Penetration by the miracidia of S. mansoni into the snail host

Description of embryonic development and ultrastructure in miracidia of Cardiocephaloides longicollis (Digenea, Strigeidae) in relation to active host finding strategy in a marine environment

The functional ultrastructure and embryonic development of miracidia in naturally released eggs of the trematode Cardiocephaloides longicollis were studied using light and transmission electron microscopy. This species has operculated eggs and embryogenesis occurs in the marine environment before an actively infecting ciliated miracidium hatches. Six different developmental stages were identified. The lack of pores in the eggshell indicates its impermeability and the miracidium's dependency on glycogen nutritive reserves, contained in numerous vitellocytes in early embryos. As the development advances, these merge into larger vitelline vacuoles that encircle the miracidium and may aid its hatching. Tissue and primary organ differentiation were observed in advanced stages, i.e., terebratorium, glands, cerebral ganglion, peripheral sensory endings, and eyespots. The anterior part of the body contains a single apical and paired lateral glands, as well as two types of sensory endings, which permit location, adhesion, and penetration of the host. No previous studies describe the embryonic development and ultrastructure of miracidia in strigeids, however, some of the structural features shared with other, well described species with unknown life cycles are emphasised. This study highlights that ultrastructural data have to be interpreted in relation to parasite biology to understand the structural requirements of specific parasite strategies.

GPCR and IR genes in Schistosoma mansoni miracidia

Background

Schistosoma species are responsible for the disease schistosomiasis, a highly prevalent helminthic disease that requires a freshwater snail as intermediate host. The S. mansoni free-living miracidium must utilize olfaction to find a suitable snail host, and certain types of rhodopsin G protein-coupled receptors (GPCRs) and ionotropic receptors (IRs) have been identified as olfactory receptors in other animal phyla. The Schistosoma genome project, together with the recent availability of proteomic databases, allowed for studies to explore receptors within S. mansoni, some of which may contribute to host finding.

Results

We have identified 17 rhodopsin-type GPCR sequences in S. mansoni belonging to four subclasses, including ligand-specific GPCRs (i.e. neuropeptide and opsin). RT-PCR demonstrated the expression of nine out of the 17 GPCRs in the free-living miracidia, each of which have been characterized for homology to S. haematobium. Among the nine GPCRs, two are predicted as Gq-opsins. We also describe the characterization of a Schistosoma-encoded IR based on similarity with other species IR and conservation of IR-like domains. Schistosoma mansoni IR is expressed in miracidia at 3 and 6 h post-hatch.

Conclusions

The identification of receptors in S. mansoni miracidia, presented here, contributes not only to further understanding of Schistosoma biology and signal transduction but also provides a basis for approaches that may modify parasite behaviour.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1837-2) contains supplementary material, which is available to authorized users.

The Attachment and Penetration Apparatus of the Miracidium of Schistosoma

Preliminary electron microscope observations have shown that in the miracidium of Schistosoma mattheei the surface of the apical epidermal plate consists of branching and anastomosing microvilli. It is suggested that this arboreal arrangement serves to attach the miracidium to the body surface of snail hosts during attempts to penetrate; the apical papilla can therefore be regarded as an attachment organ, functionally analogous to the suckers of the adult schistosome. It is also suggested that the degree of ‘fit’ between the attachment organ and the snail body surface may be an important factor determining the success of attempts by the miracidium to penetrate.

Electron microscopy has also shown that the so-called penetration and apical glands are single flask-shaped cells. No endoplasmic reticulum and very few ribosomes were seen in these cells and it is therefore suggested that, in the mature (free-swimming) miracidium, the “glands” are not functional as such; they are simply sacs full of fluid. If they contain histolytic substances they must either have been synthesized at an earlier stage in the life history of the organism or they are synthesized elsewhere and passed into the “glands”.

Histochemical attempts to identify leucine aminopeptidase and mucin in the contents of the “glands” of S. mansoni miracidia were unsuccessful. It is concluded that these substances probably do not play any role in the penetration into or attachment on snail intermediate hosts by S. mansoni miracidia.

Ultrastructural studies on the miracidium ofSanguinicola inermis(Digenea: Sanguinicolidae)

The miracidium ofSanguinicola inermiscontains a stylet and rodlet complex located within the single apical gland, which lies between a pair of lateral glands. The miracidial stylet consists of an arrangement of hollow tubes, composed of microtubules, connected at their base to large secretory granules and opening anteriorly through the apical papilla. A single rodlet lies adjacent to the stylet base and at least 8 rodlets are arranged at the apical tip of the stylet. The surface plasma membrane of the apical papilla is drawn into knob-like projections and 2 ciliated sensory endings are associated with the papilla. The ultrastructure of the apical gland, especially the stylet and rodlet complex, is related to the requirement for sanguinicolid miracidia to penetrate the double barrier of fish and snail hosts.

Light and scanning electron microscopy of the miracidium of Echinostoma paraensei (Trematoda, Echinostomatidae)

Echinostoma paraensei was described 1967 by Lie and Basch. Recently its natural definitive host, the aquatic rodent Nectomys squamipes, endemic in Brazil, was identified. As most of the echinostomatids, this species presents a zoonotic potential. The morphology and topography of the E. paraensei miracidium obtained from adult worms collected from the natural definitive host N. squamipes is described by light and electron microscopy. The arrangement and the dimensions of the epidermal plates are given. The eyespots are composed by two pairs of lenses measuring 6.03 microm. SEM observations shows that the miracidium body is covered by cilia except at the terebratorium region and the presence of alpha-tubulin in the cilia of the larvae is first recorded by immunelabeling. Nineteen papilla-like structures arranged in three axes and four groups were observed at the terebratoriun, this structure is retractable, presenting folds and a corrugated surface with profiles of cytoplasmic expansions, forming network of anatomizing folds.

Ultrastructural immunolocalization of two circulating egg antigens in miracidia of Schistosoma mansoni

In the present study ultrastructural immunocytochemical techniques have been used to localize the cells producing two soluble egg antigens present in the hatching fluid of Schistosoma mansoni, using two recently described monoclonal antibodies (MAbs). In the miracidium of S. mansoni, the one apical and the two lateral unicellular glands showed immunoreactivity for the egg antigens, using a post-embedding immunogold technique. Immunoreactivity was associated with the contents of vesicles for both the MAbs, although the intravesicular distribution was not identical. This difference in ultrastructural staining pattern provides additional evidence that the two MAbs are binding with different epitopes of the same antigen or with different antigens altogether. The finding of free unbound label around the embedded miracidium is consistent with secretion having been released from the glands.

Ultrastructural changes in the body wall of Schistosoma mansoni during the transformation of the miracidium into the mother sporocyst in the snail host Biomphalaria pfeifferi

The ultrastructure of the body wall of the free miracidium of Schistosoma mansoni and the changes occurring within 48 h after penetration into the intermediate host Biomphalaria pfeifferi are described. Within 2 h after penetration the ciliated plates are shed into the haemolymph of the snail and phagocytized by amoebocytes. At the same time the narrow ridges between the plates of the free miracidium expand to form the continuous outer layer of the sporocyst. Within 48 h the entire tegumental structure, consisting of a thin outer layer, connected with sunken nucleated areas, develops to its full extent. The observations are compared with those on Fasciola hepatica.

[Contributions to the micromorphology of the miracidium of Schistosoma mansoni. I. Fine structure of the tegument and its "associated structures" (author's transl)]

The body wall of the miracidium of S. mansoni has been studied by light and electron microscope. It has been found that the tegument layer contains so-called membrane-bound bodies. The tegument layer will be separated from muscle layer by means of a basal membrane. Special attention was focused on the associated structures of the tegument; these are cilia, microvilli-like appendices amounting to six in number at our test-organisms and finally two types of sensory papillae on the so-called terebratorium. After treatment of the miracidium in antiserum, fine granulated precipitate was formed around the cilia.

The penetration of Fascioloides magna miracidia into the snail host Fossaria bulimoides. A scanning electron microscope study

Snails of the species Fossaria bulimoides were exposed to miracidia of Fascioloides magna for given periods of time. The course of penetration was followed by means of scanning electron microscopy. Topographical features of the miracidial morphology were studied; the apical organ, epidermal plates and their shedding, and body shapes during penetration. Preliminary data were obtained concerning the nature of the lesions formed by the miracidia. Inferences were drawn concerning the mode and duration of penetration.

Stereoscan observations of the miracidium and early sporocyst of Schistosoma mansoni

Whole miracidia of Schistosoma mansoni, miracidia vibrated in an ultrasonic cleaner, and the miracidium-sporocyst transition were studied in the stereoscan electron microscope. After vibrating, the cilia broke off near the bases and the epidermal cells, intercellular ridge and sensory structures were revealed. The apical papilla had a folded surface with penetrating sensory cilia. The number of epidermal cells varied between 17 and 22. The lateral papillae appeared as bulbous projections on either side between the first and second tiers of epidermal cells. There was a ciliated pit nerve ending close to each lateral papilla. A few ciliated pits were found between the cells in the first tier, and up to twelve ciliated pits with long cilia could be found between the second and third tiers. Miracidia placed in haemolymph from Planorbarius corneus cast off the apical ciliated part of the epithelial cells, and large scars appeared where the ciliated plates had been. Later, the syncytial intercellular ridge dispersed throughout the surface of the mother sporocyst, and small cytoplasmic knobs appeared on the surface. The apical papilla and the lateral papillae were still observed a few hours after shedding the ciliated plates, but the ciliated pits disappeared shortly after the ciliated plates were lost.

Attachment and penetration of miracidia observed by scanning electron microscopy

Scanning electron microscopy can be utilized to understand more clearly many aspects of the parasite-host relationship of schistosome miracidia and their molluscan intermediate hosts. Specialized structures on the apical papilla of the miracicium, used for attachment and penetration, become visible in greater detail.

Observations on the epidermis of the miracidium and on the formation of the tegument of the sporocyst of Fasciola hepatica

The relationship between the ciliated epidermal cells and the subepidermal layer of the miracidium of Fasciola hepatica has been described. Non-ciliated ridge-like extensions of the subepidermal layer separate the ciliated epidermal cells from each other. The sunken portions of the subepidermal layer, each containing a nucleus, lie below the outer body wall muscles of the miracidium and open into the ridge by narrow neck-like connexions. Elongate vesicles, which may be a source of stored plasma membrane similar to that which occurs in the transitional epithelium of other animals, fill most of the ridge. In addition, characteristic round electron dense granules are found in the ridge but the majority are found in the sunken portions of the subepidermal layer.

The development and origins of the tegument of the sporocyst of F. hepatica have been described at the ultrastructural level. When the miracidium is in the process of penetrating the snail host, large vacuoles appear between the ciliated epidermal cells and the basal lamina which overlies the muscles of the body wall. These vacuoles have the effect of loosening the epidermal cells from the basal lamina of the body wall of the miracidium. Possible mechanisms involved in the formation of such vacuoles are suggested and discussed.

During penetration of the snail the ciliated epidermal cells of the miracidium are lost; the ridge, a syncytial layer between the epidermal cells which is connected with the subepidermal layer, spreads over the basal lamina and exposed body wall muscles of the metamorphosing sporocyst to form the new outer covering of the sporocyst.

Cytoplasm passes from the subtegumentary layer into the tegument during this stage of the development of the body wall of the sporocyst. Muscular contraction and microtubules may be involved in the outward movements of this cytoplasm. The nuclei of the subtegumentary layer remain below the muscles of the body wall.

Twenty-four hours after penetration of the snail the outer plasma membrane of the tegument forms folds, which greatly increase the surface area.

Sixty hours after penetration involutions between the folds, which may indicate pinocytosis, are present, and it is suggested that pinocytosis may play a role in food absorption.

The fully formed tegument is a syncytial layer containing numerous electron dense granules, vacuoles, mitochondria and lipid droplets.

The results on the formation of the tegument of the sporocyst have been discussed with reference to the controversy about the origins and terminology of the outer covering of the Platyhelminths.