An ultrastructural study of oogenesis in a marine triclad

The ultrastructural features of oocyte differentiation were studied in the marine triclad Cercyra hastata. Oocytes at several stages of maturation, each surrounded by follicle cell projections, are present within each of the two ovaries. A pre-vitellogenic and a vitellogenic stage have been detected in the oogenesis of C. hastata. The pre-vitellogenic stage is mainly characterized by an increase in the nuclear and nucleolar volume and activity, and the appearance and development of cortical granule precursors which are elaborated by the Golgi complex. In early phases of the vitellogenic stage, intense delamination and blebbing of the nuclear envelope occurs which probably contributes to an increase in number of cytoplasmic membranes and to transfer of nuclear material to the cytoplasm. The rough endoplasmic reticulum is extensively developed and often assumes a ‘whorl’ array. Several areas of yolk precursor formation appear in the whorls. Numerous 2–5 μm protein yolk globules are subsequently formed which appear surrounded by a double membrane (cisternae of the smooth endoplasmic reticulum) and become randomly distributed throughout the cytoplasm of mature oocytes. The peripheral ooplasm is occupied by a monolayer of electron-dense cortical granules. Finally, the evolutionary significance of the autosynthetic mechanism of yolk production is discussed.     

Ultrastructural study of oogenesis in Monocelis lineata (Turbellaria,Proseriata)

Summary

Oogenesis in the marine turbellarian proseriat Monocelis lineata was investigated at the ultrastructural level. Oocyte differentiation is not synchronous so that successive stages of germ cell maturation were simultaneously detected in each of the two ovaries. Each developing oocyte is enveloped by follicle cell projections which are presumably involved in a morphologically undetectable support of vitellogenesis. The main features evidenced during oocyte differentiation are: (1) The synthesis of cortical granules by the rough endoplasmic reticulum and Golgi complex, occurring in the earlier stages of oogenesis; (2) The synthesis of yolk globules by the rough endoplasmic reticulum (RER) and Golgi complex, occurring in the later stages of oogenesis, namely late meiotic prophase I. Neither morphologically visible endocytotic activity, nor the presence of intercellular bridges, nor even the development of microvilli were observed at the oolemma or cortical ooplasm, so that the sole mechanism of vitellogenesis appears to be autosynthetic. The significance of these findings is discussed in relation to the taxonomic position of M. lineata and more generally in relation to the phylogenetic history of the class Turbellaria.     

Ultrastructural features of cocoon-shell globules in the vitelline cells of neoophoran platyhelminths

The Neoophora is characterized by the presence of complex female gonads composed of both germaria and vitellaria. The vitellaria are made up of vitelline cells that differentiate to produce and accumulate reserve substances (proteins, lipid, glycogen) and precursors of the egg capsule or cocoon shell (phenolic proteins). A comparative ultrastructural and cytochemical investigation of shell-forming globules from the vitelline cells of some neoophoran platyhelminths shows that the internal structures of the shellforming globules can be grouped into two (or three) main types. In the first, globule content is characterized by a more or less homogeneous electron-dense substructure evidently arising from repeated coalescence of small vesicles produced by the rough endoplasmic reticulum and Golgi complex. A variant of this type of globule shows intermingling (or concentric) electron-dense and translucent areas producing a pattern resembling brain convolutions (convoluted pattern). The second type of shell-globule structure shows a multigranular pattern, presumably resulting from repeated fusions of Golgian vesicles followed by incomplete coalescence of the electron-dense content. Comparison of shell-globule structure in different taxa could be useful in elucidating some complex and still-unclear phylogenetic relationships among Platyhelminthes.     

Ultrastructure of oogenesis in the bluefin tuna, Thunnus thynnus

Ovarian ultrastructure of the Atlantic bluefin tuna (Thunnus thynnus) was investigated during the reproductive season with the aim of improving our understanding of the reproductive biology in this species. The bluefin, like the other tunas, has an asynchronous mode of ovarian development; therefore, all developmental stages of the oocyte can be found in mature ovaries. The process of oocyte development can be divided into five distinct stages (formation of oocytes from oogonia, primary growth, lipid stage, vitellogenesis, and maturation). Although histological and ultrastructural features of most these stages are similar among all studied teleosts, the transitional period between primary growth and vitellogenesis exhibits interspecific morphological differences that depend on the egg physiology. Although the most remarkable feature of this stage in many teleosts is the occurrence of cortical alveoli, in the bluefin tuna, as is common in marine fishes, the predominant cytoplasmic inclusions are lipid droplets. Nests of early meiotic oocytes derive from the germinal epithelium that borders the ovarian lumen. Each oocyte in the nest becomes surrounded by extensions of prefollicle cells derived from somatic epithelial cells and these form the follicle that is located in the stromal tissue. The primary growth stage is characterized by intense RNA synthesis and the differentiation of the vitelline envelope. Secondary growth commences with the accumulation of lipid droplets in the oocyte cytoplasm (lipid stage), which is then followed by massive uptake and processing of proteins into yolk platelets (vitellogenic stage). During the maturation stage the lipid inclusions coalesce into a single oil droplet, and hydrolysis of the yolk platelets leads to the formation of a homogeneous mass of fluid yolk in mature eggs.     

Reproductive biology of the polychaete Kefersteinia cirrata Keferstein (Hesionidae). I. Ovary structure and oogenesis

The ultrastructure of the ovary and the developing oocytes of the polychaete Kefersteinia cirrata have been described. The paired ovaries occur in all segments from the 11th to the posterior. Each consists of several finger-like lobes around an axial genital blood vessel. Oogenesis is well synchronised, young oocytes start to develop in September and vitellogenesis begins in January and is completed by May.

The young oocytes are embedded among the peritoneal cells of the blood vessel wall which have accumulations of glycogen and other storage products. Each oocyte becomes associated with a follicle cell with abundant rough endoplasmic reticulum. Yolk synthesis involves the accumulation of electron dense granules along the cisternae of the abundant rough endoplasmic reticulum. Active Golgi complexes are present and are involved in the production of cortical alveoli. The oocyte has branched microvilli, which contact the follicle cells or blood sinuses between the follicle cells and peritoneal cells. In post-spawning individuals the lysosome system of the follicle cells is hypertrophied and the cells play a role in oocyte breakdown and resorption.     

The importance of turbellarians in the marine meiobenthos: a review

Recent data and opinions on meiofaunal ecology are briefly reviewed; and from scattered data, the place of turbellarians in the meiobenthic community is discussed. Turbellarian diversity, density, and biomass are higher in sandy habitats than in muddy bottoms. In sand, turbellarian diversity is of the same magnitude as that of other important meiofaunal taxa, while densities range between 7–25% of the total meiofauna. Mean individual turbellarian dry weight seems to be four times that of nematodes and in sandy habitats turbellarian biomass may be equal to or excede that of nematodes. Most turbellarian species may be considered as predators and in this respect may take the place occupied by macrofaunal species in muddy sediments. mens.

---

     

Ultrastructural studies of early stages of oogenesis in a trichuroid nematode,Trichuris muris

Oogenesis within the hologonic ovary of the trichuroid nematode, Trichuris muris, was observed by light and electron microscopy. Early germinal stages in the form of oogonia and young primary oocytes were characterised by a high nuclear-cytoplasmic ratio, numerous ribosomes and several mitochondrial clusters. Previtellogenic primary oocytes contained a prominent nucleus with a nuclear envelope punctuated by pores. They also contained increased amounts of granular endoplasmic reticulum (GER), often arranged as annulate lamellae, several Golgi complexes and limited amounts of lipid. The appearance of three types of cytoplasmic inclusion, in the form of lipid, dense yolk granules and reticulate granules, indicated the onset of vitellogenesis. At this stage of oogenesis, all three types were distributed throughout the ooplasm. The possible role of the granules is discussed. During passage along the oviduct the oocyte was coated by an additional unit membrane and associated fibrillar layer external to the oolemma. It is suggested that this may be synthesised by the oocyte.     

Ultrastructure of muscle cells from a few selected turbellarians: possible correlations between form and function

A comparative ultrastructural study of muscle cells from several turbellarian species revealed a basic smooth type of construction, with thick and thin myofilaments. However, the distribution of contractile elements, "dense bodies", smooth reticulum, mitochondria, as well as junctional specializations was found to vary within different systems. This variability is probably related to greater or lesser physical demands on the musculature. From a mechanical viewpoint, the most ‘powerful’ muscular system studied was represented by the pharyngeal bulb of Geocentrophora applanata, in which muscle fibers were organized into a compact, square reticulum, hemidesmosomes occurred in large numbers, and myofilaments were grouped in patches reminiscent of A and I bands of striated muscles. At the other extreme, muscular fibrils of the catenulids studied were thin and regularly spaced along the body wall, and the branched ends of radial myofibrils in the parenchyma were connected to the epithelial layer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ultrastructural features of the epidermis of the planarian Artioposthia triangulata (Dendy)

The epidermis of the land planarian Arthioposthia triangulatawas examined by scanning and transmission electron microscopy. Thisinvestigation revealed that the flatworm was covered entirely withcilia and was especially densely populated on the ventral surface.In all regions the epidermis consisted of a one-layered columnarepithelium resting on a prominent basement membrane, but lacking aterminal web. Various secretions were found in the epidermistogether with epidermal rhabdoids. Below the basement membraneother secretory material was visible and this included thecytoplasmic lamellated granules and adenal rhabdites. The basementmembrane consisted of fibrils with a beaded appearance and thesewere arranged parallel to the epidermal layer but did not displaycross-banding. The secretory cells above and below the basementmembrane were compared and their products characterized on thebasis of shape, size and location. Their possible function isdiscussed.     

Ultrastructural study of oogenesis and yolk formation in an opisthobranch mollusc, Runcina

Runcina is a hermaphroditic opisthobranch mollusc of small size. It produces large eggs, rich in yolk substances, and it is thus ideal for studying the mechanisms of vitellogenesis. The rough endoplasmic reticulum and the Golgi apparatus appear to be involved in early build up of yolk precursors. Endocytotic vesicles carrying yolk proteins, taken up from the haemolymph, dominate yolk formation at a later stage. The findings presented in this study enhance the proposition of a dual pathway of auto- and heterosynthesis in yolk formation. It is not found that the yolk bodies in Runcina acquire lysosomal enzymes in order to digest the perivitelline fluid, as has been described for some pulmonates. The importance of the role played by the follicle cells in oocyte development is discussed.     

细角螺卵子发生的细胞学和超微结构的研究

利用组织切片和透射电镜观察细角螺卵细胞发育的显微和超微结构,结果表明:细角螺卵原细胞期细胞核体积较大,呈椭圆形,核膜明显且有不规则的凹陷,细胞质内出现大量的线粒体和高尔基体.根据卵黄颗粒物的多少和大小可将卵母细胞分为前、中、后三个时期:前期卵母细胞细胞核内染色质浓缩,核仁可见,并出现核周间隙;中期卵母细胞内细胞核移向细胞的一端,核内染色质仍呈高电子密度状态,核仁不明显或消失;后期卵母细胞内的细胞核受挤压形状变得不规则,细胞质内可见少量的线粒体,大量的卵黄颗粒聚集在细胞质中并融合成很大的卵黄球.成熟期卵母细胞卵黄物质多且有较大的脂滴.     

Early oogenesis in the ascidian Ciona savignyi

Summary

Morphology of the germinal epithelium and the early follicular oocyte in the ascidian Ciona savignyi as examined by electron microscopy. The oogenetic part of the germinal epithelium contains oocytes at two different stages and the dark and clear cells. The smaller oocyte contains synaptonemal complexes. The larger oocyte in the initial phase of growth has a conspicuous nucleolus, electron-dense materials and some mitochondria close to the nuclear envelope. The nucleus of the larger oocyte is round and has the smooth contour. The dark cell contains a relatively large nucleus and is sometimes connected to each other by an intercellular bridge. Therefore, the dark cell, which has been suggested to be the progenitor cell of two kinds of accessory cells, may be also the oogonium. The early follicular oocyte just after migration from the germinal epithelium retains most of cytological features similar to those of the larger oocyte. However, the nuclear contour of the early follicular oocyte is uneven. This difference in the nuclear contour probably indicates that such a follicular oocyte is in the second phase of growth.     

Ultrastructural features of the trophonema and oogenesis in the starlet sea anemone, Nematostella vectensis (Edwardsiidae)

Abstract. The starlet sea anemone, Nematostella vectensis Stephenson 1935, is a burrowing, estuarine species that has become a model organism for fundamental studies of cnidarian and metazoan development. During early oogenesis, oocytes appear in the basal region of the gastrodermis in the reproductive mesenteries and gradually bulge into the adjacent connective tissue space (mesoglea) where the majority of oocyte growth and vitellogenesis occurs. However, oocytes do not physically contact the cellular and amorphous matrix of the mesogleal compartment due to a thin, intervening basal lamina. Oocytes retain limited contact with the basal gastrodermal epithelium via groups of ultrastructurally modified gastrodermal cells called trophocytes. Trophocytes are monociliated accessory cells of somatic origin that collectively form a structure called the trophonema, a unique accessory cell/oocyte association not observed outside the Cnidaria. The trophonema consists of 50–60 trophocytes that maintain contact with <1% of the oocyte surface and forms a circular, bowel‐shaped depression on the luminal surface of the gastrodermis as they sink into the mesoglea with the oocyte. The oocyte remains highly polarized throughout oogenesis with the germinal vesicle positioned near the trophonema and presumably representing the future animal pole of the embryo. Contact between the trophonema and the oocyte is restricted to cell junctions connecting peripheral trophocytes and narrow extensions from the oocyte. Previous studies suggest that the trophonema plays a role in transport of extracellular digestive products from the gastrovascular cavity to the oocyte, and the ultrastructural features described in this study are consistent with that view. Vitellogenesis is described for the first time in a sea anemone. Yolk synthesis involves both autosynthetic and heterosynthetic processes including the biosynthetic activity of the Golgi complex and the uptake of extraoocytic yolk precursors via endocytosis, respectively.     

Ultrahistology of oogenesis and vitellogenesis in the spider mite Tetranychus urticae

Histology of the ovary of the spider mite Tetranychus urticae is described light and electron microscopically with special reference to oogenesis and vitellogenesis of this mite. Morphology of the ovary is comparable to the typical sac-like chelicerate ovary with oocytes protruding from the ovarian surface, thus resulting in a grape-like appearance. According to different oogenetic stages, a germ, pre-vitellogenic and vitellogenic region can be observed. Oogonia and primary oocytes characterized by extranuclear material or 'yolk nuclei' are situated in the germ region. Primary oocytes develop into three-nucleated nurse cells situated in the periphery of the pre-vitellogenic region, and into pre-vitellogenic oocytes protruding from the ovarian surface. Growth of oocytes is performed while they are in ovarian pouches by uptake of nurse cell cytoplasm and following extraovarian yolk precursors. Intraoocyte yolk synthesis interpreted from altered cytoplasmic organelles also occurs. Processes taking place during oogenesis and vitellogenesis in T. urticae are compared to published information on yolk synthesis of other animal species.     

Ultrastructural study of oogenesis in Phoronopsis harmeri (Phoronida)

Temereva, E.N., Malakhov, V.V. and Yushin, V.V. 2011. Ultrastructural study of oogenesis in Phoronopsis harmeri (Phoronida). —Acta Zoologica (Stockholm) 92 : 241–250. The successive stages of oogenesis in Phoronopsis harmeri were examined by electron microscopy methods. During the oogenesis, each oocyte is encircled by vasoperitoneal (coelomic) cells forming a follicle. The previtellogenic oocytes are small cells which accumulate ribosomes for future synthesis; their cytoplasm contains characteristic clusters of mitochondria and osmiophilic particles resembling a germ plasm of other metazoans. The cytoplasm of the vitellogenic oocytes includes numerous mitochondria, cisternae of the rough endoplasmic reticulum, Golgi bodies and annulate lamellae. The synthesis of three types of inclusions was observed: strongly osmiophilic granules (lipid droplets) as a prevalent component, distinctly larger granules surrounded by membrane (proteinaceous yolk) and numerous large vesicles with pale flocculent content. No inclusions which could be unequivocally interpreted as the cortical granules were detected. The surface of the vitellogenic oocytes is covered by microvilli which increase in number and length during development. The oogenesis in Phoronida may be interpreted as follicular because of close association of oocytes with the vasoperitoneal tissue. However, well‐developed synthetic apparatus together with a strongly developed microvillous surface and absence of endocytosis indicate a clear case of autosynthetic vitellogenesis. Thus, in phoronids, there is a combination of simply developed follicle and autosynthesis that, apparently, is plesiomorphic character.     

Ultrastructural features of oogenesis in a free-living marine Platyhelminth, Vorticeros luteum

The female gonad of the prolecithophoran turbellarian Voticeros luteum has a heterocellular organization as is typical in the neoophoran Platyhelminthes. It is composed of an ovarian and a vitellarian area that are not well separated from one another and from the surrounding tissues by a tunica. Each oocyte during maturation becomes completely surrounded by a single accessory cell whose function has been hypothesized on the basis of the present investigation. The main feature in oocyte differentiation appears to be the synthesis and accumulation of small electron-dense inclusions; these are about 1-2 mum in size, are elaborated by the Golgi apparatus and have a content which reacts partially to the cytochemical test for polyphenols. The genesis, composition and cortical localization of the inclusions suggest that they could be homologous to the egg-shell granules present in the oocytes of 'archoophoran' species. A comparative analysis and discussion of the oocyte inclusions observed to date in other turbellarians is undertaken and the hypothetical use of these structures as characters providing some new insights in clarifying the phylogenetic relationships among free-living Platyhelminthes is finally discussed.     

泥螺卵子发生的超微结构研究

利用透射电镜观察了泥螺卵子发生过程。结果表明 ,泥螺的卵子发生可划分为卵原细胞、卵黄发生早期、卵黄发生中期及卵黄发生后期卵母细胞 4个时期。卵原细胞核大而圆 ,胞质内分布有少量的线粒体和高尔基囊泡 ,细胞表面具微绒毛。卵黄发生早期的卵母细胞 ,胞质中各类细胞器发达 ,并出现数量较多的类朦胧子。卵黄发生中期的卵母细胞胞体迅速增大 ,核伸出伪足状突起 ,卵质中各种细胞器活动活跃 ,并参与形成卵黄粒和脂滴。此期还可观察到卵母细胞与滤泡细胞间的物质交换现象。卵黄发生后期的卵母细胞体积增至最大 ,细胞器数量减少。本文就卵黄发生前后卵母细胞内部构造的变化、意义及滤泡细胞与卵母细胞蛋白来源间的关系作了探讨     

The nucleolus and its modifications during oogenesis of Torpedo marmorata

The structural organisation of the nucleolar apparatus during oogenesis of the spotted ray Torpedo marmorata was investigated. The observations showed that unlike other cartilaginous fishes, in T. marmorata the nucleolar apparatus was always represented by one or two conspicuous nucleoli, whose organization significantly changed during oocyte development. In the smallest follicles (follicles <300 μm in diameter) the nucleolus was made up of granular and fibrillar components, and actively incorporated ³H uridine; later it becomes more and more electron‐dense so in follicles of 400 μm in diameter its components and ³H uridine incorporation were no longer evident. These results indicate that in T. marmorata the nucleolar apparatus significantly changes and undergoes a possible impairment in rRNA synthesis. After nucleolus inactivation, the synthesis of rRNA may be substained by granulosa.     

Effects of elimination of Wolbachia on oogenesis of the uzifly Exorista sorbillans,a parasitoid of the silkworm Bombyx mori

Maternally inherited bacteria of the genus Wolbachia are known to affect the physiology and reproduction of several arthropod and nematode hosts. This paper presents the effects of using tetracycline to eliminate Wolbachia on the oogenesis of its host uzifly Exorista sorbillans (Diptera: Tachinidae), a parasitoid of silkworm Bombyx mori L. Earlier we reported cytoplasmic incompatibility, nucleo‐cytoplasmic conflict, male biased sex ratio and reduction in fecundity upon curing of Wolbachia infection using an antibiotic. This paper presents the abnormal growth and proliferation of oocytes in the uninfected/cured uziflies. The anomalies include abnormal/arrested oocyte production with inhibition of normal functioning of nurse cell nuclei, malformation and agglutination in egg chambers, empty egg chambers due to arrested vitellogenesis and undeveloped ovarian tubules. These anomalies further lead to reduction in growth of the ovarian tubules, which resulted in a 20% reduction of fecundity. It is hypothesized that normal oocyte production in uzifly requires the infection of Wolbachia bacteria. Further, in the uzifly, infection is at transition stage from a facultative to an obligatory mode of symbiosis. In light of this, this paper discusses the relationship between Wolbachia and oocyte production.     

普通针毛蕨颈卵器和卵的发育

蕨类植物卵发生是有性生殖研究的重要内容。真水龙骨类II的卵发生尚无人研究, 本文对该类群中的普通针毛蕨(Macrothelypteris torresiana)卵发生过程进行了光镜和透射电镜研究。结果显示: 卵细胞刚形成时, 与腹沟细胞紧密相连, 随着卵进一步发育, 卵细胞和腹沟细胞之间逐渐形成分离腔, 但孔区处卵细胞和腹沟细胞始终相连。随后, 卵细胞上表面有不定型物质堆积在质膜外, 形成一层加厚的卵膜, 孔区处没有卵膜覆盖的位置最后形成受精孔。在卵发育后期, 卵细胞核变得不规则, 近成熟时卵核产生大量核外突。卵发育过程中卵膜的出现、受精孔的产生以及核外突等特征, 与进化的真水龙骨类中其他蕨类植物的卵发生研究结果相似, 与薄囊蕨类中原始基部类群的卵发生现象有显著差别。由此可见, 普通针毛蕨属于进化类型。依据卵发生中卵膜和受精孔等特征推测原始薄囊蕨类经过里白类、桫椤类, 最终演化为水龙骨类。