perineurium

The perineurium is the connective tissue sheath that surrounds each fascicle of axons within a peripheral nerve. It consists of several concentric layers of flattened perineurial cells embedded in a basement membrane, forming a distinct envelope around the bundle of nerve fibers and separating it from the surrounding endoneurial space and epineurium.

A key feature of the perineurium is the presence of tight junctions between adjacent perineurial cells. These junctions contribute to the blood-nerve barrier, regulating the movement of ions, metabolites, and macromolecules between the bloodstream and the nerve fibers. The thickness and robustness of the perineurial layers vary with nerve size and function.

Functionally, the perineurium provides mechanical protection and structural integrity to nerve fascicles, helping to maintain a stable microenvironment for axons and their myelin. It also participates in nerve regeneration by forming a guiding tubular structure that supports and directs regrowing axons after injury.

Clinically, the perineurium is relevant in regional anesthesia and surgical contexts, where injections around a nerve fascicle (perineurial block) can influence the spread of anesthetic agents and improve block efficacy. Pathological changes to the perineurium, such as thickening or disruption, can be associated with nerve injury, inflammation, or certain neuropathies.

In summary, the perineurium is the protective, barrier-forming sheath around nerve fascicles, essential for nerve function, protection, and regeneration within the peripheral nervous system.