Etude au microscope electronique de la cuticule et de ses structures associees chez quelques Cestodes. Essai d'histologie comparee

The functional significance of scolex retraction and subsequent cyst formation in the cysticercoid larva of Hymenolepis microstoma

The cysticercoid type of larval development, involving the retraction of the scolex and the associated development of a cyst, is of widespread occurrence within cyclophyllidean cestodes, but previously its possible adaptive value has been unexplained. Scolex retraction in Hymenolepis microstoma is an active, muscular process which takes only about a minute to complete. Following its retraction the scolex develops adult structures. The cyst enclosing the scolex also differentiates further, involving the secretion of three layers of collagen fibres and the development of a myelin-like layer adjacent to the retracted scolex. The cells within the cyst, including the muscle, become condensed or are autolysed causing a decrease in thickness of the cyst and an increase in the proportion of fibrous tissue. The collagenous part of the cyst provides mechanical protection for the scolex during grinding by the teeth on entry into the mouse host. The myelin-like layer insulates the scolex from the low pH conditions of the stomach. Values of lower than pH 3·0 are lethal to newly excysted worms. The myelin-like layer loses its insulatory capacity following exposure to bile salts.

The ultrastructure of the scolex–neck syncytium, neck cells and frontal gland cells of Caryophyllaeus laticeps (Caryophyllidea: Cestoda)

The syncytium of the scolex and neck regions of Caryophyllaeus laticeps differs from that of the remainder of the body in a number of ways. The microtriches have short shafts and lamellated spines and the surface cytoplasm is characterized by numerous elongated granules, occasional lipid-like bodies and, in the scolex region, is penetrated by ducts from the frontal glands. The elongated granules arise from the tegumentary neck cells and pass into the syncytium via narrow cell processes possessing extensions of the neck cell microtubule system. The granules have a peripheral sub-structure of regularly spaced electron-lucent areas. Evidence suggests that the granular material becomes more diffuse before extrusion. The spherical, lipid-like bodies also arise from the neck cells, but extrusion from the syncytium was not observed. Homology of the neck cells with the tegumentary cells of the rest of the body is discussed. The frontal glands are restricted to the scolex apex and secrete spherical or sub-spherical, homogeneous granules. The cells, with microtubule-lined ducts, open to the surface via pores which possess an electron-dense ring at which point the microtubule system terminates. Sensory bulbs are also present and will be described in a subsequent communication. The inter-relationships of the gland distribution, scolex morphology and host pathology are discussed.

The ultrastructure of the cysticercoid of Tatria octacantha Rees, 1973 (Cyclophyllidea: Amabiliidae) from the haemocoele of the damsel-fly nymphs Pyrrhosoma nymphula, Sulz and Enallagma cyathigerum, Charp

The cysticercoid of Tatria octacantha Rees, 1973 from the haemocoele of damsel-fly nymphs consists of an outer and inner cyst. The tegument of the outer surface of the outer cyst bears long slender microvilli which increase the surface area presumably for absorption. The tegument on the inner surface bears electron-dense microvilli and contains vesicles embedded in an electron-dense material. The contents of the vesicles resemble those of the central cavity in which the inner cyst floats freely. Nutrients absorbed by the outer cyst pass into the central cavity where they are available to the inner cyst.

Amplification of the cestode surface: a stereological analysis

A stereological study was undertaken on the length, diameter and numbers of microtriches and on the structural and functional amplification factors of the adult cestodes, Hymenolepis diminuta, Hymenolepis nana, Eubothrium crassium, Proteocephalus pollanicoli, the plerocercoid of Schistocephalus solidus and the cysticercus of Taenia crassiceps. The study showed wide variations in these stereological parameters between species, stages and regionally in the same species. A comparison of the present results with previous studies has shown a large measure of agreement but some differences are evident and possible explanations of these differences are proposed.

Ultrastructure of Archigetes sieboldi (Cestoda: Caryophyllidea): relationship between progenesis, development and evolution

Ultrastructural characteristics of progenetic and monoxenic Archigetes sieboldi Leuckart, 1878 from the oligochaete Limnodrilus hoffmeisteri Claparède are described. Our observations demonstrate that progenetic Archigetes sieboldi shares characteristics of both larval (progenetic) and adult stages. The primary larval characteristics are: the presence of a cercomer; a surface filamentous coat covering the whole worm; the presence of the penetration glands and the absence of tegumental ones; wide sarcoplasmic processes connecting the circular and longitudinal external tegumental muscles; the absence of the dense homogenous zone of the basal lamina beneath the epithelial cytoplasm of all reproductive organs and ducts; non-functional gonopores; and an orthogonal plan of nervous system with three pairs of longitudinal nerve trunks. The principle adult characteristics are: oogenesis, spermiogenesis and vitellogenesis that produce fertilized eggs; the uterine glands; a well-developed longitudinal tegumental muscle layer between tegumental cytons; and the presence of different microtriches. As a result of this progenetic development there has been a secondary reduction in the life cycle of A. sieboldi. It is postulated that a similar process of progenesis may have played a major role in the early evolution of the Caryophyllidea by first appearing in a plerocercoid stage of an ancestral strobilate cestode from fish.

Cytoskeletal features of the syncytial epidermis of the tapeworm Hymenolepis diminuta

A hallmark feature of parasitic platyhelminths is a cytoarchitecturally unusual syncytial epidermis composed of a peripheral layer of continuous cytoplasm (the ectocytoplasm) connected to underlying nucleated cell bodies by small cytoplasmic bridges. The helminth epidermis, or tegument, plays important roles in protection and nutrient acquisition; cestodes, in fact, completely lack a gastrointestinal tract and absorb all nutritive material through the tegument. Perhaps not surprisingly, the cestode tegument bears certain resemblances to the mucosal epithelium of the vertebrate small intestine, including the possession of a microvillous brush border upon the surface of the ectocytoplasm. In contrast to the intestinal epithelial cell, however, very little is known concerning the nature and organization of the cytoskeleton within the helminth epidermis. Therefore, a number of different microscopical preparative techniques were used to examine the tegument of the tapeworm Hymenolepis diminuta for the presence and distribution of microfilaments, intermediate filaments, and microtubules. It was found that both actin-containing microfilaments and intermediate-sized filaments are present but are restricted to specific locations along the plasmalemmae of the ectocytoplasm. In contrast, microtubules are found throughout the tegument, and are concentrated in the supranuclear regions of the perikarya and in the cytoplasmic bridges interconnecting the perikarya and ectocytoplasm. Unlike brush borders of most other epithelia, the cestode epidermal brush border lacks a filamentous terminal web and is instead associated with microtubules. A network of fine filaments, 5-8 nm in diameter but distinct from actin-containing microfilaments, runs throughout the ectocytoplasm and appears to interlink tegumental vesicles. These fine filaments may represent the primary "skeletal" system responsible for maintaining the structure of the tegumental cytoplasm.

An ultrastructural study of the microtriches of adult Proteocephalus tidswelli (Cestoda: Proteocephalidea)

Ultrastructural observations on adult Proteocephalus tidswelli revealed a marked microtrichial polymorphism. Structural and dimensional variations of microtriches between different regions of the strobila and scolex, as well as within the same region, were observed. The authors suggest that microtriches are involved in a diversity of functions, and possible functional activities are discussed.

Ultrastructural study of the tetrathyridium of Mesocestoides corti Hoeppli, 1925: tegument and parenchyma

The study of dividing and non-dividing tetrathyridia using electron microscopy shows that the mode of multiplication by antero-posterior fission of these larvae is due to a particular tissue which is called the "apical massif". The apical massif is a part of the tegumental syncytium. It is located at the top of the scolex. It represents a polynucleated cell mass which has cytomorphogenetic power. During asexual multiplication, it differentiates into tegumental syncytium, sub-tegumental muscles, glycogen-storing parenchyma cells, and other cell types. Parts of it remain undifferentiated. The hypothetic origin of the apical massif is discussed. Longitudinal growth of the tetrathyridia occurs by invasion of migrating cells into the tegumental syncytium. These cells also originate from the apical massif. During asexual multiplication and longitudinal growth, filamentous microtriches are synthesized below the plasmalemma of the superficial cytoplasm of the tegumental syncytium. It is supposed that the blade-like microtriches derive from filamentous forms.

Ultrastructure of the embryonic syncytial epithelium in a cestode Hymenolepis diminuta

The embryonic epithelium in Hymenolepis diminuta appears in the early preoncosphere stage. Inside the embryo there is a binucleate cell connected by a cytoplasmic strand with an epithelial layer spreading over the embryonic surface. After the embryo has become covered by tine epithelium the latter delammates into three layers. A basal layer resting on the basal lamina accumulates dense bodies. These bodies are spheroid and membrane-bound in the early embryo. In the late preoncosphere stage rodlike bodies remain in the basal epithelial layer. The basal membrane forms long invaginations into the basal layer. The intermediate epithelial layer is rich in polysomes and it is postulated that secretes extracellular material which cements the intermediate and peripheral layers and the ‘oncospheral membrane’. The continuity in embryonic, larval and adult cestode epithelium is discussed.

The ultrastructure of the parasite-host interface of three tetraphyllidean tapeworms of the elasmobranch Raja naevus

Ultrastructural examination of the parasite-host interface of three tetraphyllidean cestodes ofRaja naevus(sectioned attached to the intestine) indicates that while the strobilar tegument of each closely follows the typical cestode pattern that of the bothridia has several modifications probably associated with attachment. Microtriches similar to those of the strobilum are absent, although small spines onEcheneibothriumsp. andPhyllobothrium pirieiand wart-like structures onAcanthobothrium quadripartitumbothridia are considered to represent the osmiophilic tips of specialized microtriches. The bothridia of the three species have no apparent function in the absorption of food but evidence suggests that the apical pad of the myzorhynchus ofEcheneibothriumis specialized for the uptake of nutrients directly from the submucosa of the host intestine to which it is applied. Attachment and sensory roles by the myzorhynchus are also indicated.EcheneibothriumandP. pirieibothridia severely damage the host mucosal surface, while those ofA. quadripartitumdo little harm. This difference is probably associated with the size, mobility and mode of attachment of the cestodes. There are indications that the bothridia ofEcheneibothriumupset the metabolism of mucosal cells in the attachment region. The tips of the strobilar microtriches ofA. quadripartitumare shown to function in the attachment of the strobilum to the host mucosa, and in the protection of the microthrix proximal surfaces by a filtering action.