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Fascicles are bundles of muscle fibres surrounded by perimysium, which contains capillaries, nerve endings, and neuromuscular spindles. Each fascicle is then surrounded by another layer of connective tissue called the endomysium. The endomysium is, in turn, surrounded by a thick outer layer of connective tissue called the fascia. Fascial layers can be classified as superficial, deep, visceral, or parietal. Superficial fascia is found directly under the skin, while deep fascia surrounds muscles, bones, nerves, and blood vessels. Visceral fascia surrounds organs in cavities, and parietal fascia lines the walls of body cavities.
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Connective tissue epimysium
The epimysium is the outermost connective tissue sheath of skeletal muscle, surrounding the entire muscle. It is a dense irregular connective tissue, predominantly composed of type I collagen fibres. It helps to define the muscle's volume and prevents friction between neighbouring muscles. The epimysium is continuous with the perimysium and endomysium deep to it, and all three layers converge and blend with the connective tissue of the muscle's tendon.
The epimysium is one of the three distinct connective tissue layers that surround muscle tissue, the other two being the perimysium and endomysium. The perimysium surrounds muscle fibre bundles, or fascicles, and the endomysium surrounds individual muscle fibres. The connective tissue layers are responsible for many of the functional properties associated with muscle, such as tissue structure, the elasticity of the tissue, vascular supply to the muscle tissue, and water-holding capacity.
The epimysium is also considered a type of fascia. Fascia is made up of sheets of connective tissue that are found below the skin. Fascia can be classified as superficial, deep, visceral, or parietal. Deep fascia surrounds bones, muscles, nerves, and blood vessels. It is commonly more fibrous and rich in hyaluronan compared to the other subtypes. The epimysium can sometimes blend with the deep fascia between adjacent muscles, or even the superficial fascia.
During muscle contractions, muscle fibres pull on the epimysium, which transmits the force to the bone that the tendon is inserting into, producing movement. If the muscle does not have a tendon of insertion and instead inserts directly onto bone, the epimysium will blend into the periosteum of the bone. Blood vessels and nerve fibres also pierce the epimysium to supply the muscle.
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Perimysium
The perimysium is a layer of dense irregular connective tissue that surrounds individual muscle fascicles (bundles of muscle fibres) and separates them from other fascicles within the skeletal muscle. It is continuous with the endomysium, which wraps around individual muscle fibres, and the epimysium, which encloses the entire muscle. The perimysium contains a rich network of blood vessels and nerves, known as neurovascular bundles, that branch out to supply muscle fibres of each fascicle with nutrients and oxygen, while also facilitating signal transmission.
The perimysium is an important structure for the diagnosis of muscle hernias, which manifest as soft tissue masses that may be constant or intermittent and primarily involve the lower extremity. Ultrasound findings include thinning and slight elevation of the fascia, evident when the muscle is contracted. Serous myositis, a nonspecific form of myositis, is characterised by an underlying infiltration of the perimysium, resulting in subsequent thickening of the muscle fascicles.
The perimysium is an extension of the epimysium, a dense connective tissue layer that surrounds individual muscles. The epimysium is hyperechoic and contiguous with its tendon on long-axis views. The perimysium appears as bright echogenic dots on transverse imaging, distributed through the hypoechoic background of the muscle fascicles.
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Endomysium
The endomysium is a sheath of connective tissue that surrounds each individual skeletal muscle fibre. It is composed primarily of type III and type IV collagen, with minor amounts of type I and type V collagen also present. The endomysium is the site of metabolic exchange between blood and the skeletal muscle fibre, as it contains capillaries that supply the muscle fibre.
The endomysium is one of three collagenous sheaths that unite and fuse where muscles connect to adjoining structures such as tendons. The other two sheaths are the epimysium and the perimysium. The epimysium is the connective tissue sheath that surrounds the entire muscle, and the perimysium surrounds bundles of muscle fibres, known as fascicles.
Within each fascicle, there are many individual muscle fibres, which are the basic structural units of skeletal muscle. These muscle fibres are surrounded by the endomysium. The endomysium, perimysium, and epimysium unite into tendons or tendinous layers, or may insert directly into the periosteum or dermis.
The endomysium has been shown to be involved in certain inflammatory disorders that affect muscle, such as serous myositis, where there is an underlying infiltration of the perimysium, with subsequent thickening of the muscle fascicles. Ultrasound assessments of muscle require multiple focal zones and adjustments of depth, depending on the size and location of the muscle, due to the varying thicknesses of the different layers of connective tissue.
In addition to its role in muscle structure and function, the endomysium is also relevant in the context of certain diseases. For example, anti-endomysial antibodies (EMA) are present in celiac disease.
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Superficial fascia
The knowledge of superficial fascia is essential in surgery, as it creates borders for infectious processes. An increase in pressure may require a prompt fasciotomy to prevent compartment syndrome. Surgeons suture superficial fascia together following a procedure to ensure wound integrity, and it can be used as a surgical patch.
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Deep fascia
The main function of deep fascia is to support and protect muscles and other soft tissue structures. It also provides a barrier against the spread of infection from the skin and superficial fascia into muscle compartments. Deep fascia is relatively fixed to the structures it covers and provides good fixation points for soft tissue repositioning and surgical procedures.
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Frequently asked questions
A fascicle is a bundle or cluster of muscle fibres surrounded by perimysium.
A fascia is a sheet of connective tissue that surrounds a muscle.
The endomysium, a sheet of connective tissue, covers each fascicle.
Fasciae attach, stabilize, impart strength, maintain vessel patency, separate muscles, and enclose different organs.
Fascia can be classified as superficial, deep, visceral, or parietal.
