membrane system of sarcolemmal 2+ invaginations that are analogous to T system to deliver Ca to the cytoplasm d. Contractile Apparatus i. Myofilaments: 1. actin and myosin ii. Lack of striation 1. myofilaments are not regularly arranged, instead, they are oriented obliquely to the long axis of the cell 2. actin filaments attach to sarcolemma via dense bodies, which function as Z disk analogues iii. Gap junctions are present for united contraction 2+ III. Ca regulation of contraction a. At the level of the thick filament 2+ b. Ca -calmodulin complex leads to the phosphorylation of myosin, leading to contraction IV. How to differentiate smooth muscle vs. dense irregularly-arranged connective tissue a. Color i. Smooth muscle is more purple because of combination of actin/myosin and RNA ii. Connection tissue is more red because of the purity of the collagen b. Nuclei i. Smooth muscle nuclei are inside the cell ii. Fibroblast nuclei are in between collagen fibers c. Homogeneity i. Smooth muscle is composed only of muscle ii. Connective tissue contains fibroblasts, macrophages, lymphocytes, and will therefore have a ragged appearance Muscle Type Skeletal Smooth Cardiac Location Muscles of skeleton, Viscera (GI system, Heart (duh!), visceral striated respiratory system, Superior and inferior (tongue, esophagus, etc), blood vessels, vena cava, and diagraphm) organs pulmonary veins Nucleus Polynucleated, Single central Single central peripheral Type of Voluntary Involuntary Involuntary Innervation Striation Yes No Yes Shape Long and elongate Short and elongate Short with branching Practice Questions: 1) Which of the following statements is FALSE? a. The band marked “A” does not include actin filaments. b. The M line is found in the band marked “A”. c. During contraction, the length of the band marked “A” remains the same. d. During contraction, the length of the band marked “B” is shortened. 2) Which of these statements is FALSE about the muscle type at the pointer? a. The cells in this muscle type are electrically coupled. b. This muscle type utilizes a T-tubule system for calcium delivery. c. This muscle type is not capable of regeneration. d. Calcium regulation occurs at the level of thick filaments. 3) Which figure(s) demonstrates smooth muscle? a. Figure A&C b. Figure A&D c. Figure B&C d. Figure B&D 4) The structure at the pointer is composed of all of the following except: a. desmosomes b. tight junctions c. gap junctions d. fascia adherens 5) The structure at the pointer is a: 2+ a. site of calcium-binding protein (calsequestrin) and a Ca -activated-ATPase. b. site of glycogen storage and degradation to glucose 2+ c. site from while Ca is passively transported during muscle relaxation d. site into which calcium is released during muscle relaxation e. form of rough endoplasmic reticulum MUSCLE ANSWERS 1) a. This is skeletal muscle. A marks the A band and B marks the I band. The A band consists of myosin and some actin that overlaps the myosin. Thick filaments of myosin anchor at the M line, which is found in the middle of the H zone. The figure below demonstrates how the length of the A band stays the same during contraction, while the I band gets shorter. 2) d. This is cardiac muscle. The cells are electronically coupled via gap junctions, thus allowing contractile signals to pass from cell to cell and allowing cardiac muscle to behave as a syncytium. T tubules are used for calcium delivery in both skeletal and cardiac muscle. Smooth muscle cells have no T tubule system. Cardiac muscle is not capable of regeneration (mature cardiac muscle cells do not divide). If injury to cardiac muscle tissue leads to death of cells, fibrous connective tissue forms (scarring) with consequent loss of cardiac function. Since tropomyosin and troponin wind around the actin filaments, calcium regulation does not occur at the level of the thick filament. 3) b. A is smooth muscle and B is skeletal muscle (from the esophagus). C is an EM of skeletal muscle and D is an EM of smooth muscle. Skeletal muscle has striations whereas smooth muscle does not. The EM in Figure C shows a higher magnification of the H band, A band, I band, and Z line in a striated muscle fiber. Figure C also shows a triad of T tubules and flanking terminal cisternae, which is only found in skeletal muscle. Whereas skeletal muscle has multiple, peripherally-located nuclei, smooth muscle has a single, centrally located nucleus, as seen in Figure D. 4) b. Pointer is at an intercalated disc. An intercalated disc does not have tight junctions. The desmosomes are on both the transverse and lateral components of the intercalated disc, while the fasciae adherens are on the transverse. 5) a. This is an EM of skeletal muscle. The pointer is at the sarcoplasmic reticulum. The sarcoplasmic reticulum (SR) provides a mechanism for the muscle cell to regulate the concentration of cytosolic calcium. It is a modified smooth endoplasmic reticulum that serves alternatively as a storage site and a source of cellular calcium. Calcium is 2+ actively transported from the cytosol to the SR through the activity of a Ca - dependent ATPase. Calsequestrin is a calcium-binding protein found in the SR that, as it’s name implies, functions in the sequestion of calcium. Calcium is released from the SR during muscle contraction and stored during relaxation. Glycogen is stored as particles or droplets in the cytoplasm, which contains the enzymes required for the synthesis and breakdown of glycogen.
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