Ultrastructure of the tegument in daughter sporocyst and cercarial embryos of Meiogymnophallus minutus (Digenea:Gymnophallidae)

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.

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.

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.

Ultrastructure study of the intramolluscan stages of Meiogymnophallus minutus (digenea: gymnophallidae) in Scrobicularia plana (bivalvia) from Portugal

The ultrastructure of the tegument of the sporocyst and contained cercariae of Meiogymnophallus minutus found in Scrobicularia plana collected in Ria de Aveiro estuary (Portugal) were studied by scanning (SEM) and transmission electron microscopy (TEM). TEM revealed microvilli extending from the surface of the sporocyst tegument that contributed to the lacetrimmed surface morphology observed by SEM. The cercarial surface was papilliform and covered densely with single-pointed tegumental spines. On TEM, besides the spines and several mitochondria, translucid and electrondense spherical secretory vesicles were evident in the matrix of the tegument but were not seen on the sporocyst tegument. Uniciliate sensory receptors found in the cercaria of M. minutus were examined under SEM and TEM. Structural differences between the oral and the ventral sucker were observed. On the lip of the ventral sucker, large and strong spines were arranged in concentric rows, and two groups of six sensory papillae were placed concentrically and equidistantly. The oral sucker, surrounded by a crown of dense and long microvilli, exhibited a ring of uniciliate sensory papillae and some tegumental spines along the lip. The surface morphology of the M. minutus cercaria is compared with that of other Gymnophallidae cercariae.