Formation, histochemistry and ultrastructure of the metacercarial cyst wall of plagiorchis species 1 (trematoda, plagiorchiidae)

Histochemical Study of the Progenetic Trematode Alloglossidium renale

A histochemical study of the progenetic trematode Alloglossidium renale has demonstrated the absence of any secreted material between the adult worm and the host (freshwater shrimp) antennal gland tubules. Host tissue is affected only by the compression, abrasion, and ingestion by the parasite, and host tubule cells near the worm have the same staining patterns as those distant from the parasite. The trematode sometimes dies within the host, leaving a necrotic mass whose histochemical staining differs significantly from the living organism. In the necrotic mass, the only recognizable features were the ova and the vitellarium, which atrophied and resulted in tyrosine-positive staining within the mass. A melanin reaction was not observed in the host using a specialized ferro-ferricyanide stain. The only apparent host response to infection was a layer of damaged squamous host cells adhering to the necrotic worm. The results confirm benign host-parasite effects and a highly evolved relationship between the host and parasite, perhaps bordering on commensalism.

Development of the alimentary tract during morphogenesis of the metacercariae of Levinseniella brachysoma

For the first time the development of the alimentary tract of Levinseniella brachysoma metacercaria (Trematoda: Microphallidae) obtained experimentally from Gammarus oceanicus has been described. The foregut primordium in 16-day-old metacercariae is represented by a syncytial cylindrical cord, resulting from the fusion of embryonic cells. Non-fused parts of the plasma membranes of adjacent cells are revealed as gap cavities within the cord. Later (30th day post infection) the lumen of the foregut is formed as a result of both partial vacuolization of the cytoplasm and by a broadening of the gap cavities, resulting from a thinning of the cytoplasmic spaces between them. Besides the usual organelles, the foregut of the mature metacercaria (42nd day p.i.) contains dense secretory granules in the apical cytoplasm region and numerous microtubules in basal areas. The cellular gastrodermis is formed later than the foregut syncytium (on day 30 p.i.); its large cells contain well-developed Golgi complexes, RER and mitochondria. A noteable inclusion of the gastrodermal cells of mature metacercariae are spherical granules of moderate electron density measuring up to 3 μm in diameter. On the basis of an analysis of the ultrastructural data the possible functioning of the metacercarial alimentary tract is discussed.

Hatching mechanism of the metacercaria ofPlagiorchisspecies 1 (Trematoda: Plagiorchiidae)

Metacercariae ofPlagiorchisspecies 1 were observed to hatch by an active mechanism in a medium free of enzymes. A hatching opening in the bilayered cyst wall was formed by the combined action of caecal fluid extruded by the activated larva through the mouth opening and an internal pressure due to the tendency of the cyst wall to contract on hatching, resulting in an explosive expulsion of part of the metacercarial body. The cyst wall was apparently pierced at any place where the larva delivered its caecal fluid. After excystation the hatching medium contained high phosphatase and proteinase activities and was able to dissolve the inner walls of empty cyst envelopes. The phosphatase activity assayed on 4-methylumbelliferyl phosphate was optimal at pH 3–5. The proteolytic activity was demonstrable on photographic film, Azocoll, and synthetic chromogenic and fluorogenic peptides. A preference for peptides was found which are also susceptible to plasmin. The proteolytic activity was optimal at pH 3–9 and 40–45°C and, when assayed on Suc-Ala-Phe-Lys-MCA, was only due to thiol proteinase(s) according to inhibitor studies. It is suggested that the proteinase(s) represent the hatching enzyme(s) of the metacercaria, because (a) only proteolytic activity was detectable in the pH range optimal for excystment, (b) the inner cyst wall is stabilized by proteins, and (c) the inner wall is dissolved by other proteinases such as trypsin. Enzyme histochemical investigations of metacercariae showed that, in the caeca, acid phosphatase was present mainly before hatching and non-specific esterase developed after hatching. Proteolytic activity was not localized with the methods used although it was suggested that it derived from the caecal fluid. A possible relationship of the thiol proteinase(s) detected in the hatching medium to haemoglobin-digesting proteinases from the gut of schistosomes is discussed.

In vitro excystment of the metacercaria of Cryptocotyle concavum from the common goby Pomatoschistus microps

The metacercaria of the heterophyid digenean Cryptocotyle concavum (Creplin, 1825) Lühe 1899, as seen by light and transmission electron microscopy, from the common goby (Pomatoschistus microps) is enclosed within a rigid cyst wall composed of four layers of parasite origin surrounded by a host-derived capsule. The encysted parasite contains anterior glands with secretion-filled ducts opening at the anterior extremity of the ‘head’. Excystment in vitro occurred most readily at 40–42°C on treatment with alkaline bile salts following pretreatment with acid pepsin. After 1 min in pepsin, the metacercaria became intermittently active, periods of rotation lasting 1–3 sec alternating with quiescent periods of 20–40 sec. Metacercariae excysted, some in less than 1 min, after transfer to bile salts. The metacercaria emerged through a hole, which appeared in the cyst wall at the site of application of the ‘head’ of the parasite. Some of the anterior gland ducts of excysted metacercariae were empty, indicating that their secretions are involved in penetrating the cyst wall. Following pretreatment in acid pepsin, no excystment occurred in EBSS (Earle's balanced salt solution) ranging in pH from 2.0 to 4.0, but above pH 4.0, excystment increased, reaching a maximum between pH 7.5 and 8.5. Pepsin and bile salts were not essential for excystment and metacercariae emerged, although less readily, in alkaline EBSS with or without acid EBSS pretreatment. Some emerged in distilled water with no pretreatment. Although no excystment occurred in acid pepsin or in EBSS at pH 2.0 and already-excysted metacercariae were rapidly killed by acid (pH 2.0), acid pepsin pretreatment enhanced excystment in alkaline bile salts.

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.

Plagiorchis muris: recovery, growth and development in albino rats

Metacercariae of Plagiorchis muris, obtained from naturally infected dragonflies, Sympetrum eroticum, successfully established in 4-week-old albino rats up to 14 days post-infection (p.i.) but by day 28 p.i. the recovery rate had significantly decreased. The genital primordia in excysted metacercariae were differentiated into those of metraterm, Mehlis' gland, ovary and cirrus pouch, with the primordial testes appearing on day 1 p.i. The vitellaria and eggs in the uterus were present in flukes on days 2 and 4 p.i., respectively. Mature flukes were established in the lower part of the small intestine on day 5 p.i., with the peak of egg production occurring on day 14 p.i. Growth of the flukes continued up to day 28 p.i.

The structure and formation of metacercarial cysts in the trematode family Microphallidae travassos 1920

This study deals with the formation of the metacercarial cysts of four microphallid trematodes, Maritrema subdolum, M. arenaria, Levinseniella brachysoma and Microphallus claviformis. The first observable cyst was present around Maritrema arenaria 18 h p.i. (post-infection). The other species had not developed a cyst by day 8 p.i. but their cysts were apparent by day 16 p.i. These were bi-layered and that of M. subdolum was thicker than those of L. brachysoma and Microphallus claviformis of the same age. The structure of older cysts varied substantially between the four species. Microphallus claviformis and Maritrema subdolum cysts were fully formed at 30 days p.i. Like those of M. arenaria they were bi-layered, the outer layer (up to 3 microm thick) being electron-dense and the inner one (up to 7 microm thick) being less electron-dense. The cysts of fully formed L. brachysoma metacercariae were much more complex, composed of four layers, one of which was divisible into three sub-layers. It was concluded that the outer cyst layer was the product of secretory granules which were previously identified in cercarial tegument. The inner, thicker layer was derived from several sources. These included small tegument vesicles produced over the entire surface of the metacercariae, larger fragments of tegument released from the anterio-ventral region and material liberated from the metacercarial excretory bladder. This heterogeneous material accumulated in the cyst lumen for some time before becoming polymerized to form the thick inner layer or layers of the metacercarial cysts.