Download Histology of muscle tissue

MESILEYA AYODEJI OLUWAFEMI
14/MHS01/079
MEDICINE AND SURGERY
200 LEVEL
HISTOLOGY OF BASIC TISSUES (ANA 203)
Histology of muscle tissue
Muscle tissue is often categorized on the basis of a functional property: the ability of its
cells to contract. In muscle tissue, the bulk of the cytoplasmic volume consists of the contractile
protein fibrils actin and myosin. Muscle is responsible for movement of the body and changes in
the size and shape of internal organs. Muscle cells are of mesodermal origin and differentiate by a
gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and
myosin. They are generally referred to as muscle fibres. Muscle fibres are typically arranged in
parallel arrays, allowing them to work together effectively. Histologically, there are three types of
muscle tissue: skeletal muscle, cardiac muscle and smooth muscle. The fibres of skeletal and
cardiac muscles exhibit cross striations at the light microscope level and they are both referred to
as striated muscle. The smooth muscle is non-striated.
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Skeletal muscle- it is striated and voluntary;
Cardiac muscle- it is striated and involuntary;
Smooth muscle- it is non-striated and involuntary
Types of Muscle
- SKELETAL MUSCLE: They consist of muscle fibers that are long, cylindrical multinucleated
cells. Skeletal muscle constitutes the muscle that is attached to the skeleton and controls motor
movements and posture. There are a few instances where this type of muscle is restricted to soft
tissues such as the tongue, pharynx, diaphragm and upper part of the esophagus. Skeletal muscle
fibres (cells) are actually a multinucleated syncytium formed by the fusion of individual small
muscle cells or myoblasts, during development. They are filled with longitudinally arrayed
subunits called myofibrils. These myofibrils are made up of the myofilaments myosin (thick
filaments) and actin (thin filaments). The striations reflect the arrangement of actin and myosin
filaments and support structures. The individual contractile units are called sarcomeres. A
myofibril consists of many sarcomeres arranged end to end. The entire muscle exhibits crossstriations because sarcomeres in adjacent myofibrils and muscle fibers are in register. The most
obvious feature in longitudinal sections of skeletal muscle is the alternating pattern of dark and
light bands, called respectively the A (anisotropic) and I (isotropic) band. The I band is bisected
by a dense zone called the Z line, to which the thin filaments of the I band are attached.
The nuclei are located peripherally, immediately under the plasma membrane
(sarcolemma). The thickness of individual muscle fibres varies (depending for example on location
in the body and exercise) but each fibre is of uniform thickness throughout its length. Skeletal
muscle fibres do not branch.
Connective tissue elements surround muscle fibres. Individual muscle fibres are
surrounded by a delicate layer of reticular fibres called the endomysium. Groups of fibres are
bundled into fascicles by a thicker CT layer called the perimysium. The collection of fascicles that
constitutes one muscle is surrounded by a sheath of dense CT called the epimysium, which
continues into the tendon. Blood vessels and nerves are found in the CT associated with muscle.
The endomysium contains only capillaries and the finest neuronal branches.
- CARDIAC MUSCLE: Cardiac muscle is the type of muscle found in the heart, and at the base
of the venae cavae as they enter into the heart. Cardiac muscle is intrinsically contractile but is
regulated by autonomic and hormonal stimuli. Cardiac muscle exhibits striations because it also
has actin and myosin filaments arranged into sarcomeres. Generally these striations do not appear
as well-defined as in skeletal muscle. (At the ultrastructural level, some differences in the
arrangement of the sarcoplasmic retiuculum and T tubules can be seen. Cardiac muscle also has a
much greater number of mitochondria in its cytoplasm. More details on the anatomy and
physiology of muscle will be discussed in H&D and Cardiovascular Blocks).
At the light microscope level, a number of features distinguish cardiac from skeletal
muscle. Cardiac muscle cells have only one or two nuclei, which are centrally located. The
myofibrils separate to pass around the nucleus, leaving a perinuclear clear area (not always evident
in standard preparations). This clear area is occupied by organelles, especially mitochondria
(which are of course not visible in LM). As in skeletal muscle, individual muscle fibres are
surrounded by delicate connective tissue. Numerous capillaries are found in the connective tissue
around cardiac muscle fibres.
Cardiac muscle cells are joined to one another in a linear array. The boundary between two
cells abutting one another is called an intercalated disc. Intercalated discs consist of several types
of cells junctions whose purpose is to facilitate the passage of an electrical impulse from cell to
cell and to keep the cells bound together during constant contractile activity. Unlike skeletal
muscle fibres, cardiac muscle fibres branch and anastomose with one another. Although made up
of individual fibres, heart muscle acts as a functional syncytium during contraction for the efficient
pumping of blood.
Specialized fibres, called purkinje fibres, arise from the atrioventricular node and travel
along the interventricular septum toward the apex of the heart, sending branches into the
ventricular tissue. Purkinje fibres are of larger diameter than ordinary cardiac fibres, with fewer
myofibrils and an extensive, well-defined clear area around the nucleus. They conduct impulses at
a rate about four times faster than that of ordinary cardiac fibres and serve to coordinate the
contraction of the atria and ventricles.
- SMOOTH MUSCLE: Smooth muscle is the intrinsic muscle of the internal organs and blood
vessels. It is also found in the iris and ciliary body of the eye and associated with hair follicles
(arrector pili). No striations are present in smooth muscle due to the different arrangement of actin
and myosin filaments. Like cardiac muscle, smooth muscle fibres are intrinsically contractile but
responsive to autonomic and hormonal stimuli. They are specialized for slow, prolonged
contraction.
Smooth muscle fibres are generally arranged in bundles or sheets. Each fibre is fusiform in
shape with a thicker central portion and tapered at both ends. The single nucleus is located in the
central part of the fibre. Fibres do not branch. They range enormously in size, from 20 (in wall of
small blood vessels) to 500 (in wall of uterus during pregnancy) micrometers. Smooth muscle
fibres lie over one another in a staggered fashion (tapered part of one fibre over thicker part of
another). In longitudinal sections, it is often not possible to distinguish the fibre boundaries, and
smooth muscle may closely resemble connective tissue (bundles of collagen). Where smooth
muscle bundles are interlaced with bundles of connective tissue (eg. in the uterus), one can
distinguish the smooth muscle by the orientation of the nuclei (all oriented in the same direction),
and the greater abundance of nuclei per unit area (every smooth muscle cell has a nucleus,
fibroblast nuclei are more scattered in bundles of CT). Also, smooth muscle nuclei often have a
corkscrew shape in longitudinal section due to contracton of the muscle fibre during fixation. In
cross section, smooth muscle appears as profiles of various sizes, depending on whether the cut
went through the thick central part or tapered end of any individual fibre. Nuclei are seen only in
the thicker profiles.
One distinguishing physiological feature of smooth muscle is its ability to secrete
connective tissue matrix. In the walls of blood vessels and the uterus in particular, smooth muscle
fibres secrete large amounts of collagen and elastin.