perineurial

Perineurial is an adjective used in anatomy to describe structures related to the perineurium, the connective tissue sheath that surrounds nerve fascicles in the peripheral nervous system. The perineurium encases each nerve fascicle, separating it from surrounding tissue, and is distinct from the epineurium, which covers the nerve as a whole, and from the endoneurium, which surrounds individual axons.

The perineurium is formed by multiple concentric layers of specialized perineurial cells. These cells are arranged as flat, layered sheets and are joined by tight junctions, with a continuous basement membrane surrounding the sheath. The layered organization and junctional complexes create a diffusion barrier that helps regulate the intrafascicular environment.

Functionally, the perineurium provides mechanical protection to nerve fascicles and contributes to the blood-nerve barrier, limiting the entry of large molecules into the endoneurial space and helping to maintain ionic balance essential for nerve conduction.

Perineurial cells are specialized cells associated with nerve fibers and are integral to fascicle integrity. The perineurial sheath develops during nerve maturation and continues to reinforce the endoneurial compartment.

Pathologically, tumors arising from perineurial cells are called perineuriomas. They are rare, typically benign soft tissue tumors presenting as slow-growing masses. Immunohistochemistry commonly shows positivity for epithelial membrane antigen (EMA) and other perineurial markers, with variable or negative S-100 staining.