Fine structure and development of the cuticle of Intoshia variabili (Orthonectida)

The body of free-swiming mature Intoshia variabili (Orthonectida) is covered by a thin cuticle, 0.3 μm thick. The cuticle is formed at the time when the orthonectid embryos develop in the plasmodium. The process of cuticle formation begins just after the first cilia begin to appear at the surface of the ciliated cells. At first, small extensions of the cell membrane appear at the surface of the cell, more or less parallel to the cell surface. As they develop futher, they stand up, and amorphic material begins to appear between them. The extensions then become microvilli and obtain their final shape, with a small subdistal swelling and a narrower distal part. They are situated very regularly on the surface of the cell. After the microvilli have obtained their final form, material between them begins to get its final structure typical of the adult form. During the period when the mature orthonectid begins to leave the plasmodium and emerge from the host, the regular microvilli begin to disappear, and only small irregular extensions are present under the cuticle on the surface of the cell. During the process of cuticle formation a large amount of smooth endoplasmic reticulum develops in the cells, but once the cuticle is formed it gradually disappears.     

Muscle formation in the sexual genetation of Intoshia variabili (Orthonectida)

Muscle formation in Intoshia variabili (Orthonectida) has been studied in the course of development of the sexual (free-living) specimen. The muscle system originates in the early embryogenesis as a distinct continuous layer located between the outer cell layer and the inner cell mass. Later this cell layer disintegrates into separate muscle strips. The presence of a distinct muscle system in Orthonectida and the pattern of its development evidences for placing this group into Triploblastica rather than into Diploblastica.     

Fine Structure of the Female Intoshia variabili(Alexandrov & Sljusarev) (Mesozoa: Orthonectida)

The morphology and fine structure of female Intoshia variabili, new combination for Rhopalura variabiliAlexandrov & Sljusarev, 1992, were studied with transmission electron microscopy. The body surface is covered with a 3-layered cuticula, under which is a layer of ciliated + non-ciliated cells arranged in alternating rings around the body. Ciliated cells have lateral extensions that intercalate with the non-ciliated cells. The kinetosome of each cilium has two longitudinally oriented cross-striated rootlets. The outer surface of the ciliated cells is covered with small tubercles, and the cytoplasm of these cells contains granules, vacuoles, mitochondria, fibrillar structures and lamellary bodies. A band of dense fibrils passes through the upper part of each ring of cells, going from one cell junction to another, encircling the entire body. Between the layer of ciliated + non–ciliated cells and the oocytes, elongated contractile cells from 4–5 longitudinal columns and 1 ring, the latter at the level of ciliated rings 7–9. The contractile cells contain thick and thin longitudinally oriented fibrils. The oocytes contain a large nucleus, numerous mitochondria, electron–dense granules and 1–2 spherical structures. An anteriorly situated, ciliated goblet–like receptor, not described for any other orthonectids, consists of three closely apposed cells, the upper part of which contains densely packed cilia. The genital pore opens through a non–ciliated cell and is surrounded by several cells with granules.     

Nuclei in the plasmodium of Intoshia variabili (Orthonectida) as revealed by DAPI staining

DAPI staining of wholeamounts was used to reveal the parasitic plasmodium of the orthonectid Intoshia variabili in its host, the turbellarian Macrorhynchus crocea. The nuclei of the parasite differ drastically from those of the host in size, morphology, and the estimated DNA content. Our findings indirectly support the idea that the orthonectid plasmodium is a distinct parasitic organism, rather than modified host cells.     

The structure of the muscular and nervous systems of the male Intoshia linei (Orthonectida)

Orthonectida is a small group of parasites, which, according to recent studies, may be phylogenetically close to Annelida. Here, we describe the musculature and serotonin-like immunoreactive (SLIR) nervous system of male adults of Intoshia linei (Orthonectida) using immunohistochemistry and confocal laser scanning microscopy. The whole muscular system consists of four outer longitudinal and eight pairs of inner semicircular muscle fibres. Immunohistochemistry revealed six serotonin-like cells at the anterior part of the body, and two backward lateral longitudinal nerves, merging at the posterior end. Compared to females, the organization of the nervous system is modified and its progenetic origin seems unlikely. The general neuromuscular organization corresponds to the pattern of small-sized annelids, suggesting their possible phylogenetic affinity. Free-living males and females of the orthonectid I. linei may present a good example of a highly simplified Bilaterian with fully functioning nervous and muscular systems. This simplicity is secondary and is caused by two factors—the parasitic life style and miniaturization of free-living sexual stages.     

Fine Structure of the Mature Plasmodium of Intoshia variabili (Phylum Orthonectida), a Parasite of the Platyhelminth Macrorhynchus crocea

The fine structure of the mature plasmodia of Intoshia variabili, found in the kalyptorhynch flatworm Macrorhynchus crocea, was studied with transmission electron microscopy. As the Plasmodium matures it forms long finger-like extensions inside the host. These extensions eventually take the place of the genital system of the host turbellarian. The mature Plasmodium is separated from the host tissues by a continuous membrane. The entire Plasmodium is surrounded by modified host cells, which are thin and overlapping. These cells also have a rather homogenous cytoplasm with limited endoplasmic reticulum. The surface of the Plasmodium is mostly smooth and has numerous small extensions. Within the cytoplasm of the Plasmodium, different stages of development of both males and females could be observed. The cytoplasm also contains numerous small nuclei with distinct nucleoli. Scattered throughout the cytoplasm, in addition to nuclei, are mitochondria, various granules, multivesiculate bodies with lamellar structures, and a large number of small vesicles. The nature of the formation of the Plasmodium is discussed. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

The fine structure of red and white myotomal muscle fibres of the coalfish (Gadus virens)

Summary The fine structure of the red and white myotomal muscles of a marine teleost, the coalfish Gadus virens, has been examined and ultrastructural measurements and analyses carried out. The sarcomere lengths of the red and white fibres were found to be 1.60 minimum, 1.82 maximum and 1.70 minimum, 1.85 maximum, respectively. No significant difference was found between the red and white fibres in their percentage of sarcoplasmic reticulum and T system. Both were found to have regularly occurring triads at the Z disk level, to have distinctive M lines and to be multiply innervated. Ultrastructurally the two fibres can be distinguished by the thicker Z line and more abundant mitochondria of the red fibre, and by the ribbon-shaped peripheral myofibrils of the white fibres. The structure of the fibres in these two types of muscle is discussed in relation to their possible role in swimming.This work was supported by a research grant from the National Environmental Research Council.     

The fine structure of the female reproductive system of Zorotypus caudelli Karny (Zoraptera)

The general structure of the female genital system of Zorotypus caudelli is described. The ovarioles are of the panoistic type. Due to the reduction of the envelope (tunica externa) the ovarioles are in direct contact with the hemolymph like in some other insect groups, Plecoptera included. The calices are much larger in Z. caudelli then in Zorotypus hubbardi and their epithelial cells produce large amounts of secretions, probably protecting the surface of the eggs deposited on the substrate. Eggs taken from the calyx bear a series of long fringes, which are missing in the eggs found in the ovariole, and in other zorapteran species. The long sperm of Z. caudelli and the long spermathecal duct are likely related to a sexual isolating mechanism (cryptic female choice), impeding female re-mating. The apical receptacle and the spermathecal duct - both of ectodermal origin - consist of three cell types. In addition to the cells beneath the cuticle lining the lumen, two other cell types are visible: secretory and canal cells. The cytoplasm of the former is rich in rough endoplasmic reticulum cisterns and Golgi complexes, which produce numerous discrete dense secretory bodies. These products are released into the receiving canal crossing the extracellular cavity of secretory cells, extending over a series of long microvilli. The secretion is transported towards the lumen of the apical receptacle of the spermatheca or to that of the spermathecal duct by a connecting canal formed by the canal cells. It is enriched by material produced by the slender canal cells. Before mating, the sperm cells are enveloped by a thick glycocalyx produced at the level of the male accessory glands, but it is absent when they have reached the apical receptacle, and also in the spermathecal duct lumen. It is likely removed by secretions of the spermatheca. The eggs are fertilized at the level of the common oviduct where the spermathecal duct opens. Two micropyles at the dorsal side of the equator level possibly facilitate fertilization. The presence of these two micropyles is a presumably derived feature shared with Phasmatodea. The fine structure of the female reproductive system of Z. caudelli does not allow to assess the phylogenetic position at the present stage of knowledge. The enlarged calyx and the temporary presence of long fringes on the eggs are potential autapomorphies of Z. caudelli or may indicate relationships with other Zorotypus species.     

The fine structure of the gamont of Pterospora floridiensis (Apicomplexa: Eugregarinida)

Transmission electron microscopy of the gamont stage of Pterospora floridiensis has revealed a number of features. The gamont's surface varies from smooth to crenulate, with numerous pockets and folds. The pellicle is composed of an outer membrane, a middle lucent region, and an inner dense layer comprised of two tightly appressed membranes. Short ridges on the pellicle are 200-300+ nm long, 75-100 nm wide, and have a height of approximately 50 nm. The thickness of the pellicle is 100 nm when measured from the inner membrane to the top of a ridge. The ridges are formed by the plasma membrane and an underlying structure that is circular in cross-section. The surface folds and the pellicular ridges are distributed over the soma and the cell's unusual branching arms, though both are reduced near the junction between two gamonts in syzygy, and are absent at the central area of the junctional site. The cell has numerous active Golgi complexes associated with vesicles, as well as scattered dense mitochondria, lipid droplets, and paraglycogen granules. The nucleus has a large (13 microm) endosome, eccentrically located, and peripheral chromatin along the inner nuclear membrane.     

The fine structure of the frontal filament complex of barnacle larvae (Crustacea: Cirripedia)

Cell and Tissue Research - The frontal filaments comprise two regions, the internal vesicles and the external filaments. Dendrites of extra-optic protocerebral origin pass ventrally from the brain,...     

The female genital system of Ooperipatellus decoratus (Onychophora, Peripatopsidae): an ultrastructural study

The female genital system of the oviparous peripatopsid Ooperipatellus decoratus consists of an ovary, oviducts equipped with receptacula seminis and additional pouches, uteri, and a vagina. It is examined using transmission and scanning electron microscopy. The ovary is made up of paired ovarian tubes united anteriorly and posteriorly and differentiated into a sterile dorsal part and a fertile ventral part with exogenous oocytes. Fertilization presumably occurs in the oviducts once the oocytes pass the receptaculum seminis. Although the receptacula seminis have been reported to occur in juvenile O. decoratus females only, the present study reveals that they are present in adult females as well. Their wall consists of a cuboidal epithelium covered with a thin collagen-muscle layer. The additional pouches are projections of the oviducts facing the receptacula seminis. They are distally closed to the haemocoel by a flattened epithelium and lack external muscle cells. A thin collagen layer is only found proximally. The uteri are characterized by a columnar epithelium with folded cell membranes allowing extension of the uteri, thus facilitating the passage of the large uterine eggs towards the vagina. Another dominating feature of the uteri is a distally increasing secretory production, which probably contributes to chorion development. Cilia occurring along the entire length of the uteri are considered to assist in the transport of eggs towards the vagina.     

The pedal muscle of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata): an ultrastructure approach

M. Cristina Faccioni-Heuser, Denise M. Zancan, Christiane Q. Lopes and Matilde Achaval. 1999. The pedal muscle of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata): an ultrastructure approach. — Acta Zoologica (Stockholm) 80: 325–337
The ultrastructure of the pedal muscle of the Megalobulimus oblongus is described. This muscle consists of transverse, longitudinal and oblique bundles ensheathed in collagenous tissue. Each muscle cell is also ensheathed by collagen. The smooth muscle cells contain thin and thick filaments; the thin filaments are attached to dense bodies. These cells contain a simple system of sarcoplasmic reticulum, subsarcolemmal caveolae and mitochondria with dense granules in the matrix, and glycogen. Three types of muscle cells were identified. Type A cells exhibited densely packed myofilaments, abundant glycogen rosettes, numerous mitochondria and sarcoplasmic reticulum profiles. Type B cells exhibited scanty glycogen and mitochondria, few cisternae of sarcoplasmic reticulum and large intermyofibrillar spaces. Type C cells exhibited intermediate characteristics between type A and type B cells. Neither nexus nor desmosomes were observed between the muscle cell membranes. The muscle contains well developed connective tissue and blood vessels. These structures and the distribution of muscle cells are probably involved in the muscular-hydrostat system. The muscle is richly innervated, having neuromuscular junctions with clear and electron-dense synaptic vesicles. The clear vesicles probably contain acetylcholine because the axons to which they are connected arise from acetylcholinesterase positive neurones of the pedal ganglion. The other vesicles may secrete monoamines such as serotonin and/or neuropeptides such as substance P.     

The fine structure of the brain of Gastrocotyle trachuri (Monogenea: Platyhebninthes)

Summary The fine structure of the brain of the monogenean Gastrocotyle trachuri (Platyhelminthes) is described. The brain consists of a central neuropile surrounded by a layer of cell bodies. The neuropile is composed of a fine meshwork of naked neurites which contain various types of vesicles and other organelles although microtubules have not been found. Five kinds of vesicles; three clear types and two dense types, were found within the neuropile.Two types of neuronal cell body were identified on the basis of their vesicle contents although it is possible that these two types represent the extremes of a single cell type. A characteristic feature of the neuronal perikarya of Gastrocotyle is the presence of deep infoldings into the cell of the outer membrane. These infoldings often contain fibrous interstitial material and in a number of cases hemidesmosome-like structures have been found in the distended, distal end of the infoldings.     

The fine structure of the conducting system of the monkey heart (Macaca mulatta)

Summary In the sino-atrial (S-A) node of the monkey heart two types of muscle cells occur: 1. typical nodal cells which are the predominant cells and form the nodal fibers. 2. Intercalated clear cells with various diameters (4 to 12 m) and containing poorly developed myofibrils, rich in glycogen and demonstrating poor staining properties. These latter cells are dispersed, few in number, and never form discrete fibers of themselves, but are intercalated between the cell rows of the typical nodal fibers. Such intercalated clear cells become more numerous at the periphery of the node. Interconnection between the S-A node and the conventional atrial muscle is established by a progressive transformation of nodal fibers into atrial fibers producing an intermediate (or junctional) type of fiber at the nodal periphery. However, in addition, few nodal fibers make direct contact with the atrial cardiocytes. Our light and EM studies have failed to prove the existence of truly specialized internodal pathways. Nevertheless intercalated clear cells, nodal-like cells, junctional or intermediate type of cells are relatively frequent in valvular regions (Thebesian, Eustachian, A-V, fossa ovalis) and less frequent in other regions of the atrial wall.This study was conducted in part in the Department of Histology and Embryology of the Medical University in Budapest.     

The fine structure of the leaf nodules of Ardisia crispa (Thunb.) A.DC. (Myrsinaceae)

The fine structure of the leaf nodules of Ardisia crispa is described. The bacterial endophyte in mature nodules is extracellular, forming a compact mass which is encompassed within a 5–6 cell thick sheath of modified mesophyll cells. Processes from these sheath cells invade the bacterial mass providing a host surface across which exchange of metabolites may take place. Considerable pleomorphism is exhibited by the bacteria, paralleling that found in other symbiotic associations in which the host can influence the morphology of the microsymbiont. The spherical, highly granate chloroplasts, rich in starch, which are characteristic of the spongy mesophyll leaf cells, are replaced, in the modified sheath cells, by a degenerate plastid form containing membrane whorls. Lipid reserves appear to replace the starch in the sheath cells. The significance of these changes is discussed.     

The fine structure of the leaf nodules of Ardisia crispa (Thunb.) A.DC. (Myrsinaceae)

The fine structure of the leaf nodules of Ardisia crispa is described. The bacterial endophyte in mature nodules is extracellular, forming a compact mass which is encompassed within a 5–6 cell thick sheath of modified mesophyll cells. Processes from these sheath cells invade the bacterial mass providing a host surface across which exchange of metabolites may take place. Considerable pleomorphism is exhibited by the bacteria, paralleling that found in other symbiotic associations in which the host can influence the morphology of the microsymbiont. The spherical, highly granate chloroplasts, rich in starch, which are characteristic of the spongy mesophyll leaf cells, are replaced, in the modified sheath cells, by a degenerate plastid form containing membrane whorls. Lipid reserves appear to replace the starch in the sheath cells. The significance of these changes is discussed.     

群居型和散居型东亚飞蝗雌成虫飞行肌的超微结构

应用电子显微镜对群居型和散居型东亚飞蝗Locusta migratoria manilensis（Meyen）雌成虫背纵肌进行了比较观察。结果表明：群居型和散居型成虫背纵肌具有类似的亚细胞结构,飞行肌的肌原纤维具有1∶3粗细丝比例,每根粗丝由6根细丝环绕排列成六角形结构。飞行肌的发育和线粒体的形成均是渐进的过程,在不同日龄成虫间存在差异。肌节长度为2.1～3.4 μm;7和10日龄时群居型成虫肌节长度小于散居型;7日龄群居型肌原纤维直径显著大于散居型。背纵肌内线粒体含量约占肌纤维的20%～43%,两型飞蝗之间存在着显著的差异,7日龄时群居型线粒体占肌原纤维的比例高达42.96%,而散居型的只有22.45%;10日龄群居型线粒体含量为41.32%,散居型线粒体29.98%。上述差异可能是东亚飞蝗群居型成虫飞行能力显著强于散居型成虫的重要原因之一。     

Morphology and fine structure of the gravid uterus of three hymenolepidid tapeworm species (Platyhelminthes: Cestoda)

Summary

The ultrastructure of the uterus in gravid proglottids of Hymenolepis citelli, Vampirolepis nana and Vampirolepis microstoma was examined using transmission and scanning electron microscopy. The cellular and subcellular structures were similar in all three species, consisting basically of a syncytial layer attached to a basal extracellular matrix. All nuclei were juxtaluminal and each contained a single large centrally located nucleolus and prominent masses of central and peripheral heterochromatin. The cytoplasm showed evidence suggesting a high level of protein synthesis and secretion. It consisted primarily of granular endoplasmic reticulum with moderately dilated cisternae; most cisternae were completely filled with an electron-lucent flocculent material, but others contained electrondense granules. Free ribosomes and mitochondria were also present. The apical plasma membrane and a small amount of enclosed cytoplasm were folded into long microlamellae that extended into the uterine lumen. Larger epithelial folds and villi consisting of folded portions of the entire epithelium projected into the uterine lumen. The uterine epithelium along with its basul extracellular matrix and underlying cellular parenchyma were folded into the lumen at several points, forming epitheliomesenchymal folds and villi that closely apposed many of the eggs.