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The perineurium is a protective layer of tissue located around nerves in the body and the internal organs. It is composed of concentric layers of connective tissue that form a protective sheath around bundles of nerve fibers. This structure is a transparent tube-shaped layer that is easily pulled away from the bundled nerves. Perineurium nerve coverings are a part of the peripheral nervous system (PNS) which is responsible for transmitting messages from the central nervous system (CNS) in the brain to the arms, legs, and internal organs.
A perineurium is part of the three layers of tissue that protect the nerves throughout the body. First, the individual nerve fibers in the peripheral nervous system are covered by a layer called the endoneurium. Many of the endoneurium-covered fibers are then clustered together in groups called fascicles and covered with another protective layer of connective tissue, which is the perineurium. Several of these perineurium-coated bundles are then grouped with blood vessels and adipose tissue, and covered with an additional protective sheath called the epineurium.
Fibroblasts within the body form perineurium cells. Flattened cells are arranged in layers and surrounded by collagen fibers and a basement membrane. These specially shaped cells are able to prevent the stretching of the nerve fibers and help to create the blood-nerve barrier that further provides protection to the bundled nerve fibers.
Benign perineuriomas may form within the nerve coverings. These small growths on the protective nerve sheath are extremely uncommon. Some people with neurofibromas or a malignant peripheral nerve sheath tumor may have diagnostic test results that show perineurial cell involvement. Very rarely, malignant perineuriomas may develop on this layer along with other spindle cell neoplasms in a person predisposed to cancer.
If the perineurium layer is damaged or torn, the nerves may transmit very painful signals to the central nervous system. The unprotected fascicles may grow together, causing nerve adhesions to form that impair the nerve’s functionality. A very thin layer of a poly-lactide (PLA) film with a porous patterned surface on one side is sometimes used to prevent nerve adhesions from developing after surgery. Nerves that are protected by the artificial sheath during the healing process exhibit a greater functional recovery than ones that were not treated with this method.
When these adhesions do develop, a surgical procedure called neurolysis may be required. A longitudinal incision into the perineurium is used to separate the adhesion from the nerve fibers and the outer sheath. Thin layers of the poly-lactide film may be implanted over the exposed nerve fibers once the adhesion has been removed.