Musculoskeletal Module - Histology and Cell Biology PDF

Musculoskeletal Module Lecture presentation Histology and Cell Biology OBJECTIVES At the end of the lecture, students should be able to: 1. Describe the histological features of skeletal muscle at L/M and E/M levels 2. Differentiation between the histological structure of skeletal, ca...

Musculoskeletal Module Lecture presentation Histology and Cell Biology OBJECTIVES At the end of the lecture, students should be able to: 1. Describe the histological features of skeletal muscle at L/M and E/M levels 2. Differentiation between the histological structure of skeletal, cardiac and smooth muscle CONNECTIVE TISSUE COMPONENT  Epimysium: covering the whole muscle, dense irregular collagenous connective tissue  Perimysium: surrounds each bundle (fascicle) of muscle, a less dense collagenous connective tissue  Endomysium: surrounds each muscle fiber, a delicate layer of CT composed mainly of external lamina and reticular  fibers.  Blood vessels penetrate the muscle within the connective tissue septa and form rich capillary network that runs between and parallel to the muscle fibers. The capillaries are of the continuous type and lymphatic vessels are also found in the C.T. T.S. skeletal muscle at L.M. T.S. skeletal muscle at L.M. DEVELOPMENT OF SKELETAL MUSCLE The skeletal muscle consists of muscle fibers arranged in regular bundles of very long (up to 40 mm) cylindrical cells with a diameter of 10-100 µm.  Their nuclei are multiple, oval and peripheral in position.  The cytoplasm: acidophilic striated  longitudinal section of skeletal muscle fibers shows that the sarcoplasm of each muscle fiber is filled with long filamentous bundles called myofibrils.  Cross section of skeletal muscle, showing the bundles of myofibrils that appear as polygonal dark areas called Cohenheim's areas. Diagram of skeletal muscle (L.S. &T.S.)  Unique (special)terms are often used in describing the components of muscle cells: 1.The cell membrane = Sarcolemma. 2.The cytoplasm = Sarcoplasm. 3.The S. E. R = Sarcoplasmic reticulum. 4.Mitochondria = Sarcosomes. - L/M - Skeletal muscle fibers show its characteristic striations of alternating - dark bands (A band) - light bands (I band) - The center of A band is occupied by a pale area called H bands which is bisected by a thin (M) line. - Each I band is bisected by a dense line called Z line or disk.  NB.By polarized microscope dark band A (anisotropic) they cause double refraction to the polarized light Light band I (isotropic) they cause single refraction to the polarized light, EM: Sarcoplasm contains myofibrils and myofilaments In addition to: Golgi complex Many free ribosomes Few rER Glycogen dense granules Abundance of the oxygen-binding protein myoglobin Mitochondria Well developed sER …form Triad Myofilaments (bundles of protein filaments) Myofibrils: Run parallel to the long axis of the muscle fibers. Each myofibril is formed of large number of myofilaments which are of two types:  Thin (Actin) filaments.  Thick (Myosin) filaments. EM: Each dark band (A band) is divided into two equal parts by a pale region called Hensen's disc or H band and within this a very fine dark middle strip or M line is present, the major protein of it is creatine kinase. Each light band (I band) is divided into two equal parts by dark membrane called Z-line. The Z lines divided the myofibrils into contractile units called the sarcomeres.  The sarcomere it is the unit of contraction of the muscle fiber. It is the distance between two successive Z lines. It contains two types of myofilatnents; thick myofilaments (myosin) are present in the dark (A) bands and thin myofilaments (actin) are present in the light (I) bands and extend for a short distance in the dark bands. The region of the sarcomere which contain both actin and myosin filaments appears darker than the region which contains actin filaments only. The H band appears slightly lighter because it has only myosin filaments  N z z A I H M  STRUCTURE OF SARCOMERE Sarcomere is the distance between two successive Z lines EM: Each myofibril is surrounded by a network of sarcoplasmic reticulum (smooth endoplasmic reticulum). At the regions of the A-I junction the sacs are wider and are called the Terminal cisternae. Between each 2 neighboring terminal cisternae there is a transverse tubule; T-tubule which is formed by the invagination of the sarcolemma. It extends transversely to encircle the sarcomeres like collars at the A-I junction. Each two terminal cisternae with a T-tubule in between are together called the triad. Types of skeletal muscle fibers: Type I fibers (red fibers): they have small diameter, contain large amount of myoglobin which is responsible for the red color of this type of fibers. They are highly vascular, contract slowly and can sustain contraction for a long time without fatigue (slow twitch). Type II fibers (white fibers): they have large diameter, contain small amount of myoglobin and contract faster but they become easily fatigued (fast twitch). Intermediate type CARDIAC MUSCLE  LM  It is formed of short cylindrical cells which are joined end to end to form cardiac muscle fibers.  cardiac muscle fibers … branch and anastomose forming a network  15 um in diameter, 80-100 um in length  One or double central nuclei  Acidophilic satiated cytoplasm EM: have no triads, only diad  have short, wide T tubules  Not well developed sER  are aerobic (high in myoglobin, mitochondria 40%)  have intercalated discs as dark transverse line between the joining fibers Figure 10.10a  SMOOTH MUSCLE Smooth muscle fibers are non-striated and involuntary. They are present in: The wall of blood vessels. The viscera of various systems (digestive, respiratory and genitourinary, etc.). Figure 9.24 SMOOTH MUSCLE  LM  Fusiform in shape with single central nucleus.  They have the smallest diameter of all muscle fibers (about 3 m).  Theirlength varies from 30 m as in the wall of blood vessels to 500 µm in the wall of pregnant uterus.  Their cytoplasm appears homogeneous and acidophilic  They are arranged in layers, which may be circular, longitudinal, oblique or spiral.  Contractile filaments: they are Actin , Myosin, Intermediate filaments (vimentin and desmin)  Dense Bodies  Located in both cytoplasm and sarcolemma.  They may be similar to Z line in its function.  They formed of  actinin.  Intermediate filaments and thin filaments are inserted into it.  Sarcolemmal vesicles (caveoli): Present along the periphery of the cell and termed caveoli.  They are invagination of cell membrane (Similar to T tubules).  They might function to take up and release calcium ions.  Smooth endoplasmic reticulum  Less developed and sparse but closely associated with sarcolemmal vesicles.  Gap Junction : Between smooth muscle fibers to facilitate the spread of excitation.  Glycogen granules. No satellite cells in spite of their capability of mitosis and regeneration.  REFERENCES  Pawlina, W. (2016): A text of Atlas with cell and molecular biology. 7th ed. Lippincott, Williams and Wilkins.  Junqueira, L.C.; Carneiro, J. and Kelly, R.O.(2014): Basic Histology. 11th ed. Appleton and Lange. New York.  Gartener, L.P. and Haiat, J.L. (2011): C.V.S. In: Colour Text Book of Histology, 4th ed. Saunders Elsevier.