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Skeletal Muscle Cells
Skeletal Myocytes—Myoblast Fusion, Myotube
Organization in Skeletal Muscle Tissue and
Satellite Cells
Each skeletal muscle fiber is a single skeletal muscle cell, also known as
a skeletal myocyte ("myo-" refers to "muscle" and "-cyte" refers to
"cell"), that is formed from the fusion of precursor cells. As described
before, cell fusion leads to multinucleation of each mature muscle fiber.
Each myoblast, the embryonic cell type that differentiates into muscle,
contributes one nucleus when the muscle fiber is formed during
development.
During development, individual myoblasts ("-blast" refers to "building" …
like osteoblasts), migrate to different regions in the body and then fuse
to form a myotube. A myotube is a type of syncytium, which is the term
used for a group of fused cells. Skeletal muscle cells are multinucleate
because the syncytium ("syn-" means "same" and "cyt" refers to
"cytoplasm") fusion retains the nucleus of each contributing myoblast.
This syncytium leads to the collective sarcoplasm and sarcolemma,
described above.
Mature muscle does not grow by this process. Mature cells can change in
size, but new cells are not formed when muscles grow. Instead,
structural proteins are added to muscle fibers in a process called
hypertrophy. The reverse, when structural proteins are lost and muscle
mass decreases, is called atrophy. Cellular components of muscles can
also undergo changes in response to changes in muscle use.
Although the number of muscle cells is set during development, satellite
cells help to repair skeletal muscle fibers. Satellite cells are similar to
myoblasts in that they are able to divide, fuse and differentiate. These
satellite cells are located outside the muscle fibers and are stimulated to
grow and fuse with muscle cells by growth factors that are released by
muscle fibers under certain forms of stress. Satellite cells can regenerate
muscle fibers to a very limited extent, but they primarily help to repair
damage in living cells. Satellite cells facilitate the protein synthesis
required for repair and growth. If a cell is damaged to a greater extent
than can be repaired by satellite cells, the muscle fibers are replaced by
scar tissue in a process calledfibrosis. Because scar tissue cannot
contract, muscle that has sustained significant damage loses strength
and cannot produce the same amount of force or endurance as it could
before being damaged.