Histology of Muscle Tissue

Muscle Tissue

• Muscle tissue is composed of cells that optimize the universal cell property of contractility.
• There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal Muscle

• Skeletal muscle consists of long, cylindrical, multinucleated cells with diameters of 10-100 μm and cross-striations.
• Contraction is quick, forceful, and usually under voluntary control.
• Elongated nuclei are found peripherally just under the sarcolemma, a characteristic nuclear location unique to skeletal muscle fibers.
• Muscle satellite cells are a small population of reserve progenitor cells that remain adjacent to most fibers of differentiated skeletal muscle.

Organization of Skeletal Muscle

• The entire muscle is surrounded by the epimysium, an external sheath of dense irregular connective tissue.
• The perimysium is a thin connective tissue layer that surrounds each bundle of muscle fibers (fascicle).
• The endomysium is a very thin, delicate layer of reticular fibers and scattered fibroblasts that surrounds the external lamina of individual muscle fibers.
• Collagens in these connective tissue layers serve to transmit the mechanical forces generated by the contracting muscle cells.

Ultrastructure of Skeletal Muscle Fibers

• Longitudinally sectioned skeletal muscle fibers show striations of alternating light and dark bands.
• The sarcoplasm is highly organized, containing primarily long cylindrical filament bundles (myofibrils) that run parallel to the long axis of the fiber.
• Myofibrils consist of an end-to-end repetitive arrangement of sarcomeres.
• The dark bands on the myofibrils are called A bands, and the light bands are called I bands.
• Mitochondria and sarcoplasmic reticulum are found between the myofibrils.

Sarcomeres

• The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin myofilaments, composed of myosin and F-actin, respectively.
• The thick myosin filaments occupy the A band at the middle region of the sarcomere.
• Myosin is a large complex with two identical heavy chains and two pairs of light chains.
• The myosin heads bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity).
• The thin, helical actin filaments are each 1.0-μm long and 8-nm wide and run between the thick filaments.

Sarcoplasmic Reticulum and Transverse Tubule System

• The membranous smooth ER, called sarcoplasmic reticulum, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils.
• The sarcolemma has tubular infoldings called transverse or T-tubules.
• The triad complex (T-tubule with two terminal cisternae of sarcoplasmic reticulum) allows depolarization of the sarcolemma in a T-tubule to affect the sarcoplasmic reticulum and trigger release of Ca2+ ions into the cytoplasm around the thick and thin filaments, which initiates contraction of sarcomeres.