Topography and ultrastructure of the tegument of adult Schistosoma mekongi

Fifty years of the schistosome tegument: discoveries, controversies, and outstanding questions

The unique multilaminate appearance of the tegument surface of schistosomes was first described in 1973, in one of the earliest volumes of the International Journal for Parasitology. The present review, published almost 50 years later, traces the development of our knowledge of the tegument, starting with those earliest cytological advances, particularly the surface plasma membrane-membranocalyx complex, through an era of protein discovery to the modern age of protein characterization, aided by proteomics. More recently, analysis of single cell transcriptomes of schistosomes is providing insight into the organisation of the cell bodies that support the surface syncytium. Our understanding of the tegument, notably the nature of the proteins present within the plasma membrane and membranocalyx, has provided insights into how the schistosomes interact with their hosts but many aspects of how the tegument functions remain unanswered. Among the unresolved aspects are those concerned with maintenance and renewal of the surface membrane complex, and whether surface proteins and membrane components are recycled. Current controversies arising from investigations about whether the tegument is a source of extracellular vesicles during parasitism, and if it is covered with glycolytic enzymes, are evaluated in the light of cytological and proteomic knowledge of the layer.

Unique ultrastructural characteristics of the tegument of the digenean blood fluke Aporocotyle simplex Odhner, 1900 (Digenea: Aporocotylidae), a parasite of flatfishes

This paper includes the first transmission electron microscopical (TEM) study of the tegument of a member of the basal digenean family Aporocotylidae. Scanning electron microscopical investigations of the fish blood fluke Aporocotyle simplex show that each boss on the lateral body surface bears 12-15 simple, uniform spines which extend from 0.5-2.7 μm above the surface of the boss. TEM observations revealed that these spines reach deep beneath the distal cytoplasm of the tegument for much of their length (9-12 μm) and are surrounded by a complex of diagonal muscles in each boss. This is the first record of any digenean with so-called 'sunken' spines. The results suggest that aporocotylid spines arise from within the sarcoplasm of the boss diagonal muscles. The sunken cell bodies (perikarya) of the tegument are connected to the distal cytoplasm via ducts (specialised processes lined by microtubules); this in contrast to other digeneans studied, where they are connected via non-specialised cytoplasmic processes. Within the distal cytoplasm, the tegumental ducts of A. simplex are surrounded by invaginations of the basal membrane and release their cytoplasmic inclusions into the distal cytoplasm. These apparently unique morphological features of the tegument, especially the deep origin of the spines, may represent useful characteristics for understanding aporocotylid relationships, especially in view of the known variation in the spine patterns of aporocotylids.

Transmission electron microscopic observations on ultrastructural alterations in Schistosoma mansoni adult worms recovered from C57BL/6 mice treated with radiation-attenuated vaccine and/or praziquantel in addition to passive immunization with normal and vaccinated rabbit sera against infection

Although the current treatment of schistosomiasis relies largely on praziquantel (PZQ), it has not been successful in significantly reducing the overall rate of disease cases, one of the suggested reasons being the inevitable resistance to PZQ. Previous studies showed that radiation-attenuated vaccine provides protection against Schistosoma mansoni in a host of various species. In the present study, we evaluated the effect of various vaccination strategies in C57BL/6 mice, including single or multiple vaccination strategy, subcurative dose (20 mg/kg) of PZQ, and a combination of single vaccination with subcurative dose of PZQ. Treatment either with subcurative dose of PZQ or with a single vaccination of attenuated cercariae (500 per mouse), caused significant reduction in total worm burden, hepatic, and intestinal ova counts of 43.03, 73.2, and 59.5 and 37.97, 52.02, and 26.3%, respectively. Furthermore, tegumental changes were observed. In multiple vaccinated group, there was an extensive lysis in tegumental layers. High deformations in gastrodermis, testis cells, vitelline cells, and oocytes were recorded. Also, this study is to explore the role of humoral immunity using highly resistant rabbits that had been exposed to three immunizations with ultraviolet (UV)-irradiated cercariae (8000 per rabbit in each immunization), and their sera were tested for their ability to transfer protection. The reduction in challenge worm burden had reached 32.76-43.64% when compared with recipients of normal serum or no serum. The reduction in hepatic and intestinal ova counts reached to 74.4 and 71.08% in group immunized with vaccinated rabbit sera. Swelling and extensive lysis of tegumental layers, gastrodermis lumen, spermatocytes, and deformation of oocytes were recorded with more severity than that recorded in normal rabbit sera group. Our findings recorded that multiple vaccination strategy is the most effective strategy then passive transfer of vaccinated rabbit. This gives guiding in the design the appropriate therapeutic strategy.

Morphology and surface topography of the schistosome Bivitellobilharzia nairi from the Asian elephant (Elephas maximus maximus) in Sri Lanka

Bivitellobilharzia nairi was first recorded from an Indian elephant (Elephas maximus) in Berlin. Infections with this parasite have become increasingly important in E. maximus maximus populations in Sri Lanka. The present work is the first morphological description of this schistosome from Sri Lanka. A number of adult worms were recovered from a dead Asian elephant near the elephant orphanage, Pinnawala, in Sri Lanka. The observed clinical features of the infected elephant included emaciation, subventral oedema and anaemia. Post-mortem results indicated that the liver was enlarged and adult schistosomes were found in the blood vessels of the liver parenchyma. The total number of worms recovered from a portion of the liver was 129,870, which is an average of 22 worms per 100 g of liver. The present study uses both light microscopic and scanning electron microscope (SEM) techniques for the morphological and topographical characterization of this parasite and to permit comparison with other species of schistosomes. Morphologically, these worms correspond very well to the description of B. nairi by Dutt & Srivastava (1955). Moreover, it is clear that B. nairi is a distinctive species easily differentiated from other schistosomes. The SEM study of the tegument of male worms shows that the surface of B. nairi is smoother than in other schistosomes.

Cubic crystal protein inclusions in the neodermis of the pancreatic fluke, Eurytrema pancreaticum, and Eurytrema coelomaticum

Light microscopy of Eurytrema pancreaticum and Eurytrema coelomaticum collected from cattle in Japan, China, Thailand, and Brazil showed many cubic crystal inclusions in the neodermis (tegument) of all flukes. The crystal inclusions were histochemically positive for protein. Scanning electron microscopy showed many cubic protrusions containing cubic crystal protein inclusions on the surface of the neodermis. Transmission electron microscopy showed that cubic crystal protein inclusions appeared in the perikarya of subtegumental parts, passed through the cytoplasmic bridge, moved into the syncytial neodermal cytoplasm, and then protruded from, and finally separated from, the neodermal cytoplasm. Cubic crystal protein inclusions were hexahedral with each side 2-18 microm long. High-resolution microscopy of ultrathin sections of crystal inclusions showed a lattice fringe at spacings of about 0.52 nm by using a filtering processing. Diffractograms were obtained by Fourier transform of the images. The lattice structure of the crystal protein inclusions was shown by inverse Fourier transform, indicating that the cubic crystal protein inclusions were single crystals. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis estimated the molecular weight of protein in the cubic crystal inclusion as 36.6 kDa. Energy-dispersive X-ray spectroscopy proved that the cubic crystal protein inclusions were composed of protein and sulfur.

Making sense of the schistosome surface

The syncytial cytoplasmic layer, termed the tegument, which covers the entire surface of adult schistosomes, is a major interface between the parasite and its host. Since schistosomes can survive for decades within the host bloodstream, they are clearly able to evade host immune responses, and their ability is dependent on the properties of the tegument surface. We review here the molecular organization and biochemical functions of the tegument, combining the extensive literature over the last three decades with recent proteomic studies. We have interpreted the organization of the tegument surface as bounded by a conventional plasma membrane overlain by a membrane-like secretion, the membranocalyx, with which host molecules can associate. The range of parasite proteins, glycans and lipids found in the surface complex is evaluated, together with the host molecules detected. We consider the way in which the tegument surface is formed after cercarial penetration into the skin, and changes that occur as parasites develop to maturity. Lastly, we review the evidence on surface dynamics and turnover.

Schistosoma mekongi: the in vitro effect of praziquantel and artesunate on the adult fluke

The efficacy and tolerance of 80 microg/ml praziquantel (PZQ) and 40 microg/ml artesunate (ATS) against adult stage Schistosoma mekongi in vitro were investigated after 3, 6, 12, and 24h incubation by monitoring worm motility and compared tegumental changes using scanning electron microscopy (SEM). Thirty mice were infected with S. mekongi cercaria for 49 days. Adult worms were collected by perfusion method and prepared for in vitro study. Contraction and decreased motor activity were observed after as little as 3h incubation with PZQ and ATS. Some of the worms were immobile 12h after exposure, and died within 24h. The tegument of S. mekongi showed severe swelling, vacuolization and disruption, fusion of the tegumental ridges, collapse and peeling. After 12-24h incubation, PZQ induced similar but they less severe, tegumental changes to those observed after exposure to ATS. The direct observation of the fluke motility and SEM study suggest that ATS is more effective than PZQ in causing tegumental damage in adult S. mekongi, and provides a basis for subsequent clinical trials.

Effects of praziquantel and artesunate on the tegument of adult Schistosoma mekongi harboured in mice

The effects of praziquantel and artesunate on the tegument of adult Schistosoma mekongi harboured in mice were compared using scanning electron microscopy (SEM). Forty-two mice infected with S. mekongi for 49 days were treated intragastrically with either 300 mg/kg praziquantel or 300 mg/kg artesunate. Mice were sacrificed 1 or 3 days post-treatment. Worms were collected by perfusion and examined by SEM. One to 3 days after administration of artesunate, the tegument of S. mekongi showed severe swelling, vacuolization, fusion of the tegumental ridges and loss or shortening of the spines on the trabeculae, collapse and peeling. Praziquantel induced similar tegumental alterations as those observed after administration of artesunate, but they were less severe. Three days post-treatment, there was evidence of recovery only in the case of praziquantel. The results of our study suggest that artesunate is more effective than praziquantel in causing tegumental damage in adult S. mekongi, and provides a basis for subsequent clinical trials.

The cytoskeleton and motor proteins of human schistosomes and their roles in surface maintenance and host-parasite interactions

Schistosomes are parasitic blood flukes, responsible for significant human disease in tropical and developing nations. Here we review information on the organization of the cytoskeleton and associated motor proteins of schistosomes, with particular reference to the organization of the syncytial tegument, a unique cellular adaptation of these and other neodermatan flatworms. Extensive EST databases show that the molecular constituents of the cytoskeleton and associated molecular systems are likely to be similar to those of other eukaryotes, although there are potentially some molecules unique to schistosomes and platyhelminths. The biology of some components, particular those contributing to host-parasite interactions as well as chemotherapy and immunotherapy are discussed. Unresolved questions in relation to the structure and function of the tegument relate to dynamic organization of the syncytial layer.

The ultrastructural architecture of the adult Schistosoma japonicum tegument

The tegument of the adult blood fluke Schistosoma japonicum is in direct contact with the host blood and immune systems. A comprehensive understanding of the ultrastructure of the tegument is crucial to the understanding of how the parasite maintains itself within the mammalian host. Important functions such as nutritional uptake and immune evasion are suspected functions of the tegument and this review discusses these aspects and presents some insights into some of these crucial functions. Transmission electron microscopy has allowed the identification of ultrastructural features of the adult S. japonicum, some of which differ from the reported features of other schistosome species. Morphological differences within the tegument of the adult S. japonicum are noted between sexes, among different regions of the worms and between aspects along the length of the parasite. Differences included variations in the ultrastructure, size and number of tegumental bodies and mitochondria within the matrix, and differences in the relative area of the apical surface of the tegument. Functions of the various components of the tegument matrix and specialised functions of different regions of the male and female parasites are discussed based on ultrastructural findings and previously reported biochemical and molecular data.

Opisthorchis viverrini: changes of the tegumental surface in newly excysted juvenile, first-week and adult flukes

The tegumental surface of the newly excysted juvenile, first-week and adult stages of a human liver fluke, Opisthorchis viverrini, was studied by scanning electron microscopy. The surface of the newly excysted juvenile is characterized by circumferentially arranged ridges alternated with troughs which bear rows of spines encircling the body. These spines are characterized by the shapes of their edges: the first type with serrated edge appears on the anterior part, and the second type with single sharp edge appears on the middle part. There are no spines on the posterior part of the body. Three types of presumed sensory papillae are present: type A is a single small cone-shaped ciliated papilla; type B is a group of ciliated papillae on a common dome-shaped base; and type C is a large papilla with nonciliated bulb. Type A papillae are scattered evenly on the surface but are more concentrated around the oral and ventral suckers, as well as the excretory pore. Pairs of both type A and B papillae are located in rows along the lateral surfaces of the body. Type C papillae are located solely on the lip of the ventral sucker. In the first-week fluke the tegumental surface appears corrugated and covered with short developing microvilli, and spines are mostly lost. In the fully-grown adult fluke the surface appears highly corrugated with wave-like folds encircling the body which are covered in turn with closely packed stubby microvilli. Microvilli are more numerous and are taller on the ventral than on the dorsal surface. Among microvilli on the surface of both older stages three types of sensory papillae as in the newly excysted juvenile were observed, but they are of larger sizes. The number of type A papilla increases whereas that of type B papilla decreases during maturation. The distribution of papillae in both stages is similar to the newly excysted stage.