Histology of Skeletal Muscles I PDF

Histology of Skeletal Muscles I By: Dr. Moustafa Al Sawy Dr. Shaimaa Mohamed Amer MBBCH, M.SC. M.D HISTOLOGY MBBCH, M.SC. M.D HISTOLOGY Associate Professor of Histology & Cell Biol...

Histology of Skeletal Muscles I By: Dr. Moustafa Al Sawy Dr. Shaimaa Mohamed Amer MBBCH, M.SC. M.D HISTOLOGY MBBCH, M.SC. M.D HISTOLOGY Associate Professor of Histology & Cell Biology Associate Professor of Histology & Cell Biology Certified Medical Educator Learning objectives Knowledge: Learning Objectives At the end of the lecture , each student will be able to: 1.List types of the muscles. 2. Describe the light and electron microscopic features of skeletal muscle. 3.Discuss regeneration and growth of skeletal muscles.  Muscular Tissue *General characteristics It is one of the four basic tissues which has the property of contractility. CON NECTIV ETIS SU E It is mesodermal in origin. ❖Three types of muscle tissue: A. Skeletal muscle B. Cardiac muscle C. Smooth muscle  Classification of Muscular Tissue A) According to morphology : 1-Striated muscles show regular pattern of alternating light and dark bands (skeletal and CON NECTIV ETIS SU E cardiac muscles). 2-Non-striated muscles lack cross striations (smooth muscles). B) According to function : 1-Voluntary under the control of will as striated skeletal muscles. 2-Involuntary not under the control of will as cardiac and smooth muscles.  General Features of Skeletal Muscle *It consists of parallel skeletal muscle fibers, arranged in bundles or fascicles and separated CON NECTIV ETIS SU E by thin C.T. septa, the perimysium. *The fibers are connected together by C.T., endomysium. It is a very thin, delicate layer of reticular fibers and scattered fibroblasts. It surrounds the external lamina of individual muscle fibers. *The whole muscle is covered by a dense irregular C.T. covering the epimysium.  Epimysium Perimysium Endomysium Skeletal muscle  General Features of Skeletal Muscle The connective tissue is continuous from one part to another and consists of CON NECTIV ETIS SU E collagen, elastic & reticular fibers and contains fibroblasts, macrophages and fat cells. Functions of C.T.: 1-It carries blood vessels (for nourishment) and nerves to the muscle. 2-It aids in integrating & transmitting force of muscular contraction through binding muscle units together.  Skeletal muscle fibers 1-General characteristics: They are non-branched except in face and tongue. Site: CON NECTIV ETIS SU E Skeletal muscle fibers are present in skeletal muscles, tongue and pharynx.  Skeletal muscle fibers 2-Shape & structure: A-Light microscope (LM): Shape: CON NECTIV ETIS SU E *Each skeletal muscle fiber is an elongated cell (10- 100 um in diameter and 1-3 cm in length). *Its cell membrane is called sarcolemma. Nucleus: *Skeletal muscle fiber is cylindrical multinucleated cell has up to 100 peripherally located nuclei. *Cytoplasm (Sarcoplasm) :is acidophilic with clear transverse striations.  Skeletal muscle fibers In adult muscle: *There is precursor cells (Satellite cells) which act as stem cells to replace damaged muscle cells. *These cells are small spindle-shaped lying beneath the external lamina of a muscle fiber.  Skeletal muscle fibers B-Electron microscope (E/M): Skeletal muscle fibers are metabolically active, contain organelles (membranous and non- membranous) and inclusions. Membranous organelles: *Numerous Golgi bodies. *Sarcoplasmic reticulum (modification of smooth endoplasmic reticulum, serves as a store for calcium ions). *Mitochondria arranged in rows between the myofibrils. *Negligible amount of RER.  Skeletal muscle fibers Non membranous organelles: *Well developed parallel myofibrils, run the length of the muscle fiber. *Each myofibril shows a banding pattern ( alternating dark and light bands) similar to the whole muscle fiber. Negligible amount of ribosomes (low level of protein synthesis). Inclusions: Abundant myoglobin (oxygen binding protein similar to hemoglobin). Large amount of glycogen that serves as a depot of energy during muscle contraction.  Myofibrils *They are contractile threads (organelles), arranged longitudinally in sarcoplasm of the muscle fiber. *Each myofibril consists of longitudinal fine myofilaments of two types: 1-Thick filaments formed of protein myosin. Each filament has thickened mid-portion and tapering ends, occupy central portion of sarcomere. 2-Thin filaments are formed mainly of actin and regulatory proteins (troponin & tropomyosin)  Myofibrils Dark bands: A Band Formed of two types of filaments (thick & thin).Its center is pale & formed only of thick filaments called Hensen's zone (H zone) which is divided by dark membrane (M line). Light bands: I bands Formed of thin myofilaments (Actin) It is divided into two equal portions, by a dark membrane, Z line which anchors the actin filaments. Sarcomere is the distance between two successive Z lines and is the functional unit of the muscle fiber. *I band and the H zone are light because they have only one type of myofilaments. H zone contains thick filaments (myosin).  Triad or Tubular System Shape & structure (E/M): It is present at the junction of A & I bands. The sarcoplasmic reticulum (SR) at the junction of A & I bands forms a pair of transverse dilated sarcotubules (terminal cisternae). One pair of terminal cisternae serves A band and forms an irregular network over it. The other pair serves I band in a similar way. The sarcolemma sends transverse invaginations into sarcoplasm, the T-tubules which form collars around the myofibrils.  Triad or Tubular System The two cisternae of the sarcoplasmic reticulum & T-tubule in-between form the triad tubular system of skeletal muscle. Function: *It transmits impulses from exterior of the fiber to all the myofibrils causing a coordinated response. *Passage of electric impulses to sarcoplasmic reticulum results in passive release of calcium ions from reticulum to the contractile elements causing their contraction. *When the impulse ends the sarcoplasmic reticulum actively takes calcium ions back into the cisternae and the muscle relaxes.  Regeneration of skeletal muscle fibers It is carried by the satellite cells. They have a limited regenerative potential. After injury of the muscle fibers: A. In case of mild injury: The necrotic areas are removed by the macrophages. The satellite cells replace the degenerated part. B. In case of severe lesion: Fibrous scar tissue will be formed.  Growth of skeletal muscle fibers The skeletal muscle fibers cannot divide, so increase muscle mass is due to enlargement of existing muscle fibers (hypertrophy). This occurs by: 1.Increase the width: of muscle fibers: by synthesis of new myofibrils. 2.Increase the length: of the fibers: by activation of satellite cells-----myoblasts------ fused with skeletal muscle fibers.  Case Scenario An investigator is studying the microstructure of skeletal muscle fibers to identify new targets for skeletal muscle relaxation. He obtains a biopsy from the brachioradialis of a test subject and observes it under a transmission electron microscope. A photomicrograph of the structures seen during microscopy is shown. Which of the following parts contains the greatest proportion of actin filaments? A B C D E Any Questions? References: 1.Basic Histology: Text & Atlas. Editor: Luiz Carlos Junqueira, MD, PhD; Jose Carneiro, MD, PhD. 14th Ed. 2. Wheatear’s functional histology: A text & color atlas.15th Ed. 3. AMBOSS platform. Thank You