Glial cells are cells of the nervous system composed of a number of support schemes which are essential for the normal functionality of the nervous system. The structure of glial cells is quite different from that of neurons. Unlike neurons, they neither possess an axon or dendrites. They also lack the capacity to conduct nerve impulses. They are significantly smaller and three times more abundant than the neurons in the nervous system. Glial cells have a number of functions in the nervous system. Some of them include, provision of support for the brain, insulting neurons, aiding in the repair and maintenance of the nervous system, provision of metabolic functions for the neurons, and lastly taking part in the development of the nervous system (Beckstead, 1996). However, the functions of glial cells might vary in different organisms in different environmental settings. This paper focuses on the structure and functions of glial cells in hydrothermal vent endemic worms.
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The environmental conditions under which hydrothermal vent endemic worms survive is quite extreme, hence the need for unique innovations. Hydrothermal vent environment is characterized by high levels of toxic agents within the abiotic area responsible for the rapid changes and uncontrolled mixing of sea waters and toxic gases. The deep-sea environment is also characterized by hypoxic conditions, an abundance of heavy metals, low pH, lethal metabolites and hydrogen sulfide. These environmental conditions compromise the survival of many organisms. To survive such conditions, hydrothermal vent organisms, such as Alvinella pompejana and Paralvinella sulfinicola have developed distinctive characteristics, including significant structural modifications in their nervous system (Shigeno et al., 2014). A number of studies have proven that such organisms exhibit exceptional thermos-tolerance in the functioning of the essential enzymes responsible for anti-oxidative processes and high protein stability in the above species (Dilly et al., 2012). However, there is still very little information regarding how the nervous system has adapted to this environment to be able to survive such adverse conditions.
A recent study found out that there is a distinct structural modification of the nervous system supportive tissue of the hydrothermal vent endemic worms responsible for their unique adaptations. The study was conducted on an endemic polynoid scale-worm, Branchinotogluma japonica. The species was of interest due to their free-living nature in addition to being fast-moving predators with active locomotive organs. Scale-worm species have a wide variety of habitats ranging from diffuse hydrothermal venting fields to higher temperature chimney walls. Frequent observations made suggest that the scale-worm share the same niche as that of the alvinellid worm, Paralvinella hessleri. However, the habitat of the scale worm is quite different from that of deep-water mussel Bathymodiolus septemdierum, the galatheid lobster Shinkaia crosnieri, and the alvinocarid shrimp Alvinocaris longirostris (Shigeno et al., 2015). This similarities and differences suggest that the nervous system of scale-worms is quite different and might possess tolerance mechanisms for their survival in the toxic hydrothermal vent environments.
In the same study, it was discovered that members of the scale-worm species, Branchinotogluma japonica, possess two distinct organizations in their nervous system. The first distinctive characteristic was that all the neuronal cell bodies of the brain, with exception of the globuli cells, are ensheathed by multiple layers of membrane. There is still a lot of speculations on the functional significance of the wrapping of the cell body. However, it is expected that the highly developed glial cell network is responsible for the reception of a variety of mechanical, chemical and electrical signals through intracellular information coding and extracellular ionic regulation, just in the same way as it had been previously observed in different glial cell types. Different studies show that the breakdown of the glial cell membrane, in the human blood-brain barriers, is the first step that takes place under hypoxia. The glial cell membranes multi-layer characteristic could, therefore, be the main adaptive feature for the protective mechanisms required to maintain the active neuronal cells in the hypoxia (Treherne, & Moreton, 1970). The same feature might also be responsible for maintaining the activity of neuronal cells in the extreme environment of hydrothermal vents. Globuli cells are however not ensheathed unlike other cells in the scale-worm brain. This calls for further research on the cell-type-specific mechanisms that are present to support the normal functions of the neural tissues.
Secondly, it was discovered that there were abundant fibrous bodies in the axonal bundles and the neuropils. These irregular microfilaments are spatially homogeneous, with entities which are electron dense, directly amalgamated with axons. Intracellular fibers with similar characteristics, known as tonofilaments, were also discovered in cells that were highly electron dense. Tonofilaments were however abundant in the subepithelial regions in the gills of alvinellids which are endemic to the extreme environment of hydrothermal vents. The epithelial cells of gills of alvinellids are involved in the storage of less toxic compounds, providing effective access to oxygen and detoxifying sulfides (Tsuchida et al., 2011). The functional significance of tonofilaments in these organisms is however unclear.
In the same way as the scale-worm, different studies have discovered the existence of similar fibrous matrices in the nervous system of several marine polychaetes, such as Nereis diversicolor. The filaments are intermingled in the neurons forming part of the cytoplasmic component of the interstitial glial cell. Most of these fibers, however, end at the desmosomes in the glial cell membranes. Although, the fine fiber matrix of scale worm is quite different as that of the nervous system of marine polychaetes. In as much as the fibers seem to be situated in the intracellular space, in the same way, they are directly connected to the epithelial cuticle in the joint region. The tonofilaments, electron-dense penetrative filaments, are also located in the same position for both the scale-worm and the marine polychaetes, in the cytoplasm of the sheath cells. There is, however, some difference in the location of the axonal bundles that surround the gigantic axons of the ventral nerve cord in the polychaetes.
Besides, the penetrative fiber matrix of the scale worm can be compared to the ones of the mammalian meningeal organization and their correlation to copepods. To begin with, the scale-worm fibrous matrix and the mammalian meningeal structure both comprise of an organized blockade of neural coverings. The second comparable feature is the electron dense fibroblasts of mammalian meninges and the “dark cells” of the copepods. Both have similar organization and integrated in the same way from the outside layer to the neural tissues. Lastly, the meningeal cells of mammals possibly contribute to neural stem cells production, morphogenesis, learning and memory, neural growth, and protection of the nervous system. All the three comparisons suggest that the specialization of the scale-worm glial cell types may be of significance when it comes to the adaptation of the organism in the extreme environmental conditions of hydrothermal vents (Zierenberg et al., 2014). However, despite the numerous structural and functional similarities, the tonofilament-like intracellular fibers that are found in the scale-worms are not significant in the mammalian meningeal structure. More research needs to be conducted to find out whether such features in different species are related to their adaptive characteristics species in different ecological settings.
Considering a variety of organisms from different species, there is still no common evolutionary origin when it comes to neural supportive cells. However, specialization of glial cells within the different species is still under investigation, with most studies majoring on the reasons behind lack of glial cells in the basal species of most animals from major phyla. This leads to the assumption that comparative analyses of the adaptive features to extreme environmental conditions of the nervous system of different organisms might provide insight into rooting the evolution of specialized glial cell types (Exposito et al., 2002). Taking into account Polynoids, scale-worms may be one of the most highly-mobile organisms in the hydrothermal vents. Additionally, distinctive tetra-domain hemoglobin found in hydrothermal vent scale-worms has been reported to be one of the main adaptive features in high redox environments. A number of crustaceans have also been recognized for their fast-moving characteristic and adaptation to a similar ecological niche.
In as much as one-layer cell body wrappings comprising of multiple cytoskeletons have been observed in the thoracic ganglia of members of crustacean family such as shallow-water crab, multi-layer glial cells wrappings of the neuronal cell bodies have not been identified or reported. This information is still lacking in most studies conducted on the adaptive features of crustaceans which are abundant in extreme environments of hydrothermal vents. There are however ongoing studies on organisms such as the squat lobster Shinkaia crosnieri and alvinocarid shrimp Shinkaicaris leurokolos. It is important to observe that presence of different adaptive features to the same environment vary among animals from different species (Jessen & Richardson, 2001). This observation is supported by the fact that Paralvinella hessleri, which possess a less-developed adaptive glial cell, is a vent-endemic species belonging to the family of alvinellid worms and has high thermos-tolerant characteristics and redox environmental capacities. The same wrappers are absent in the giant tube worm ventral nerve cord especially in organisms such as Riftia pachyptila. The difference in the species adaptive features may portray difference in habitats, and mobility (Ronaldson, & Davis, 2012). Without a doubt, the scale-worms has proved to represent the most active animal, with well-developed locomotor appendages, higher sensory centers, and big brains. The same features are limited when it comes to organisms such as vestimentiferan tube worms and alvinellids, which usually live as sessile animals and are very slow crawlers.
Mammalian sensory ganglia found in the peripheral nervous system possess satellite glial cells that cover the neuronal cell body surface. The satellite glial cells play a major role in some pathological changes such as inflammatory pain through receptors for numerous neuroactive agents and neurotransmitters. In contrast, the glial cells of scale-worms discussed in this paper are far much cytomorphologically comparable to the myelinating Schwann cells, which together form a spiraled sheath and a multi-layer, rather than a satellite glial cells of a few layer sheath and short cellular processes. Regardless of such differences, future studies on glial cells and neuronal interactions of organisms endemic to extreme environments of hydrothermal vents may lead to the discovery of a variety of resource of cellular protection for medical and scientific applications (Matsas et al., 1999).
In conclusion, the multilayer properties, in addition to the epithelial support cells, offer an additional advantage to the functions of the glial cells. The two properties may provide physicochemical homeostatic regulations, responsible for the filtration of toxic heavy metals that are present in the hydrothermal vent environment. Additionally, they may prevent the breakdown of the integrity of glial cell membrane under significant oxidative stress as a result of hypoxia the same extreme hydrothermal vent environment. Studies have also proved that similar functions have been observed in the mammalian blood-brain barrier, in addition to the human stroke. Generally, adaptive features in the glial cells are responsible for the survival of hydrothermal vent endemic worms.
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