HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 9 MUSCLE

A INTRODUCTION

l Most muscular tissue is derived from mesoderm, by a modification of the cells into elongated muscle fibres.
2 The muscle fibre is itself a cell, and muscle has relatively little extracellular tissue.
3 Each fibre has a special cytoplasm (sarcoplasm), in which lie contractile filaments, which can contract the fibre or cell along its long axis.
4 The three kinds of muscle found are adaptations to the requirements of:
... (a) strong contractions of short duration (skeletal muscle);
... (b) rhythmic, strong unfaltering contractions (cardiac);
... (c) contractions of longer duration and greater cell shortening (smooth).
These varieties differ also in their innervation and the way in which their forces are applied. Powerpoint
5  (a) Skeletal/striated/voluntary;              |   are the various
   (b) Cardiac/heart/(striated);                 |-  names, of the
   (c) Smooth/unstriated/involuntary/visceral;   |   three kinds.
6 Sarcoplasm stains pink with eosin, and selectively purple with Masson's trichrome stain, or yellow with van Gieson's.

B SKELETAL MUSCLE

l Connective tissue (CT) sheaths and subdivisions
... CT epimysium encloses the whole muscle;
... CT perimysium encloses each fasciculus (bundle) of fibres;
... CT endomysium encloses each muscle fibre.

Connective tissue carries blood vessels, lymphatics and nerves, and serves to harness and direct to the attached tendons the force developed by contraction.

2 Individual skeletal muscle fibre
l Outside lies a connective tissue endomysium with some fibroblasts, collagen fibrils, and capillaries.
2 Cell membrane is the sarcolemma.
3 Directly under the sarcolemma, i.e., peripherally, lie elongated nuclei. The cell, as another product of cell fusion, is multinucleated.
4 In one place, the sarcolemma is modified to take a nerve fibre's terminal motor-end-plate/ myoneural junction (Chapter l2.C.l.2.).
5 The interior of the fibre has sarcoplasm with orderly myofibrils.
6 Fibre is large and cylindrical, with a diameter between l0 and l00 µm and a length between l and 40 mm.
7 Regularly along the length of the fibre a cross-banding of light and dark lines is seen.
8 Fainter longitudinal lines, the myofibrils, are also visible.

3 Myofibril
The fibre is cross-banded because the many constituent myofibrils are banded, and lie side by side with their dark areas in register. High power light microscopy reveals the repetitive sequence (see Fig. l) along the myofibril.

   Light I band            Dark A Band             Light I band
    0.8µm wide              1.5µm wide               0.8µm wide
  with central dark                                with central dark
        Z line                                         Z line
          |                                               |
          |                                               |
          |            Sarcomere extends                  |  
          |                                               |  
          |-------- from one Z line ---- to the next -----|                               
          |                                               |
                      Resting fibre has a paler
                      zone H (Hensen's) in the
                      centre of the A band. It 
                      disappears in contraction.
                      The A band stays the same 
                      during contraction.

The I bands narrow in contraction, and thus the sarcomere (Z to Z) shortens

4 Red and white fibres
Groups of only one kind of fibre can be identified by colour with the naked eye in some fresh, unstained muscles. They differ physiologically with red, richer in myoglobin and mitochondria, providing slower responses, but being less prone to fatigue. Histochemistry reveals further subtypes: white, intermediate, red (fast-twitch), and red (slow).
(The classification of muscle fibre types is used in assessing muscular disease, but the classification by roman numeral can be based on the profiles of contractile proteins, or on the metabolic behaviour of the fibre, so one needs to ask the particular criteria for any types encountered.)

C CARDIAC MUSCLE COMPARED WITH SKELETAL

  1. Cross-banded, with the same repetitive sequence as in Figs. 1 and 2, but the banding is weaker.
  2. Sometimes a Z line's place is taken by a dark line across the width of the fibre - intercalated disc.
  3. Intercalated discs mark a strong end-to-end cell connection. The muscle thus pulls upon itself during contraction.
  4. Each cell has only one or two nuclei lying centrally, elongated, but with blunt ends.
  5. Fibres are narrower, around 9-22 µm in diameter.
  6. Fibres branch and anastomose and, until intercalated discs were discerned using EM, the muscle was believed to be syncytial - one huge cell.
  7. EM shows the intercalated discs to be extensive, interdigitated cell junctions with gap junctions, fasciae adhaerentes, where the myofibrils attach, and desmosomes.
  8. Mitochondria are more numerous.
  9. There is less CT.
  10. Cardiac myofilaments are not clearly aggregated into myofibrils.

D ELECTRON MICROSCOPY OF SKELETAL AND CARDIAC MUSCLE

1 Sarcolemma is the plasmalemma, outside which is a basal lamina.
2 Sarcoplasm contains glycogen granules, lipid droplets, many mitochondria and soluble proteins. A Golgi complex lies by some nuclei.
3 T-system and sarcoplasmic reticulum/SR (smooth)
The sarcolemma extends down, at the junction between A and I bands (or at the Z lines in cardiac muscle), into the fibre as centrotubules of the T-system. Cisternae of junctional SR lie immediately adjacent to the T-system, and from these a tubular and vesicular system of free SR extends on either side along, and wrapping around, the myofibrils. A centro-tubule and the two lots of cisternae on either side constitute a triad of separate compartments, but ones joined by densities (feet).
4 Myofilaments (contractile elements). Thick (of myosin) and thin (of actin protein) lie densely packed in orderly array in each myofibril (Fig. 2). Cross-sections revealing sites with only thin (at I band), only thick (at H), and thick and thin filaments (A band outside the H zone), and differences between the resting and contracted states, led Huxley to propose a `sliding filament' theory of contraction. In contraction, the thin filaments are induced to slide in further between the static thick ones, with their many heads/lateral projections of heavy meromyosin.
                             _________________ A __________________
                            |                                      |
                 ____ I  ___                                         ____ I ____
                |           |                                       |           |

                                               M
                   N     N                     |
          |-----      |      ------------------|---------------------     |
          |-----------|-------------------     |     ---------------------|--------------
Relaxed  _|-----      |      ------------------|---------------------     |
myofibril |-----------|-------------------     |     ---------------------|--------------
          |-----      |                        |                          |
                      |                   |_________|                     |
                      Z                      H zone                       Z

                      <--------------------- Sa ------------------------->

Fig. 2 Key
A .. Anisotropic band (bright in polarized-light microscopy; dark in bright-field)
I .. Isotropic band (dark in polarized light, light in bright-field microscopy)
Z .. Zwischenscheibe (between disc): the Z line or band to which the thin actin myofilaments attach
Sa .. Sarcomere is the contractile unit of the fibril extending longitudinally between one Z line and the next Z line
M .. Mittelscheibe (middle disc): the M line or band bisecting the A band
H .. Hensen's pale zone or band devoid of actin myofilaments
N .. Nebenscheiben (accessory discs): the N bands on either side of the Z line

5 Some events and chemistry of contraction

6 Some other muscle molecules
(a) a-actinin is a non-contractile Z-line actin-attachment protein.
(b) Nebulin is a giant ruler-like molecule to align, and establish the length of, the actin filaments.
(c) The ryanodine receptor - a calcium channel - is the principal protein of the SR feet, linking the T-tubule to the sarcoplasmic reticulum. Other proteins are at the complex.
(d) Desmin is the intermediate filament of muscle.
(e) Dystrophin lies just under the sarcolemma to help attach it to the cytoskeleton. Its lack causes one type of muscular dystrophy.
(f) Titin is another huge, long molecule, connecting the Z disc with the material of the M-line. It provides an elastic framework for filament movement, and helps actinin anchor the actin filaments in the Z disc.
(g) The contractile and regulatory proteins exist as isoforms, characteristic of slow, fast, and cardiac muscles. The isoforms change during development and in disease, e.g., muscular dystrophy, cardiac hypertrophy.
(h) Caveolin is associated with the inwardly budding sarcolemmal protrusions - caveolae - of cardiac and smooth muscle that are part of the signal transduction machinery for contraction.

E SMOOTH MUSCLE

l The fibres are spindle-shaped (fusiform) with one, central, cigar-shaped nucleus, and are usually 200 µm or less long, but in the hypertrophic uterus they may reach 0.5 mm. Width is around 6 µm.
2 Fibres show no cross-banding, but have many fine filaments.
3 Cells are firmly attached by gap junctions, and elsewhere by glycoprotein external laminae (like basal lamina). Diverse patterns of attachment and contraction occur in gut, vessel walls, genital organs, etc.
4 Fibres are usually packed to form a sheet or bundle. Reticular fibres enfold the muscle fibres, assist in holding them together and carry blood vessels, and fine autonomic nerve fibres going to inconspicuous myoneural junctions (Chapter l2.C.2).
5 The nuclei may be wrinkled in the contracted state.
6 EM shows thin and thick filaments, but the thick are labile and not easily preserved. These filaments connect with Z line-like densities in the cytoplasm, or at the cell membrane. Desmin intermediate filaments help to structure the filamentous arrays.
By the nucleus lie mitochondria and the Golgi body.
7 Peripheral vesicles are part of a vesicular and tubular Ca2+-holding sarcoplasmic reticulum. These organelles, and inward protrusions of cell membrane - caveolae - function similarly to the better-defined SR and T-tubules of striated muscle.
8 Contraction is triggered by a Ca2+-dependent phosphorylation of myosin light chain by smooth-muscle MLC kinase. This is the primary control, fine-tuned by the calcium-mediated binding of caldesmon and calponin to actin in ways which interfere with actomyosin force-generation and ATPase activities.
9 Myoepithelial cells, wrapped around glandular secretory or duct cells, have contractile processes resembling smooth muscle cells.
10 Vascular smooth muscle cells also can make elastin and collagen during development.

F MYOGENESIS (skeletal muscle)

l Mesodermal cells of the myotome become elongated premyoblasts.
2 These multiply, acquire more cytoplasm and elongate further, becoming granular, with many mitochondria and ribosomes.
3 Filaments and microtubules appear in the cytoplasm of the myoblasts. New myoblasts fuse with the more mature ones accumulating myofilaments to build long, multinucleated cells.
4 Filaments aggregate into myofibrils near the sarcolemma, leaving a paler central core, with a row of nuclei (myotube stage).
5 The fibrils develop prominent striations; nuclei move to the periphery of the fibre; and mitochondria and SR order themselves in relation to the myofibrils.
6 Some cells stay in a peripheral position to lie within the basal lamina as a regenerative reserve of satellite cells.

G TENDON

l Musculo-tendinous junction entails no continuity between myofibrils and collagen fibres: the sarcolemma intervenes.
2 Myofibrils pull on the tapering sarcolemma at the muscle fibre's extremity, and its contraction is conveyed by the muscle's CT to the tendon with which the muscle CT merges.
3 Tendon is composed of: 4 Freedom of movement is provided for some tendons by enclosing them in lubricated synovial sheaths, or interposing a synovial bursa between the tendon and a bony prominence or ligament.
5 Tendons and skeletal muscle have nervous proprioceptors - Golgi tendon organs and muscle spindles.

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159