Collagen Fibrils in the Peripheral Nerves

Peripheral nerves have three layers of connective tissue sheaths, i.e., endoneurium, perineurium and epineurium. Each of them has some proper cells and contains different types of collagen fibrils. Endoneurium consists of thin collagen fibrils whose diameters are uniform. These thin collagen fibrils are made of collagen type III which are produced by Schwann cells. The quantity of the endoneurial collagen fibrils depends on the sorts of peripheral nerves. Generally there is some space without collagen fibrils in the endoneurium, but the endoneurial space of facial nerve is filled up with abundant thin collagen fibrils. It is thought that these collagen fibrils protect the nerve fibers against external forces. On the other hand, the optic nerve has a central nervous system-like morphology and no collagen fibrils are seen between axons. At the beginning of the development, thin collagen fibrils appear not only in the endoneurial space but also in peri- and epineurial spaces. During development endoneurial collagen fibrils do not change their diameter, but peri- and epineurial collagen fibrils increase in their diameter. Finally, epineurium consists of the typical thick collagen fibrils which are the same as the collagen fibrils in the skin. Epineurial collagen fibrils are mainly made of type I collagen. The perineurium consists of both thin and thick collagen fibrils, displaying a wide spectrum of the fibril diameter. It is thought that they contain both type I and III collagens. They show characteristics intermediate between endo- and epineurium.