endomysial

Endomysial refers to anything related to the endomysium, the thin connective tissue layer that surrounds each individual skeletal muscle fiber within a muscle fascicle. The endomysium lies between the individual muscle fiber’s membrane (sarcolemma) and the surrounding extracellular matrix, and it is continuous with the perimysial and epimysial connective tissues that organize fibers into fascicles and the whole muscle, respectively.

Structure and contents: The endomysium is composed mainly of reticular fibers (type III collagen) with small amounts of type I collagen and occasional elastic fibers. It houses capillary networks that supply each muscle fiber and intramuscular nerve fibers, as well as satellite cells, which sit at the periphery of fibers and participate in growth and repair. The endomysium also interacts with the basal lamina that surrounds each muscle fiber.

Function: The endomysium provides a flexible, supportive matrix that helps transmit the contractile force of individual fibers to the surrounding tissue. It serves as a conduit for capillaries and nerves and creates a microenvironment for satellite cells, contributing to muscle maintenance and regeneration.

Clinical relevance: Inflammatory and degenerative muscle diseases can involve the endomysium. Endomysial inflammation is a histopathologic feature described in polymyositis, characterized by CD8+ T-cell infiltrates within the endomysium. Chronic disease can lead to endomysial fibrosis, reducing muscle elasticity and contributing to weakness; such fibrosis is seen in various chronic myopathies and muscular dystrophies. The term endomysial is also used more broadly to describe structures or changes within this tissue layer, such as endomysial capillaries or endomysial fibrosis.

Etymology: The term comes from Greek endon (within) and mys (muscle), forming a descriptor for structures within the muscle tissue.