The Fine Structure of the Colonial Kinetoplastid Flagellate Cephalothamnium cyclopum Stein

Cell structure, cell adhesion, and stalk formation have been examined by electron microscopy in the colonial flagellate, Cephalothamnium cyclopum. Each cell is obconical or spindle-shaped, pointed posteriorly and truncated anteriorly. The cell membrane is underlain by epiplasm 0.1 μm thick in the posterior region, but bands of microtubules support the anterior region which is differentiated into a flagellar pocket, oral apparatus and contractile vacuole. Each of 2 flagella, joined a short way above their bases by an interflagellar connective, has a paraxial rod and mastigonemes. One flagellum is free and is important in food gathering while the other is recurrent and lies in a shallow groove on the ventral cell surface but projects posteriorly into the stalk. The basal bodies of these flagella are bipartite structures connected by a pair of striated rootlets with accessory microtubular fibers. The oral apparatus consists of a funnel-shaped buccal cavity and cytostome. It is supported by helical and longitudinal microtubules and also has nearby striated and microtubular fibers. Possible roles of associated oral vesicles in relation to ingestion are discussed. A reticulate mitochondrion houses a massive kinetoplast which has a fibrillar substructure resembling that of dinoflagellate chromosomes. Adjacent flagellates adhere by laminate extensions of their posterior regions and attach by their recurrent flagella to a communally secreted stalk composed of finely fibrillar material. This study indicates that Cephalothamnium belongs in the order Kinetoplastida, and has many features in common with members of the family Bodonidae.     

Unusual Structures in the Flagellar Apparatus of a Strain of Trypanosoma cruzi*

Certain structures, associated with the flagellum, and which had hitherto been described as appearing occasionally in some species of trypanosomes, were found very frequently in epimastigote forms of strain F of Trypanosoma cruzi: (a) a group of tubular elements in an electron-dense mass enclosed within a swelling of the flagellar membrane as the flagellum emerges from its reservoir; (b) an expansion of the flagellar membrane at the point of the above swelling, which in cross-sections appears as a ring; and (c) an electron dense band in the body of the organism alongside the border of the flagellar pocket. The possible significance of these structures and the fact that so far they have been found only in one strain of T. cruzi are discussed.     

Observations on the Fine Structure of the Zoospore of Oedogonium cardiacum with Special Reference to the Flagellar Apparatus

Parts of the flagellar apparatus both inside and outside thecell have been investigated in a preliminary way by sectionsand whole mounts. Zoospores from male plants of O. cardiacumpossess about 120 flagella, the bases of which are held togetherin a ring by means of a characteristically patterned fibrousband and some differently arranged more homogeneous material.Between each pair of bases a compound root passes backwardsinto the cell. Each root has two radially superposed components:the outer component consists of three fibres starting at thefibrous ring and passing backwards close to the cell surface;the inner component is shorter and stouter, extending backwardsfor an uncertain distance but penetrating forward below thefibrous ring, to end within the substance of the colourlessapical dome of the cell. The material of the inner componentof a root displays a regular crossbanding of alternate thinand thicker lines spaced at approximately 140A for each unitof pattern (i.e. two lines). The orientation of the free partsof the flagella with respect to the fibrous ring is such thattheir two central strands when seen in cross-section near thebase are obliquely vertical.     

The Fine Structure of the Centriolar Apparatus and Associated Structures in the Flagellates Deltotrichonympha and Koruga. II. Division

SYNOPSIS. At division of Deltotrichonympha operculata and Koruga bonita from the Australian termite, Mastotermes darwiniensis , the 2 centriolar bodies separate, each becoming a mitotic center. Spindle microtubules develop from the lower end of each centriolar body and radiate towards the elongating nucleus. A new rostrum is formed in association with each centriolar body. Thus, centriolar bodies which lack the structure of typical centrioles can nevertheless function as division centers during mitosis.     

Cytochemical Nature and Fine Structure of the Gregarine Zeylanocystis burti Dissanaike, with Special Reference to Microfilaments, Microtubules and Movement

SYNOPSIS. The mature trophozoite of the acephaline gregarine Zeylanocystis burti Dissanaike, parasitic in the seminal vesicles of the Sri Lankan earthworm Pheretima peguana Rosa, was studied by cytochemical methods and by electron microscopy. Some observations were also made on gametocysts and oocysts. The trophozoite has a peculiar saucer shape, unlike other monocystid gregarines, and has marginal papillae with cytoplasmic hairs. Its fine structure conforms broadly to that of other gregarines, but differs with respect to its fibrillar organization and in some details of cytoplasmic organelle structure. The pellicle is composed of 2 parallel unit membranes elevated into a number of stumpy epicytary folds, more or less evenly distributed on both surfaces of the gregarine. Some of these are associated with adjacent accessory cells and may have a nutritive role. Bundles of fine microfilaments (5–6 nm) were detected both in the ectoplasm and deep in the endoplasm; these are possibly the main contractile elements (“myonemes”) involved in movement. Microtubules are larger (22–24 nm) and are found predominantly in papillae and cytoplasmic hairs, but extend also in small bundles beneath the pellicle—they appear to be more skeletal in nature. The significance of these findings is discussed in the light of recent work on the roles of microfilaments and microtubules in nonmuscle cells. Mitochondria are located superficially and have a complex organization. The endoplasmic reticulum is poorly developed, but ribosomes are abundant. Golgi lamellae-like membranes and vesicles akin to lysosomes were observed. Typical paraglycogen granules were found together with blobs of lipid and glycogen. The nucleus had a wrinkled envelope, a homogeneous matrix, and a spherical nucleolus. A variety of staining reactions and cytochemical tests were carried out. The distribution of lipids, polysaccharides, nucleic acids, calcium, and melanin were studied. Succinate dehydrogenase was detected in mitochondria, thiamine pyrophosphatase in Golgi bodies, and acid phosphatase in lysosomes. Golgi structures were found to be chemically very complex. Gametocysts and oocysts were associated with extraneous cells which probably contribute to the formation of their walls. The gametocyst wall is thin and consists of 2 membranes of the unit type. The oocyst wall is thicker and composed of 2 chemically different layers. Telolysosomes were seen in the disorganized residual cytoplasm within gametocysts.     

The Jakobid Flagellates: Structural Features of Jakoba, Reclinomonas and Histiona and Implications for the Early Diversification of Eukaryotes

ABSTRACT. Jakobid flagellates are small, free-living, bacterivorous heterotrophs, with similar morphology, asexual reproduction, and nucleocytoplasmic ultrastructural features at interphase and during cell division. Jakoba is typified by aloricate trophic cells, each containing a branched mitochondrion with prominent nucleoids and flattened cristae. Reclinomonas and Histiona are characterized by loricate trophic cells, each with an unbranched mitochondrion without prominent nucleoids or otherwise differentiated regions and bearing tubular cristae. An undescribed genus is typified by aloricate cells, each with an unbranched mitochondrion bearing discoidal cristae. I propose, on the basis of cytoskeletal architecture and other structural and developmental features, that jakobids are ancestral mitochondrial protists, sister taxa to the amitochondnal retortamonads and ancestral to diverse lineages of mitochondrial eukaryotes. Application of different classification paradigms produces different family-level taxonomies for jakobids.     

Ultrastructure of the Harmful Unarmored Dinoflagellate Cochlodinium polykrikoides (Dinophyceae) with Reference to the Apical Groove and Flagellar Apparatus

ABSTRACT. The external and internal ultrastructure of the harmful unarmored dinoflagellate Cochlodinium polykrikoides Margalef has been examined with special reference to the apical groove and three‐dimensional structure of the flagellar apparatus. The apical groove is U‐shaped and connected to the anterior sulcal extension on the dorsal side of the epicone. The eyespot is located dorsally and composed of two layers of globules situated within the chloroplast. A narrow invagination of the plasma membrane is associated with the eyespot. The nuclear envelope has normal nuclear pores similar to other eukaryotes but different from the Gymnodinium group with diagnostic nuclear chambers. The longitudinal and transverse basal bodies are separated by approximately 0.5–1.0 μm and interconnected directly by a striated basal body connective and indirectly by microtubular and fibrous structures. Characteristic features of the flagellar apparatus are as follows: (1) a nuclear extension projects to the R1 (longitudinal microtubular root) and is connected to the root by thin fibrous material; (2) fibrillar structures are associated with the longitudinal and transverse flagellar canal; and (3) a striated ventral connective extends toward the posterior end of the cell along the longitudinal flagellar canal. We conclude, based on both morphological and molecular evidence, that Cochlodinium is only distantly related to Gymnodinium.     

The Structure and Morphogenesis of the Ventral Ciliature in Paraurostyla hymenophora*

SYNOPSIS. The structure and morphogenesis of the ventral ciliature of Paraurostyla hymenophora (Stokes) are described. The oral primordium apparently originates in association with transverse cirrus #6, from which it migrates anteriorly simultaneous with kinetosomal proliferation. The primordium eventually forms an elongate ciliary field from which the future opisthe's fronto-ventro-transverse (FVT) and undulating membrane primordial fields arise. Concomitantly, the future proter's FVT primordial field is initiated by the disaggregation of frontal cirri #4, #5, and #6. Primordia then develop simultaneously within marginal and ventral cirral rows by a disaggregation of cirri within the respective rows, and do not give rise to new cirri until the FVT fields complete segregation into discrete cirri. Near the completion of cirral production from the FVT primordia, each ventral cirral primordium (VCP) forms the 2 rightmost transverse cirri. Segregation of new cirri within the marginal cirral primordia and VCP then occurs, eventually replacing all old cirri within their respective marginal and ventral cirral rows. At the end of cortical morphogenesis, all old ciliary organelles, with the exception of the adoral zone of membranelles, are either reorganized or replaced. These results suggest an evolutionary affinity between the ventral and marginal cirral rows and raise questions about the control of the developmental competence of individual primordia.     

Ultrastructure and large subunit rDNA-based phylogeny of Sphaerodinium cracoviense, an unusual freshwater dinoflagellate with a novel type of eyespot

Sphaerodinium cracoviense was collected near Cracow, Poland, and analysed by light microscopy, scanning electron microscopy, and serial-section transmission electron microscopy. Thecae showed a peridinioid type of plate arrangement with unusual numbers in the anterior intercalary and postcingular plate series: 4 and 6, respectively. The apical pore of S. cracoviense differed from the typical arrangement seen in many thecate forms and included a furrow with knob-like protuberances reminiscent of the apical area of some woloszynskioids. The flagellar apparatus included the three microtubular roots that extend to the left of the basal bodies and a striated root connective between the transverse striated root and the longitudinal microtubular root. Both the single-stranded root that associates with the right side of the longitudinal basal body in peridinioids and gonyaulacoids, and the layered connective typical of peridinioids were absent. The eyespot was formed by a layer of vesicle-contained crystal-like units underlain by layers of variably fused globules not bounded by membranes, and represents a novel type. The pusular system included a long canal with a dilated inner portion with radiating tubules. Bayesian and maximum likelihood analyses based on large subunit rDNA placed Sphaerodinium as a sister taxon to a group of woloszynskioids and relatively far from Peridinium and its allies.     

The Structure of the Salmonella typhimurium Type III Secretion System Needle Shows Divergence from the Flagellar System

The type III secretion system (T3SS) is essential for the infectivity of many pathogenic Gram-negative bacteria. The T3SS contains proteins that form a channel in the inner and outer bacterial membranes, as well as an extracellular needle that is used for transporting and injecting effector proteins into a host cell. The homology between the T3SS and the bacterial flagellar system has been firmly established, based upon both sequence similarities between respective proteins in the two systems and the structural homology of higher-order assemblies. It has previously been shown that the Shigella flexneri needle has a helical symmetry of ∼ 5.6 subunits/turn, which is quite similar to that of the most intensively studied flagellar filament (from Salmonella typhimurium), which has ∼ 5.5 subunits/turn. We now show that the Sa. typhimurium needle, expected by homology arguments to be more similar to the Sa. typhimurium flagellar filament than is the needle from Shigella, actually has ∼ 6.3 subunits/turn. It is not currently understood how host cell contact, made at the tip of the needle, is communicated to the secretory system at the base. In contrast to the Sa. typhimurium flagellar filament, which shows a nearly crystalline order, the Sa. typhimurium needle has a highly variable symmetry, which could be used to transmit information about host cell contact.     

Fine Structure of the Cone of Heterodera schachtii with Emphasis on Musculature and Fenestration

Fine structure of the posterior cone of monoxenically cultured Heterodera schachtii is examined. The cone is not evident at the end of the fourth molt, but as the female matures the cone elongates, vulval lips enlarge, and cuticular patterns on the lips are modified. Body wall cuticle (BW) of the cone includes layers A and B, but C is modified or replaced by a network of fibers which correspond to the semifenestrae. Vaginal lining is continuous with the BW and terminates at the cuticular underbridge near the uterus. Vaginal musculature includes 48 dilatores vaginae (DV) as well as a sphincter vaginae (SV). The DV include a contractile and noncontractile region with abundant actin and glycogen. A distinct anal depressor muscle is present. In the cyst, only bullae, the underbridge, vagina lining, and traces of the SV muscle persist. Detailed morphology of the cone of H. schachtii provides insight into characters which, when compared with other heteroderines, will be useful in phylogenetic analysis of Heteroderinae.     

On the Fine Structure of Valvognathia pogonostoma gen. et sp.n. (Gnathostomulida, Onychognathiidae) with Special Reference to the Jaw Apparatus

Valvognathia pogonostoma gen. et sp.n. is described as belonging to the family Onychognathiidae Sterrer, 1972. An integrated picture of the jaw apparatus is presented, based on interference phase contrast (Nomarski) and electron microscopy. The teeth are shown to be continuous with lamellae, apophyses, fibulae, and even jugum. These structures are all extracellular secretions covering the apical surfaces of the jaw apparatus epithelium. They constitute a single, interconnected, cuticular structure. Spiral-ciliary-organs and rhabditoid organelles are described in the epidermis.     

Discotricha papillifera: Structure and Morphogenesis of a Marine Interstitial Ciliate*

SYNOPSIS. Discotricha papillifera Tuffrau, a marine interstitial ciliate, is redescribed with the aid of light, scanning, and transmission electron microscopy from cells collected at a New Hampshire beach. Presence of primitive membranelles as well an an advanced stomatogenesis is demonstrated. The ultrastructure, including a unique membrane-bounded septate structure, is described. The cell is tentatively placed in the nassulid suborder Microthoracina, but affinities with other groups are discussed.     

Fine Structure of an Unidentified Protozoon in the Epithelium of Rainbow Trout Exposed to Water with Myxosoma cerebralis*

SYNOPSIS. An intracellular protozoon was discovered in the epithelium of young rainbow trout (Salmo gairdneri) exposed for as short a time as 1 hr to water known to contain infective stages of Myxosoma cerebralis. Light- and electron-microscopic examination of this tissue revealed what appeared to be a proliferative stage (presumptive schizont) of a sporozoon; other possible stages in the life cycle were also observed. The relationship of this unidentified protozoon to M. cerebralis remains unresolved.     

The Fine Structure of the Cytostome of the Apostomatous Ciliate Hyalophysa chattoni*

SYNOPSIS. The fine structure of the organelles concerned with the ingestion of exuvial fluid by the trophont of the apostome ciliate, Hyalophysa chattoni, has been examined. One of the taxonomic characteristics of the order Apostomatida is that cytostomes of ciliates within the taxon are reduced and evolving toward astomy. When examined by electron microscopy the cytostome of H. chattoni appears as a small region of active pinocytosis which is continuous with a very large cortical area, the extended cytostome. The fine structure of the extended cytostome resembles that of the cytostomes of ciliates from other orders in that it is covered by a single membrane underlain with microtubular ribs. Beneath the extended cytostome are accumulations of peculiar organelles that may represent stored membrane for recycling during food vacuole formation. Associated with the site of pinocytosis is a complex fiber that may be contractile.     

Adaptation of the Photosynthetic Apparatus of Chlorella and Ankistrodesmus to Blue and Red Light

The mode of adaptation of the photosynthetic apparatus of three unicellular green algae, Ankistrodesmus braunii, Chlorella fusca and Chlorella saccharophila to red and blue light are documented by the fluence-rate curves of photosynthetic oxygen evolution. For all three algae tested photosynthetic capacity, respiration and light compensation point were higher for cells grown under red light, while the chlorophyll content increased in blue light-grown cells. Blue light-adapted cells have a lower chlorophyll a to chlorophyll b ratio and more chlorophyll in the light-harvesting system than red light-adapted cells, as shown in the electrophoretic profile of the pigment-protein complexes. It is concluded that the action of red light resembles that of high levels of white light, while blue light causes the same effects as low levels of white light. In agreement with previous publications these findings indicate that the mode of adaptation to different light qualities is ubiquitous in unicellular green algae.     

Histology and ultrastructure of the gut epithelium of the neotenic cave salamander, Proteus anguinus (Amphibia, Caudata)

Histological, histochemical, and ultrastructural features of the gut of the European endemic cave salamander Proteus anguinus were studied. The gut is a relatively undifferentiated muscular tube lined with a simple columnar epithelium containing numerous goblet cells. The mucosa and underlying lamina propria/submucosa are elevated into a number of high longitudinal folds projecting into the lumen. The enterocytes are covered apically with uniform microvilli. Irregularity in the arrangement of microvilli was observed. Occasionally, irregular protrusions of the cytoplasm appear between groups of microvilli. Pinocytotic activity occurs at the bases of the intermicrovillous space. Mitochondria are numerous in the apical cytoplasm and basally beneath the nuclei. The supranuclear cytoplasm contains most of the cell organelles. The lateral plasma membranes of adjacent cells interdigitate and are joined by junctional complexes. The periodic acid-Schiff (PAS) reaction, indicating neutral mucosubstances, is positive only in the apical brush border of enterocytes and in goblet cells. The goblet cells also stained with Alcian blue (AB), at pH 2.5, thus revealing the presence of carboxylated glycosaminoglycans. Compact aggregations of AB- and PAS-negative cells are situated directly below the epithelium. Mitotic figures are present in individual clusters of cells. The fine structure of cells in these clusters indicated that these cells could be responsible for renewal of intestinal epithelium. Numerous endocrine-like cells could also be seen. The closely packed smooth muscle cells and amorphous extracellular material with collagen fibrils constitute a net-like structure under the basal lamina that is very closely associated with the epithelium. There are numerous acidophilic granular cells between epithelial cells, in the lamina propria/submucosa, and between cells aggregations. They seem to be associated with nematode infections and possibly constitute a humoral defense mechanism.     

Fine Structure of Oocyst Transformation and the Sporozoites of Leucocytozoon dubreuili*

SYNOPSIS. The sporogonic stages of Leucocytozoon dubreuili in the midgut and salivary glands of the simuliid vectors was studied by electron microscopy. Young uninucleate oocysts have a pellicle that initially resembles that of the ookinetc. Numerous electron-dense bodies and microtubules in the peripheral cytoplasm may be involved in the formation of the cyst wall. The dense bodies appear to give rise to the amorphous material of the wall. The tubules which run circumferentially beneath the oocyst's boundary probably serve as a skeletal support for the cell surface during deposition of the wall material. A subcapsular “space” which provides area for expansion of the developing sporozoites is formed in early multinucleate oocysts. The subcapsular “space” appears to be formed through a condensation of the peripheral cytoplasm, resulting in an osmotic gradient across the oocyst's limiting membrane. Consequently water diffuses out, creating a fluid-filled space. Sporozoite formation begins with localized thickenings on the oocyst's limiting membrane. Subsequent extension of the thickened regions into the subcapsular “space” marks the onset of sporozoite budding. The process is highly synchronized, and culminates with the production of up to 150 sporozoites about the sporoblastoid body. The structure of sporozoites from mature oocysts and of the salivary glands of the vector is basically similar, although salivary gland sporozoites are more elongate and have numerous electron-dense micronemes. The paired rhoptries in the latter sporozoites are more elongate and uniformly electron-dense than in oocyst sporozoites.