Histology of Ganglia
A ganglion (pl. ganglia) is a mass of nerve cell bodies found outside of the central nervous system (CNS) along with some glial cells and connective tissue. Ganglia have both afferent and efferent nerve fibers. As they exist outside of the CNS, they are sometimes referred to as peripheral ganglia.
Peripheral ganglia can be divided into two subtypes: sensory ganglia and autonomic ganglia. Sensory ganglia comprise the soma of sensory neurons. A classic example of this type of ganglia are the dorsal root ganglia. These are clusters of sensory nerve bodies found at the base of the spine which can respond to mechanical, chemical, and thermal stimuli. Meanwhile, autonomic ganglia comprise the cell bodies of postsynaptic neurons conducting impulses to glands, smooth and cardiac muscles.
The two types of autonomic ganglia are sympathetic and parasympathetic ganglia. Sympathetic ganglia form part of the sympathetic trunk, whereas parasympathetic lie close to or within the walls of the viscera. Another important type of ganglia are the enteric ganglia of the the enteric nervous system (ENS). Often referred to as a second brain, the ENS is separate from the CNS but has the ability to communicate with it. Whilst most ganglia occur outside of the CNS, there is one exception to the rule in the form of basal ganglia. These ganglia are found within the CNS, at the base of the forebrain. They are quite different from peripheral ganglia, as they are better described as a collection of nuclei rather than cells bodies.
In terms of the histology, this type of peripheral ganglia contains cell bodies of postganglionic multipolar neurons. The ganglia are surrounded by dense connective tissue capsule. On H&E staining most ganglia appear pale and foamy due to the presence of myelinated nerve fibers which wash away during the staining procedure. Meanwhile the cytoplasm of the cell bodies show basophilia due to high concentrations of Nissl substance, while the nuclei lie eccentrically with prominent nucleoli.
Surrounding the neuronal cell bodies are flattened satellite cells which are derived from neural crest cells, and form a single layer of cells. This layer of satellite cells around each neuronal cell body is just like satellites around a planet. These satellite cells are flat in shape. They are modified Schwann cells and have heterochromatic nuclei. These nuclei appear smaller than those of the neurons. The outer regions of the satellite cells are enveloped in a basement membrane.
It may be useful to note that when ganglia are stained as paraffin sections, artifacts can lead to extra space between the neuronal soma and satellite cell. In histological staining, the hematoxylin and eosin (H&E) stain is commonly used to visualise the structure of peripheral ganglia tissues.
The histological features of the sensory ganglia are similar to those of the autonomic ganglia apart from a few key differences. For example, the cells of sensory ganglia are usually unipolar or pseudounipolar with centrally placed nucleus. Along with the Nissl substance some of the cells also contain golden brown lipofuscin pigment in their cytoplasm.
Each unipolar cell is surrounded by two cellular layers, the inner layer is of satellite cells which are flat cells with small spherical nuclei and outer layer of capsule cells of connective tissue. There are also many fibrocytes within the surrounding connective tissue. Another important feature of the sensory ganglia is that they do not have synapses.