Free pre-tetrathyridia of Mesocestoides sp. are described, for the first time, from samples obtained from the coelomic cavity of a ground skink, Scincella lateralis , from Oklahoma. Closer examination of these early-stage tapeworms revealed they were transitional metamorphosis stages between a post-hexacanth procercoid form to the full metacestode of Mesocestoides . A series of transitional stages was found that span the full period of sucker and apical organ development. However, we did not see any fully developed tetrathyridia, i.e., having classic Mesocestoides morphology but with the apical sucker absent following developmental atrophy. This is the first time that metamorphic pre-tetrathyridial stages of a Mesocestoides sp. have been reported in vivo from a natural infection. These observations corroborate earlier reports of such stages of Mesocestoides vogae developed in vitro, though the previously reported isolate of M. vogae is asexually proliferative, and the species from the present study showed no sign of asexual proliferation. The fact that these immediately post-hexacanth stages can occur in a single lizard intermediate host may suggest that Mesocestoides spp. might develop through a simple 2-host life cycle rather than an obligate 3-host cycle that has been speculated to occur by most previous authors. DNA sequence comparisons and phylogenetic analyses based on mitochondrial 12S , cox1 , and nad1 genes have demonstrated that our specimens from S. lateralis represent a species clearly distinct from all previously sequenced Mesocestoides and closely related to 2 forms from domestic dogs and Channel Island fox in California previously published as Mesocestoides sp. C.

abstract Tetrathyridia of Mesocestoides sp. were collected from the body cavities of the shrews (Insectivora), Crocidura russula , in Valencia, Spain and Crocidura suaveolens on the Mediterranean island of Corsica, France. Specimens were processed by routine microscopic and histological techniques, including examination with brightfield, phase-contrast, and differential-interference-contrast optics. Most tetrathyridia were clustered together inside host-derived fibrotic capsules, but some occurred free in the body cavity. All specimens examined from both locations had solid hindbodies, i.e., lacking a primary lacuna, thus conforming to the plerocercoid metacestode type; all possessed a single normal tetra-acetabulate scolex. All metacestodes from C. russula in Valencia were normal tetrathyridia. Those from C. suaveolens in Corsica were either normal tetrathyridia or had aberrant deep convolutions of an unusually elongated hindbody. No tetrathyridium from either location or host showed tegumental or excretory duct anomalies such as those reported by several authors from aberrant tetrathyridia and spargana in some other locations. No definitive evidence of asexual proliferation was visible in any of the tetrathyridia, but those with abnormally convoluted hindbodies from a single C. suaveolens in Corsica suggest the potential for asexuality by fission of the hindbody. These results add to our understanding of morphological and developmental variation among metacestodes in this widespread and variable genus.

Cellular details of early embryogenesis have been studied extensively among cyclophyllidean cestodes, but have been reported for only 2 species of the order Proteocephalidea, both belonging to the genus Proteocephalus . Thus, we performed a detailed ultrastructural analysis of early embryos of a second species, Corallobothrium fimbriatum , including early events in the formation of the embryonic envelopes. Adult worms were collected from the small intestine of brown bullhead catfish, Ameiurus nebulosus , from the St. Lawrence River in North America and processed by standard methods for transmission electron microscopy. The vitelline capsule consists of 2 closely apposed electron-dense membranous layers, separated by a more electron-lucent material. The 2 vitellocytes that accompany each oocyte contain numerous ribosomes, vesicles, and lipid droplets. These fuse to form a vitelline syncytium, which elongates and almost completely encircles the cleaving embryo by the 4-blastomere stage, forming a partial lipid-rich cellular envelope that undergoes apoptosis as cleavage continues. This envelope is later replaced by outer and inner embryonic envelopes. The outer envelope derives from the fusion of the vitelline syncytium with the cytoplasm of macromeres, whereas the inner envelope originates from 3 mesomeres. Simultaneous to the formation of the embryonic envelopes, other blastomeres multiply and differentiate, while some micromeres undergo degeneration or apoptosis. In most respects, ultrastructural features of early C. fimbriatum embryos closely resemble those of previously studied Proteocephalus longicollis , but differ somewhat from those of other orders. This demonstrates that, despite marked ultrastructural heterogeneity within some orders such as the Cyclophyllidea, some embryonic traits distinguish cestode orders from each other.

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.

Spermiogenesis and the ultrastructural characters of the spermatozoon of Mesocestoides lineatus are described by means of transmission electron microscopy, including cytochemical analysis for glycogen. Materials were obtained from a golden hamster ( Mesocricetus auratus ) after experimental infection with tetrathyridia metacestodes obtained from naturally infected lizards ( Anolis carolinensis ) from Louisiana. Spermiogenesis in M. lineatus is characterized by the orthogonal growth of a free flagellum, a flagellar rotation, and a proximodistal fusion. The zone of differentiation contains 2 centrioles associated with striated rootlets and a reduced intercentriolar body. The mature spermatozoon of M. lineatus lacks a mitochondrion, and it is characterized by the presence of (1) a single, spiraled, crested body 150 nm thick; (2) a single axoneme of the 9+‘1’ pattern of trepaxonematan Platyhelminthes; (3) a parallel and reduced row of submembranous cortical microtubules; (4) a spiraled cordon of glycogen granules; and (5) a spiraled nucleus encircling the axoneme.

The ultrastructure of the infective oncosphere of the cestode Mosgovoyia ctenoides (Anoplocephalidae) is described. The surface of the infective oncosphere is covered by a thin cytoplasmic layer of tegument connected by a narrow cytoplasmic process with the binucleate subtegumental cell, situated deeper in the body. Below the basal matrix of the cytoplasmic layer of the tegument are situated wide bands of the peripheral, somatic musculature responsible for body movements. The 3 pairs of hooks and their muscles form a complex hook muscle system, responsible for coordinated hook action. Five major types of cells have been distinguished: (1) a binucleate subtegumental cell, (2) a binucleate penetration gland, (3) 2 nerve cells, (4) numerous somatic cells, and (5) about 6 germinative cells. The approximate number of cells is 24 (26 nuclei, including 2 syncytial structures). The results of this study, when compared with other published reports from other cestode taxa, support previous hypotheses that the progressive reduction of oncosphere cells is an adaptive feature in cestode evolution.

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.