Why skeletal muscle fibres are multinucleated
Skeletal muscle fibres are very long cells made when many myoblasts (immature muscle cells with one nucleus each) fuse, and their many nuclei supply gene copies for protein.
Remember
Many myoblasts fuse during development to make one very long skeletal muscle fibre, and its many nuclei give gene copies along its length rather than dividing inside the fibre.
A skeletal muscle fibre is a single muscle cell, and it is very long: in the longest muscles it can reach up to about 30 cm. A nucleus is the compartment that holds a cell's DNA, and the genes there are read to make the instructions for proteins. One nucleus is poorly placed to supply a cell this big, so the fibre is multinucleated: it has many nuclei, and therefore many copies of the genes for making proteins.
The fibre's original nuclei arise in early development. Many myoblasts (immature muscle cells, each with one nucleus) fuse with up to hundreds of other myoblasts, and the single large cell that results keeps all of their nuclei in one shared cytoplasm. The fibre has to make and maintain huge amounts of contractile protein (actin and myosin, the filaments that slide past each other to shorten the fibre), and nuclei spaced along its length mean protein can be made close to where it is needed.
The nuclei do not come from division inside the fibre. Once a myoblast has fused, its nucleus is post-mitotic, which means it can no longer divide. When a muscle grows or repairs itself, new nuclei are supplied by satellite cells, small stem cells lying against the fibre, which fuse in and donate their nuclei.
Cardiac muscle cells have one to two nuclei, and smooth muscle cells have a single nucleus. Striations (a striped pattern from regularly arranged actin and myosin filaments) are present in skeletal and cardiac muscle but absent from smooth muscle. Many nuclei in one long fibre is the feature that best distinguishes skeletal muscle from both cardiac and smooth muscle.
Key facts
- Length
- a skeletal muscle fibre is a single cell that can be up to about 30 cm long.
- Purpose of many nuclei
- they give many copies of the genes, so a very long cell can make the large amounts of protein it needs.
- Origin
- many single-nucleus myoblasts fuse during development to form one multinucleated fibre.
- Position
- the nuclei lie around the edge of the fibre.
- Renewal
- the nuclei of a mature fibre are post-mitotic, and new nuclei come from satellite cells.
- Other muscle types
- cardiac muscle cells have one to two nuclei, and smooth muscle cells have a single nucleus.
Exam traps
- Nuclei dividing repeatedly without the cell splitting would give one cell with many nuclei, but skeletal muscle fibres do not form that way: they form when many single-nucleus myoblasts fuse together.
- Skeletal muscle can repair itself after injury, but the nuclei inside a mature fibre are post-mitotic, so they have lost the ability to divide: new nuclei come from satellite cells, which are muscle stem cells that sit beside the fibre.
- Regular striations do not separate skeletal muscle from cardiac muscle, because cardiac muscle is striated too; they separate both of them from smooth muscle.
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