Histology of Muscle Tissue PDF

“Histology of Muscle Tissue” Prof./ Sahar Khalil Objectives: By the end of the lecture, you should: 1. List characteristic features of muscular tissue 2. Identify different types of muscles 3. Describe LM & EM structure of different types of muscles (skeletal & smooth) 4. Differentiat...

“Histology of Muscle Tissue” Prof./ Sahar Khalil Objectives: By the end of the lecture, you should: 1. List characteristic features of muscular tissue 2. Identify different types of muscles 3. Describe LM & EM structure of different types of muscles (skeletal & smooth) 4. Differentiate between LM & EM structure of different types of muscle fibers (skeletal & smooth) Muscle Terminology: Muscle fiber: muscle cell Sarcolemma: the plasma membrane of muscle cell Sarcoplasm: the cytoplasm of muscle cell Sarcoplasmic reticulum: the smooth endoplasmic reticulum Types of Muscle Tissue 1. Skeletal muscle 2. Cardiac muscle 3. Smooth muscle Objectives: By the end of the lecture, you should: 1. List characteristic features of muscular tissue 2. Identify different types of muscles 3. Describe LM & EM structure of different types of muscles (skeletal & smooth) 4. Differentiate between LM & EM structure of different types of muscle fibers (skeletal & smooth) Skeletal Muscle Striated (cross-striations present), Voluntary Site: attached to the skeleton, tongue and pharynx. Shape: long, parallel, cylindrical fibers of almost uniform diameter (~100 µm), non-branching fibers except in tongue and face). LM structure: Nucleus: Multiple, flattened oval or rod-shaped, peripherally situated under the sarcolemma Skeletal Muscle LM structure: Sarcoplasm: deeply acidophilic. o L.S.: Transverse striations of alternating light (I) and dark (A) bands. o T.S.: Rounded or polygonal muscle Fibers surrounded by a B.M, Myofibrils appear as dark acidophilic dots arranged in groups (Cohenheim areas) Satellite cells found between the sarcolemma and the B.M. Transverse section Skeletal Muscle (LM structure) L.S.: Transverse (cross) striations of alternating light (I) and dark (A) bands Each (A) band is bisected by pale disc called H zone Each (I) band is bisected by dark line called (Z) line (A) band (H) zone nuclei Z line (I) band Skeletal Muscle (EM structure) Organoids: 1. Myofibrils 2. Transverse tubules (T-tubules) 3. Sarcoplasmic reticulum (sER) The Traid 4. Mitochondria Inclusions: 1. Myoglobin 2. Glycogen Skeletal Muscle (EM structure) Myofibrils: thread-like myofilaments of 2 types (thick and thin). Thick filaments (myosin protein): M line - Present in (A) band, - Each myosin molecule resembles a “golf club”. - Myosin molecules arranged in an antiparallel way. I-band A-band I-band Z line Z line Z line Myosin filaments formed of 2 strands: 1. Heavy meromyosin (includes head and neck) 2. Light meromyosin (includes tail)  Thin filaments: - Present in (I) band and (A) band, - Composed of actin protein, tropomyosin and troponin. - Actin filaments arranged longitudinally, one of their ends attached to Z line and the other end interdigitating freely in between myosin filaments. - Each actin filament is composed of: ❖ G-actin molecules, ❖ Tropomyosin, ❖ Troponin molecules ( 3 subunits: Troponin I, Troponin C and Troponin T) Sarcomere: ✓ Distance between 2 successive (Z) lines is named a “Sarcomere” ✓ “Sarcomere” is the contractile unit of the skeletal muscle fiber. ✓ “Sarcomere” includes whole A-band and ½ of I-band on each side Sarcomere: Desmin ✓ Alignment of myofibrils results in a characteristic banding (alternating dark & light bands). ✓ The alignment is done by intermediate filament called desmin; that tether Z disks of adjacent myofibrils together. Skeletal Muscle (EM structure) Transverse tubules (T-tubules): Invaginations of the sarcolemma at the level of A-IA-I junction junction. Their lumina are continuous with the extracellular space Function: conduction of depolarization waves to all parts of the sarcoplasmic reticulum resulting in uniform A-I junction contraction of skeletal muscle fibers. Z line Sarcoplasmic reticulum (sER): It is the sER. Present between 2 successive T- tubules. Formed of terminal cisternae and sarco-tubules. The triad: Formed of one T-tubule and 2 terminal cisternae Function: play a pivotal role in muscle contraction. Skeletal Muscle (EM structure) Mitochondria Plenty of mitochondria. Arranged peripherally & in-between the myofibril. Z line Skeletal Muscle (EM structure) Inclusions: Myoglobin: An oxygen binding protein pigment. Responsible for the red colour of some muscle. It takes up O2 from blood and stores it in the muscle fibers. According to the concentration of myoglobin, the muscle fibers are classified into 3 types: 1. Red fibers (small diameter, abundant amount of myoglobin, numerous mitochondria, slow contraction & almost non-fatigable) 2. White fibers (large diameter, less myoglobin, fewer mitochondria, fast contraction only for short period & fatigued rapidly) 3. Intermediate fibers.  Red fibers Intermediate White fibers Fiber size Small in diameter Intermediate Large in diameter Myoglobin content Large amount of Intermediate Less amount of myoglobin. myoglobin. Mitochondria Many mitochondria. Intermediate Few mitochondria. Oxidative enzymes Rich in oxidative enzymes. Intermediate Poor in oxidative enzymes. Main ATP source Gets energy from aerobic Gets energy from Gets energy from oxidative phosphorylation aerobic oxidative glycolysis of FAs. phosphorylation of FAs. Glycogen content Low Intermediate High Contractile speed Slow continuous Fast continuous Fast (sprinting), and in (example) contraction over time contraction over time fingers(playing music & (muscles of posture of the (major muscles of legs). computer games) back). Fatigability Not easily fatigued Not easily fatigued Easily fatigued Mechanism of Skeletal muscle contraction (Sliding filament model) The sarcomere & whole myofibril are shortened. https://youtu.be/BVcgO4p88AA Growth of the skeletal muscle fibers: 1. Growth in width (hypertrophy) due to increase number of myofibrils. 2. Growth in length due to addition of dividable satellite cells. Regeneration of the skeletal muscle fibers: By stimulation of satellite cells in small defects only. Connective tissue coverings of skeletal muscle whole muscle muscle fascicle muscle fiber Connective tissue and fascicles: Myofibril: composed of bundles of myofilaments Endomysium: loose CT covering each muscle fiber and binding it to its neighbors. Perimysium: dense CT covering muscle fascicles. Fascicles: bundles of muscle fibers surrounded by perimysium. Epimysium: covering of dense CT surrounding the entire muscle. Skeletal Muscle Objectives: By the end of the lecture, you should: 1. List characteristic features of muscular tissue 2. Identify different types of muscles 3. Describe LM & EM structure of different types of muscles (skeletal & smooth) 4. Differentiate between LM & EM structure of different types of muscle fibers (skeletal & smooth)  Smooth Muscle Site: wall of blood vessels and wall of viscera (GIT, respiratory, urinary systems). LM structure: Not striated (no cross-striations present), involuntary Shape: fibers are spindle shaped overlapping each other, arranged in bundles, surrounded by loose C.T. that carries B.V. and nerves. Nuclei Nucleus: Single, flattened oval or rod-shaped, Situated in the widest part of the muscle fiber. Sarcoplasm: acidophilic, contains dark granules called dense bodies (formed of α-actinin and believed to be analogous to Z line in function). Smooth Muscle (EM structure): Sarcoplasm: contains the usual organoids and inclusions needed to perform its function. Myofilaments: there are 3 types; - Thin filaments: Actin, possess tropomyosin and caldesmon (mask the active site of the actin) - Thick filaments: Myosin, - Intermediate filaments include Desmin (in all smooth muscle) and Vimentin (additionally in smooth muscle of B.V.) All filaments are arranged irregularly and overlapping each other, giving the appearance of dense bodies. Contraction occur due to sliding of thin filaments over thick filaments. Smooth Muscle (EM structure) Transverse tubules (T-tubules): Not found. There are subsurface vesicles open into the cell surface known as caveolae. Caveolae regulate Ca pump needed for muscle contraction. Sarcoplasmic reticulum (sER): Formed of narrow sarco-tubules with no terminal cisternae (poorly developed). The jap junction: Present between the muscle fibers to allow rapid conduction of impulses and contract together in groups. Growth of the smooth muscle fiber: 1. Hypertrophy: to increase in width. 2. Proliferation by mitotic division. Regeneration of the smooth muscle fiber: By division of smooth muscle fibers.  Differentiate between LM & EM structure of skeletal muscle fibers and smooth muscle fibers Skeletal muscle Smooth muscle Shape of fibers Control Striations Nuclei T-tubules Sarcoplasmic reticulum Gap junctions Locations Growth/renewal Quiz Thank You