Endoneurium

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Histological appearance of the peripheral nerves.

The endoneurium also called intrafascicular connective tissue or Henle's sheath is a connective tissue sheath that directly surrounds individual Schwann cell–axon units (myelinated axon surrounded by Schwann cells) of peripheral nerves and accompanying endoneurial capillaries that supply the nerve axons.

The endoneurium is composed mainly of loosely arranged type III collagen (reticulin) fibers that lie parallel to the nerve fibres. Its cellular components are primarily Schwann cells and endothelial cells with scattered fibroblasts, macrophages and mast cells. Endoneurial fluid bathes both the fibrous and cellular components of the endoneurium.

In large peripheral nerves, endoneurium covered axons are grouped into separate bundles called fascicles which are wrapped by a layer of connective tissue called the perineurium. These fascicles are in turn surrounded by a superficial connective tissue called the epineurium.

The main function of the endoneurium is to electrically isolate individual nerve axons. It also plays an important role in maintaining endoneural fluid pressure.