Histology of Muscle Tissue
.
Introduction
• Muscle is a soft tissue of animals made up of
muscle cells containing protein filaments that
slide past one another to produce a contraction
that changes both the length and the shape of
the cell.
• Muscles function to produce force and cause
motion.
Introduction cont…
• They are primarily responsible for
maintenance of and changes in posture,
locomotion, movement of internal organs, i.e.
contraction of the heart and movement of food
through the alimentary canal by peristalsis.
• Muscles form the flesh of the body and walls
of hollow organs such as the gut, blood
vessels.
Introduction cont…
DEFINITION OF TERMS USED IN
MUSCLES.
 Sarcolemma: the cell membrane
 Sarcoplasm: the cytoplasm of the muscle
 Sarcoplasmic reticulum: endoplasmic
reticulum of the muscle
Introduction cont…
 Myocyte: a muscle cell
 Sarcosome: the mitochondria of a muscle cell
 Sarcomere: the contractile or functional unit
of muscle
 Striated muscle: is marked by transverse dark
and light bands, is made up of elongated
usually multinucleated fibers, and includes
skeletal and cardiac muscles of vertebrates.
Functional characteristics of muscle
 Contractility:
-Unidirectional; generates pulling force
 Excitability:
-Innervated; propagation of electrical impulses
 Extensibility:
-Stretch with contraction of an opposing muscle
 Elasticity:
-Recoils passively after being stretched
Classification of Muscles
3 types of muscles, based on microscopic
appearance of the muscle cells;
The skeletal Muscles
The Cardiac Muscles
The Smooth Muscles
THE STRUCTURE OF THE MUSCLES
The Skeletal Muscles
• Attached to skeletons by tendons.
• Features of skeletal muscle cells;
– Elongated, cylindrical in shape
– Contain multiple peripheral nuclei
• Also they have specialized sER which stores
Calcium
• Estimated; length 1-40mm, thickness 10100µm
CLASSIFICATION OF SKELETAL MUSCLES
 Skeletal muscles are divided into two broad
types:
Type I, slow twitch fibers and
 Type II, fast twitch fibers
Slow Twitch Fibers
• Type I m. /Slow twitch m. / Red m.
• Is dense with capillaries and is rich in
mitochondria and myoglobin, giving the
muscle tissue its characteristic red color.
• It can carry more oxygen and sustain aerobic
activity using fats or carbohydrates as fuel.
• Slow twitch fibers contract for long periods of
time but with little force.
• Abundant in muscles responsible for
maintenance of posture e.g. those in the legs,
thigh, trunk, back and gluteal region.
Fast twitch fibers
• Type II / Fast twitch m. / White m.
• Anaerobic, glycolytic, "white" muscle with
few mitochondria and myoglobin.
• Contract quickly and powerfully but easily
fatigue
Comparison of Types of Skeletal Muscle Fibers
Characteristics
Red Muscle Fibers
White Muscle Fibers
Vascularization
Rich vascular supply
Poorer vascular supply
Innervation
Smaller nerve fibers
Larger nerve fibers
Fiber diameter
Smaller
Larger
Contraction
Slow but repetitive; not easily fatigued; Fast but easily fatigued; stronger
weaker contraction
contraction
Sarcoplasmic reticulum
Not extensive
Extensive
Mitochondria
Numerous
Few
Myoglobin
Rich
Poor
Enzymes
Rich in oxidative enzymes; poor in
adenosine triphosphatase
Poor in oxidative enzymes; rich in
phosphorylases and adenosine
triphosphatase
Organization of Skeletal Muscle
• Epimysium: an external sheath of dense
connective tissue surrounding the entire
muscle
• Perimysium: thin septa of connective
tissue around each bundle of muscle fibers
• Endomysium: a delicate layer of
connective tissue surrounding each muscle
fiber. It is composed mainly of a basal
lamina and reticular fibers.
Organization of Skeletal Muscle
Skeletal Muscle Cells
Organization of Skeletal Muscle Fibers
• Has light and dark bands as seen under light
microscope
• A bands: darker bands
• I bands: lighter bands
• Z line: a dark transverse line that bisects each I
band
• Sarcomere: the smallest repetitive subunit of
the contractile apparatus, extends from Z line
to Z line.
Organization cont..
Myofibrils:
• Long cylindrical filamentous bundles filling
the sarcoplasm
• They run parallel to the long axis of the muscle
fiber
• Consist of an end – to end chainlike
arrangement of sarcomeres
• Sarcomeres in myofibrils causes the entire
muscle fiber to exhibit a characteristic pattern
of transverse striations.
Organization cont..
• The pattern of sarcomere is mainly due to the
presence of two types of filaments:
• Thick filaments
– 1.6 m long and 15 nm wide;
• Thin filaments
– 1.0 m long and 8 nm wide.
Organization cont..
• Thick filaments: occupy the A bands, the
central portion of the sarcomere.
• Thin filaments: run between and parallel to
the thick filaments and have one end attached
to the Z line.
• I bands consist of the portions of the thin
filaments that do not overlap the thick
filaments.
Organization cont..
• H band: lighter zone on the center of the A band.
Corresponds to a region consisting only of the
rodlike portions of the myosin molecule
• M line: bisects the H band. It is a region at
which lateral connections are made between
adjacent thick filaments.
• The major protein of the M line is Creatine kinase
which catalyzes the transfer of a phosphate group
from phosphocreatine ( a storage form of high –
energy phosphate groups) to ADP, thus supplying
ATP for muscle contraction.
Sarcomere
S
A
I
H
Z
M
= Sarcomere, unit of muscle function
= A-band, region of myosin
= I-band, region of actin
= H-zone, region of just myosin
= Z-line, sarcomere boundary
= M-line, sarcomere center
• Portion of a myofibril, showing sarcomere
structure
• The four main proteins found in striated
muscle filaments include:
– Actin
– Tropomyosin
– Troponin
– Myosin
• Myosin and actin together represent 55% of
the total protein of striated muscle
• Thin filaments are composed of actin,
tropomyosin and troponin
• Thick filaments are composed of myosin
• Actin is present as long filamentous (F-actin)
polymers consisting of two strands of globular
(G-actin) monomers,
• Each G-actin monomer contains a binding site for
myosin
• Actinin and desmin (an intermediate filament
protein) are believed to tie adjacent sarcomeres
together, thus keeping the myofibrils in register.
• Tropomyosin, a long, thin molecule about 40
nm in length, contains two polypeptide chains.
• Troponin is a complex of three subunits:
– TnT, which strongly attaches to
tropomyosin;
– TnC, which binds calcium ions;
– TnI, which inhibits the actin and myosin
interaction.
A troponin complex is attached at one specific
site on each tropomyosin molecule
• Myosin,
• Is a much larger complex (molecular mass 500
kDa),
• Small globular projections at one end of each
heavy chain form the heads, which have ATPbinding sites as well as the enzymatic capacity
to hydrolyze ATP (ATPase activity) and the
ability to bind to actin.
• The myosin molecules are composed of a rodlike portion (light meromyosin) and twin
rounded heads (heavy meromyosin).
• Sarcoplasmic Reticulum & Transverse
Tubule System
• To provide for a uniform contraction, skeletal
muscle possesses a system of transverse (T)
tubules
• Adjacent to opposite sides of each T tubule are
expanded terminal cisternae of the
sarcoplasmic reticulum.
• This specialized complex, consisting of a T
tubule with two lateral portions of
sarcoplasmic reticulum, is known as the triad
• At the triad, depolarization of the sarcolemmaderived T tubules is transmitted to the
sarcoplasmic reticulum membrane.
Cardiac Muscles
• Cardiac muscles are involuntary muscles
found in the heart and in some walls of large
blood vessels associated with the heart.
• The cells are mononucleated with centrally
located nucleus
• The cells have small amount of cytoplasm
containing many mitochondria with closely
parked cisternae, golgi apparatus and lipid
droplets, with large amount of glycogen
Cardiac muscle cont…
• The cells have less prominent connective
tissue, but extensive cappillary and lymphatic
network
• A unique and distinguishing characteristic of
cardiac muscle is the presence of intercalated
disks represent junctional complexes found at
the interface between adjacent cardiac muscle
cells
INTERCALATED DISC
o There
are
three
main
junctional
specializations within the Intercalated disk:
o Fasciae adherentes, serve as anchoring
sites for actin filaments of the terminal
sarcomeres.
o Essentially, they represent hemi-Z bands.
o Maculae adherentes (desmosomes) present
in the transverse portion and bind the
cardiac cells together, preventing them from
pulling apart under constant contractile
activity.
o On the lateral portions of the disk, gap
junctions provide ionic continuity between
adjacent cells.
o The significance of ionic coupling is that
chains of individual cells act as a syncytium,
allowing the signal to contract to pass in a
wave from cell to cell.
• Lengths of individual cardiac muscle cells vary,
on average they are 15 μm in diameter and 80
μm in length
• Muscle cells of the atria are somewhat smaller
than those of the ventricles.
• These cells also house granules (especially in
the right atrium) containing atrial natriuretic
peptide, a substance that functions to lower
blood pressure
• This peptide acts by decreasing the capabilities
of renal tubules to resorb (conserve) sodium
• Organelles
• The extracellular fluid is the primary calcium
source for cardiac muscle contraction.
• The sarcoplasmic reticulum of cardiac muscle
does not form terminal cisternae and is not
nearly as extensive as in skeletal muscle;
instead, they form a diad. (one T tubule and
one sarcoplasmic reticulum cisterna)
• The diads in cardiac muscle cells are located in
the vicinity of the Z line.
Smooth Muscles
• Type of muscle which exhibit no striations
• Have cells that are elongated, spindle shaped,
with a single elongated(CORK SCREW
SHAPED) centrally located nucleus
• Myosin and actin are longitudinally placed and
anchored by desmin
• They are found in the walls of hollow viscera (e.g
GIT, some of the reproductive tract, and the
urinary tract), walls of blood vessels, larger ducts
of compound glands, respiratory passages, and
small bundles within the dermis of skin.
• Are involuntary muscles
Smooth Muscles
• Regulated by the autonomic nervous system,
hormones , and local physiological conditions.
• Smooth muscle cells contains numerous
invaginations of the plasma membrane called
caveolae which allows calcium ions from
extracellullar space into cells.
-They are capable of regeneration (contains
undifferentiated mesenchymal cells)
-Specialized for slow but prolonged
contraction and are resistant to fatigue.
Functions of Smooth Muscles
• Contractile functions
• Exogenous protein synthesis.
– Among the substances manufactured by
smooth muscle cells for extracellular
utilization are collagen, elastin,
glycosaminoglycans, proteoglycans, and
growth factors
Light Microscopy View of Smooth
Muscle Fibers
• Light microscopy reveals that smooth muscle
fibers are short, spindle-shaped cells with a
centrally placed nucleus.
• Smooth muscle fibers are fusiform, elongated
cells
• Each smooth muscle cell is surrounded by an
external lamina, which invariably separates
the sarcolemma of contiguous muscle cells
Fine Structure of Smooth Muscle
• The perinuclear cytoplasm of smooth muscle
cells, especially the regions adjacent to the two
poles of the nucleus, contains numerous
mitochondria, Golgi apparatus, rough
endoplasmic reticulum (RER), smooth
endoplasmic reticulum (SER), and inclusions
such as glycogen
• An extensive array of interweaving thin
filaments (7 nm) and thick filaments (15 nm)
is present.
Fine Structure of Smooth Muscle
• The thin filaments are composed of actin (with
its associated caldesmon), and tropomyosin,
with the notable absence of troponin).
• The thick filaments are composed of the same
myosin II that is present in skeletal muscle.
Feature
Skeletal Muscle
Cardiac Muscle
Smooth Muscle
Sarcomeres and myofibrils
Yes
Yes
No
Nuclei
Multinucleated; peripherally
located
One (or two); centrally located
One; centrally located
Sarcoplasmic reticulum
Well-developed with terminal
cisterns
Poorly defined; some small
terminals
Some smooth endoplasmic
reticulum
T tubules
Yes; small, involved in triad
formation
Yes; large, involved in dyad
formation
No
Cell junctions
No
Intercalated disks
Nexus (gap junctions)
Contraction
Voluntary; "all or none"
Involuntary; rhythmic and
spontaneous
Involuntary; slow and forceful; not
"all or none"
Calcium control
Calsequestrin in terminal cisternae
Calcium from extracellular sources Calcium from extracellular sources
and the sarcoplasmic reticulum
(via caveolae) and the
sarcoplasmic/endoplasmic
reticulum
Calcium binding
Troponin C
Troponin C
Calmodulin
Regeneration
Yes, via satellite cells
No
Yes
Mitosis
No
No
Yes
Nerve fibers
Somatic motor
Autonomic
Autonomic
Connective tissue
Epimysium, perimysium, and
endomysium
Connective tissue sheaths and
endomysium
Connective tissue sheaths and
endomysium
Distinctive features
Long; cylinder-shaped; many
peripheral nuclei
Branched cells; intercalated disks;
one or two nuclei
Fusiform cells with no striations;
single nucleus
REGENERATION OF MUSCLE
• Although skeletal muscle cells do not have the
capability of mitotic activity, the tissue can regenerate
because of the presence of satellite cells.
• These cells may undergo mitotic activity, resulting in
hyperplasia, subsequent to muscle injury.
• Under certain other conditions, such as "muscle
building," satellite cells may fuse with existing muscle
cells, thus increasing muscle mass during skeletal
muscle hypertrophy.
• Skeletal muscle cells regulate their number and their
size by the secretion of a member of the transforming
growth factor-β (TGF-β) superfamily of extracellular
signaling molecules, myostatin.
REGENERATION OF MUSCLE
• Cardiac muscle is incapable of regeneration.
Following damage, such as a myocardial
infarct, fibroblasts invade the damaged
region, undergo cell division, and form fibrous
connective tissue (scar tissue) to repair the
damage.
REGENERATION OF MUSCLE
• Smooth muscle cells retain their mitotic capability to
form more smooth muscle cells.
• This ability is especially evident in the pregnant
uterus, where the muscular wall becomes thicker both
by hypertrophy and hyperplasia
• Small defects, subsequent to injury, may result in
formation of new smooth muscle cells.
• These new cells may be derived via mitotic activity of
existing smooth muscle cells, as in the
gastrointestinal and urinary tracts, or from
differentiation of relatively undifferentiated pericytes
accompanying some blood vessels
Referrences
1.
K.L. Moore (2008). The Developing Human; Clinically Oriented Embryology
8th Edition
2. Richard Snell, Clinical Neuroanatomy, 6th Edition
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http://anatomyalmanac.blogspot.com/2008/01/from-archive-musclecomes-from-latin.html. Retrieved October 3, 2012.
4. Douglas Harper (2012). "Muscle“, Online Etymology Dictionary.
http://www.etymonline.com/index.php?allowed_in_frame=0&search=musc
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5. Marieb, EN; Hoehn, Katja (2010). Human Anatomy & Physiology (8th ed.).
San Francisco: Benjamin Cummings. p. 312. ISBN 978-0-8053-9569-3
6. McCloud, Aaron (30 November 2011). "Build Fast Twitch Muscle Fibers“.
Complete Strength
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8. Color Textbook of Histology
9. Past presentations
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tissue)