endomysium

Endomysium is the innermost layer of connective tissue in skeletal muscle, surrounding each individual muscle fiber (myocyte). It lies external to the basal lamina that surrounds the sarcolemma and consists mainly of thin reticular fibers rich in type III collagen, with some type I collagen and a proteoglycan-rich ground substance. The endomysium also contains small blood vessels and nerve fibers that innervate the individual fibers, as well as satellite cells positioned between the sarcolemma and the basal lamina.

Functionally, the endomysium provides mechanical support to individual muscle fibers and helps transmit contractile forces to the surrounding connective tissue. Its vascular and neural elements allow diffusion of nutrients and removal of waste, as well as the control of muscle fiber activity. The endomysium also houses satellite cells, which participate in muscle growth and repair.

Structurally, the endomysium is part of a hierarchical series of connective tissue layers that organize muscle into fascicles (perimysium) and into the whole muscle (epimysium). The endomysium forms an intimate, continuous network that interfaces with the basal lamina and connects to the outer layers, contributing to the overall elasticity and integrity of muscle tissue.

Clinical relevance: changes in the endomysium, such as inflammation or fibrosis, can occur in various muscular diseases and inflammatory myopathies, reflecting involvement of the microvasculature and connective tissue. The endomysium thus plays a key role in muscle structure, function, and repair.