Peripheral Nerve Histology
Sections
Summary
Nerve vs Nerve fibers (axons)
- A peripheral nerve encompasses bundles of nerve fibers (axons) and has various connective tissue coverings (superficial epineurium, perineurium, and endoneurium).
Nerve Coverings
Outside to Inside:
Superficial epineurium
- Superficial epineurium is the outer covering of the nerve.
- It is a supporting coat: a cylindrical, dense connective tissue sheath.
Perineurium
- Perineurium encases separate nerve fascicles.
- It is a mechanically strong sheath that is dense and forms a protective barrier around the nerve fascicle: a blood-nerve barrier.
- Perineurium comprises a flattened form of epithelial cells that are joined by special junctions, which helps it withstand tremendous pressure.
Endoneurium
- Endoneurium is a loose connective tissue; it comprises collagenous fibers.
Additional, less often addressed terminology:
Deep epineurium
- Deep epineurium accounts for the connective tissue sandwiched between the nerve fascicles.
- We find vasculature in this region.
Perineurial septa
- Perineurial septa pass through the nerve fascicles and carry vasculature to the nerve fibers.
Nerve fibers (Axons)
- The nerve fiber is the nerve cell axon.
- Myelin sheath surrounds myelinated axons.
- Schwann cells: each myelinates at most one axon internode.
Myelin
Unmyelinated cell
In an unmyelinated cell, we see:
- The Schwann cell nucleus intermixed with axons within Schwann cell cytoplasm.
- The mesaxon is the zone of apposition for the Schwann cell membrane and the axon.
- Multiple axons can pass through a Schwann cell, when the nerve fiber is unmyelinated.
Myelinated cell
In a myelinated cell, we see:
- The Schwann cell nucleus lies off to the side of the axon, which is enveloped in concentric circles of myelin: a myelinated sheath.
Schwann cells vs oligodendrocytes
- Unlike oligodendrocytes, which can myelinate up to 50 individual axons, a Schwann cell only myelinates one axon, as shown here (specifically only one peripheral nervous system internode).
Dorsal root ganglion
We highlight the following:
- Ganglion cell
- Cell nucleus
- Surrounding satellite cells (which support the ganglionic neurons).
- Satellite cells are derived from neural crest cells in the same way that Schwann cells are.
Peripheral nerve in longitudinal view
We highlight the following:
- Schwann cells
- Myelin sheath
- The myelin sheath is vacuolated because its high lipid content creates this artifact upon paraffin embedding.
- Deep stained nerve fibers (axons).
Clinical Correlation
Full-Length Text
Nerve Coverings
Overview
- Here, we'll learn the histology of the peripheral nerves.
- To begin, let's draw a cross-section of a peripheral nerve.
Superficial Epineurium
- First, draw the outer covering of the nerve: the superficial epineurium.
Perineurium
- Within the nerve cross-section, draw two separate nerve fascicles, which are are encased in perineurium, which is a mechanically strong sheath that is dense and forms a protective barrier around the nerve fascicle: a blood-nerve barrier.
Endoneurium
- Then, immediately internal to it, draw the endoneurium, which is a loose connective tissue; intermixed in the endoneurium, draw various groups of nerve fibers.
Nerve fiber (axon)
- Indicate the nerve fiber axon.
Myelin
- Then draw the myelin sheath that surrounds myelinated axons.
Schwann cell
- Then, include a Schwann cell, each of which myelinates at most one axon internode.
Additional Terminology
Deep epineurium
- First, specify the deep epineurium, which accounts for the connective tissue sandwiched between the nerve fascicles.
- Add a representative artery and vein to this region.
Perineurial septa
- Then, draw perineurial septa, which pass through the nerve fascicles and carry vasculature to the nerve fibers.
Histology
Now, let's look at these histological features in actual slides.
- In one slide, stained with H&E, we see two nerve fascicles.
- On the other, a darkly stained section (stained with osmium tetroxide), one see one major nerve fascicle.
- On the H&E slide, label the perineurium of one of them; we see that it comprises a flattened form of epithelial cells.
- They are joined by special junctions, which helps it withstand tremendous pressure.
- And label its endoneurium, which mainly comprises collagenous fibers.
- Then, label the superficial epineurium, which is a supporting coat: a cylindrical, dense connective tissue sheath.
- And the deep epineurium that lies between the fascicles.
- Label its perineurium.
- Then, the endoneurium.
- Next, the peineurial septa, which pass through the nerve fascicle.
- Next, label the surrounding superficial epineurium.
- Then, the deep epineurium.
Myelination
Now, let's address Schwann cells and show how they myelinate cells.
Unmyelinated cell
- Draw the Schwann cell nucleus.
- Then, draw a few axons.
- Now, surround the nucleus and axons in the Schwann cell cytoplasm.
- Indicate mesaxon for the zone of apposition for the Schwann cell membrane and the axon.
- Importantly, we show that multiple axons can pass through a Schwann cell, when the nerve fiber is unmyelinated.
Myelinated cell
- Next, draw the cytoplasm of another Schwann cell.
- Then, draw an axon.
- Show that it's enveloped in concentric circles of myelin – a myelinated sheath.
- Draw its nucleus off to the side.
Schwann cells vs oligodendrocytes
- Unlike oligodendrocytes, which can myelinate up to 50 individual axons, a Schwann cell only myelinates one axon, as shown here (specifically only one peripheral nervous system internode).
Myelin Histology
Let's look at a higher magnification section of our osmium tetroxide stained slide.
- Point out a typical, black staining myelin sheath.
Dorsal Root Ganglion
Now, let's look at a dorsal root ganglion.
- Identify a ganglion cell.
- Then, the nucleus.
- Then, the surrounding satellite cells, which support the ganglionic neurons.
- We show them, now, because they are derived from neural crest cells in the same way that Schwann cells are.
Longitudinal view of a peripheral nerve
Finally, let's examine two slides of a peripheral nerve in longitudinal view.
- Label a Schwann cell in each slide, then show myelin sheath, which is vacuolated because its high lipid content creates this artifact upon paraffin embedding.
- Then, label deep stained nerve fibers.
Sensory Receptors
This information is not within the present tutorial but is available, here:
Overview
Major Sensory Receptor Types
- There are six major forms of sensory receptors (from superficial to deep):
- Free nerve endings
- Merkel cells
- Meissner corpuscles
- Ruffini corpuscles & Krause end bulbs
- Pacinian corpuscles
Major Classes of Sensory Receptors
- There are three major classes of sensory receptors:
- Mechanoreceptors detect mechanical deformation
- Nociceptors detect pain
- Thermoreceptors detect temperature
Sensory Receptors by Location within the skin
EPIDERMIS
Free nerve endings
- Unmyelinated nerve endings.
- Detect pain and temperature.
BASAL EPIDERMIS
Merkel cells
- Discs at the ends of unencapsulated nerve fibers.
- Detect tactile sensation: light touch/hair movement.
SUPERFICIAL DERMIS
Meissner corpuscles (aka tactile corpuscles)
- Elliptical shaped.
- Lie perpendicular to the epidermis, within the dermal papillae.
- Detect light touch; hence, "tactile corpuscle".
DERMIS
Ruffini corpuscles
- Detect pressure.
Krause end bulbs
- Detect pressure and low-frequency vibration.
DEEP DERMIS & HYPODERMIS
Pacinian corpuscles (aka lamellated corpuscles)
- Pacinian corpuscles are large and oval-shaped with a very characteristic concentric, lamellated histological appearance.
- They are encapsulated mechanoreceptors that detect touch, pressure, vibration.
- Capsule distortion works to amplify the signal of a mechanical stimulus.
Histological Images
Histological images modified with permission from:
- http://medsci.indiana.edu/a215/virtualscope/docs/chap6_3.htm
- http://medsci.indiana.edu/c602web/602/c602web/toc.htm