endoneurium

{{Short description|Connective tissue layer around myelinated nerve fibers in peripheral nervous system}}

{{Infobox anatomy

| Name = Endoneurium

| Latin =

| Image = Gray636.png

| Caption = Transverse section of human tibial nerve.

| Image2 = Illu nerve structure.jpg

| Caption2 = Nerve structure

| System = Nervous system

| part_of = myelin sheath of nerves

}}

The endoneurium (also called endoneurial channel, endoneurial sheath, endoneurial tube, or Henle's sheath) is a layer of delicate connective tissue around the myelin sheath of each myelinated nerve fiber in the peripheral nervous system.{{cite web |title=Neurohistology lecture |url=http://vanat.cvm.umn.edu/NeuroLectPDFs/NeurohistologyLectI.pdf |access-date=27 April 2019}} Its component cells are called endoneurial cells.{{cite book | author = Elaine N. Marieb and Katja Hoehn | title = Human Anatomy & Physhyphysiology (7th Ed.) | publisher = Pearson | pages = 498–9 | year = 2007 | isbn = 978-0-8053-5909-1}} The endoneuria with their enclosed nerve fibers are bundled into groups called nerve fascicles, each fascicle within its own protective sheath called a perineurium.{{Citation|last1=Barral|first1=Jean-Pierre|title=Chapter 1 - Some preliminary thoughts|date=2009-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780702031007500045|work=Manual Therapy for the Cranial Nerves|pages=1–5|editor-last=Barral|editor-first=Jean-Pierre|place=Edinburgh|publisher=Churchill Livingstone|language=en|doi=10.1016/b978-0-7020-3100-7.50004-5|isbn=978-0-7020-3100-7|access-date=2020-11-18|last2=Croibier|first2=Alain|editor2-last=Croibier|editor2-first=Alain}}{{Citation|last=Sheikh|first=Kazim A.|title=25 - AMAN: What It Teaches Us about Mechanisms Underlying Axonal Injury|date=2005-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780127387611500262|work=Multiple Sclerosis As A Neuronal Disease|pages=355–371|editor-last=Waxman|editor-first=Stephen G.|place=Burlington|publisher=Academic Press|language=en|doi=10.1016/b978-012738761-1/50026-2|isbn=978-0-12-738761-1|access-date=2020-11-18}} In sufficiently large nerves multiple fascicles, each with its blood supply and fatty tissue, may be bundled within yet another sheath, the epineurium.

The endoneurium contains a liquid known as endoneurial fluid, which contains little protein. In the peripheral nervous system the endoneurial fluid is notionally equivalent to cerebrospinal fluid in the central nervous system. Peripheral nerve injuries commonly release increased amounts of endoneurial fluid into surrounding tissues; these can be detected by magnetic resonance neurography, thereby assisting in locating injuries to peripheral nerves.

The endoneurium runs longitudinally along the nerve fiber, but with discontinuities where septa pass inward from the innermost layer of the perineurium. It contains fine bundles of fibrous connective tissue, primarily collagen, embedded in a matrix of ground substance.{{Citation|last1=Reina|first1=Miguel A.|title=Chapter 7 - Microscopic Morphology and Ultrastructure of Human Peripheral Nerves|date=2015-01-01|url=http://www.sciencedirect.com/science/article/pii/B978012410390000007X|work=Nerves and Nerve Injuries|pages=91–106|editor-last=Tubbs|editor-first=R. Shane|place=San Diego|publisher=Academic Press|language=en|doi=10.1016/b978-0-12-410390-0.00007-x|isbn=978-0-12-410390-0|access-date=2020-11-18|last2=Sala-Blanch|first2=Xavier|last3=Arriazu|first3=Riánsares|last4=Machés|first4=Fabiola|editor2-last=Rizk|editor2-first=Elias|editor3-last=Shoja|editor3-first=Mohammadali M.|editor4-last=Loukas|editor4-first=Marios}} This structure serves to support capillary blood vessels, arranged so as to form a network of elongated meshes. Similar structures occur around some nervous components elsewhere in the body, for example around the Schwann cells on the peripheral side of the transitional zone on the auditory nerve.{{cite journal |author=Fraher JP |title=The transitional zone and CNS regeneration |journal=J. Anat. |volume=196 |issue= 1|pages=137–58 |year=2000 |pmid=10697296 }}