Formation of the cyst wall and related changes in the structure of the tegument of Bucephalus haimeanus (Lacaze-Duthiers, 1854) during its metamorphosis from the cercarial to the metacercarial stage

Topography and ultrastructure of the tegument of Bucephalus anguillae (Digenea: Bucephalidae), a parasite of the European eel Anguilla anguilla (Osteichthyen: Anguillidae)

The tegumental ultrastructure of the intestinal fluke Bucephalus anguillae was studied with the use of scanning and transmission electron microscopy. The surface of the tegument is covered by transverse ridges from which protrude numerous closely packed, digitated, and claw-shaped spines. Cobblestone-like units of the tegument were observed on the crescent-shaped formation of the rhynchus and at the posterior part of the body. Three types of sensory structures were examined, i.e., 2 uniciliated receptors and 1 without cilia. As anterior-posterior differences were observed, particular attention was given to spines and sensory receptors. Spine insertion zones and average cilia length are variable between anterior and posterior tegument areas. Ultrastructural study revealed that the tegument of B. anguillae has a typical syncytial organization with a distal cytoplasm lying over a basal matrix and cytons below. Cytoplasmic bridges allowed transit of secretory vesicles and granules. Diagrams of spines and sensory receptors were made to help in understanding the nature of these structures.

In vitro excystment of the metacercaria of Bucephalus haimeanus from the common goby (Pomatoschistus microps)

The structure of the metacercarial cyst wall and excystment in vitro were studied in the bucephalid digenean Bucephalus haimeanus. Optimum excystment occurred in a mixture of 1% trypsin and 1% bile salts at pH 7.8, following a 15 min pretreatment with 1% pepsin at pH 2.0. Enzymes (pepsin, trypsin) facilitated excystment, but were not essential, since some of the active metacercariae perforated the cyst wall in their absence. Other distinctive features were the weak development of the cyst wall, the occurrence of some excystment in strongly acidic media, ability to survive for at least 2 h in acidic media (pH 2.0) and total inactivity and failure to excyst in bile salts. The possible significance of these features is considered in relation to the location of the cysts in the liver of the second intermediate host, the common goby (Pomatoschistus microps), and conditions experienced by the cysts when the goby is eaten by the definitive host, the bass (Morone labrax). Excystment occurred readily at temperatures from 10 to 40°C, but percentage and speed of excystment were reduced at 5°C.

Functional morphology of the metacercarial cyst of Bucephaloides gracilescens (Trematoda: Bucephalidae)

The metacercarial cyst wall of Bucephaloides gracilescens is bilayered. It comprises an inner granular layer of pepsin-resistant carbohydrate and protein material secreted by the parasite, and an outer fibrous and cellular capsule susceptible to pepsin digestion and elaborated by the host. The capsule of cysts from the orbits and nasal regions of the whiting host are substantially thicker than those from the cranial cavity. Cells in the outer portion of the capsule resemble fibrocytes engaged in the synthesis and secretion of collagen; collagen fibres form a loose meshwork in the intercellular space. The cells incorporated [3H]proline in vitro, but not [3H]tyrosine. The cells of the inner portion of the capsule are closely packed and possess pyknotic nuclei; desmosomes and cytofilaments are numerous and probably impart strength and elasticity, respectively, to the cyst wall. Host cells are necrotic at the interface with the inner cyst wall, possibly due to compression or isolation from oxygen and nutrients. Differences in the levels of incorporation of trace-labelled substrates by encysted metacercariae were related to the site of infection and to the thickness and permeability of the cyst wall.

Developmental and functional ultrastructure of Ornithodiplostomum ptychocheilus diplostomula (Trematoda: Strigeoidea) during invasion of the brain of the fish intermediate host, Pimephales promelas

We examined tegumental development of the diplostomulum of Ornithodiplostomum ptychocheilus, with respect to structural transformations that have functional relevance to the invasion, migration, and site establishment processes in the brain of the fish second-intermediate host, Pimephales promelas. Using a combination of brightfield, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and confocal microscopy (CM), we demonstrated that the diplostomula become established in the outer region of the optic lobes within 24-48 hr of penetration and continue to grow and transform over a period of 4-14 days. During this period, the J-shaped body consists of 2 distinct regions: (1) a highly motile prosoma with distinctive tegumental spines and (2) an opisthosoma, the tegument of which is elaborated into a dense uniform layer of long, thin microvilli. The prosoma is alternately invaginated into and everted from the opisthosoma, thus constituting a protrusible proboscis. By day 14 postinfection (PI), the body has lost this bipartite structure and has taken on the uniformly flattened form characteristic of metacercariae. The transitory complex structure of the diplostomula appears to be well suited to burrowing through host tissues (primarily by action of the prosoma), followed by rapid dissociation of host tissue and nutrient accumulation (primarily by action of the opisthosoma) in preparation for metacercaria encystment.

Cellular aspects of early development of Ornithodiplostomum ptychocheilus metacercariae in the brain of fathead minnows, Pimephales promelas

Trematode metacercariae typically are regarded as nonfeeding and metabolically inactive. However, the metacercariae of many trematode species undergo complex and prolonged periods of development within their intermediate hosts. In the present study, we used electron microscopy to document chronological changes in development of the tegument of Ornithodiplostomum ptychocheilus metacercariae recovered from the brains of experimentally infected fathead minnows (Pimephales promelas). Commencing at 4 days postinfection (PI), the smooth, thin, syncytial tegument transforms into a complex microlamellar and microvillar system that encircles the entire body surface. The microvilli are oriented in parallel in an extended pattern, reaching directly away from the parasite and toward the receding host tissue. The microvilli disappear at approximately 28 days PI, followed by deposition of the cyst wall and further transformation of the tegument into the spinose, a glandular structure typical of an immature adult. To our knowledge, the progressive disaggregation of host cells at the leading edge of elongating parasite microvilli has not been demonstrated previously for any trematode. These results provide morphological evidence that the metacercariae of some trematode species undergo complex developmental changes associated with feeding in their intermediate host.

Cycle évolutif de Bucephalus anguillae Spakulová, Macko, Berrilli & Dezfuli, 2002 (Digenea, Bucephalidae) parasite de Anguilla anguilla (L.)

The species of Bucephalus from the eel Anguilla anguilla of the north-eastern Tunisian lagoons was identified as B. anguillae which was recently described by Spakulová et al. (2002) from the Adriatic coast of Italy. In order to confirm that this eel digenean is distinct from B. polymorphus von Baer, 1827 present in other freshwater fishes, we investigated the life-cycle of this species. Surveys in the wild and experimental studies have shown that the life-cycle of B. anguillae is completed in brackish environments, with the lamellibranch Abra tenuis as the first intermediate host and the cyprinodontid fish Aphanius fasciatus as the second intermediate host. This life-cycle differs from that of B. polymorphus and can be considered as an additional argument for the distinction of these two morphologically similar species.