Satellite glial cell

Satellite glial cells are glial cells that cover the surface of nerve cell bodies in sensory, sympathetic, and parasympathetic ganglia. Both satellite glial cells (SGCs) and Schwann cells (the cells that ensheathe some nerve fibers in the PNS) are derived from the neural crest of the embryo during development. SGCs have been found to play a variety of roles, including control over the microenvironment of sympathetic ganglia. They are thought to have a similar role to astrocytes in the central nervous system (CNS). They supply nutrients to the surrounding neurons and also have some structural function. Satellite cells also act as protective, cushioning cells. Additionally, they express a variety of receptors that allow for a range of interactions with neuroactive chemicals. Many of these receptors and other ion channels have recently been implicated in health issues including chronic pain and herpes simplex. There is much more to be learned about these cells, and research surrounding additional properties and roles of the SGCs is ongoing. Satellite glial cells are glial cells that cover the surface of nerve cell bodies in sensory, sympathetic, and parasympathetic ganglia. Both satellite glial cells (SGCs) and Schwann cells (the cells that ensheathe some nerve fibers in the PNS) are derived from the neural crest of the embryo during development. SGCs have been found to play a variety of roles, including control over the microenvironment of sympathetic ganglia. They are thought to have a similar role to astrocytes in the central nervous system (CNS). They supply nutrients to the surrounding neurons and also have some structural function. Satellite cells also act as protective, cushioning cells. Additionally, they express a variety of receptors that allow for a range of interactions with neuroactive chemicals. Many of these receptors and other ion channels have recently been implicated in health issues including chronic pain and herpes simplex. There is much more to be learned about these cells, and research surrounding additional properties and roles of the SGCs is ongoing. Satellite glial cells are the principal glial cells found in the peripheral nervous system, specifically in sensory, sympathetic, and parasympathetic ganglia. They compose the thin cellular sheaths that surround the individual neurons in these ganglia. In an SGC, the cell body is denoted by the region containing the single, relatively large nucleus. Each side of the cell body extends outward, forming perineuronal processes. The region containing the nucleus has the largest volume of cytoplasm, making this region of the SGC sheath thicker. The sheath can be even thicker if multiple SGCs are layered on top of one another, each measuring 0.1 micrometres (3.9×10−6 in). Despite their flattened shape, satellite glial cells contain all common organelles necessary to make cellular products and to maintain the homeostatic environment of the cell. The plasma membrane of SGCs is thin and not very dense, and it is associated with adhesion molecules, receptors for neurotransmitters and other molecules, and ion channels, specifically potassium ion channels. Within individual SGCs, there is both rough endoplasmic reticulum and smooth endoplasmic reticulum, but the latter is much less abundant. Most often the Golgi apparatus and the centrioles in an SGC are found in a region very close to the cell’s nucleus. On the other hand, mitochondria are found throughout the cytoplasm along with the organelles involved in autophagy and other forms of catabolic degradation, such as lysosomes, lipofuscin granules, and peroxisomes. Both microtubules and intermediate filaments can be seen throughout the cytoplasm, and most often they lie parallel to the SGC sheath. These filaments are found in greater concentrations at the axon hillock and at the beginning portion of an axon in an SGC of the sympathetic ganglia. In some SGCs of the sensory ganglia researchers have seen a single cilium that extends outward from the cell surface near the nucleus and into the extracellular space of a deep indentation in the plasma membrane. The cilium, however, only has the nine pairs of peripheral microtubules while it lacks the axial pair of microtubules, making its structure very similar to the cilia of neurons, Schwann cells, and astrocytes of the CNS. Satellite glial cells in sensory ganglia are laminar cells that most often an envelope of multiple SGCs completely surrounds each sensory neuron. The number of SGCs that make up the sheath increases proportionately with the volume of the neuron which it surrounds. Additionally, the volume of the sheath itself increases proportionately with the volume and surface area of the neuron’s somata. The distance of extracellular space between the sheath and the neuronal plasma membrane measures 20 nanometres (7.9×10−7 in), allowing the neuron and its SGC sheath to form a single anatomical and functional unit. These individual units are separated by areas of connective tissue. However, there are some sensory neurons that occupy the same space within connective tissue and are therefore grouped together in a “cluster” of two or three neurons. Most often each individual neuron in a cluster is still surrounded by its own SGC sheath, but in some cases it is missing. Some sensory neurons have small projections called microvilli that extend outward from their cell surfaces. Due to their close proximity to the SGC sheath, these microvilli of the neuronal plasma membrane reach into the grooves of the sheath, allowing for possible exchange of materials between the cells. In the sympathetic ganglia, satellite glial cells are one of three main types of cells, the other two being the sympathetic ganglion neurons and small intensely fluorescent (SIF) cells. SIF cells of sympathetic ganglia are separated into groups, each of which is surrounded by an SGC sheath. The SGCs of the sympathetic ganglia come from the neural crest and do not proliferate during embryonic development until the neurons are present and mature, indicating that the neurons signal the division and maturation of the SGCs. The SGCs of sympathetic ganglia follow the same basic structure as the SGCs of sensory ganglia, except that sympathetic ganglia also receive synapses. Therefore, the SGC sheath of sympathetic neurons must extend even further to cover the axon hillock near the somata. Like the regions of the sheath near the glial nucleus, the regions of the sheath at the axon hillocks are thicker than those surrounding the rest of the neuron. This indicates that the SGCs play a role in the synaptic environment, thereby influencing synaptic transmission. Many people liken SGCs to the astrocytes of the CNS because they share certain anatomical and physiological properties, such as the presence of neurotransmitter transporters and the expression of glutamine synthetase. However, there are distinguishing factors that put SGCs in their own distinct category of glial cells. SGCs most often surround individual sensory and parasympathetic neurons with a complete, unbroken sheath while most neurons of sympathetic ganglia lack a completely continuous SGC sheath, allowing for limited direct exchange of materials between the extracellular space of the neuron and the space within the connective tissue where the SGCs are situated. Furthermore, gap junctions exist between SGCs in the sheaths of adjacent neurons as well as between SGCs in the same sheath (reflexive gap junctions). These gap junctions have been identified through the use of electron microscopy and weight tracer markers, such as Lucifer yellow or neurobiotin. The degree to which SGCs are coupled to SGCs of another sheath or to SGCs of the same sheath is dependent on the pH of the cellular environment. From studies on rats and mice, researchers have found that satellite glial cells express many neurotransmitter receptors, such as muscarinic acetylcholine and erythropoietin receptors. In order to differentiate between SGCs and other glial cells researchers have used markers to identify which proteins are found in different cells. Although SGCs express glial fibrillary acidic protein (GFAP) and different S-100 proteins, the most useful marker available today for SGC identification is glutamine synthetase (GS). The levels of GS are relatively low at rest, but they greatly increase if the neuron undergoes axonal damage. Furthermore, SGCs also possess mechanisms to release cytokines, adenosine triphosphate (ATP), and other chemical messengers. Research is currently ongoing in determining the physiological role of satellite glial cells. Current theories suggest that SGCs have a significant role in controlling the microenvironment of the sympathetic ganglia. This is based on the observation that SGCs almost completely envelop the neuron and can regulate the diffusion of molecules across the cell membrane. It has been previously shown that when fluorescent protein tracers are injected into the cervical ganglion in order to bypass the circulatory system, they are not found on the neuron surface. This suggests that the SGCs can regulate the extracellular space of individual neurons. Some speculate that SGCs in the autonomic ganglia have a similar role to the blood–brain barrier as a functional barrier to large molecules.