An ultrastructural study of reproduction in the sporocysts of Prosorhynchoides gracilescens (Digenea: Bucephalidae)

Germinal development in embryonic rediae of the hemiuroid digenean Bunocotyle progenetica: an ultrastructural study

Embryonic development of reproductive organs in rediae of the digenean Bunocotyle progenetica was studied using transmission electron microscopy. The germinal primordium becomes morphologically distinct in early embryos as a weakly separated cell mass with a forming cavity. It consists of undifferentiated, differentiating, and supporting cells. As embryos develop, the supporting cells form a wall around the enlarging cavity. Other cells of the germinal primordium are incorporated into the wall as solitary cells or as small cell aggregations. Those situated posteriorly give rise to an incipient germinal mass functioning during postembryonic development. Undifferentiated and differentiating cells in the middle and the anterior part of the primordium ensure a considerable growth of the cavity wall, which incorporates solitary germinal cells. In advanced embryonic rediae, these cells mature, cleave, and give rise to germinal balls, which enter the forming brood cavity. In the most mature embryonic rediae, all these early cercarial embryos reside in a brood cavity, which is lined by that time with a syncytium continuous with the supporting tissue of the incipient germinal mass. Based on our results and the literature data, we suggest that the morphogenesis of the reproductive apparatus of the daughter parthenitae in hemiuroid digeneans may be characterized by (1) emergence of an incipient brood cavity within the germinal primordium, (2) formation of the cavity lining from the cells of the germinal primordium, (3) fragmentation and uneven distribution of the germinal material of the germinal primordium around the cavity, and (4) an anticipatory development of some of this germinal material.

The fine structure of the germinal mass, brood cavity and birth canal of the rediae of the monoxenous hemiuroid digenean Bunocotyle progenetica Chabaud & Buttner, 1959

Bunocotyle progenetica is a hemiuroid digenean whose sexual adults become fully developed and lay their eggs inside the rediae in the molluscan host. In this study, the fine structure of the germinal mass, brood cavity and birth canal in the B. progenetica rediae was examined using transmission electron and confocal microscopy. The large germinal mass attached to the body wall has a cellular composition typical for this organ. The characteristic traits of this germinal mass are weakly developed supporting tissue and the presence of deep lacunae opening into the brood cavity. These lacunae presumably participate in feeding the deeply lying embryos and facilitate their release into the brood cavity. The germinal mass is also characterized by intensive degeneration of cellular elements, which may represent a mechanism controlling the offspring number, limited in this species by the size of the redial brood cavity. The brood-cavity lining consists of flattened cells bearing lamellar projections and is connected anteriorly with the epithelium of the birth canal. The brood-cavity musculature, which is well developed in other hemiuroid digeneans, is significantly reduced in B. progenetica, most likely because their cystophorous cercariae remain inside the rediae, removing the need for muscle contractions pushing them through the brood cavity. The birth canal comprises three regions distinguished by the structure of the lining and muscle arrangement. The comparison of rediae of B. progenetica with parthenitae of other digeneans has shown that the organization of the redial reproductive apparatus in this species may have been influenced by life-cycle modification.

Reproduction of trematodes in the molluscan host: an ultrastructural study of the germinal mass and brood cavity in daughter rediae of Tristriata anatis Belopolskaia, 1953 (Digenea: Notocotylidae)

This study describes the fine structure of the germinal mass in daughter rediae of Tristriata anatis. The germinal mass consists of undifferentiated cells, germinal cells and supporting cells and contains numerous cercarial embryos up to tail bud stage. Supporting cells and their outgrowths form a tight meshwork of the germinal mass. In its basal part, this meshwork serves as scaffolding for undifferentiated and germinal cells, naked cell aggregates and early germinal balls. More mature embryos are located apically. The hypertrophied supporting tissue appears to be involved in an intensive transport of substances, as indicated by abundant gap junctions between cell outgrowths and numerous pinocytotic vesicles and microtubules in their cytoplasm. Germinal cells contain annulate lamellae and the nuage, typical organelles of animal oocytes. In young rediae containing embryonic cercariae at the tail bud stage, the supporting tissue starts to degenerate in the apical part of the germinal mass, and a primordial brood cavity emerges though it develops fully only in mature rediae containing late embryonic cercariae. An unusual feature of the germinal mass in T. anatis rediae is an enhancement of the embryo brooding function. At the same time, the performance of this function by the brood cavity is reduced. This is the first time such a redistribution of the embryo brooding function between the germinal mass and the brood cavity has been reported.

Trematode reproduction in the molluscan host: an ultrastructural study of the germinal mass in the rediae of Himasthla elongata (Mehlis, 1831) (Digenea: Echinostomatidae)

The germinal mass in Himasthla elongata rediae was studied in detail using transmission electron microscopy. It was shown to be a specialized reproductive organ consisting of germinal cells at various maturation stages, supporting cells and stem cells. The germinal mass also contains early cercarial embryos emerging as a result of cleavage division of mature germinal cells. The stem cells that give rise to germinal cells have heterochromatin-rich nuclei with distinct nucleoli and scarce cytoplasm containing mainly free ribosomes and few mitochondria. The differentiating germinal cells undergo a growth, which is accompanied by an emergence of annulate lamellae and the nuage in their cytoplasm, a noticeable development of RER and Golgi apparatus and an increase in the number of mitochondria. The mitochondria form a large group at one of the cell poles. During differentiation, the nucleus and nucleolus of the germinal cell enlarge while the chromatin becomes gradually less condensed. The supporting tissue of the germinal mass is made up of cells connected by septate junctions. These supporting cells are distinctly different in cellular shape and nuclear ultrastructure. Their outgrowths form a tight meshwork housing stem cells, germinal cells and early cercarial embryos. The cytoplasm of the supporting cells in the mesh area is separated into fine parallel layers by labyrinthine narrow cavities communicating with the intercellular space. The supporting tissue contains differentiating and degenerating cells which indicates its renewal. The results of this ultrastructural study lend support to the hypothesis that the germinal cells of digeneans are germ line cells.

Morpho-functional specialization of the branching sporocyst of Prosorhynchoides borealis Bartoli, Gibson & Bray, 2006 (Digenea, Bucephalidae)

Sporocysts of Prosorhynchoides borealis were obtained from the marine bivalves Abra prismatica and studied using transmission electron microscopy. The sporocyst body consists of a mass of branching and intertwining hollow tubules that ramify through the host's digestive gland and gonads. This study investigated the ultrastructure of the sporocyst branches which comprise alternate distended areas (brood chambers) with a relatively thin body wall, narrower portions with a thicker body wall (constricted areas) and terminal regions. Pronounced differences between these areas were revealed in the structure of their tegument and body cavity lining, as well as in the cellular composition of the subtegumental layers. Body wall composition in distended areas was consistent with the specialization for cercarial nurture in the brood chambers. The structure of the constrictions suggested a dual role of nutrient absorption and physical separation of adjacent brood chambers. Two types of terminal region were identified, one specialized for the investigation and penetration of host tissues and the other, in which the germinal cells are formed, for cercarial production. The overall structure of the sporocyst branches helps explain why this linear modular system, i.e. brood chambers and constrictions continuously growing into the host tissue, enables the sporocyst's long-term existence and can continuously produce cercariae in numbers comparable with those produced by rediae and/or daughter sporocyst infrapopulations in other digeneans. The origin of the nuclei in the outer tegumental layer of some branching bucephalid sporocysts is also discussed.

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.