Schistosoma mansoni: the development of the cercarial tegument

An atlas of the germ ball-cercaria-schistosomulum transition in Schistosoma mansoni, using confocal microscopy and in situ hybridisation

Schistosomes are complex platyhelminth parasites with a genome comprising ∼12,000 protein-coding genes, three distinct generations, and at least seven distinct phenotypes. We chart here cellular and gene expression changes associated with development of the cercaria, in the intramolluscan daughter sporocyst, and its transformation into the skin stage schistosomulum upon infection of the mammalian host. We describe the morphology of the early daughter sporocyst and the increasing complexity of cellular organisation in germ balls as they rapidly develop into cercariae. We show how individual myocytes differentiate and combine to create the complex musculature of the head capsule and body wall. In situ hybridisation reveals that some transcripts encoding the secretory proteins, released during skin penetration, are expressed in gland-cell precursors very early in germ ball development. However, those for the projected anti-inflammatory protein Sm16-stathmin are widely expressed in germ ball tissues, suggesting the protein has intracellular functions. Transcripts for smkk7 are expressed in six cells of the larval body, while the KK7 protein is present throughout the peripheral nerve net, including sensory nerve bulbs, providing a marker for the nerve net in adult worms. We also note that the cercaria-schistosomulum transformation is accompanied by tissue remodelling without growth.

Nanomedicine Approaches Against Parasitic Worm Infections

Nanomedicine approaches have the potential to transform the battle against parasitic worm (helminth) infections, a major global health scourge from which billions are currently suffering. It is anticipated that the intersection of two currently disparate fields, nanomedicine and helminth biology, will constitute a new frontier in science and technology. This progress report surveys current innovations in these research fields and discusses research opportunities. In particular, the focus is on: (1) major challenges that helminth infections impose on mankind; (2) key aspects of helminth biology that inform future research directions; (3) efforts to construct nanodelivery platforms to target drugs and genes to helminths hidden in their hosts; (4) attempts in applying nanotechnology to enable vaccination against helminth infections; (5) outlooks in utilizing nanoparticles to enhance immunomodulatory activities of worm-derived factors to cure allergy and autoimmune diseases. In each section, achievements are summarized, limitations are explored, and future directions are assessed.

Schistosome feeding and regurgitation

Schistosomes are parasitic flatworms that infect >200 million people worldwide, causing the chronic, debilitating disease schistosomiasis. Unusual among parasitic helminths, the long-lived adult worms, continuously bathed in blood, take up nutrients directly across the body surface and also by ingestion of blood into the gut. Recent proteomic analyses of the body surface revealed the presence of hydrolytic enzymes, solute, and ion transporters, thus emphasising its metabolic credentials. Furthermore, definition of the molecular mechanisms for the uptake of selected metabolites (glucose, certain amino acids, and water) establishes it as a vital site of nutrient acquisition. Nevertheless, the amount of blood ingested into the gut per day is considerable: for males ∼100 nl; for the more actively feeding females ∼900 nl, >4 times body volume. Ingested erythrocytes are lysed as they pass through the specialized esophagus, while leucocytes become tethered and disabled there. Proteomics and transcriptomics have revealed, in addition to gut proteases, an amino acid transporter in gut tissue and other hydrolases, ion, and lipid transporters in the lumen, implicating the gut as the site for acquisition of essential lipids and inorganic ions. The surface is the principal entry route for glucose, whereas the gut dominates amino acid acquisition, especially in females. Heme, a potentially toxic hemoglobin degradation product, accumulates in the gut and, since schistosomes lack an anus, must be expelled by the poorly understood process of regurgitation. Here we place the new observations on the proteome of body surface and gut, and the entry of different nutrient classes into schistosomes, into the context of older studies on worm composition and metabolism. We suggest that the balance between surface and gut in nutrition is determined by the constraints of solute diffusion imposed by differences in male and female worm morphology. Our conclusions have major implications for worm survival under immunological or pharmacological pressure.

Embryological development of the cercarial tegument of Paramphistomum epiclitum in the planorbid snail, Indoplanorbis exustus

Cercariae develop from individual germinal cells occurring freely within the posterior body cavity of rediae. Individual germinal cells give rise to a germinal ball which becomes enveloped by increasing numbers of cytoplasmic extensions originating from specialized parenchyma-like cells, termed nursc cells. Up to eight cytoplasmic layers of nurse cells invest larger germinal balls. These layers may provide mechanical support for developing embryos and/or play a role in the provision of nutrients to them. The cercarial tegument develops from superficially located somatic cells in the germinal ball. Cytoplasmic extensions of presumptive tegumental cells fuse laterally to form a syncytial layer beneath the encapsulating nurse cell layers. As the cercarial tegument differentiates further, the cytoplasm of the nurse cell layers becomes vacuolated and ultimately these layers degenerate. The surface tegumental syncytia of intra-redial cercariae and newly released extra-redial cercariae are nucleated. Separate subtegumental perikarya develop with further differentiation of extra-redial cercariae.

Development of the tegument and alimentary tract in a trematode, Fellodistomum fellis

Formation of the tegument and alimentary tract in developing Fellodistomum fellis cercariae has been examined by electron microscopy. Morphogenesis of the tegument is a two-stage event: (1) ectodermal cells at the germ ball periphery coalesce beneath a closely apposed and transitory primitive epithelium of sporocyst origin, forming a syncytial boundary of nucleated tegument. The primitive epithelium is then lost and the nuclei in the surface layer of the early tegument degenerate; (2) cells in the mesenchyme of the cercarial embryo differentiate as secretory cells and connect with the anucleate layer to form the true tegument. The foregut develops as an apical invagination of early nucleated tegument, with the primordial digestive cells differentiating behind the oesophagus. When fully formed, the surface anucleate layer of oesophagus tegument extends posteriorly over the apposed surfaces of the developing digestive cells, thus isolating them from each other and from the gut lumen. An apical cavity develops in each of the digestive cells and later communicates with the gut lumen through a pore in the overlying caecal tegument. Lamellated extensions of digestive cell cytoplasm project into the cavity, and development of GER and a Golgi apparatus in the cell mark the onset of digestive secretion.

An ultrastructural study of the early cercarial development in Prosorhynchoides borealis (Digenea: Bucephalidae) with special reference to formation of the primitive epithelium

Primitive epithelium and outer tegumental layer formation during early cercarial development was studied in Prosorhynchoides borealis using electron microscopy. It demonstrated that germinal cells freely floating in the sporocyst body cavity divide to give rise to naked cell aggregates. These early embryos are highly irregular in outline and are composed of blastomeres differing in size and structure. In embryos consisting of about 12-14 cells a few (possibly only two) superficial macromeres become concave and produce thin extensions which envelop the embryonic mass before fusing to form a syncytial primitive epithelium. This primitive epithelium forms syncytial connections with underlying embryonic cells. Primordial tegumental cells become apparent in late germinal balls below the primitive epithelium. These cells expand and fuse to give rise to an embryonic nucleated tegument. The embryonic tegument is connected to peripheral embryonic cells by thin cytoplasmic bridges until the basement lamina is formed. Subsequently, the primitive epithelium is shed by the embryos and the nuclei in the embryonic tegument undergo pyknotic degeneration. These results are analysed and compared with data from studies on other trematode species and it is concluded that the primitive epithelium is derived from the embryo in at least the majority of digeneans.

Ultrastructure of the Schistosoma mansoni cercaria

The cercaria of the schistosome parasite is a short-lived, free-swimming larval stage that is infective for the mammalian, definitive host. This atlas describes the ultrastructure of the cells that comprise the cercaria of Schistosoma mansoni, a leading causative agent of human schistosomiasis. In addition to the cells which make up the various organ systems, such as the nervous, tegumental, osmoregulatory, muscular and primordial digestive systems, also we show the ultrastructure of those cells whose organization or location are not as well defined structurally but are essential nevertheless for the success of the parasite. These latter include the various support cells, and those cells that, upon differentiation into the adult worm, serve reproductive functions. A description is also given of the cells whose sole functions are realized only at the cercarial stage, chiefly involved in the vigorous act of skin penetration. Although we include a detailed review of the ultrastructure of S. mansoni cercariae, much of the information reported has not been previously published. In summary, this paper brings together an ultrastructural description of all the cell types presently known that make up the much studied larval stage of this medically important trematode.

Ultrastructural studies on the sporocyst wall of Diplostomum pseudospathaceum Niewiadomska, 1984 (Digenea, Diplostomidae)

The sporocyst wall of the daughter sporocyst of Diplostomum pseudospathaceum is composed of many elements. It is covered by a syncytial layer of cytoplasm connected to sunken nucleated perikarya (cytons), lying underneath the outer-circular and the inner longitudinal muscle layers. The outer tegumental part forms numerous microvilli covered by glycocalyx and contains many microfilaments, single ribosomes, electron-dense granules and short electron-lucid cisternae. No other cell organelles were seen in this layer. The cytons include all the organelles needed for cell metabolism. The sporocyst wall also contains the sarcoplasmic parts of the muscle cells; large vacuolar cells filled with electron-transparent vacuoles; macrophagic cells containing numerous phagosomes and sending extensions separating the sporocyst wall from the brood chamber; flame cells and reproductive cells. Nerve fibres, filled with neurosecretory granules, are seen among the sporocyst wall cells. The uniciliate sensory receptors are inserted in the outer tegumental part.

Schistosoma mansoni: development of the cercarial glycocalyx

The development of the cercarial glycocalyx of Schistosoma mansoni was studied by transmission electron microscopy and immunofluorescence light microscopy employing antibodies raised against extracted and chromatographed glycocalyx. By electron microscopy, cercariae present in the brood chamber of daughter sporocysts were surrounded by an electron-dense granular and fibrillar matrix. This material appeared structurally distinct from the glycocalyx which was coarsely fibrillar and located only on the surface of organisms that had developed a final tegument. The thickness of the glycocalyx apparently increased with the maturation of the tegument, since teguments that had many spines also had the thickest glycocalyx. Immunofluorescent staining of frozen sections of infected snail hepatopancreas showed that glycocalyx antigens were present on the surface of the cercariae and not in the matrix within the brood chamber or in snail tissues. Immunofluorescent staining of isolated larval cercariae showed staining of some but not all parasites with partially elongated tails. These studies suggest that the glycocalyx develops late in cercarial development (late in Stage 6 or in Stage 7 of Cheng and Bier), is made by the cercariae themselves, and is not a product of either the sporocyst wall cells or snail hepatopancreas.

Effect of praziquantel on larval stages of Schistosoma japonicum

The effect of praziquantel on S. japonicum mother sporocysts, daughter sporocysts and cercariae was studied. At concentrations of 3 X 10(-7), 3 X 10(-6) and 3 X 10(-5) M and treatment times of 24 or 48 h, mother and daughter sporocysts and young cercarial embryos were not affected but nearly mature cercariae were killed and dissociated. The resistance of young cercariae could support the suggestion that the primitive cercarial epithelium arises from the sporocyst tegument. Treatment with praziquantel always stopped cercarial emission; this cessation lasted for a few days with the lowest concentration and for up to 25 d with the highest. The duration of treatment slightly affected the pattern of reappearance of cercariae but markedly affected the long-term reduction in numbers. Free cercariae treated with praziquantel lost their tails in 10 to 60 min, depending on the concentration.

Inhibition of human peripheral blood mononuclear cell proliferative responses by released materials from Schistosoma mansoni cercariae

During in vitro transformation of Schistosoma mansoni cercariae into schistosomula, surface and glandular materials are released into the culture medium. Extracts of these materials, termed cercarial released extracts 1 and 2 (CRE-1 and CRE-2), were analysed and found to consist primarily of protein and carbohydrate at ratios of 5:1 (CRE-1) and 7:1 (CRE-2). It was observed that inclusion of either CRE-1 or CRE-2 in cultures of human peripheral blood mononuclear cells (PBMN) led to decreased cell proliferation. This was true whether the cells were resting, control cultures or were stimulated with either phytohaemagglutinin or an antigenic preparation from adult S. mansoni worms. The inhibition was equally effective with PBMN of patients with active schistosomal infection or PBMN from uninfected individuals. Since these materials are released spontaneously during cercarial-to-schistosomular transformation they may have a putative immunosuppressive effect in decreasing antischistosomular activities early after cercarial penetration.

Ion fluxes changes during early stages of Schistosoma mansoni. Evaluation of complement effect

The relationship between the average membrane potential (delta psi av) and sensitivity to complement action of the Schistosoma mansoni parasite was explored. The average membrane potential was estimated by measuring the uptake of [3H]tetraphenyl phosphonium ([3H]Ph4P+). The parasites take up Ph4P+ indicating the existence of a negative internal plasma potential which is in part dependent on the transmembrane K+ gradient, maintained by an active Na+/K+-ATPase. Values for Ph4P+ uptake could be corrected for mitochondrial accumulation by employing the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP), which collapses the mitochondrial potential. The plasma membrane potential derived by this technique was in the range of -60 mV. Transformation of this parasite, from its early cercaria stage to the adult worm, was associated with changes in the average membrane potential. The apparent hyperpolarization, which accompanies transformation, may be related to changes in ionic permeability and morphology which occur concomitantly. Complement acting through both the classical and alternative pathways was found to affect the potential of the parasite in its early development stages. The correlation between effects on delta psi av and sensitivity to complement action, indicates that the complement-induced changes in delta psi av are indeed tightly associated with its mode of action. Treatment of the parasite with complement resulted in net hyperpolarization of the membrane indicating that hyperpolarization rather than depolarization of the membrane is linked to the primary non-lethal action of complement.

Schistosoma mansoni: a new parenchymal cell in cercariae

A new type of cell has been identified in cercariae of Schistosoma mansoni. The perikarya (cell bodies) of these cells were located in the body (midsegment), in an area oral to the acetabulum (ventral sucker). Cytoplasmic processes extending from the perikarya ramified throughout the parenchyma of the anterior organ (oral sucker), body, and tail segments by following the path of the nerve processes from the neuropile. The perikarya of these cells had heterochromatic nuclei and a predominance of particulate material and granules (240-360 nm) in their cytoplasm. Aggregates of granules (240-360 nm) and associated vesicles (34 nm) were scattered throughout the cytoplasmic processes of the cells and formed distinct varicosed areas. These processes often connected to the tegument in the midsegment (body) of the cercariae. The granules and associated vesicles reacted (became electron dense) with fixatives reported to be detectors of biogenic amines: The glutaraldehyde/osmium tetroxide fixation procedure rendered the granules electron dense while the glutaraldehyde/chromate/osmium tetroxide fixation procedure rendered the granules and the associated vesicles electron dense. The chromate solution of the latter procedure was responsible for the electron density of the associated vesicles. The morphology of these cells (their long ramifying cytoplasmic processes) and their reaction to chromium suggests that they are probably biogenic aminergic sensory cells.

Ultrastructure of the daughter sporocyst and developing cercaria of Schistosoma japonicum in experimentally infected snails, Oncomelania hupensis hupensis

Ultrastructure of daughter sporocysts and cercariae of Schistosoma japonicum were studied 2 and 4 months after infection of Oncomelania hupensis hupensis. The body walls of daughter sporocysts are similar at all infectious stages. They consist of an external syncytial tegument on a basement membrane, and an internal cellular subtegument surrounding a body cavity containing developing cercariae. The cercariae embryos develop 2 months after infection from germinal balls in the brood chamber of the daughter sporocyst. They are at first enveloped by a primitive epithelium rising from the daughter sporocyst. Four months after infection, the cercariae were almost fully developed and the primitive epithelium had degenerated. The body wall of the cercaria consists of a thin tegument covered by a surface coat of fibrous material and connected to the subtegumental cells by cytoplasmic processes. The matrix of the tegument contains numerous dense bodies, vacuoles, and spines. Two types of sensory structures - uniciliated and multiciliated - are found at the anterior tip of the cercaria. There are five pairs of penetration gland cells of two distinct types differentiated by the morphology of secretory granules. Flame cells are found in both daughter sporocysts and in cercariae. The cilia of the flame cells are characterized by the typical 9 and 2 cilium pattern.

Human neutrophils endocytose multivalent ligands from the surface of schistosomula of Schistosoma mansoni before membrane fusion

Human buffy coat cells adhering to schistosomula of Schistosoma mansoni that were preincubated in fluorochrome-conjugated concanavalin A (Con A), wheat germ agglutinin, lentil lectin, or purified IgG from a hyperimmunized rabbit, were examined by fluorescence and transmission electron microscopy and by freeze-fracture. All four fluorochrome-conjugated multivalent ligands were homogeneously distributed on the parasite surface after preincubation. Within 1-3 h after the addition of cells, large areas of nonfluorescence, 10-20 micrometer in diameter, were seen on the parasite surface. In addition, the fluorochromes were observed in granules within the cells. Electron microscope autoradiography of worms preincubated with 125I-Con A showed silver grains evenly distributed over the tegumental membrane. After the addition of cells, grains were seen over phagolysosomes in the cytoplasm of neutrophils adhering to the parasites. In addition, no grains were present over large areas of the tegumental membrane, which still retained its normal architecture, or over fusions between the neutrophil plasma membrane and the outer tegumental membrane. Rabbit IgG formed an electron-dense layer on the tegumental membrane which was endocytosed by neutrophils. Both neutrophils and eosinophils fused with the parasite in areas containing no electron-dense material on the surface. It is concluded that human neutrophils will endocytose a variety of multivalent ligands from the surface of schistosomula, which probably accounts for the failure of neutrophils to kill the parasite and acts to clear the parasite surface of both antigen and antibody. Presumably, the components of the parasite surface which have originally bound the ligands are also endocytosed since surface components labeled by galactose oxidase and NaB3H4 are taken into cells when examined by light microscope autoradiography. Finally, membrane fusion occurs in areas devoid of multivalent glands, which suggests that these ligands serve to bring the cells and parasites close together, but the actual fusigens probably reside in the lipids in the outer tegumental membrane.

The development of daughter sporocysts inside the mother sporocyst of Schistosoma mansoni with special reference to the ultrastructure of the body wall

The development of the mother sporocyst and the differentiation of the daughter sporocyst of Schistosoma mansoni in Biomphalaria pfeifferi are described. The tegumental structure of the mother sporocyst, consisting of an outer layer connected to internally situated nucleated cell bodies, forms extensions which enwrap the germinal cells. The parenchyma cells, in which the germinal cells were embedded before, degenerate. When daughter sporocyst embryos develop from germinal cells they are enveloped by a primitive epithelium which is formed by fusion of the extensions of the tegumental structure of the mother sporocyst. Somatic cells located peripherally in the developing daughter sporocyst expand and coalesce beneath the primitive epithelium to form the future outer layer of the tegumental structure of the daughter sporocyst. The primitive epithelium degenerates, the newly-formed layer looses its nuclei, and becomes connected to internally situated nucleated cell bodies. Further developments in the tegumental structure of the daughter sporocyst include the formation of microvillus-like projections, a surface coat, spines, and a basement membrane.

Intermediate host antigens associated with the cercariae of Schistosoma haematobium

Bulinus (Physopsis) africanus antigens were shown to be associated with the cercarial glycocalyx of Schistosoma haematobium using immunofluorescence and the Cercarienhüllen Reaktion. It is proposed that this snail antigen may sensitise the definitive host and that resistance to further invasion by cercariae could be induced in this manner.

The origin and development of the epidermis and associated structures in the cercaria of Cryptocotyle lingua (Creplin) (Digenea: Heterophyidae) from Littorina littorea (L.)

Cercariae of Cryptocotyle lingua develop from intraredial germinal cells which divide to form ‘naked’ cell aggregates and later germ balls covered, first, by a syncytial primitive epithelium and later a syncytial epidermis formed, in sequence, from superficial cells of the embryo. The primitive epithelium is soon lost. The original nuclei of the syncytial epidermis degenerate when the first series of epidermal cell bodies, formed immediately below and having the characters of protein synthesizing cells, become connected with it. The first cell bodies are replaced by a series of five types of epidermal (secretory) cell bodies developing in the parenchyma and giving off cytoplasmic processes which become connected, in sequence, with the outer cytoplasmic layer. Secretion bodies from four of the five types are discharged into the outer cytoplasmic layer, before the cercaria leaves the molluscan host, and remain there in the free swimming cercaria. The secretions of the fifth type are retained in their epidermal (secretory) cell bodies. The arrangement of the secretion bodies in the outer cytoplasmic layer and their histochemical reactions suggest possible functions concerned, later, with entry into and encystment within the second intermediate fish host.

Factors affecting surface changes in intact cercariae and cercarial bodies of Schistosoma mansoni

The effect of different incubation media and of temperature on the induction of water sensitivity has been investigated in intact and tailless Schistosoma mansoni cercariae. Removal of the cercarial tail by vortex stirring and elevation of the temperature of the medium from 27 to 37 degrees C resulted in the rapid onset of permeability changes in the larvae. The rate of change was greater in water than in TC-199 or Hanks' BSS media. Lowering the pH of the medium or increasing the concentration of Ca2+ ions decreased the rate of permeability change: raising the pH of the medium or the addition of 10(-5) M EDTA enhanced the rate. Raising the temperature of the medium also increased the rate of permeability change in intact cercariae although the rates obtained varied with the different media tested, being greatest in TC-199. It is concluded that both temperature elevation and loss of the cercarial tail influence the onset and rate of permeability changes in cercarial bodies during the transformation to schistosomula.

The development of the primitive epithelium and true tegument in the cercaria of Schistosoma mansoni

The formation of the final cercarial tegument of Schistosoma mansoni is preceded by that of a so-called primitive epithelium. The primitive epithelium is derived from the tegument of the daughter sporocyst. The final cercarial tegument is formed from peripherally located somatic cells of the cercarial embryo, which expand and coalesce beneath the primitive epithelium. The primitive epithelium degenerates and disappears. The ultrastructure of both epithelia in the course of the development of the cercaria is described in detail. Possible functions are discussed.

An electron microscope study of the tegument of the metacercaria and adult of Leucochloridiomorpha constantiae (Trematoda: Brachylaemidae)

The tegumentary ultrastructure of Leucochloridiomorpha constantiae metacercariae and adults has been described. A filamentous glycocalyx invests the tegument of metacercariae and large numbers of biconcave, disk-shaped secretion vesicles are found in its outer zone, cytoplasmic bridges and cytons. Mitochondria within the outer tegumentary zone of metacercariae are restricted to its basal half. The transformation of metacercariae to adults involves the loss of the glycocalyx, a decrease in the number of disk-shaped secretion vesicles, a more uniform distribution of mitochondria through the outer tegumentary zone, an apparent degeneration of cytons, a thinning of the fibrous basal lamina and a decrease in the number of visible cytoplasmic bridges traversing it. The significance of these events for a metacercaria which does not encyst within the molluscan host is discussed. In addition, crystalline, spine-like inclusions in the outer tegumentary zone of adult L. constantiae are described and their possible function discussed.

Epidermal fine structure and development in the oncomiracidium larva of Entobdella soleae (Monogenea)

The development of the epidermis of the oncomiracidium larva of Entobdella soleae was studied in embryos dissected from the egg and processed for electron microscopy. At first (7- to 8-day embryos at 15 °C) the embryo is covered with a nucleated primary epidermis of flattened cells which are closely associated with the viteUine cells and may take up nutrients from them. This layer is either replaced by or develops into the secondary epidermis which consists of ciliated cellular regions joined by an apparently syncytial interciliary cytoplasmic layer. Both ciliated cells and interciliary cytoplasm are at first nucleated but later both lose their nuclei. There may be a turnover of ciliated epidermal cells at the surface of young embryos. Between days 16–20 a discontinuous presumptive adult epidermis appears beneath the ciliated cells which has connexions to cell bodies lying in the parenchyma. This layer, apparently fuses with the syncytial (?) interciliary regions which have by this time lost their nuclei. After shedding of the ciliated cells, the presumptive adult epidermis spreads out to form a continuous syncytial covering to the post-larva. The ciliated epidermal cells apparently lose their nuclei between days 20 and hatching. This may be associated with the ‘programmed life’ of the ciliated cells. Details of the food reserves and morphology of the ciliated cells are discussed in connexion with the energetics of these cells.