MUSCULAR TISSUE/
HISTOLOGY
Learning Objectives
At the end of the lecture, students should be able to;
1. Differentiate the three types of muscles under the light and
electron microscope levels.
2. Identify the distinctive features of each type of muscle fiber
under the microscope.
 Smooth Muscle.
 Skeletal Muscle.
 Cardiac Muscle.
1. Describe the basic structure of muscle striations
Classification of Muscles
According to morphological classification;
 Striated
 Non striated or smooth
According to functional classification;
 Voluntary
 Involuntar
Microscopic Classification
• According to the presence or absence of cross striations --- 2
main variety:
A. Non-striated muscle -----Smooth muscle.
B. Striated muscle ---further divided into 2 sub-types, on basis of
morphological and functional characteristics:
1. Skeletal muscle.
2. Cardiac muscle.
Skeletal Muscle
• Makes flesh or meat of animal
• In fresh state --- Pink colour.
• Muscle fibers grouped into bundles – Fasciculi.
• Each muscle is covered by dense CT- Epimysium
• Each fasciculus is covered by Perimysium, serves as route of large
blood vessels.
• Each muscle fiber is surrounded by a thin layer of delicate reticular
fibers called Endomysium, which contains capillaries.
• Shape – cylindrical.
• Length – variable ( 1 – 40 mm).
• Diameter – vary from 10 to 100 µm.
• Sarcolemma – thick glycocalyx on outer surf. invaginations of it at
variable distance into the fiber are called T- tube.
• Nuclei – peripherally located numerous (135/mm), ovoid in shape.
• Sarcoplasm – mainly occupied by special organelles called
myofibrills, these are long cylindrical parallel filamentous elements.
• Special SER, called sarcoplasmic reticulum.
• Also a small GA, numerous sarcosomes, few ribosomes, glycogen
granules & few fat droplets
Sarcomere
Electron micrograph of skeletal muscle of a tadpole.
Note the sarcomere with its A, I, and H bands and Z line. The position of the thick
and thin filaments in the sarcomere is shown schematically in the lower part of
the figure. As illustrated here, triads in amphibian muscle are aligned with the Z
line in each sarcomere.
In mammalian muscle, however, each sarcomere exhibits 2 triads, one at each
A—I band interface.
Cardiac Muscle
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Found only in the myocardium.
Involuntary but striated.
Cells are aligned in the form of chains with complex junctions b/w their ends.
Cells within a chain often branch and make junctions with the cells in adjacent chains.
Endomysium CT containing a rich capillary network.
Cardiac Muscle Fibers
• They are elongated, branching cells with irregular contours at their
junctions.
They show a cross striated banding pattern.
Size – 100 µm & 15 µm.
Nucleus – usually a single large oval central.
Sarcolemma – T- tubes, not regularly arranged, and at Z- lines.
Sarcoplasm - more abundant.
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In addition to myofibrils, it contains:
• Organelles – numerous mitochondria, SER & GA.
• Inclusions – glycogen granules, fat droplets, lipofuscin
pigment & membrane bound granules.
Intercalated discs (disks)
• Under L/M, L/S of cardiac muscle shows a unique & distinguish
point i.e. 0.5 – 1 µm thick darkly staining transverse lines that cross
the chains of cardiac cells at irregular intervals called “Intercalated
discs”.
• It often follows an irregular or step-like course.
• Two regions can be distinguish:
• Transverse portion.
• Lateral portion.
Junctional specializations making up the intercalated disk. Fasciae (or
Zonula)adherents (A) in thetransverse portions of the disk anchor actin
filaments of the terminal sarcomere to the plasmalemma. Maculae
adherentes, or desmosomes (B), found primarily in the transverse
portions of the disk, bind cells together, preventing their separation
during contraction cycles. Gap junctions (C), restricted to longitudinal
portions of the disk–the area subjected to the least stress–ionically
couple cells and provide for the spread of contractile depolarization.
Heart, muscular interventricular septum
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Note the acidophilic fibers that change direction frequently. Observes the striations present in cardiac
muscle.
observe fiber bundles which are cut in longitudinal , oblique and transverse section
Also the centrally located nuclei of cardiac muscle.
Smooth muscle
• Mainly found in the wall of blood vessels & hollow viscera.
Smooth muscle fiber (cell)
• Shape – Fusiform
• Size – Length 15 - 200 µm and Diameter 3 – 8 µm
• Cell membrane – where the adjacent smooth muscle cells
come in close contact to each other, their cell membranes
shows gap junctions.
• Nucleus – single centrally located.
• Cytoplasm – contains thin (actin) & thick (myosin) filaments.
• These filaments form bundles, which criss cross obliquely through the cell, forming a
network.
Summary ;Muscle Types
Structure of the 3 muscle types.
Skeletal muscle is composed of large, elongated, multinucleated fibers.
Cardiac muscle is composed of irregular branched cells bound together
longitudinally by intercalated disks.
Smooth muscle is an agglomerate of fusiform cells.
The density of the packing between the cells depends on the amount of extracellular connective
tissue present.
REFERENCES
BASIC HISTOLOGY BY JUNQUEIRA