Muscle tissue Department of Histology and Embryology Medical college in Three Gorges University

Muscle tissue can be divided into three types: Skeletal muscle Cardiac muscle Smooth muscle

Skeletal muscles

Skeletal muscle is composed of extremely elongated, multinucleate contractile cells, often described as muscle fibres, bound together by collagenous supporting tissue.

The connective tissue associated with muscle is named according to its relationship with muscle fiber: endomysium perimysium epimysium

Endomysium the delicate layer of reticular fibers that immediately surrounds individual muscle fibers. picture

Perimysium is a thicker connective tissue layer that surrounds a group of fibers to form a bundle or fascicle.Fascicles are functional units of muscle fibers that tend to work together to perform a specific function.

Epimysium is a thicker layer connective tissue which invested the whole muscle mass, large blood vessels and nerves enter the epimysium and divide to ramify throughout the muscle in the perimysium and endomysium.

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perimycium endomycium

The histological features of skeletal muscle fibres in longitudinal section: 1)extremely elongated, unbranched cylindrical cells 2)with regular cross-striations 3)numerous flattened nuclei located at fairly regular intervals just beneath the sarcolemma (plasma membrane)..

the cross-striations result from the arrangement of the contractile proteins. Cross striation means alternating light and dark bands in the longitudinal sectioned skeletal muscle.

Transverse section perimycium endomycium

This micrograph of skeletal muscle cut in transverse section shows the extreme peripheral location of the nuclei of skeletal muscle fibres. In cross-section muscle fibres appear polyhedral with flattening of adjacent cells.The cell is spherical, polyhedral or irregular shape.

Sarcomere : the part of a myofibril situated between two consecutive Z-band is called a sarcomere. Each of them is composed of I ½+ A band + I ½. It is the stuacture and function unit of myofibrils

The sarcomere consists of two types of myofilaments, thick filaments and thin filaments. Why do the myofibril display cross striation? Myofilaments Thick filaments Thin filaments

Each type remains constant in length irrespective of the state of contraction of the muscle. The thick filaments, which are composed mainly of the protein myosin, are maintained in register by their attachment to a disc-like zone represented by the M line.

Similarly the thin filaments, which are composed mainly of the protein actin, are attached to a disc-like zone represented by the Z line. The I and H bands, both areas of low electron density, represent areas where the thick and thin filaments do not overlap one another.

light band dark band Z 线 H M 肌节： 1/2I 带 + A 带 + 1/2I 带

Thick filaments Thin filaments

sacomere

Thick filaments: Myocin Thin filaments: actin, tropomyosin and troponin

Thick filaments Thin filaments Myocin actin ， tropomyocin ， troponin

Transverse tubules : These fingerlike invaginations of the sarcolemma form a complex anastomosing network of tubules that encircle the boundaries of the A-I bands of each sarcomere in every myofibrils. Function: rapidly conduct impulse for contraction to every myofibrils in the muscle fiber.

Sarcoplasmic R Transverse tubules Triad Terminal cisterna

Sarcoplasmic reticulum: Sarcoplasmic reticulum is a specialized smooth endoplasmic reticulum. It forms extensive anastomosing network which runs between and around the myofibrils in a longitudinal direction. Adjacent to opposite sides of each T tubule are expanded terminal cisternae of the sarcoplasmic reticulum

Triad: This specialized complex consist of a T tubule with 2 lateral terminal cisternae of sarcoplasmic reticulum is known as the triad.

Cardiac muscle

Cardiac muscle fibers are essentially short cylindrical cells with one or at most two nuclei, centrally located within the cell. The ends of the fibers are split longitudinally into a small number of branches.

Intercalated disc disc

Micrograph illustrates an extremely thin resin- embedded section at very high magnification. The branching cytoplasmic network is readily seen with prominent intercalated discs marking the intercellular boundaries. Note the typical cross-striations.

Summing: Feature of cardiac muscle in LM 1. short cylindrical,one or two nuclei, located in the center of the cardiac muscle. 2. cross-striations of cardiac muscle, are not very clear than that of the skeletal. 3.Intercalated disc 闰盘,

it is dark-staining transverse line that cross the chains of cardiac muscle. located between the ends of adjacent cardiac muscle cells,which can cause the adjacent cells to contract almost simultaneously.

Feature of cardiac muscle in EM 1. myofibrils is less than that of SM 2. Tranverse tubules is thick 3. Sarcoplasmic reticulum is less 4. terminal cisternae is small, 5. mitochondria is aboundant

Mitochondria Sarcoplasmic reticulum

Intercalated disc desmosome

SR Terminal cisternae T tubules dyad

Triad Sarcoplasmic R T C T tubules dyad

Smooth muscle

Smooth muscle is involuntary and unstriated. They are often seen as individual fibers or slender bundles or fascicles, however they are predominantly found lining the visceral organ and blood vessels.

Smooth muscle fibers are elongated, spindle- shaped cells with tapered ends which may occasionally be bifurcated. Smooth muscle fibers are generally much shorter than skeletal muscle fibers and contain only one nucleus which is elongated and centrally located in the cytoplasm at the widest part of the cell.

Smooth muscle fibres are bound together in irregular, branching fasciculi.Within the fasciculi, individual muscle fibres are arranged roughly parallel to one another with the thickest part of one cell lying against the thin parts of adjacent cells.

The contractile proteins of smooth muscle are not arranged in myofibrils as in skeletal and cardiac muscle, and thus visceral muscle cells are not striated.

This micrograph illustrates smooth muscle cut in longitudinal section. At this magnification, the micrograph demonstrates the spindle-shaped and elongated central nuclei of smooth muscle cells.

In this case, the fibres are arranged in a highly regular manner and packed so closely that it is difficult to identify individual cell outlines although cell shape can be deduced from that of the nuclei.