Striated Skeletal Muscle Tissue
Skeletal muscle tissue is one of the three types of muscle tissue, specialized for strong, quick, and usually voluntary contractions. It gets its "striated" name from the characteristic banding pattern visible microscopically.
- Origin: Nearly all muscle cells are of mesodermal origin. Skeletal muscle specifically develops when mesenchymal myoblasts fuse together to form multinucleated myotubes, which then differentiate into mature striated muscle fibers. Striated muscles are derived from somites, somitomeres, and somatopleure mesoderm.
- Control: It is under the voluntary control of the somatic nervous system.
Structure of Striated Muscle Fiber (Myocyte)
A skeletal muscle fiber, also known as a myocyte, is the individual cell of skeletal muscle tissue.
- Cellular Characteristics:
- Shape and Size: Muscle fibers are long, cylindrical cells, with diameters ranging from 10 to 100 μm. They can be several centimeters long.
- Nuclei: Each muscle fiber is multinucleated. These elongated nuclei are characteristically located peripherally, just under the sarcolemma (the muscle cell membrane). This is a distinguishing feature compared to cardiac and smooth muscle.
- Cytoplasm: The cytoplasm of muscle cells is often called sarcoplasm.
- Satellite Cells: A small population of muscle satellite cells, which are reserve progenitor cells, remain adjacent to the muscle fibers. These cells are crucial for the limited regenerative capacity of skeletal muscle after injury.
- Connective Tissue Organization: Thin layers of connective tissue surround and organize the muscle fibers, which are particularly well-seen in skeletal muscle:
- Epimysium: A dense connective tissue sheath that surrounds the entire muscle.
- Perimysium: A thinner connective tissue layer that surrounds bundles of muscle fibers, called fascicles. Fascicles are functional units where fibers work together.
- Endomysium: A very thin, delicate layer of reticular fibers and scattered fibroblasts that surrounds the external lamina of individual muscle fibers within fascicles. This layer contains a rich network of capillaries that bring oxygen to the muscle fibers.
- Internal Organization (Contractile Elements):
- Myofibrils: Muscle fibers are densely packed with long cylindrical structures called myofibrils, which run parallel to the long axis of the fiber. These are the contractile organelles. Mitochondria and sarcoplasmic reticulum are found between the myofibrils.
- Sarcomeres: Myofibrils consist of an end-to-end repetitive arrangement of contractile units called sarcomeres. Sarcomeres are the basic functional units of muscle contraction. They extend from one Z disc to the next. The lateral alignment of sarcomeres in adjacent myofibrils causes the characteristic transverse striations of skeletal muscle.
- Banding Pattern: The striations consist of alternating light and dark bands: