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Intrauterine embryonic development in the microphallid trematode Maritrema feliui is examined by means of transmission electron microscopy. Both fertilization and eggshell formation take place in the ootype. The eggshell is formed from a shell globule material derived from the vitelline cells combined with secretions of Mehlis' gland. The proximal uterus is packed with unembryonated eggs of the oligolecithal type, each composed of a fertilized oocyte and several vitelline cells, all surrounded by the shell. Intrauterine embryonic development of the egg is followed to the early stage of outer embryonic envelope formation, resulting in an embryo of ~20 blastomeres of three different types: macromeres, mesomeres and micromeres. The first equal cleavage division of the zygote produces two macromeres. The outer envelope is of cellular origin and formed by the cytoplasmic fusion of two macromeres, which become situated at opposite poles in the peripheral layer of the embryo just beneath the eggshell. Simultaneously, other blastomeres multiply and differentiate, whereas several micromeres exhibit clear signs of degeneration or apoptosis. These results show that the embryonic development of M. feliui starts in utero and represents an example of early stage ovoviviparity. A reduction in the number of blastomeres results from a continued degeneration of micromeres, which after autolysis and re-absorption, appear to represent an important source of nutritive reserves for the embryo. The embryonic development of this digenean is discussed in relation to its life cycle.

Fine structural features of the vitelline follicles of the monopisthocotylean monogenean Ancyrocephalus paradoxus are revealed and compared with those of other monogenean species. As in other monogeneans, each vitelline follicle of A. paradoxus is composed of a single cell type, i.e. vitellocytes at various stages of development, with no sign of any interstitial cells. There is no special isolation of the vitelline follicles from the surrounding tissue, and both heterologous (between adjacent membranes of the vitelline and surrounding parenchymal cells) and homologous (between adjacent membranes of the vitelline cells) cell junctions (zonulae occludentes) are present. Non-membrane-bound vitelline clusters of A. paradoxus contain 50-100 vitelline globules, moderately electron-dense lipid droplets and glycogen particles present in the mature cell cytoplasm. In a search for phylogenetically informative characters of the fine structure of the vitellarium, the new findings are compared with those known for trematodes, and, to enable this, additional observations were made on the structure of the vitellarium of the aspidogastrean Aspidogaster limacoides. Some new discriminatory traits are revealed in A. limacoides; these include the presence of a single type of cellular component within the vitellarium, sarcoplasmic processes filling the space between and around the vitellocytes and zonulae occludentes between adjacent membranes of vitelline and muscle cells. On the basis of ultrastructural features of the vitellarium, a relationship between the Monogenea and the Aspidogastrea is indicated.

Intrauterine embryonic development in the caryophyllidean tapeworm Khawia sinensis has been investigated using transmission electron microscopy and cytochemical staining with periodic acid-thiosemicarbazide-silver proteinate for glycogen. Contrary to previous light microscopy findings that reported the release of non-embryonated eggs of K. sinenesis to the external environment, the present study documents various stages of embryonation (ovoviviparity) within the intrauterine eggs of this cestode. At the initial stage of embryonic development, each fertilised oocyte is accompanied by several vitellocytes that become enclosed within the operculate, electrondense shell. Cleavage divisions result in formation of blastomeres (up to about 24 cells) of various sizes. Mitotic divisions and apparent rosette arrangment of the blastomeres, the latter atypical within the Eucestoda, are observed for the first time in the intrauterine eggs of K. sinenesis. The early embryo enclosed within the electrondense shell is surrounded by a thin membraneous layer which in some enlarged regions shows presence of nuclei. Simultaneously to multiplication and differentiation, some of the blastomeres undergo deterioration. A progressive degeneration of the vitellocytes within eggs provides nutritive reserves, including lipids, for the developing embryo. The possible significance of this atypical timing of the intrauterine embryonic development to (1) the ecology of K. sinensis and that of a recent introduction of another invasive tapeworm, the caryophyllidean Atractolytocestus huronensis Anthony, 1958 to Europe; and (2) the affiliation of caryophyllideans with other lower cestodes, are discussed.

During vitellogenesis in the microphallid trematode Maritrema feliui, we distinguished four stages: (I) a stem cell stage of the gonial type; (II) an early differentiation stage with the main cell activity concentrated on the initiation of protein synthetic activity and the beginning of shell globule formation; (III) an advanced differentiation stage concentrated on a rapid intensification of protein synthetic activity, the progressive fusion of individual shell globules into large shell globule clusters and the formation of saturated lipid droplets and a small amount of β-glycogen particles in the peripheral cytoplasm, considered as a store of nutritive reserves for the developing embryos; and (IV) the mature vitellocyte. Early vitellocyte maturation is characterised by: (1) an increase in cell volume; (2) extensive development of large, labyrinth-like cisternae of GER that produce proteinaceous granules; (3) the development of Golgi complexes engaged in packaging this material; and (4) a continuous enlargement of proteinaceous granules within vacuoles and their transformation into shell globule clusters composed of the heterogeneous material observed during vitellocyte cytodifferentiation. Mature vitelline cells are very rich in two types of cell inclusions accumulated in large amounts in their cytoplasm: (1) shell globule clusters, which play an important role in eggshell formation; and (2) a few osmiophobic lipid droplets of a saturated nature that undoubtedly represent nutritive reserves for the developing embryos. In addition, there are small numbers of β-glycogen particles in the peripheral cytoplasm of mature vitellocytes of this species. The general pattern and ultrastructure of vitellogenesis in M. feliui greatly resembles those observed in another microphallid trematode, Maritrema linguilla, in other digeneans and in some lower cestodes. Quantitative and qualitative variations in lipids (saturated and unsaturated) and glycogen (α-glycogen rosettes and β-glycogen particles) during platyhelminth vitellogenesis between the different species of trematodes and some lower cestodes are identified and discussed.

The ultrastructure of the uterine epithelium and underlying extracellular matrices has been studied in several cyclophyllideans, but relatively little has been published on uterine fine structure in other cestode orders. To develop comparative data, we initiated a study focusing on a common and widespread genus of the order Proteocephalidea. Adult specimens of Corallobothrium fimbriatum were removed from the small intestine of brown bullhead catfish, Ameiurus nebulosus, from the St. Lawrence River in North America. Early gravid proglottids were examined by light microscopy as whole mounts or diced in buffered glutaraldehyde, processed by standard techniques and examined by as stained sections by light microscopy and by transmission electron microscopy. Voucher specimens were deposited in the Museum of Comparative Zoology at Harvard University. The uterus consisted of an epithelial wall supported basally by an extracellular matrix and parenchyma cells, which together formed epitheliomesenchymal villi at many points. The uterine epithelium consisted of a single thin syncytial layer, folded distally into long epithelial villi that protruded into the lumen at some points. The epithelium's numerous nuclei were very close to the lumen, but bulged along with the perinuclear cytoplasm proximally into the surrounding medullary parenchyma. Each nucleus contained small amounts of evenly dispersed heterochromatin and euchromatin, with a central nucleolus. Numerous mitochondria occurred in the cytoplasm, which was densely filled with free ribosomes but contained scant endomembrane components and showed little evidence of secretory activity. The uterine cells possessed few of the apical microvilli that are typical of some cyclophyllidean species, but the apical plasma membrane and cortical cytoplasm were extensively developed into long microlamellae similar to other species. The basic syncytial nature of the epithelium is similar to that of other cestodes studied previously, but the specific combination of features is unlike any previously described. Based on this one proteocephalidean species, we discerned no pattern that would distinguish between Proteocephalidea and Cyclophyllidea based on uterine structure.

The ultrastructural characters of the mature spermatozoon of Taenia taeniaeformis are described by means of transmission electron microscopy. Materials were obtained from a naturally infected road-killed cat (Felis catus) from La Palma (Canary Islands, Spain). The mature spermatozoon of T. taeniaeformis is a filiform cell, which is tapered at both extremities and lacks mitochondria. It is characterised by the presence of (1) a single spirallised crested body about 140 nm thick, (2) a single axoneme of the 9+'1' pattern of trepaxonematan Platyhelminthes, (3) a twisted (40 degrees ) layer of submembranous cortical microtubules, (4) a periaxonemal sheath surrounding the axoneme, (5) transverse intracytoplasmic walls and (6) a spirallised nucleus encircling the axoneme. The mature spermatozoon of T. taeniaeformis is also characterised by the presence of an apical cone in its anterior extremity and by the disorganisation of the axoneme in its posterior extremity. The ultrastructural characters of the mature spermatozoon of T. taeniaeformis are compared with those of other cestodes studied to date, with particular emphasis on other representatives of the family Taeniidae.

This is the first ultrastructural study of the spermiogenesis and the mature spermatozoon of a cyclophyllidean cestode of the family Gryporhynchidae. The spermiogenesis of Valipora mutabilis begins with the formation of a differentiation zone delimited by arching membranes and containing two centrioles. One of the centrioles develops an axoneme that grows directly into the cytoplasmic protrusion. The other centriole remains situated in a cytoplasmic bud and is subsequently aborted (type IV of cestode spermiogenesis). The mature spermatozoon of V. mutabilis is a filiform cell, tapered at both extremities and lacks mitochondria. The anterior extremity is characterised by the presence of an apical cone and a single helicoidal crested body. The axoneme is of 9 + '1' trepaxonematan pattern, with a periaxonemal sheath. The cortical microtubules are twisted at an angle of about 45 degrees to the spermatozoon axis. The nucleus is electron dense and spirally coiled around the axoneme. The cytoplasm is electron lucent and contains numerous granules of electron-dense material. In contrast to a recent opinion for close phylogenetic relationships, these ultrastructural data demonstrate the distant position between gryporhynchids and the family Taeniidae. The most similar pattern in the ultrastructure of the spermiogenesis and the mature spermatozoon has been described for dilepidids, some hymenolepidids and some anoplocephalids.

The ultrastructural organization of the mature spermatozoon of the trypanorhynch cestode Dollfusiella spinulifera is described. The spermatozoon is a long filiform cell, tapered at both ends, which lacks mitochondria. Its cytoplasm contains (1) two axonemes of different lengths of the 9+'1' pattern of trepaxonematan Platyhelminthes, (2) two rows of parallel cortical microtubules, each of which is adjacent to the opposite sides of the plasma membrane, (3) the nucleus, and (4) glycogen in the form of both alpha-glycogen rosettes and beta-glycogen particles. Unlike the majority of cestode spermatozoa, the spematozoon of D. spinulifera lacks crested bodies and periaxonemal sheath. In view of the present results, the postulated synapomorphic value of crested bodies for the Eucestoda is questioned.

In this study the ultrastructural aspects of egg envelope formation in the anoplocephalid cestode Mosgovoyia ctenoides are described. In the early stage of oncospheral morphogenesis, formation of three following primary embryonic envelopes takes place: (1) the capsule, (2) the outer envelope, and (3) the inner envelope. The capsule is formed from the vitellocyte material. Two macromeres contribute to the formation of the outer envelope and three mesomeres take part in the formation of the inner envelope. The three primary envelopes undergo further differentiation and transformation into the secondary envelopes, the so-called oncospheral or egg envelopes. In the advanced preoncospheral phase, the inner envelope undergoes differentiation into three sublayers: (1) a thick extra-embryophoral cytoplasmic layer; (2) an electron-dense embryophore, as a stiff pyriform apparatus; and (3) a thin intra-embryophoral cytoplasmic layer containing mesomere nuclei. The oncosphere is located in the extended cupule-like part of the pyriform apparatus. The two embryophoral horns elongate and fuse, thus forming a rigid cone. Four egg envelopes surround the mature infective oncosphere of M. ctenoides: (1) a thick capsule; (2) the outer envelope; (3) the inner envelope with a characteristic embryophore, in the form of the pyriform apparatus; and (4) the oncospheral membrane. The differentiation and ultrastructure of the egg envelopes of M. ctenoides are compared, in particular to those described in other anoplocephalids, and in general to the oncospheres of other cestode species.

The present work involves an ultrastructural study of the mature spermatozoon of the anoplocephalid cestode Gallegoides arfaai (Mobedi and Ghadirian, 1977) Tenora and Mas-Coma, 1978, obtained from the small intestine of naturally infected wood mice, Apodemus sylvaticus Linnaeus, 1758 (Rodentia, Muridae). The mature spermatozoon of G. arfaai is a filiform cell, tapered at both ends and lacking mitochondria. It is characterized by the presence of a 1,000-nm-long apical cone and two 140-nm-thick crest-like bodies in its anterior extremity. The axoneme, of the 9 + '1' trepaxonematan pattern, lacks a periaxonemal sheath, and disorganization occurs at the level of the nuclear region of the sperm cell. The cortical microtubules form two to four fields according to the different regions of the male gamete. They are twisted at an angle of about 35 degrees , becoming parallel towards the posterior extremity of spermatozoon. The nucleus, spiralled around the axoneme, shows an irregular shape in both longitudinal and cross-sections. Numerous electron-dense granules were observed, which transform into an electron-dense material in the posterior extremity of the cell. Moreover, we describe for the first time the total length of the anterior region of sperm containing the helical crest-like bodies. This anterior extremity measures around 15 mum and presents two helical crest-like bodies of different lengths that describe 13-14 turns around the sperm body. Our ultrastructural results on the G. arfaai spermatozoon are compared with the ultrastructural organization of the spermatozoa of other previously studied species, with particular emphasis on the anoplocephalids.