The perineurium is composed of multiple concentric single-cell layers enclosing individual nerve fascicles. Each layer has a thickness equivalent to the width of a perineurial cell. Groups of these cells join by means of tight junctions and desmosomes to form layers that function as a barrier against diffusion of particles across them. Perineurial internal layers have more of these specialized unions among perineurial cells, which are proximal to nerve fascicles [1–4].

Perineurial cells have basal laminas constituted by glycoproteins such as fibronectin, as well as heparin sulfate and laminin, and their nuclei are flattened. The cytoplasm is granular and contains few mitochondria and pinocytic vesicles. Measurements of the thickness of the perineurium are related to the number of layers contributing to its shape, which ranges from 8 to 16 perineurial layers. Extracellular matrix, together with collagen fibers and a few fibroblasts, occupies the spaces between perineurial laminas [1–6].

The membrane allows the axons a certain degree of mobility within the nerve fascicles, and it becomes progressively thinner as the number of fascicles increases. Tension exerted on the perineurium is transmitted successively toward the endoneurium, affecting the intracellular pressure in the axon. The perineurium, together with endothelial cells of intrafascicular capillaries, contributes to the blood–nerve barrier (Figs. 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10, 4.11, 4.12, 4.13, 4.14, 4.15, 4.16, 4.17, 4.18, 4.19, 4.20, 4.21, 4.22, 4.23, 4.24, 4.25, 4.26, 4.27, 4.28, 4.29, 4.30, 4.31, and 4.32) [7–10].