Peripheral nerve: Investments

The peripheral nervous system, located outside the central nervous system, consists of peripheral nerves, as well as their ganglia, nerve endings and ensheathments.  Individual axons are surrounded by an endoneurium, not easily seen at this magnification.  Groups of axons form a fascicle enclosed by a perineurium.  Groups of fascicles form major nerve trunks enclosed by an epineurium.  100x

The perineurial cells form a compact wrapping around fascicles that contributes to the protective blood-nerve barrier. These cells are squamous, exhibiting epithelial features, such as individual basal laminae and tight junctions. However, they also contract, possessing large numbers of actin filaments, and synthesize collagen. Thus, they also resemble smooth muscle cells and fibroblasts.

The epineurium consists of a dense irregular connective tissue of collagen bundles that blends with the surrounding connective tissue. Epineurium surrounds larger trunks of peripheral nerves and is lost as peripheral nerves decrease in size.