satellite glial cell satellite glial cell satellite glial cells, formerly called amphicytes, are glial cells that cover the surface of neuron cell bodies in ganglia of the peripheral nervous system. thus, they are found 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. for the progenitor cells in muscle tissue, see satellite cell. satellite glial cellsatellite glial cells surround the cell bodies of sensory neuronsdetailslocationsurface of neuron bodies in sensory, sympathetic and parasympathetic gangliaidentifierslatingliocytus ganglionicusneurolex idsao792373294thh2.00.06.2.02002 anatomical terms of microanatomy structure satellite glial cells are expressed throughout the sympathetic and parasympathetic ganglia in their respective nervous system divisions. satellite glial cells are a type of glia 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 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. 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. in sensory ganglia satellite glial cells in sensory ganglia are laminar cells that wrap around sensory neurons. 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 sympathetic ganglia 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. 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. 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. function 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. sgcs role as a regulator of neuronal microenvironment is further characterized by its electrical properties which are very similar to those of astrocytes. astrocytes have a well studied and defined role in controlling the microenvironment within the brain, therefore researchers are investigating any homologous role of sgcs within the sympathetic ganglia. an established mode of controlling the microenvironment in sensory ganglia is the uptake of substances by specialized transporters which carry neurotransmitters into cells when coupled with na+ and cl−. transporters for glutamate and gamma-aminobutyric acid (gaba) have been found in sgcs. they appear to be actively engaged in the control of the composition of the extracellular space of the ganglia. the enzyme glutamine synthetase, which catalyzes the conversion of glutamate into glutamine, is found in large amounts in sgcs. additionally, sgcs contain the glutamate related enzymes glutamate dehydrogenase and pyruvate carboxylase, and thus can supply the neurons not only with glutamine, but also with malate and lactate. molecular properties unlike their adjacent neurons, sgcs do not have synapses but are equipped with receptors for a variety of neuroactive substances that are analogous to those found in neurons. axon terminals as well as other parts of the neuron carry receptors to substancesas acetylcholine (ach), gaba, glutamate, atp, noradrenaline, substance p, and capsaicin that directly affect the physiology of these cells. current research is revealing that sgcs are also able to respond to some of the same chemical stimuli as neurons. the research is ongoing and sgcs role in injury repair mechanisms is not yet fully understood. molecular characteristics of sgcs moleculetype of gangliamethod of detectioncomments glutamine synthetasemouse tgihccatalyzes the condensation of glutamate and ammonia to form glutamine gfaprat drg, tgihcupregulated by nerve damage s100rat drgihcupregulated by nerve damage endothelin etb receptorrat, rabbit drgihc, autoradiographyblockers of ets are shown to alleviate pain in animal models bradykinin b2 receptorrat drgelectrophysiologyinvolved in the inflammatory process p2y receptormouse tgca2+ imaging, ihccontributes to nociception ach muscarinic receptorrat drgihc, mrna (ish)role not well defined in sensory ganglia ngf trka receptorrat drgimmuno-emmay play a role in response to neuronal injury tgfαrat drgmrna (ish), ihcstimulates neural proliferation after injury erythropoietin receptorrat drgihc tnf-αmouse drg, tgihcinflammatory mediator increased by nerve crush, herpes simplex activation il-6mouse tgihccytokine released during inflammation, increased by uv irradiation erkrat drgihcinvolved in functions including the regulation of meiosis, and mitosis jak2rat drgihcsignaling protein apart of the type ii cytokine receptor family somatostatin sst1 receptorrat drgihcsomatostatin inhibits the release of many hormones and other secretory proteins gaba transporterrat drgautoradiography glutamate transporterrat drgmrna (ish), ihc, autoradiographyterminates the excitatory neurotransmitter signal by removal (uptake) of glutamate guanylate cyclaserat drg, tgihc for cgmpsecond messenger that internalizes the message carried by intercellular messengers such as peptide hormones and no pgd synthasechick drgihcknown to function as a neuromodulator as well as a trophic factor in the central nervous system clinical significance chronic pain 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. secretion of bioactive molecules sgcs have the ability to release cytokines and other bioactive molecules that transmit pain neuronally. neurotrophins and tumor necrosis factor α (tnfα) are other cellular factors that work to sensitize neurons to pain. sgcs are present in the pns in fewer numbers than other more well-known types of glial cells, like astrocytes, but have been determined to affect nociception because of some of their physiological and pharmacological properties. in fact, just like astrocytes, sgcs have the ability to sense and regulate neighboring neuronal activity. first, after a period of nerve cell injury, sgcs are known to up-regulate gfap and to undergo cell division. they have the ability to release chemoattractants, which are analogous to those released by schwann cells and contribute to the recruitment and proliferation of macrophages. additionally, several research groups have found that sgc coupling increases after nerve damage, which has an effect on the perception of pain, likely