Fine structure of glial cells in the central nervous system of the tapeworm Grillotia erinaceus (Cestoda: Trypanorhyncha)

The problem of glial cells existing in parasitic and free living flatworms is correlated with organization of parenchyma in platyhelmintes. In the contrary to the widespread opinion that myelin-like envelopes and glial cells do not exist in the nervous system of parasitic flatworms, it has been shown by ultrastructural researches that Amphilina foliacea (Cestoda, Amphilinidea) has well developed glial cells and myelin-like envelopes in the ganglia and main cords, which include both glial cells and intercellular components. The aim of our research was to reveal and investigate in details structural components corresponding to the concept of the glial cell in the CNS of Grillotia erinaceus (Cestoda: Trypanorhyncha). Three types of glial cells have been found. The first type is the fibroblast-like glial cells; cells locate in the cerebral ganglion, contain in cytoplasm and extract out fibrillar matrix, form desmosomes and have supporting function. The glial cells of the second type form myeline-like envelope of the giant axons and bulbar nerves in scolex and have laminar cytoplasm. These cells are numerous and exceed in number the neurons bodies into the nerve. The glial cells of the third type form multilayer envelopes in the main nerve cords; extra cellular fibers and gap-junctions take place between the layers. There are contacts between the glial cells of the third type and excretory epithelium but specialized contacts with neurons have been not found. The existing of glial cells in free living and parasitic flatworms is discussed.     

Ultrastructure of glial cells in the nervous system of <Emphasis Type="Italic">Grillotia erinaceus</Emphasis>

Until now, there has been no answer to the question of whether specialized glial cells exist in the nervous system of platyhelminths. The identification of these cells in parasitic flatworms is difficult due to their organization as parenchymal animals. The goal of this study was to reveal and describe structural elements corresponding to the term glia in the CNS of the parasitic flatworm Grillotia erinaceus (Cestoda: Trypanorhyncha). Three types of glial cells are revealed. The first type consists of fibroblast-like cells located in the cerebral ganglia that contain fibrils and excrete onto the surface fibrillar material and possess desmosomes; the presumable function of fibroblast-like glial cells is the isolation and support of ganglionar neurons. Glial cells of the second type form a myelin-like envelope of giant axons and bulbar nerves of the scolex and have laminar cytoplasm; they are numerous and exceed the number of neurons in the composition of nerves. Glial cells of the third type form multilayer envelopes in the main nerve cords and make contacts with the excretory epithelium; however, specialized junctions with neurons were not found. The existence of glia in other free living and parasitic flatworms is discussed.     

Protofilament number in microtubules in cells of two parasitic nematodes

The parasitic nematodes, Ascaridia galli and Trichostrongylus colubriformis, were prepared for electron microscopy with fixatives containing tannic acid, which allowed their microtubule protofilament number to be examined. In contrast to many mammalian tissues, the nematodes did not contain microtubules with 13 protofilaments. Ascaridia galli contained microtubules with 11 protofilaments in all tissues examined, including nerve, intestinal, pharyngeal, and hypodermal cells. Trichostrongylus colubriformis contained nerve cells, known as microtubule cells, with bundles of larger microtubules (approximately 30 nm in diameter) with 14 protofilaments. The microtubules in these cells did not appear to be continuous for the entire length of the axon. Other cells examined in T. colubriformis, including nerve, intestinal and pharyngeal cells, contained two distinct types of microtubules, one with 11 protofilaments and an approximate diameter of 25 nm, and one with 12 protofilaments and an approximate diameter of 27 nm. All cell types examined contained both types of microtubules.     

Ontogenetic development of the brain of the platyhelminth Fasciola hepatica

During ontogenetic development in the definitive host, the cerebral ganglia of the parasitic flatworm Fasciola hepatica lose their cell rind integrity and develop specialized nerve processes. The organization and cytological features of the central nervous system were examined during three developmental stages in the parasitic life cycle of F. hepatica to determine when the changes occur. The cerebral ganglion cell bodies of migrating juvenile worms (5 days post-infection) are organized into a one-cell-thick rind that surrounds a central neuropile composed of small unmyelinated nerve processes (less than 3 microns in diameter). In young, sexually-immature adult worms (30 days post-infection), the cell bodies of the ganglia are no longer organized into a complete or tight cell rind around the ganglia. In addition, large diameter ('giant') unmyelinated nerve processes (greater than 12 microns) are found in the neuropile area. These giant nerve processes are also found in the transverse commissure and the longitudinal nerve cords. In mature adult worms (4-6 months post-infection), the rind of nerve cell bodies has completely disappeared and cell bodies are scattered around and within the neuropile. More than half of the volume of the mature adult neuropile is composed of giant nerve processes. The three developmental stages of the parasite that were used in this study differ significantly in their sizes, behaviours and microhabitat locations in the host. The results suggest that the organizational and morphological changes in the ganglia reflect selective adaptations to changes in the parasitic microenvironment.     

Some ultrastructural observations on the microfilaria of Breinlia sergenti--nervous system

The amphids, cephalic papillae, phasmids, excretory complex, anal vesicle, rectal cells, G cell and the nerve ring are described in the microfilaria of Breinlia sergenti. The significance of these various structures in playing a nervous role in this organism is discussed, and a working hypothesis presented to explain the nervous co-ordination in this parasitic stage.     

The nervous system of parasitic cnidarian Polypodium hydriforme

The nervous system of intracellular parasitic cnidarian Polypodium hydriforme at various stages of its life cycle has been studied by the immunocytochemical method using antibodies to FMRF-amide and by electron microscopy. Neurosecretory, sensory, and ganglion cells have been identified both at the parasitic stage (planula and stolon stages, when body layers are inverted) and in free-living animals. These cells are characterized by the presence of round neurosecretory granules about 80–120 nm in diameter. Gap junctions have been detected between nerve cells. Most of the neurosecretory and sensory cells have been observed in the epidermis of sensory tentacles of free-living animals. Sensory cells possess immobile flagella. The chains of ganglion cells are located under the epidermis and penetrate mesoglea. A centriole encircled by a fragment of nuclear envelope, which is a marker of ectodermal lineage cells in Polypodium, has been described in the cytoplasm of the sensory cells, thus proving the ectodermal nature of the nervous system. Like in most cnidarians, the nervous system of Polypodium hydriforme is a network containing FMRF-amide-like neuropeptides. Neither sense organs, nor ring-shaped nerve concentrations have been observed.     

Variations in the presence of chloride cells in the gills of lampreys (Petromyzontiformes) and their evolutionary implications

Although confined to fresh water, non‐parasitic species of lampreys and the landlocked parasitic sea lamprey, all of which were derived relatively recently from anadromous ancestors, still develop chloride cells, whose function in their ancestors was for osmoregulation in marine waters during the adult parasitic phase. In contrast, such cells are not developed by the non‐parasitic least brook lamprey Lampetra aepyptera, which has been separated from its ancestor for >2 million years, nor by the freshwater parasitic species of the genus Ichthyomyzon. The length of time that a non‐parasitic species or landlocked parasitic form or species has spent in fresh water is thus considered the overriding factor determining whether chloride cells are developed by those lampreys.     

Inhibition of the development of the reproductive tract in parasitized snails

Summary

Myoblasts, muscle cells with the capacity to divide, have been detected in “Anlagen” of the male copulation organ of Lymnaea stagnalis. They only occur in the apical part of the penis. Here they could be found throughout life. Mitotic activity of these cells can be demonstrated by using an antiserum to a S-phase specific cell cycle marker, PCNA [see, e.g., Baserga (1991)]. The number/percentage of PCNA positive myoblasts is a good parameter for growth of this male copulation organ and hence also for inhibition of its growth and development as occurs in parasitized snails. In transplantation experiments, “Anlagen” of the copulation organ were used from snails 7–9 weeks after being parasitized as they can be excised in this stage and transplanted into either parasitized or nonparasitized snails. These experiments have indicated that humoral, parasitic excretory/secretory factors can be responsible for the inhibition of growth and differentiation of the copulation organ in parasitized snails as reflected by a relatively low number of PCNA positive myoblasts compared to the controls. Data obtained in in vitro experiments showed a significant decrease of the number of myoblasts in “Anlagen” cultured in the presence of parasitic E/S products. The fact that no significant effect was found on the relative low number of PCNA positive myoblasts is discussed. The effect of parasitic E/S products on these myoblasts appeared to be exerted in a direct way, not mediated by CNS-derived factors or by factors from cells in the connective tissue sheath around the CNS. Although it appears possible to use transplantation and/or in vitro culturing of these “Anlagen” as a bioassay for identification of the parasitic factor(s) responsible for the inhibitory effects on myoblasts, the methods are very laborious and do not seem very appropriate for testing many fractions of E/S products.     

A role for the enteric nervous system in the response to helminth infections

The enteric nervous system (ENS) in the gut contains a particularly high concentration of nerve cells, and effectively functions as an independent 'minibrain'. Interactions between nerve, endocrine, immune and other cell types allow the sophisticated regulation of normal gut physiology. They can also bring about a co-ordinated response to parasitic infection, possibly leading to expulsion of the parasite. In this review, Derek McKay and Ian Fairweather will consider, in brief, data pertaining to changes in the ENS following intestinal helminth infections and speculate on the role that these alterations may have in the expulsion of the parasite burden and the putative ability of the parasite to modulate these events.     

Effects of seasonality and moult cycle on the proliferation of nerve cells and on the labelling of ecdysone receptors in an estuarine crab

Decapod crustaceans show proliferation of the nerve cells in the olfactory lobe throughout their lives. However, the regulation of this process is still poorly understood, since it may vary with endogenous and exogenous factors. The objective of the present investigation was to quantify the proliferation of nerve cells and number of nerve cells with ecdysone receptors in the clusters of the central olfactory system in Neohelice granulata, according to moult stages and in different seasons (summer and winter). Three injections of bromodeoxyuridine (BrdU) were administered to the crabs. Brains were sectioned by microtome and fixed on slides for immunohistochemistry with anti-BrdU and anti-EcR antibodies. The proliferation of nerve cells was higher in winter than in summer, probably because in winter the crabs do not breed and the premoult and postmoult periods are longer. Crabs in postmoult exhibited more BrdU-labelled cells than crabs in premoult or intermoult in winter, because of a greater number of mitoses related to an increase in body size and addition of olfactory receptor neurons. The number of EcR-labelled cells was higher in premoult than in postmoult or intermoult in winter. The proliferation of nerve cells is regulated seasonally and according to moult stages.     

Changes in the ultrastructure of Merkel's nerve endings of the sinus hair of rats after denervation

Merkel nerve endings of sinus hairs in the upper lip of the rat were observed after having cut the nervus infraorbitalis. The nerve terminal of the Merkel nerve ending has already been degenerated 24 hours after denervation and was phagocytised by neighbouring keratinocytes. The Merkel cells did not change in structure after having lost their nerve terminal even within 169 days after nerve crush. Their position in the stratum basale of the sinus hairs remained constant; the number, size and position of the osmiophilic granules in the cytoplasm of the Merkel cells did not change. These results may show, that Merkel cells are not modified keratinocytes. The possibilities of their origin are discussed.     

The pattern of neuropeptide F and RF-amide in two tapeworms

Two years ago the first platyhelminth regulatory peptide, neuropeptide F (NPF), was isolated from the tapeworm Moniezia expansa by Maule et al. (1991). NPF is a 39 amino acid peptide with a C terminal phenylalaninamide. NPF is the first platyhelminth neuropeptide to be sequenced fully. Preabsorption with NPF quenches the immunostaining with anti-FMRF-amide and anti-bovine PP (Halton et al. 1992). As the first authentic flatworm neuropeptide, the occurence and distribution of NPF along the whole flatworm line are under investigation. Both free-living and parasitic flatworms are being studied. So far NPF-immunoreactivity has been reported from three free- living flatworms (see Grahn et al., 1995) and from four parasitic flatworms (Marks et al., 1993).FMRF- and RF-amide immunoreactive (IR) nerve cells and fibres are common in the gull-tapeworm Diphyllobothrium dendriticum. In order to test whether the patterns for NPF- and RF-immunoreactivity co- localize in the gull-tapeworm, immunostaining with anti-NPF and anti-RF were performed. To broaden the study, adult Proteochepalus exiguus from the intestine of whitefish were included in the experiment.The study was performed on whole mounts of skinned worms (Gustafsson, 1991). Anti-NPF was used in concentrations 1:500 and 1:1000. Controls included liquid phase absorption with the homologous antigen (1000 ng ml^–1).In D. dendriticum NPF-immunoreactivity occurs in nerve cells and varicose nerve fibres of larval and adult worms. The NPF-IR cell bodies are more common in the peripheral nerve cords than along the main nerve cords, which contain nerve fibres with large varicosities. The cell bodies in the PNS are often triangular in shape. Immediately beneath the tegumental surface a thin NPF-IR nerve fibre is observed. As to the co-localization of NPF and RF nothing definite can be said but the general pattern seems tobe the same. In the brain commissure of D. dendriticum one large ganglion cell stains with both antisera, indicating coexistence.In P. exiguus NPF- and RF-immunoreactivity was observed in the two main nerve cords situated laterally and in the pairs of thin dorsal and ventral longitudinal nerve cords. Numerous transverse commissures connect the longitudinal cords forming an orthogonal pattern. The cell bodies along the nerve cords are multipolar. Thin projections extend from the main nerve cords to the surface of the worm. The main nerve cords are lined with NPF-and RF-IR cell bodies. The general staining patterns of NPF and RF are very similar.     

No news on the flatworm front! Nitric oxide synthase in parasitic and free-living flatworms

The free radical nitric oxide (NO) has emerged as a simple and unique signalling molecule that can serve as neurotransmitter, paracrine substance or hormone. NO is a gas, formed by various neuronal cells, both centrally and peripherally. NO regulates cyclic GMP synthesis. The production of NO can be detected using the NADPH diaphorase (NADPH-d) histochemical stain for nitric oxide synthase (NOS). NOS was detected in two parasitic flatworms, Diphyllobothrium dendriticum and Hymenolepis diminuta, and two free-living flatworms, Planaria torva and Girardia tigrina. The staining for NOS was very strong in the nervous system of both parasitic worms. The main nerve cords, the transverse ring commmissures, nerves in association with the musculature, especially the cirrus musculature and sensory nerve endings showed NADPH-d staining. The NADPH-d staining in the free-living flatworms was much weaker. Still NOS activity was found in the neuropile of the brain and in association with the pharynx musculature. The demonstration of NOS in flatworms, indicates that NO is an old signal molecule in evolutionary terms. This revised version was published online in July 2006 with corrections to the Cover Date.     

c-fos Expression in Gracilothalamic Tract Neurons after Electrical Stimulation of the Injured Sciatic Nerve in the Adult Rat

The number of c-fos protein-like immunoreactive (Fos-LI) cells in the gracile nucleus was determined after electrical stimulation at Aα/Aβ-fiber strength of the normal and of the previously injured sciatic nerve in adult rats. No Fos-LI cells were seen after electrical stimulation of the noninjured sciatic nerve, or after sciatic nerve injury without electrical stimulation. However, stimulation 21 days after sciatic nerve transection resulted in numerous Fos-LI cells in the ipsilateral gracile nucleus. Combined Fos immunocytochemistry and retrograde labeling from the thalamus showed that the majority (76%; range = 70–80%) of the cells in the gracile nucleus that expressed Fos-LI after nerve injury projected to the thalamus. The results indicate that morphological, biochemical, and physiological alterations in primary sensory central endings and second-order neurons, which have earlier been demonstrated in the dorsal column nuclei after peripheral nerve injury, are accompanied by changes in the c-fos gene activation pattern after stimulation of the injured sciatic nerve. A substantial number of the c-fos-expressing neurons project to the thalamus.     

Ratios between number of neuroglial cells and number and volume of nerve cells in the spinal ganglia of two species of reptiles and three species of mammals

We studied the ratios between number of neuroglial (=satellite) cells and number and volume of neurons with which they are associated in the spinal ganglia of two species of reptiles (lizard and gecko) and three species of mammals (mouse, rat, and rabbit). In all five species, we found that the number of satellite cells associated with a nerve cell body increased with increasing volume of the latter. This result shows that there is a quantitative balance between neuroglia and nerve tissue in spinal ganglia. This balance seems to be maintained by a tight regulation of the number of satellite cells. We also found that the mean volume of nerve cell body corresponding to a satellite cell was lower for small neurons than for large ones. Since satellite cells metabolically support spinal ganglion neurons, the metabolic needs of small neurons are better satisfied than those of large ones. For a nerve cell body of a given size, the number of associated satellite cells did not differ between the lizard and gecko, nor between the mouse, rat, and rabbit. However, this number was significantly smaller in the reptiles than in the mammals. This result could be explained by the lower metabolic rate in the nervous system of poikilotherms than mammals, or could have a phylogenetic significance. These two interpretations are not mutually exclusive.     

The astrocytes in the retina and optic nerve head of mammals: A special glia for the ganglion cell axons

Summary The neuroglia in the retina and the intraocular portion of the optic nerve of the monkey and cat has been examined by light and electron microscopy. In the retina two types of macroglial cells can be distinguished: 1) Müller cells, and 2) astrocytes. The bipolar radial glial cells of Müller penetrate the entire thickness of the retina and their basal processes align in the nerve fibre layer to form septa that fasciculate the axons of the ganglion cells. In contrast to the Müller cells, the retinal astrocytes are not homogeneously distributed throughout the retina; their number correlates with the thickness of the nerve fibre layer. The processes of the astrocytes are confined to the ganglion cell layer and to the nerve fibre layer. In the latter, the astrocytic processes run parallel to and between the axons of a given nerve fibre bundle. According to cytological criteria, the retinal astrocytes are protoplasmic. In the intraocular portion of the optic nerve, however, the astrocytes are fibrous and their processes run perpendicular to the axon bundles of the prelaminar portion of the optic nerve. Thus, because of their intimate morphological relationship to axons of the nerve fibre layer and the intraocular portion of the optic nerve, the astrocytes in the eye of the monkey and the cat may be considered as a special glia for the axons of ganglion cells.     

VIP-, Bombesin- and Neurotensin-Like Immunoreactivity in Neurons of the Gut of the Holostean Fish,Lepisosteus platyrhincus

VIP-like immunoreactivity was found in nerve fibres in all layers of the gut wall in both stomach and intestine, and was abundant in the myenteric and submucous plexuses. A few fibres were associated with blood vessels. Nerve cells showing VIP-like immunoreactivity were found in the myenteric plexus. Neurotensin-like immunoreactivity was found in nerve cells and numerous nerve fibres in the myenteric plexus of both stomach and intestine and in nerve fibres of the circular muscle layer, while bombesin-like immunoreactivity was confined to a low number of nerve fibres in the myenteric plexus of the stomach. The results indicate that a VIP-like, a neurotensin-like and a bombesin-like peptide are present in neurons of the gut of Lepisosteus.     

Electrical Activity and Histological Change in the Degenerating Olfactory Epithelium

Electrical activity and histological changes were studied in the degenerating olfactory epithelium of the bullfrog after the olfactory nerve had been sectioned. After nerve section, the electrical responses to odors disappeared in the olfactory epithelium in 8 days in the summer, in 11 days in the early autumn, and in 16 days in the early winter. In the degenerating olfactory epithelium a striking decrease in the number of olfactory cells was found, but not of supporting cells. The ratio of the number of olfactory cells to that of supporting cells was found to decrease from 5 or 6 to below 2 after the nerve section. At a ratio below 2, the electrical responses to odor disappeared. The histological changes in the bullfrog are compared with those in the mouse and rabbit. The localization of the olfactory pigment and the electrical activity of the supporting cell are discussed. It was concluded that all three types of responses to odors originate from the activity of the olfactory cell.