The endoneurium (also called endoneurial channel, endoneurial sheath, endoneurial tube, or Henle's sheath) is a layer of delicate connective tissue made up of endoneurial cells that encloses the myelin sheath of a nerve fiber. These endoneuria are bundled up into groups called nerve fascicles, which have a protective sheath called a perineurium. Several fascicles may be in turn bundled together with a blood supply and fatty tissue within yet another sheath, the epineurium.
The endoneurium runs mostly longitudinally along the nerve fiber with septa that pass inward from the innermost layer of the perineurium, and shows a ground substance in which are embedded fine bundles of fibrous connective tissue, primarily collagen. This serves to support capillaryblood vessels, arranged so as to form a network of elongated meshes. It is found in other places as well, such as surrounding the Schwann cells on the peripheral side of the transitional zone on the auditory nerve.