The bulk of a peripheral nerve consists of nerve fibers and their supporting Schwann cells. The individual nerve fibers and their associated Schwann cells are held together by connective tissue organized into three distinctive components, each with specific morphologic and functional characteristics (Fig. 11.24; also, see Fig. 11.3). These components are
• Endoneurium, which includes loose connective tissue surrounding each individual nerve fiber
• Perineurium, which includes specialized connective tissue surrounding each nerve fascicle
• Epineurium, which includes dense irregular connective tissue that surrounds a peripheral nerve and fills the spaces between nerve fascicles
Endoneurium constitutes the loose connective tissue associated with individual nerve fibers
The endoneurium is not conspicuous in routine light microscope preparations, but special connective tissue stains permit its demonstration. At the electron microscope level, collagen fibrils that constitute the endoneurium are readily apparent (see Figs. 11.12 and 11.13). The fibrils run both parallel to, and around, the nerve fibers, functionally binding them together into a fascicle, or bundle. Because fibroblasts are relatively sparse in the interstices of the nerve fibers, it is likely that most of the collagen fibrils are secreted by the Schwann cells. This conclusion is supported by tissue culture studies in which collagen fibrils are formed in pure cultures of Schwann cells and dorsal root neurons. Other than occasional fibroblasts, the only other connective tissue cell normally found within the endoneurium is the mast cell. In general, most of the nuclei (90%) found in cross sections of peripheral nerves belong to Schwann cells; the remaining 10% is equally distributed between the occasional fibroblasts and other cells such as endothelial cells of capillaries and mast cells.
Perineurium is the specialized connective tissue surrounding a nerve fascicle
Surrounding the nerve bundle is a sheath of unique connective tissue cells that constitute the perineurium. The perineurium serves as a tnetabolically active diffusion barrier that contributes to the formation of a blood-nerve barrier. This barrier maintains the ionic milieu of the en-sheathed nerve fibers. In a manner similar to the properties exhibited by the endothelial cells of brain capillaries form
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