Satellite glial cell

Satellite glial cells, formerly called amphicytes,

Satellite glial cells are a type of glia found in the

In a 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.^[3] 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).^[9]

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,^[10] and it is associated with adhesion molecules,^[11] receptors for neurotransmitters and other molecules,^[10] and ion channels, specifically potassium ion channels.^[12] Within individual SGCs, there is both rough endoplasmic reticulum^[13] and smooth endoplasmic reticulum, but the latter is much less abundant.^[10] 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^[10] along with the organelles involved in autophagy and other forms of catabolic degradation, such as lysosomes, lipofuscin granules, and peroxisomes.^[14] 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.^[10] 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.^[15] 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.^[10]

In sensory ganglia

Satellite glial cells in sensory ganglia are laminar cells that wrap around sensory neurons.

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.^[3] SIF cells of sympathetic ganglia are separated into groups, each of which is surrounded by an SGC sheath.^[19] 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.^[4] 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.^[20] 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.

Differences from other glial cells

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.^[3] 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.^[9] Furthermore, gap junctions exist between SGCs in the sheaths of adjacent neurons as well as between SGCs in the same sheath (reflexive gap junctions).^[2] 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.^[2]

From studies on rats and mice, researchers have found that satellite glial cells express many neurotransmitter receptors, such as

Glial cells, including SGCs, have long been recognized for their roles in response to neuronal damage and injury. SCGs have specifically been implicated in a new role involving the creation and persistence of chronic pain, which may involve hyperalgesia and other forms of spontaneous pain.^[30]

Secretion of bioactive molecules

SGCs have the ability to release

Various neuronal receptors present on SGCs have been named as participants in ATP-evoked pain signals, particularly the homomultimer

SGCs also express a specific type of channel, the Kir4.1 channel, which works to maintain the desired low extracellular K⁺ concentration in order to control hyperexcitability, which is known to cause migraines. Additionally, extracellular K⁺ concentration has been found to be controlled by guanine nucleoside guanosine (Guo). Guo, which may be involved in neuron-to-SGC communication and interaction in sensory ganglia, is also a potential target that could control the alterations of extracellular K⁺ concentration associated with chronic pain.^[6]

Herpes simplex

Sensory ganglia have been associated with infections from viruses like herpes simplex, which can exist in a dormant state within the ganglia for decades after the primary infection.

The majority of the information available on the subject of SGCs comes from research which was focused on the sensory neurons that the SGCs surround rather than the SGCs themselves. In the future, researchers plan to give more time and attention to the SGCs, which have many supportive and protective functions essential for life.[2] Neurotransmitter and hormone receptors on SGCs in situ rather than in culture will likely be explored and definitively characterized.^[2] Changes in the receptors caused by various mutations and diseases will also be explored in order to determine the effect of these conditions.^[2] Additionally, the mechanisms behind neuronal-SGC communication is essentially unidentified, though it is likely that the various receptors both the neurons and SGCs have are used for chemical signaling, perhaps with P2Y.^[35] Ca²⁺ and NO and their effects must also be observed to gain further understanding of interactions between the two types of cells.^[2] Finally, the possibility of an influence of SGCs on synaptic transmission within autonomic ganglia provides another direction for future research.^[8]

See also

List of distinct cell types in the adult human body

References