The formation of the cyst wall of the metacercaria of Fasciola hepatica L

Ecology of free-living metacercariae (Trematoda)

The presence of trematodes with a free-living metacercarial stage is a common feature of most habitats and includes important species such as Fasciola hepatica, Parorchis acanthus and Zygocotyle lunata. These trematodes encyst on the surface of an animal or plant that can act as a transport host, which form the diet of the target definitive host. Although these species are often considered individually, they display common characteristics in their free-living biology indicating a shared transmission strategy, yet in comparison to species with penetrative cercariae this aspect of their life cycles remains much overlooked. This review integrates the diverse data and presents a novel synthesis of free-living metacercariae using epibiosis as the basis of a new framework to describe the relationship between transport hosts and parasites. All aspects of their biology during the period that they are metabolically independent of a host are considered, from cercarial emergence to metacercarial excystment.

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.

Gorgoderina vitelliloba: an ultrastructural study on the development of the tegument from the metacercaria to the adult fluke

The tegument of the metacercaria of Gorgoderina vitelliloba contains S1, S2, S3 and S4 secretory bodies. Between the parenchymal cells 2 types of tegumental cyton occur. Type 1 contains S1 and S3 secretory bodies, whilst S2 and S4 bodies are limited to Type 2. Type 1 cytons and S1 secretory bodies disappear at excystment. Post-metacercariae contain a third type of tegumental cyton in which a new secretory body, S5, occurs. This cyton type disintegrates 1–5 days after infection. The maintenance and development of the outer anucleate layer after this period is the responsibility of Type 2 cytons. Type 2 cytons and S2 and S4 secretory bodies have also been found within the cercarial body. When juvenile flukes migrate from the kidney to the bladder of their frog definitive host 21 days after infection, the outer anucleate layer increases in electron density. Within the bladder, however, the tegument of the adult fluke appears normal. The distribution of mitochondria within the outer anucleate layer changes during the fluke's migration. The mitochondria, which have a random distribution in the metacercaria, are located adjacent to the outer and basal plasmalemmas in the early kidney stages. In late kidney stages and bladder stages mitochondria occur only against the former membrane. The mitochondria increase in size in the adult fluke.

The uptake and distribution of radioselenium in the larvae of Fasciola hepatica and its snail host Lymnaea columella

Radiolabeled metacercariae of Fasciola hepatica were obtained in vivo by incubating infected Lymnaea columella snails with 20 muCi radioselenomethione (75Se-M) per snail in 5 ml of water for 5 h, or in vitro by incubating a batch of unlabeled F. hepatica metacercariae with 75Se-M for 24 h. Radioassay showed that only 5% of the 75Se-M was incorporated into maritas (juvenile flukes) from the in vivo labeled metacercariae. The inner cyst wall of in vivo labeled metacercariae contained 46% of the total activity, of which 21% was dissolved in the excysting medium. The outer, tan-colored cyst wall contained 49% of the radioactivity. Through diffusion/attachment, maritas from in vitro labeled metacercariae could occasionally be labeled with 0.4% of the total radioactivity. However, the activity was lost after inoculation into the body of mice. The outer and inner cyst walls of in vitro labeled metacercariae contained 92% and 7.6%, respectively, of the total activity. Microautoradiography demonstrated that 75Se-M was evenly distributed in the body of marita and the cyst wall of inner and outer layers from an in vivo labeled metacercaria. A 9 X 4 micron rectangularly-shaped aggregate of Ag degree grains was present on the outer periphery of the inner cyst wall. Microautoradiography of in vitro labeled metacercariae demonstrated a significant concentration of Ag degree grains on the cyst walls. The ventral plug contained fewer Ag degree grains per unit area compared to the other portion of the inner cyst wall. Uptake and distribution of 75Se-M in the snail host were also studied. It appeared that rediae and cercariae tended to concentrate the label in the foot, the mantle and the digestive gland. Little or no radioactivity was present in the areas where F. hepatica larvae were not found.

Stereoscan studies of rediae, cercariae, cysts, excysted metacercariae and migratory stages of Fasciola hepatica

The external surface of the redial body of Fasciola hepatica is provided with microvillus-like projections or short lamellae, and short cilium-like structures are common anteriorly. The anterior part of the cercarial body possesses a pattern of regularly arranged small depressions each containing a spine. Both long and short cilium-like structures occur anteriorly. The tail is spineless and provided with dorsolateral folds. The outer cyst wall is formed by granules secreted from the tegument all over the body apart from the ventral sucker. Most granules transform into fibrillae which form the thick outer spongy layer. The precursor of the inner cyst wall is at the beginning closely attached to the metacercarial surface, but later the membrane-like cyst wall extends, and when fully formed the metacercaria lies free in the flattened circular inner cyst. The ventral plug is formed by the ventral sucker. The tegument of newly excysted metacercariae is provided with simple pointed spines, but later during migration in the mouse the spines become flattened and multipointed. Very young migratory stages may be attached with host cells.

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.

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.

Schistosoma mansoni: the development of the cercarial tegument

Young cercariae were covered with a thin nucleated primitive epithelium which was lost when the true tegument appeared beneath it. The tegument was at first similar to the primitive epithelium in that it was a nucleated, cytoplasmic syncytium but it could be distinguished from the primitive epithelium by its thickened outer membrane and by the layer of interstitial material beneath its basal membrane. The tegumental nuclei became pycnotic and disappeared and, at about the same time, nucleated subtegumental cells became continuous with the tegument. Two types of small, dense bodies were formed in the subtegumental cells and passed into the tegument. Later in development the tegument became filled with dense, granular material and spines were formed. The fully developed cercarial tegument was covered with a surface coat of fibrous material which was specifically attached to the outer membrane of the tegument.

The observations described in this paper formed part of a thesis approved by the University of London for the Degree of Ph.D.

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.