The main difference between endoneurium perineurium and epineurium is that endoneurium covers the axon, while perineurium covers each fascicle, and epineurium is the outermost dense connective tissue covering the bundles of fascicles and spaces between nerve fibers.
An axon is a long slender projection of the nerve cell that conducts electric impulses known as action potentials away from the nerve cell body. Moreover, axons are covered by a layer, and multiple covered axons create a fascicle. A bundle of fascicles is then covered by another layer, which is surrounded by dense connective tissue. These covering tissues serve as protective coverings for the axons and nerves within.
Key Areas Covered
1. What is Endoneurium
– Definition, Characteristics, Function
2. What is Perineurium
– Definition, Characteristics, Function
3. What is Epineurium
– Outline of Common Features
4. Difference Between Endoneurium Perineurium and Epineurium
– Comparison of Key Differences
Endoneurium, Epineurium, Perineurium
What is Endoneurium
Endoneurium is a thin layer of collagenous connective tissue that surrounds individual nerve fibers. It is continuous with the fine layer of connective tissue of the nerve roots. Some other names for endoneurium are interfascicular connective tissue, Henle’s sheath, endoneurial channel, endoneurial sheath, or endoneurial tube. It directly surrounds the Schwann cell axon units, which are myelinated axons surrounded by Schwann cells. It also accompanies endoneurial capillaries that supply nerve axons.
Endoneurium has three main components: cellular components, fibers, and matrix. The cellular components of endoneurium include Schwann cells, endothelial cells, fibroblasts, macrophages, and mast cells. Fibroblasts can be seen scattered all around the tissue. Moreover, type III collagens (reticulin), which are the fibrous component of the tissue, are arranged parallel to the nerve fibers. Both unmyelinated and myelinated cells are surrounded by fibers. These collagen fibers are permeable and form clusters below the perineurium, around nerve fibers, and around blood vessels. Furthermore, fibers and cells are embedded in the fluid matrix known as endoneurial fluid.
The endoneurium is continuous with septa, which pass inward from the innermost layer of the perineurium. This is a very fragile connective tissue layer. It also covers the space between Schwan cells. The main purpose of the endoneurium is to electrically isolate individual nerve axons. In addition, it helps in maintaining endoneurial fluid pressure.
What is Perineurium
Perineurium is a sheath of connective tissue surrounding a bundle of nerve fibers (fascicles) within a nerve. Perineurium isolates groups of axon-Schwann cell units to form nerve fascicles. It consists of alternating layers of large, concentric, flattened polygonal cells, which were once thought to be related to fibroblasts but are now believed to include glial cells derived from the CNS.
According to the number and size of the fascicles in the nerve, the number of perineurial layers varies in the body. For example, large fascicles in the sciatic nerve may have 15 layers. Towards the nerve periphery, the thickness of the perineurium decreases, where the smallest fascicles may only have one or two perineurial layers. Between the neighboring perineurial cells are the pockets of the extracellular matrix. This matrix consists of fibrilla and microfibrillar collagens and fibronectin. This provides the ability to modulate external stretching forces, thus regulating the endoneurial pressure. Moreover, the perineurium constitutes the main diffusion barrier between the extra fascicular tissues and the endoneurium.
What is Epineurium
The epineurium is the outermost layer of dense irregular connective tissue surrounding a peripheral nerve. It is the outermost layer of a peripheral nerve and surrounds multiples of nerve fascicles. This belongs to the category of loose connective tissues and is the strongest layer. The composition of the epineurium mainly includes collagen fibers, type I and type III. Other constituents are lymphatics, fibroblasts, fat, blood vessels, and vasa nervorum.
Epineurium melds with adipose tissue surrounding peripheral nerves, specifically in subcutaneous tissue. The amount of epineurium tissue present varies along a nerve. It is more abundant in the joints. Moreover, epineurium’s thickness varies in different nerves as well as in different locations of the same nerve.
About 30%-75% of the cross-sectional area of the nerve is epineurium, but it is absent around the mono-fascicular nerves and at the nerve endings. Moreover, the main function of the epineurium is cushioning nerves. Additionally, it contributes to tensile strength.
Difference Between Endoneurium Perineurium and Epineurium
Endoneurium is a thin layer of collagenous connective tissue that surrounds individual nerve fibers, while perineurium is a sheath of connective tissue surrounding a bundle of nerve fibers (fascicles) within a nerve. On the other hand, the epineurium is the outermost layer of dense irregular connective tissue that surrounds a peripheral nerve.
The endoneurium covers the axon, while perineurium covers each fascicle, and the epineurium covers the bundles of fascicles and spaces between nerve fibers.
The main function of the endoneurium is to electrically isolate individual nerve axons, whereas the main function of the perineurium is to provide a protective barrier and structural support to the axons and Schwann cells within a nerve fascicle. Moreover, the main function of the epineurium is to cushion nerves and contribute to tensile strength.
In brief, endoneurium, perineurium, and epineurium cover the axons, fascicles, and nerves, and provide protection and separation from other substances electrically. The main difference between endoneurium perineurium and epineurium is that endoneurium covers the axon, while perineurium covers each fascicle, and epineurium is the outermost dense connective tissue covering the bundles of fascicles and spaces between nerve fibers.