Electron-dense Collars Research Articles

Defining the morphology and mechanism of the hemoglobin transport pathway in Plasmodium falciparum-infected erythrocytes.

Hemoglobin degradation during the asexual cycle of Plasmodium falciparum is an obligate process for parasite development and survival. It is established that hemoglobin is transported from the host erythrocyte to the parasite digestive vacuole (DV), but this biological process is not well characterized. Three-dimensional reconstructions made from serial thin-section electron micrographs of untreated, trophozoite-stage P. falciparum-infected erythrocytes (IRBC) or IRBC treated with different pharmacological agents provide new insight into the organization and regulation of the hemoglobin transport pathway. Hemoglobin internalization commences with the formation of cytostomes from localized, electron-dense collars at the interface of the parasite plasma and parasitophorous vacuolar membranes. The cytostomal collar does not function as a site of vesicle fission but rather serves to stabilize the maturing cytostome. We provide the first evidence that hemoglobin transport to the DV uses an actin-myosin motor system. Short-lived, hemoglobin-filled vesicles form from the distal end of the cytostomes through actin and dynamin-mediated processes. Results obtained with IRBC treated with N-ethylmaleimide (NEM) suggest that fusion of hemoglobin-containing vesicles with the DV may involve a soluble NEM-sensitive factor attachment protein receptor-dependent mechanism. In this report, we identify new key components of the hemoglobin transport pathway and provide a detailed characterization of its morphological organization and regulation.

Topography and ultrastructure of the tegument of Lecithochirium musculus (digenea: Hemiuridae), a parasite of the European eel Anguilla anguilla (osteichthyes: Anguillidae)

The tegumental ultrastructure of the stomach fluke Lecithochirium musculus was studied using scanning and transmission electron microscopy. The surface of the tegument was smooth and covered by transverse cytoplasmic ridges. Cobblestone-like units of the tegument were observed on the ventral surface. Invagination and evagination of the ecsoma induced variations in the tegumental surface. The ultrastructural study revealed that the tegument of L. musculus had a typical syncytial organization with a distal cytoplasm lying over a basal matrix and cytons. Two types of intra-tegumental sensory structures were observed. Type 1 sensory receptor was a domed-like fusiform structure consisting of a smooth elevation of the tegument. Four receptors of this type were observed on the anterior dorsal surface of the fluke. Three nerve bulbs filled with electron-lucent material and mitochondria composed this receptor. Hemispherical electron-dense collars were observed at the top of the nerve bulbs. Striated rootlets laid just beneath the hemispherical electron-dense collars. Type 2 sensory receptor presented two morphological variations, i.e., a bulb-like monolobed structure, and a bulb-like bilobed structure observed at two different degrees of evagination. For both variations, the nerve bulb enclosed mitochondria, electron-lucent material, and a conical electron-dense collar from which extended a striated rootlet. Numerous sensory receptors of this type were observed around the ventral sucker.

Electron microscopic observation of cytoskeletal frame structures and detection of tubulin on the apical region ofCryptosporidium parvumsporozoites

Cryptosporidium parvum is an intracellular protozoan parasite belonging to the phylum Apicomplexa, and a major cause of waterborne gastroenteritis throughout the world. Invasive zoites of apicomplexan parasites, including C. parvum, are thought to have characteristic organelles on the apical apex; however, compared with other parasites, the cytoskeletal ultrastructure of C. parvum zoites is poorly understood. Thus, in the present study, we ultrastructurally examined C. parvum sporozoites using electron microscopy to clarify the framework of invasive stages. Consequently, at the apical end of sporozoites, 3 apical rings and an electron-dense collar were seen. Two thick central microtubules were seen further inside sporozoites and extended to the posterior region. Using anti-alpha and -beta tubulin antibodies generated from sea urchin and rat brain, both antibodies cross-reacted at the apical region of sporozoites in immunofluorescent morphology. The molecular mass of C. parvum alpha tubulin antigen was 50 kDa by Western blotting and the observed apical cytoskeletal structures were shown to be composed of alpha tubulin by immunoelectron microscopy. These results suggested that C. parvum sporozoites were clearly different in their cytoskeletal structure from those of other apicomplexan parasites.

Dynasore, a Cell-Permeable Inhibitor of Dynamin

Dynamin is essential for clathrin-dependent coated vesicle formation. It is required for membrane budding at a late stage during the transition from a fully formed pit to a pinched-off vesicle. Dynamin may also fulfill other roles during earlier stages of vesicle formation. We have screened about 16,000 small molecules and have identified 1, named here dynasore, that interferes in vitro with the GTPase activity of dynamin1, dynamin2, and Drp1, the mitochondrial dynamin, but not of other small GTPases. Dynasore acts as a potent inhibitor of endocytic pathways known to depend on dynamin by rapidly blocking coated vesicle formation within seconds of dynasore addition. Two types of coated pit intermediates accumulate during dynasore treatment, U-shaped, half formed pits and O-shaped, fully formed pits, captured while pinching off. Thus, dynamin acts at two steps during clathrin coat formation; GTP hydrolysis is probably needed at both steps.

Is Cadlinella ornatissima a chromodorid? Sperm ultrastructure in an enigmatic nudibranch (Opisthobranchia, Mollusca)

Summary Sperm ultrastructure is described for the nudibranch gastropod Cadlinella ornatissima, type species of the genus Cadlinella (Thiele). Although C. ornatissima exhibits most of the sperm features characteristic of other Opisthobranchia and the Pulmonata (a small, rounded acrosomal vesicle, a complex, helical, mitochondrial derivative—partially paracrystalline, coarse fibres associated with the axoneme), it also possesses a number of previously undescribed and possibly unique features (a longitudinally inrolled acrosomal pedestal, an axial structure within the cavity of the acrosomal pedestal, an electron-dense collar at the anterior region of the acrosomal pedestal, the presence of crystalloid bodies within the glycogen helices of the mitochondrial derivative). To our knowledge this is the first report of crystalloid bodies in mature sperm of any mollusc. Collectively this evidence raises questions concerning the affinities and systematic position of Cadlinella within the Nudibranchia. The peculiar nature of the sperm differences, in comparison with other investigated nudibranchs, suggest that Cadlinella is not easily linked to either the Cadlinidae or Chromodorididae, and should be considered incertae sedis.

Ultrastructure studies of preadult Proteocephalus longicollis (Cestoda, Proteocephalidea): transmission electron microscopy of scolex sensory receptors.

The ultrastructure of five types of presumed sensory receptors in the scolex of preadults of Proteocephalus longicollis is described. Two types of nonciliate sensory receptors are situated on the inner surface of the lateral sucker. They differ from each other in the shape, presence, or absence of a large rootlet, electron-dense collars, desmosomes, microtubules, and/or vesicles. In addition, three types of ciliate sensory receptors are found along the edges of the lateral suckers. They can be differentiated by the length of the cilium, by the number of electron-dense collars (one or two), and by types of vesicles. Four types of vesicles were found inside the ciliate sensory receptors. One type of ciliate sensory receptor occurring in preadults differs markedly from any of the sensory receptors previously described in adult P. longicollis.

Ultrastructural study of the papillae and presumed sensory receptors in the scolex of the Gymnorhynchus gigas plerocercoid (Cestoda: Trypanorhyncha).

The ultrastructure of the papillae and presumed sensory receptors in the tegument of the scolex of Gymnorhynchus gigas plerocercoid were investigated by scanning and transmission electron microscopy. Four distinct types of putative sense receptor (including three uniciliated and one nonciliated) are described for the first time in G. gigas, and this investigation is the first detailed ultrastructure study of tegumental receptors carried out in trypanorhynch cestodes. Microtriches arranged in clusters with a dome-shaped pattern seemed to be papillae that contained a ciliated sensory receptor (type I). The density of these papillae was greater in the center of the bothridial adherent surface near the tentacle orifice than in the lateral margins of the bothridia or in the pars post-bothridialis. The type I receptor is characterized by a long cilium anchored in the nerve bulb by a dense basal body but lacks rootlets. The bulb contains one or two electron-dense collars, numerous electron-lucent neurovesicles, and some mitochondria. The type II receptor presents a short cilium retracted into an invagination of the tegument that arises from the basal body. The bulb contains one electron-dense collar and numerous electron-lucent vesicles, but the rootlets are absent. The type III receptor is also a ciliated receptor embedded in a bulb wider than that of types I and II, and it differs from type II in that it possesses two electron-dense collars and small rootlets associated with the basal body. No electron-lucent vesicle was found in the bulb of this type III receptor. The type IV receptor is a nonciliated receptor localized under the surface with no contact to the outside. It consists of a flattened bulb that contains two electron-dense collars and striated rootlets associated with a band of microfilaments. A comparison of the ultrastructural features of receptors in different cestodes is presented.

Ultrastructure of presumed sensory receptors in the scolex of adult Proteocephalus exiguus (Cestoda, Proteocephalidea)

The ultrastructure of six types of putative sense receptors including three nonciliate (Types I–III) and three ciliate (Types IV–VI) types found in the tegument of the scolex of adult Proteocephalus exiguus has been described for the first time. Type I has a simple tegumental sensory ending, containing one electron-dense collar, circular septate desmosome, electron-lucent vesicles and microtubules. Type II is represented by a receptor with a large rootlet. Type III is a receptor with a very long cross-striated rootlet, with two electron-dense collars and septate junction. The ciliated receptors differ in the length of the cilium, in the number of electron-dense collars and in the shape of sensory bulb. A comparison of structural features of receptors in different systematic groups within Platyhelminthes is made.

Plasmodium falciparum:Characterization of Organelle Migration during Merozoite Morphogenesis in Asexual Malaria Infections

Taraschi, T. F., Trelka, D., Schneider, T., and Matthews, I. 1998.Plasmodium falciparum:Characterization of organelle migration during merozoite morphogenesis in asexual malaria infections.Experimental Parasitology88, 184–193. Treatment of asexualPlasmodium falciparuminfections with the microtubule stabilizing agents Taxol or epothilone A prevents the depolymerization of nuclear microtubules. Serial thin sectioning of treated parasites revealed the presence of polymerized nuclear microtubule assemblies extending from spindle pole bodies into the forming merozoites in late stage infections. This organization prevented daughter merozoites from pinching off the mother schizont during merogony. An electron-dense collar was apparent at the junction of the budding parasites and the schizont plasma membrane, suggesting the presence of a contractile, actin-myosin ring. Examination of Taxol or EpA arrested parasites provided new information about the merogonic process and the control of organelle migration. Drug treatment did not affect the migration or polarity of the rhoptries and micronemes. Ultrastructural characterization of drug-treated trophozoites identified an assembly of smooth vesicles and short tubules adjacent to the parasite nuclei. During merogony, these membranes were observed as flattened cisternae with dilated rims that appeared to be coated. The morphology and location of these membranes suggest that they may be the parasite Golgi apparatus. This investigation reveals that the antimalarial activity of microtubule stabilizing agents is due to their inhibition of merozoite formation.

Surface ultrastructure of plerocercoids of Bombycirhynchus sphyraenaicum (Pintner, 1930) (Cestoda: Trypanorhyncha).

Light microscopy studies have previously shown that Bombycirhynchus sphyraenaicum is an exceptional trypanorhynch cestode, characterised by a poeciloacanthous armature and two enormous bothridia, which overlap parts of the pars bulbosa, a character combination unique within the trypanorhynchs. Plerocercoids of B. sphyraenaicum from the fish Lates calcarifer (Centropomidae) were investigated by scanning and transmission electron microscopy. Results revealed that the tegument of the anterior margin of the distal bothridial surface bears three kinds of microtriche; palmate microtriches, 8 microns high, with seven to eight digitiform processes; equalized filamentous microtriches, 5-6 microns long, with cap and base each forming 50% of the length, borne on undulations beneath the palmate microtriches, and cap-dominated filamentous microtriches, 5-6 microns long, with the cap forming 75% of the length, borne on the apex of putative sensory papillae. Integumental connections link the bases of the palmate microtriches, forming transverse girdles around the worm, which may serve to coordinate traction. At regular intervals between the palmate microtriches are papillae, covered with filamentous microtriches extending above the palmate microtriches. A cilium emerges from a bulb at the apex of each papilla; other structures in the bulb include an electron-dense cuff, and two electron dense collars. The posterior part of the bulb tapers and passes into the tegumental cytoplasm. These adorned papillae, observed for the first time in trypanorhynch cestodes, are identified as putative mechanoreceptors.

The origins of parasitism in the platyhelminthes

Symbiotic associations have arisen independently in several groups of the largely free-living turbellarians. Morphological adaptations of turbellarians to a symbiotic way of life include suckers and adhesive glands for attachment, elaborate systems of microvilli and other epidermal structures for absorption of food, glands for the formation of cysts, cocoons and cement material, and lack of a pharynx and intestine in some species. However, many species closely resemble their free-living relatives. Egg production is greatly increased at least in some species, and life cycles are always direct. Food of symbiotic turbellarians consists of host food and/or host tissue. Ectosymbiotes show fewer physiological adaptations than entosymbiotes. The major groups of parasitic Platyhehninthes (Trematoda Aspidogastrea, Trematoda Digenea, Monogenea, Udonellidea, Cestoda including Gyrocotylidea, Amphilinidea and Eucestoda), form one monophylum, the Neodermata, characterized by a neodermis (tegument) replacing the larval epidermis, epidermal cilia with a single horizontal rootlet, sensory receptors with electron-dense collars, spermatozoa with axonemes incorporated in the sperm body by proximodistal fusion, and protonephridial flame bulbs formed by two cells each contributing a row of longitudinal ribs to the filtration apparatus. The sister group of the Neodermata is unknown but is likely to be a large taxon including the Proseriata and some other turbellarian groups. Among the Neodennata, the Aspidogastrea is likely to be the most archaic group, as indicated by DNA studies, morphology, life cycles and physiology. Aspidogastreans can survive for many days or even weeks outside a host in simple media, they show little host specificity, and have an astonishingly complex nervous system and many types of sensory receptors, both in the larva and the adult. It is suggested that Aspidogastrea were originally parasites of mlluscs (and possibly arthropods and other invertebrates) and that they are archaic forms which have remained at a stage where vertebrates represent facultative hosts or obligatory final hosts into which only the very last stages of the life cycle (maturation of the gonads) have been transferred. The complex life cycles of Digenea have evolved from the simple aspidogastrean ones by intercalation of multiplicative larval stages (sporocysts, rediae) in the mollusc host, and of cercarial stages ensuring dispersal to the now obligatory final host. Monogenea may have lost the molluscan host or evolved before the early neodermatans had acquired it. Cestoda either replaced the original molluscan with an arthropod host, retained an original arthropod host or evolved from an early neodermatan before molluscan hosts had been acquired, newly acquiring an arthropod host. Horizontal gene transfer and implications for mosaic evolution in the Platyhehninthes are discussed.

Ultrastructure studies on the papillae and the nonciliated sensory receptors of adult Spirometra erinacei (Cestoda, Pseudophyllidea).

The small numerous papillae on the ventral surface of the gravid proglottid of adult Spirometra erinacei were studied by scanning electron microscopy. The arrangement of clumps of papillae was recognized on the surface of the central portion around the genital atrium, with lateral clumps being located above a pair of longitudinal nerve cords and marginal ones, on both sides of the proglottid. By transmission electron microscopy, two types of nonciliated sensory receptors were observed within the papillae. The type I, single receptor was embedded within a papilla. This dome-like sensory receptor contained two electron-dense collars and four rootlets surrounded by numerous thin filaments. The type II receptor was found arranged in groups in the area between the papillae, and the apical end was exposed to the external environment. This simple, club-like sensory receptor contained electron-lucent vesicles and microtubules. We believe that the papillae play an important role in cross-insemination.

Ultrastructure of Sensory Receptors of an Undescribed Species of Luridae (Platyhelminthes, Rhabdocoela)

Nine types of uniciliate sensory receptors are described from an undescribed species of Luridae: a collar receptor with a collar formed by 7-8 rods; a receptor with a long vertical and short horizontal rootlet and a thick ring-like electron-dense collar; a receptor with an approximately 2-mum-long cilium that has a narrow bent tip; a bulbous receptor with a short bulbous cilium; an intraepidermal receptor with a short cilium enclosed in an epidermal cavity; a receptor with a long cilium, short ciliary rootlet and electron-dense rod; pharyngeal receptors with long (at least 4 mum long) and short (0.5 mum long) cilia; and a pharyngeal receptor with a cilium of intermediate length. Centrioles in nerve cells and (in cross-section) butterfly-shaped cilia may also be of a sensory nature. Ecological implications of the large variety of sensory receptors are discussed: in the variety of stimuli to which animals have to respond for survival, interstitial beach habitats are comparable to habitats on or in hosts used by platyhelminth parasites (which have a comparable number of receptor types).

Sense receptors of larval Lobatostoma manteri (Trematoda, Aspidogastrea)

Seven types of uniciliate and two types of non-ciliate receptors were found in the larva of Lobatostoma manteri. Uniciliate receptors differ in the length of the cilium, the shape of the cilium, presence or absence of a ciliary rootlet, and the number of electrondense collars. Non-ciliate receptors differ in the shape of the ciliary rootlet. The variety of receptors is smaller than that of larval Multicotyle purvisi, possibly due to the different modes of infection: ingestion of the egg by a snail, hatching of a non-ciliate larva in the stomach and migration into the digestive gland in Lobatostoma, hatching of a ciliate larva in fresh water, inhalation by a snail and migration into the kidney in Multicotyle.

Ultrastructural diversity in the pore canal systems of amphipod crustaceans

Transmission electron microscopy was used to analyze the structure and organization of the intracuticular pore canal system in 34 species of amphipod crustaceans. Pore canals were detectable in all species, including those that, from scanning electron microscopy, had been considered to lack them. Canal structure ranges from simple transcuticular passages of uniform diameter to more elaborate systems with distal canal dilatations variously equipped with electron dense collars, tubular filaments, single or multiple channels leading to the surface and transverse partitions separating canal contents of different electron densities. Considerable branching between tubular elements of the canal system is evident in many species. In most species the canals communicate with the outside via epicuticular channels. Even in those species in which this communication was not established, their epicuticle generally contains abundant cavities with external pores. Although some consistency in structure and organization is present among a few groups of taxonomically related species, pore canal characteristics generally could not be correlated with habitat or life style. It is suggested that the structural and organizational variety present reflects a considerable array of functions among amphipod pore canal systems.

Ultrastructural studies of spermatogenesis in Antalis entalis (Scaphopoda, Mollusca)

Mature spermatozoa and spermatogenesis of Antalis entalis were studied by light, scanning and transmission electron microscopy. During spermatogenesis the spherical proacrosome arises from a single Golgi complex, migrates toward the anterior pole of the spermatid, and there differentiates to form an electron-dense collar. This is unique among examples of the simple acrosome of primitive spermatozoa. Nuclear condensation does not occur until the proacrosome and mitochondria migrate towards the future anterior and posterior poles of the spermatid. Chromatin condensation occurs by the formation of heterochromatin granules. No manchette of microtubules is observed in the course of nuclear condensation and elongation. Mitochondria achieve their mature spherical shape by decreasing in number and increasing in diameter at an early stage of spermiogenesis. No inter-mitochondrial or mitochondrianuclear (or centriolar) junctions occur. The centriolar satellite is formed in association with the distal centriole late in spermiogenesis. Earlier, the typical ‘9 + 2 ’ microtubular axoneme is coiled within the spermatid cytoplasm and the flagellum emerges only late in spermiogenesis. Beta glycogen granules appear late in the development of the spermatid, and are distributed in the centriolar and periacrosomal regions.

Uniciliate sensory receptors of larvalMulticotyle purvisi (Trematoda, Aspidogastrea)

Seven types of presumptive ciliate sensory receptors of larvalMulticotyle purvisi (Trematoda, Aspidogastrea) are described by transmission electronmicroscopy. The type 1 receptor has one electron-dense collar and a short (approx. 0.6 μm long) cilium. The type 2 receptor contains two electron-dense collars and a cilium with much dense cytoplasm around the ring of doublets of nicrotubules. In the type 3 receptor, two electrondense collars with large reticulate extensions into the receptor are present. The type 4 receptor has two electron-dense collars and a long, cross-striated ciliary rootlet. The cilium of the type 5 receptor contains a large number of single microtubules and doublets. The type 6 receptor has two electron-dense collars and a cilium at least 6 μm in length, and the type 7 receptor is characterized by two electron-dense collars and a cilium that is approx. 1.3 μm long.

Ultrastructural studies of juvenileAustramphilina elongata: transmission electron microscopy of sensory receptors

The ultrastructure of three types of presumptive sensory receptors is described in the juvenile amphilinidAustramphilina elongata infecting the freshwater crayfishCherax destructor: a non-ciliate tegumental receptor, a ciliate tegumental receptor, and a non-ciliate receptor with two basal bodies. All receptors have electron-dense collars; they differ strongly from all receptors described in free larvae and young, developing juveniles of this species.

Non-ciliate sensory receptors of larvalMulticotyle purvisi (Trematoda, Aspidogastrea)

Four types of presumptive non-ciliate sensory receptors of larvalMulticotyle purvisi (Trematoda, Aspidogastrea) are described by transmission electronmicroscopy. One pair of receptors in the mouth cavity has two to several electron-dense collars and a long crossstriated rootlet with some fibres fanning out from the basal body. Four pairs in the ventral wall of the posterior sucker and in the tegument ventral to the sucker contain numerous electron-dense collars and, diverging from the basal body, a large, round-ovoid ciliary rootlet that is cross-striated in its proximal and has numerous dense structures in its distal part. Two large bulbous receptors with much dense material in their apical parts and some desmosome-like structures occur in the lateral surface tegument at the level anterior to the posterior sucker, and similar smaller ones occur at the anterior end. A receptor with both a bulbous projection containing numerous vesicles with dense inclusions and some long cytoplasmic processes occurs at the posterior end.

Ultrastructural studies of larval and juvenile Austramphilina elongata (Platyhelminthes, Amphilinidea); penetration into, and early development in the intermediate host, Cherax destructor

Scanning and transmission electron microscopy were used to study penetration of larval Austramphilina elongata into and development in the intermediate host, freshwater crayfish. Larvae attach their anterior and posterior ends to the cuticle of crayfish. During penetration, the blades of the two serrate hooks on each side of the body are close to each other, between those of the two halberd-shaped hooks of the same side of the body. The blades of the median hooks are close to the midline, ventral to one pair of halberd-shaped hooks. The larva penetrates through the ruptured cuticle, leaving the epidermis behind. All or most sense receptors except the subtegumental ones are probably lost. Type I secretion was found in the host around the larvae, high levels of type I, II and III secretion were still present in the glands of the juvenile even 8 days after penetration. Some juveniles are already encapsulated by host tissue 24 h after penetration, such juveniles have a much thinner tegument than younger, not encapsulated juveniles. The tegument rapidly grows in thickness and develops numerous surface folds. Parts of juveniles of various ages may be without surface tegument, lined by the basal lamina, possibly indicating reversible or irreversible damage by the host. In juveniles 8 weeks after infection, clusters of a type of receptor not present in infective larvae were found: non-ciliate, with an electron-dense collar and numerous microtubules.