Histology: Skeletal Muscle Tissue

• Presented by Safinaz Salah Eldin, Professor of Histology, Histology Department
• Academic year 2024-2025, year 1, semester 2, module: locomotor system (LCS) 105

Objectives:

• Identify general characteristics and origin of muscular tissue
• Recognize the structure and sites of skeletal muscle
• Describe the histological structure (LM, EM) of skeletal muscle fibers
• Correlate the histological structure of skeletal muscle with its function
• Differentiate between types of skeletal muscle fibers
• Understand the regeneration of skeletal muscle

General Characteristics of Muscular Tissue

• Structural and functional units are specialized, elongated cells called muscle fibers
• Contractile filaments include thin actin and thick myosin
• Cell membrane is known as the sarcolemma
• Cytoplasm is known as the sarcoplasm
• Smooth endoplasmic reticulum is known as the sarcoplasmic reticulum
• Acidophilic, contains organelles such as mitochondria, SER, myofibrils, and inclusions

Origin of Muscular Tissue

• Of mesodermal origin
• Undifferentiated mesenchymal cells differentiate into myoblasts
• Myoblasts differentiate into myocytes
• Myocyte is equivalent to a muscle fiber

Muscular Tissue Types:

• Skeletal muscle
• Cardiac muscle
• Smooth muscle

Skeletal Muscle

• Mesodermal origin
• Formed by fusion of many mononucleated myoblasts
• Found attached to the skeleton, except in the face, tongue, pharynx, and upper 1/3 of the esophagus, diaphragm, and cremasteric muscle
• Voluntary control, except for the cremasteric muscle, esophagus, and pharynx

Histological Structure of Skeletal Muscle Fiber

• Cylindrical and non-branched shape, except in the face and tongue
• Diameter ranging from 10-100μ, length variable from 1-40 mm
• Acidophilic under Light Microscopy (LM)
• Characterized by multiple peripheral nuclei
• Uniformly placed clear transverse striations

Skeletal Muscle EM Observations

• Sarcolemma is thick due to fusion with surrounding external lamina
• Satellite cells are residual myoblasts involved in muscle regeneration
• Multiple peripheral oval euchromatic nuclei; Number corresponds to number of fused myoblasts
• Numerous mitochondria mainly in rows between myofibrils and around nuclei

Skeletal Muscle Inclusions

• Myoglobin: O2-binding pigmented protein, provides oxygen for oxidative reaction
• Glycogen & lipid found between the myofibrils to provide energy

Sarcoplasmic Reticulum

• Well-developed SER with a highly organized tubular network around the myofibrils
• Calcium storage occurs during relaxation
• Calcium pump works during contraction

Myofibrils

• Muscle fibers are filled with numerous myofibrils
• Run parallel to each other and to the longitudinal axis of the fiber
• Extend along the whole length of the muscle fiber
• Each shows alternating dark & light bands

Myofibril Appearance

• Under a polarizing microscope: dark band is anisotropic (A band), light band is isotropic (I band)
• The dark bands in adjacent myofibrils are arranged beside each other at the same level
• Alternates with light bands in a transverse striations appearance

Myofibril Bands

• The dark A band shows a central pale area called the H-zone
• The H-zone is bisected by a thin M-line
• The light I band is divided by a dense membrane called the Z-line

Myofibril composition

• Myofibrils are composed of thick myosin and thin actin filaments
• Myosin restricted to the A-band
• Actin filaments attach to the Z-line and pass through the I-band
• Actin filaments extend into the A-band till the beginning of the H-zone

Sarcomere

• The portion of a myofibril between two adjacent Z lines
• The basic contractile unit of striated muscle, considered the functional unit in contraction
• The I-bands are very short in contracted muscle and longer in relaxed muscle
• The length of the A-bands remains constant

Myofilament Bands

• A-band appears dark because of both myosin & actin filaments
• H-band appears paler because of myosin filaments only
• M-line is produced by interconnections of adjacent myosin filaments
• I-band appears light because of only actin filaments
• Z-line is dense due to condensed actin filaments & other proteins

Actin Filaments

• Anchored perpendicularly on the Z disc by the actin-binding protein α-actinin
• Accessory proteins maintain alignment of thin & thick filaments
• Accessory proteins include: Titin & Dystrophin

Titin

• Important accessory protein in I bands and the largest protein in the body
• Has scaffolding and elastic properties
• Supports & stabilizes the centering of the thick myofilaments
• Anchors thick myofilaments in the Z line
• Prevents overstretching of sarcomere

Dystrophin

• A large actin-binding protein
• Located just inside the sarcolemma, involved in the functional organization of myofibrils
• Duchenne muscular dystrophy: mutations of the dystrophin gene lead to defective linkages between the cytoskeleton and the extracellular matrix (ECM)
• Muscle contractions disrupt the weak linkages, leading to atrophy of muscle fibers

Triad Tubular System

• Transverse tubules (T-tubules): invaginations of the sarcolemma that encircle all myofibrils within the sarcoplasm
• Located at A-I band junctions, with lumens continuous with the extracellular space
• Pair of wide terminal cisternae of the sarcoplasmic reticulum
• Each T-tubule lies in-between a pair of them

Sarcomere Contraction

• Depolarization of the sarcolemma in a T-tubule
• Affects the sarcoplasmic reticulum
• Triggers release of Ca 2+ ions into cytoplasm around the thick and thin filaments
• Which initiates the contraction of sarcomeres

Skeletal Muscle Organization

• Consists of muscle fibers, held together and supported by connective tissue (CT)
• Epimysium: dense CT surrounding the whole muscle
• Perimysium: less dense CT descending from the epimysium, surrounding a group of muscle fibers to form a fascicle
• Endomysium: a layer of reticular fibers surrounding each muscle fiber

Connective Tissue Importance

• Essential for force transmission
• Connects the muscle fibers together
• Has rich in nerve fibers and blood vessels.

Muscle Fiber Types

• Red (slow twitch oxidative) vs. White (fast twitch glycolytic)
• Red = high myoglobin, many mitochondria, rich capillaries, low glycogen, small size, slow no fatigue, aerobic respiration
• White = low myoglobin, few mitochondria, poor capillaries, more glycogen, large size, fast easy fatigue, glycolysis anaerobic
• Fast oxidative-glycolytic fibers have intermediate features
• Major muscles of legs are fast, oxidative-glycolytic fibers
• Numerous mitochondria and capillaries with intermediate size
• Rich in myoglobin (red), Intermediate content of glycogen, fast with intermediate fatigue

Clinical Note: Myasthenia Gravis

• Is an autoimmune disorder characterized by:
• Skeletal muscle weakness
• Due to antibodies against proteins of acetylcholine receptors of skeletal muscle
• The disease follows a progressive course
• Affects the most active muscles first, (muscles of face, eyes & tongue)

Skeletal Muscle Regeneration

• Satellite cells are inactive reserve myoblasts present between sarcolemma & basal lamina
• these are activated after injury
• They proliferate and fuse to give rise to new muscle