Peripheral nervous system histology


The peripheral nervous system or PNS consists of all the nerves and ganglia outside of the central nervous system.

The central nervous system consists of the cerebellum, cerebrum, brainstem, and spinal cord.

The peripheral nerves consist of at least one bundle of nerve fibers called a fascicle, which has a surrounding layer of connective tissue called the perineurium.

This cross-section of a larger peripheral nerve was stained with Masson’s trichrome.

If we zoom in to look at just one fascicle, we can see that the thin layer of perineurium surrounding the fascicle is stained a darker blue.

At 40x magnification, we can see the individual nerve fibers, but the structures are not as easy to identify compared to a similar image stained with H&E instead.

The very thin and delicate layer of connective tissue surrounding each nerve fiber is called the endoneurium.

The endoneurium stains a slightly darker pink compared to its contents, which include the individual nerve axons at the center, as well as the Schwann cells or neurolemmocytes that form the myelin sheath around larger nerve fibers.

The myelin sheath can be seen as a thick pink outer ring that surrounds many of the nerve fibers.

The basophilic or purple round nuclei found along the outer edge of the nerves are the nuclei of schwann cells.

Alright, if we take a look at the outer edge of the peripheral nerve, we can see the outermost layer of dense irregular connective tissue called the epineurium.

The epineurium surrounds the entire nerve and also fills the space between fascicles in order to hold the fascicles together along with the blood vessels and loose connective tissue within the nerve.

Now, if we take a look at a longitudinal section of a large peripheral nerve, the nerve fibers have a wavy or zigzag pattern.

This arrangement of the nerve fibers helps to protect the nerves from damage when they’re stretched due to normal movement or if the nerve is connected to flexible tissue.

At 40x magnification, we can faintly see myelin sheaths surrounding their nerve axons.

With an H&E stain, the myelin sheaths have pink and red stripes or striae along the upper and lower portions of each nerve fiber.

The nuclei of Schwann cells can be identified by their round or oval appearance, while the nuclei of fibroblasts typically have more elongated nuclei.

In this image, a node of Ranvier can be seen as a purple line that’s perpendicular to the nerve fiber.

The nodes of Ranvier are the small gaps or junctions between neighboring Schwann cells.

We can even see how the myelin sheaths become more narrow right before they reach the node.

Alright, let’s move on to the ganglia, which are distinct groups of neuron cell bodies found outside of the CNS.

This low power image of the spinal cord shows portions of the dorsal root ganglia or spinal ganglia on each side of the spinal cord as they pass through spaces between vertebrae called the intervertebral foramina.

The dorsal root ganglia consists of sensory neurons; And if we zoom in a little closer, we can see that the sensory neuron cell bodies are mostly found along the outer portions of the ganglion.

In the center of the ganglion is where many of the sensory nerve axons exit the dorsal root ganglion.