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

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.

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.

Regional specializations of the tegument of the metacercaria ofTimoniella imbutiforme

The ultrastructure of the spinous body tegument of the metacercaria ofTimoniella imbutiforme(Molin, 1859) has recently been described. Other regions of the metacercarial tegument, including those of the oral sucker, pharynx, and nephridiopore, demonstrate considerable specializations. The oral sucker tegument had an aspinous outer syncytial layer that possessed a pimpled apical surface as well as enclosing two types of secretory bodies. The pharyngeal tegument likewise lacked spines, but possessed only one type of secretory body, and a smooth but folded outer surface. The nephridiopore tegument, however, showed the greatest degree of specialization possessing a single type of secretory body specific only to this region of the tegument. Also associated with the syncytium here was a prominent long filamentous glycocalyx, and microtubules which were observed for the first time in this region of the tegument.

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

The germinal development in Prosorhynchoides gracilescens sporocysts was studied using electron microscopy. The germinal cells proliferated and developed within multiple floating germinal masses located in the terminal portions of sporocyst branches. The germinal masses were composed of supporting and germinal cells. Supporting cells possessed numerous flattened extensions that spread around and between developing germinal cells to form three-dimensional mesh network, which maintained the integrity of the germinal mass. Morphological evidences of close interactions between supporting and germinal cells were numerous gap junctions between the two cell types and phagocytosis of small fragments of germinal cells cytoplasm by supporting cells. The germinal cells displayed structural differences that seemed to reflect their sequential developmental changes. These changes included (1) cell growth and increase of organelles number, (2) dispersion of nuclear chromatin and increase of nucleolus size, (3) polarization of the cell, (4) appearance of specific structures such as nuage and laminated inclusions. The germinal cells left the germinal masses to finish their differentiation in the body cavity and then cleaved to give rise to cercarial embryos. Ultrastructural features of the germinal elements of P. gracilescens sporocysts are discussed in the light of existing controversy concerning the nature of the germinal sacs reproduction.