Connective tissue layers of skeletal muscle: endomysium, perimysium and epimysium
Three connective tissue layers wrap skeletal muscle: endomysium around one fibre (a long muscle cell), perimysium around a fascicle (a bundle of fibres) and epimysium around the whole muscle.
Remember
Work from the inside out: endomysium wraps one fibre, perimysium wraps a fascicle (a bundle of fibres) and epimysium wraps the whole muscle.
Think of a skeletal muscle as a rope made of smaller ropes. At each level there is a layer of connective tissue (the supporting tissue rich in collagen, a tough fibrous protein), and each layer has its own name.
Each muscle fibre (a single long muscle cell) is wrapped in a thin sleeve called the endomysium. Many fibres are gathered into a bundle called a fascicle, and the sheath that wraps that bundle is the perimysium. A number of fascicles, together with blood vessels, are then collected into the epimysium, a dense outer sheath that forms the muscle as a whole.
The prefixes give a memory aid. Endo- means inner, epi- means outer, and peri- is the one in the middle (it also means around, as in perimeter). So the perimysium is the middle layer, the one that goes around a bundle.
The sarcolemma has a name that sounds like a wrapping, but it is the plasma membrane (the cell membrane) of the muscle fibre itself, so it belongs to the cell and is not connective tissue. The tendon is also not one of the three layers: it is a cord of dense collagen that joins muscle to bone, and the layers blend into it, which is how the pull of the muscle is passed on to the bone.
Key facts
- Endomysium
- the innermost layer, a thin sleeve around each individual muscle fibre.
- Perimysium
- the middle layer, wrapping each fascicle (a bundle of muscle fibres).
- Epimysium
- the outer layer, a dense sheath around the whole muscle.
- Prefixes
- endo- means inner, peri- is the one in the middle and epi- means outer.
- Sarcolemma
- the plasma membrane of the fibre itself, so part of the cell and not connective tissue.
- Tendon
- a cord of dense collagen that joins muscle to bone, with the three wrappings blending into it at the end of the muscle.
Exam traps
- The endomysium does not wrap a bundle of fibres, because it is the thin layer around each individual fibre and so lies inside the fascicle rather than around it.
- The epimysium is the dense sheath around the whole muscle, so it is not the layer around one fascicle, even though it is the outermost wrapping.
- A basement membrane does not gather many fibres into a fascicle, because it is a thin supporting sheet that works at the level of single cells.
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