NERVES – 2
The structure of nerves
Differently from very thin and delicate nerves the larger ones carry several fascicles of nerve fibers separated by layers of connective tissue.
An outermost layer of dense irregular connective tissue called epineurium (highlighted in pink) surrounds the whole nerves. Its thickness depends on the thickness of the nerve. Nerves that are positioned inside connective tissue or inside an organ may have a less organized epineurium that blends with the surrounding connective tissue.
The connective tissue of the epineurium of larger and medium sized nerve spenetrates into the nerve interior, surrounding and separating its fascicles, as shown in the figure.
The perineurium (highlighted in blue) is a thin connective tissue layer that covers each fascicle of nerve fibers. It is formed of collagen fibers and cells with elongated nuclei.
A loose connective tissue called endoneurium (highlighted in red) surrounds each nerve fiber. It contains blood capillaries, collagen fibers, fibroblasts, a few mast cells and macrophages. As the endoneurim exists in small ammouts around each fiber it is sometimes inconspicuous in histological sections.
Often, due to the tissue processing, the endoneurioum undergoes some degree of retraction resulting in artificial spaces formed between nerve fibers.
Very thin nerves are covered by a thin connective tissue layer and have an endoneurium between its fibers.
Drawing modified from “Basic Histology”, Junqueira and Carneiro, 12a Ed.
