Invertebrate glial cells

The evolutionary origins of glia

The evolutionary origins of glia are lost in time, as soft tissues rarely leave behind fossil footprints, and any molecular footprints they might have been left we have yet to decipher. Nevertheless, because of the growing realization of the importance glia plays in the development and functioning of the nervous system, lessons we can draw about commonalities among different taxa (including vertebrates) brought about either from a common origin, or from common adaptational pressures, shed light on the roles glia play in all nervous systems. The Acoelomorpha, primitive interstitial flatworms with very simple cellular organization and currently at the base of the bilaterian phylogeny, possess glia-like cells. If they indeed represent the ancestors of all other Bilateria, then it is possible that all glias derive from a common ancestor. However, basal taxa lacking convincing glia are found in most major phyletic lines: urochordates, hemichordates, bryozoans, rotifers, and basal platyhelminths. With deep phylogenies currently in flux, it is equally possible that glia in several lines had different origins. If developmental patterns are any indication, glia evolved from ectodermal cells, possibly from a mobile lineage, and even possibly independently in different regions of the body. As to what functions might have brought about the evolution of glia, by-product removal, structural support, phagocytic needs, developmental programming, and circuit modulation may be the more likely. Explaining possible cases of glial loss is more difficult, as once evolved, glia appears to keep inventing new functions, giving it continued value even after the original generative need becomes obsolete. Among all the uncertainties regarding the origin of glia, one thing is certain: that our ideas about those origins will change with every rearrangement in deep phylogeny and with continued advances in invertebrate molecular and developmental areas.

Control of carbohydrate metabolism in an anoxia-tolerant nervous system

Anoxia-tolerant animal models are crucial to understand protective mechanisms during low oxygen excursions. As glycogen is the main fermentable fuel supporting energy production during oxygen tension reduction, understanding glycogen metabolism can provide important insights about processes involved in anoxia survival. In this report we studied carbohydrate metabolism regulation in the central nervous system (CNS) of an anoxia-tolerant land snail during experimental anoxia exposure and subsequent reoxygenation. Glucose uptake, glycogen synthesis from glucose, and the key enzymes of glycogen metabolism, glycogen synthase (GS) and glycogen phosphorylase (GP), were analyzed. When exposed to anoxia, the nervous ganglia of the snail achieved a sustained glucose uptake and glycogen synthesis levels, which seems important to maintain neural homeostasis. However, the activities of GS and GP were reduced, indicating a possible metabolic depression in the CNS. During the aerobic recovery period, the enzyme activities returned to basal values. The possible strategies used by Megalobulimus abbreviatus CNS to survive anoxia are discussed.

Electron microscopy of glial cells of the central nervous system in the crab Ucides cordatus

Invertebrate glial cells show a variety of morphologies depending on species and location. They have been classified according to relatively general morphological or functional criteria and also to their location. The present study was carried out to characterize the organization of glial cells and their processes in the zona fasciculata and in the protocerebral tract of the crab Ucides cordatus. We performed routine and cytochemical procedures for electron microscopy analysis. Semithin sections were observed at the light microscope. The Thiéry procedure indicated the presence of carbohydrates, particularly glycogen, in tissue and in cells. To better visualize the axonal ensheathment at the ultrastructural level, we employed a method to enhance the unsaturated fatty acids present in membranes. Our results showed that there are at least two types of glial cells in these nervous structures, a light one and a dark one. Most of the dark cell processes have been mentioned in the literature as extracellular matrix, but since they presented an enveloping membrane, glycogen and mitochondria--intact and with different degrees of disruption--they were considered to be glial cells in the present study. We assume that they correspond to the perincurial cells on the basis of their location. The light cells must correspond to the periaxonal cells. Some characteristics of the axons such as their organization, ensheathment and subcellular structures are also described.

Morphological organization of neuropile glial cells in the central nervous system of the medicinal leech (Hirudo medicinalis)

Neuropile glial (NPG) cells in the central nervous system of the medicinal leech, Hirudo medicinalis, were studied by histological, histochemical and immunocytochemical techniques. The NPG cells are often surrounded by electron-dense microglial cells. The central cytoplasm of NPG cells shows a significant zonation. The zone around the nucleus contains mitochondria, glycogen and vesicles. The cytoplasm also contains many ribosomes, a few dictyosomes and distinct inclusions up to 2 microns in diameter. A second zone around the perinuclear region is marked by the occurrence of bundles of intermediate filaments that correspond in thickness to glial filaments of vertebrates. We found a positive reaction with polyclonal antibodies against human glial fibrillary acidic protein (GFAP), and the areas of intense fluorescence correspond to the regions where intermediate filaments were found to be abundant. The peripheral zone contains numerous membrane stacks that could not be contrasted by lanthane nitrate or tannic acid. Therefore, the membrane stacks could be part of an extensive smooth endoplasmic reticulum, which is characteristic of cells with active lipid metabolism.

Opiate signaling regulates microglia activities in the invertebrate nervous system

1. Evidence supporting the presence in the invertebrate nervous system of a class of glial cells resembling vertebrate microglia was obtained in the freshwater snail Planorbarius corneus. These cells are easily identified by their immunopositivity to anti-pro-opiomelanocortin (POMC)-derived peptide antibodies. 2. Invertebrate microglia, as in vertebrates, exhibit macrophage-like activity in vivo and in cell cultures. These cells respond to the trauma of ganglionic excision and their organotypic culture by leaving their location around neurons and moving to the lesion site from which they migrate in the culture dish. 3. In vitro, these microglia undergo conformational changes and show phagocytic properties in the presence of bacteria or lipopolysaccharide. The activated cells also express tumor necrosis factor-alpha-like material and an increase in nitric oxide synthase, as shown by immunocytochemistry. 4. The inhibitory effect of morphine on the mobility and phagocytic activity of invertebrate microglia provide additional functional evidence for a possible role of opiate-like compounds in downregulating immunoregulatory processes, as also observed in the circulating immunocytes.

Immunocytochemical evidence of PDGF- and TGF-beta-like molecules in invertebrate and vertebrate immunocytes: an evolutionary approach

Immunoreactive platelet-derived growth factor-AB and transforming growth factor-beta 1 were demonstrated in invertebrate and vertebrate immunocytes by an immunocytochemical procedure. These factors are only present in phagocytic cells among invertebrate immunocytes, whereas in vertebrate immunocytes they are found in monocytes, granulocytes, lymphocytes, thrombocytes and platelets. These results, in agreement with previous reports, represent further evidence in favour of the hypothesis that Nature has followed a conservative strategy in using a common pool of signal molecules that have been highly conserved throughout evolution.

Morphology of neuroglia in the antennal lobes and mushroom bodies of the brain of the honeybee

We investigated the distribution and anatomical organization of glial cells in the antennal lobes and mushroom bodies of the honeybee. Reconstructions from serial sections, prepared according to the ethyl gallate method, revealed the entire morphology of glial cells in neuropiles, tracts, and the soma rind. The distribution of the glial cell bodies in the neuropiles was derived from the staining of cell nuclei with a fluorescent dye. There are glial cells of different shape in the soma rind which are wrapped around the neuronal cell bodies of the antennal lobes and the Kenyon cells of the mushroom bodies. Glial cells surround neuropilar areas such as the external and lateral sides of the glomeruli of the antennal lobes. Whereas we could not detect glia in the glomerular neuropile, glial cells with long processes are located in the core of the antennal lobe. Extensions of these glial cells also invade tracts containing the olfactory projection neurons. A layer of glial cells separates the mushroom body neuropile from the surrounding protocerebral neuropile. The neuropile of the mushroom bodies is clearly compartmented by glial cells. There is a high density of astrocyte-like glia in a column of the pedunculus which can be followed to the ventral part of the alpha-lobe. A network of mushroom body intrinsic glial cells separates the alpha-lobe from the beta-lobe and the pedunculus. This anatomical description of glial cell types in olfactory information processing pathways of an insect brain provides a framework for further physiological studies of neuroglia in dissociated cell culture.

Mesenchyme cells in Fasciola hepatica (platyhelminthes): primitive glia?

Mesenchyme cells and their processes are found in the cerebral ganglia of the parasitic flatworm, Fasciola hepatica. The mesenchyme cell processes are found in two specialized associations within the ganglion: (i) as lamellae-like multilayer sheaths encircling the cerebral ganglia and separating it from the surrounding parenchyma cells, and (ii) invaginated into the surface of large diameter ('giant') nerve processes to form trophospongium-like relationships. Based on morphological criteria, these mesenchyme cells resemble general invertebrate glial cells suggesting that the mesenchyme cells of these flatworms may represent the earliest glial-like cell.

Defective glia in the Drosophila brain degeneration mutant drop-dead

To understand better the cellular basis of late-onset neuronal degeneration, we have examined the brain of the drop-dead mutant of Drosophila. This mutant carries an X-chromosomal recessive mutation that causes severe behavioral defects and brain degeneration, manifested a few days after emergence of the adult. Analysis of genetically mosaic flies has indicated that the focus of the drop-dead mutant phenotype is in the brain and that the gene product is non-cell autonomous. We examined the adult drop-dead mutant brain prior to onset of symptoms and found that many glial cells have stunted processes, whereas neuronal morphology is essentially normal. Adult mutant glial cells resemble immature glia found at an earlier stage of normal brain development. These observations suggest that defective glia in the drop-dead brain may disrupt adult nervous system function, contributing to progressive brain degeneration and death. The normal drop-dead gene product may prevent brain degeneration by providing a necessary glial function.

Morphology of glial blood-brain barriers

Glial cells, in certain situations in the CNS, may become modified to form the structural basis of the blood-brain barrier. This occurs in more primitive vertebrates, such as the elasmobranch fish, and in some higher invertebrates. In the latter, the outermost glial sheath, often called the perineurium in avascular ganglia, substitutes functionally for the vascular endothelium of higher organisms. The intercellular junctions between the lateral borders of these modified glial or perineurial cells may be of several types. In nearly all cases, adhesive and communicating (gap) junctions are found together with an occluding junctional structure. The latter is assumed to be the morphologic basis of the observed blood-brain barrier. It varies in nature and may be one in which the adjacent cell membranes fuse, partially or completely, to form a classical tight junction, or it may be one in which the cell membranes remain separated by a distinct intercellular cleft. If the latter, the cleft may be straddled by columns or septal ribbons, between which a charged matrix substance may be found. Restrictive linker junctions, recently found to be the basis of the interglial barrier in cephalopod CNS, as well as that of myriapods, are characterized by cross-striations or columns which, in combination with charged residues, inherent either in them or in the associated extracellular matrix, slow down the entry of exogenous molecules. Septate junctions, which occur between glial cells in certain other invertebrates, exhibit intercellular septal ribbons, which do not prohibit paracellular transport of all substances but may slow down the passage of some by virtue of charged moieties. There is an association of cytoskeletal components with these septate, linker, and tight junctions; the role of the cytoskeleton in tight junctions, which can be seen by freeze fracture to be based on simple ridges in insects or a more complex network of them in arachnids, may also be important in the regulation of paracellular permeability. The structural details of the junctions in different groups are summarized and their physiologic implications discussed.