Morphological characterization of the ovary and oocytes vitellogenesis of the tick Rhipicephalus sanguineus (Latreille, 1806) (Acari: Ixodidae)

This study presents the morphology of the ovary, as well as the process of the vitellogenesis in oocytes of the tick Rhipicephalus sanguineus. The ovary of these individuals is of the panoistic type; therefore, it lacks nurse cells. This organ consists of a single tubular structure, continuous, and composed of a wall formed by small epithelial cells with rounded nuclei which delimit the lumen. The oocytes in the different developmental stages in this tick species were classified into five stages (I-V). They remain attached to the ovary during vitellogenesis by a cellular pedicel and afterwards the mature oocytes (stage V) are released into the ovary lumen.     

Oogenesis in the leech Glossiphonia heteroclita (Annelida, Hirudinea, Glossiphoniidae) II. Vitellogenesis, follicle cell structure and egg shell formation

By the end of previtellogenesis, the oocytes of Glossiphonia heteroclita gradually protrude into the ovary cavity. As a result they lose contact with the ovary cord (which begins to degenerate) and float freely within the hemocoelomic fluid. The oocyte's ooplasm is rich in numerous well-developed Golgi complexes showing high secretory activity, normal and transforming mitochondria, cisternae of rER and vast amounts of ribosomes. The transforming mitochondria become small lipid droplets as vitellogenesis progresses. The oolemma forms microvilli, numerous coated pits and vesicles occur at the base of the microvilli, and the first yolk spheres appear in the peripheral ooplasm. A mixed mechanism of vitellogenesis is suggested. The eggs are covered by a thin vitelline envelope with microvilli projecting through it. The envelope is formed by the oocyte. The vitelline envelope is produced by exocytosis of vesicles containing two kinds of material, one of which is electron-dense and seems not to participate in envelope formation. The cortical ooplasm of fully grown oocytes contains many cytoskeletal elements (F-actin) and numerous membrane-bound vesicles filled with stratified content. Those vesicles probably are cortical granules. The follicle cells surrounding growing oocytes have the following features: (1) they do not lie on a basal lamina; (2) their plasma membrane folds deeply, forming invaginations which eventually seem to form channels throughout their cytoplasm; (3) the plasma membrane facing the ovary lumen is lined with a layer of dense material; and (4) the plasma membrane facing the oocyte forms thin projections which intermingle with the oocyte microvilli. In late oogenesis, the follicle cells detach from the oocytes and degenerate in the ovary lumen.     

Amblyomma triste (Koch, 1844) (Acari: Ixodidae): morphological description of the ovary and of vitellogenesis

The present study presents the morphology, histology, and the dynamics of vitellogenesis in females of the tick Amblyomma triste. The ovary in this species is of the panoistic type, therefore it lacks nurse cells. It is composed of a layer of epithelial cells that outwardly form the wall of the ovary, but also originate the pedicel, the structure that attaches the oocytes to its external margin, as well the oocytes themselves. In Amblyomma triste, the oocytes develop in four synchronic stages, which differs from the process in other tick species. The classification of the stages of the oocytes was carried out based on the presence of four morphologic characteristics: cytoplasm appearance; site of the germ vesicle; presence, quantity, and constitution of the yolk granules and presence of chorium.     

Oogenesis in the leech Glossiphonia heteroclita (Annelida, Hirudinea, Glossiphonidae). I. Ovary structure and previtellogenic growth of oocytes

Glossiphonia heteroclita has paired ovaries whose shape and dimensions change as oogenesis proceeds: during early previtellogenesis they are small and club-shaped, whereas during vitellogenesis they broaden and elongate considerably. During early oogenesis (previtellogenesis), each ovary is composed of an outer envelope (ovisac) that surrounds the ovary cavity and is filled with hemocoelomic fluid, in which a single and very convoluted ovary cord is bathed. The ovary cord consists of germline cells, including nurse cells and young oocytes surrounded by a layer of elongated follicle cells. Additionally, follicle cells with long cytoplasmic projections occur inside the ovary cord, where they separate germ cells from each other. The ovary cord contains thousands of nurse cells. Each nurse cell has one intercellular bridge, connecting it to a central anucleate cytoplasmic mass, the cytophore (rachis); it in turn is connected by one intercellular bridge with each growing oocyte. Numerous mitochondria, RER cisternae, ribosomes, and Golgi complexes are transported from the nurse cells, via the intercellular bridge and cytophore, to the growing oocytes. Oogenesis in G. heteroclita is synchronous with all oocytes in the ovary in the same stage of oogenesis. The youngest observed oocytes are slightly larger than nurse cells, and usually occupy the periphery of the ovary cord. As previtellogenesis proceeds, the oocytes gather a vast amount of cell organelles and become more voluminous. As a result, in late previtellogenesis the oocytes gradually protrude into the ovary cavity. Simultaneously with oocyte growth, the follicle cells differentiate into two subpopulations. The morphology of the follicle cells surrounding the nurse cells and penetrating the ovary cord does not change, whereas those enveloping the growing oocytes become more voluminous. Their plasma membrane invaginates deeply, forming numerous broad vesicles that eventually seem to form channels or conducts through which the hemocoelomic fluid can easily access the growing oocytes.     

嘉庚蛸雌性生殖系统组织学观察

对象山港自然海区中的嘉庚蛸(Octopus tankahkeei)雌性生殖系统的组织学结构进行了研究.结果表明,雌性生殖系统由卵巢、输卵管、输卵管腺组成.卵巢单个、球形,内包裹滤泡细胞围成的卵子,输卵管1对,开口于外套腔中部,每条输卵管中部膨大形成圆球状的输卵管腺.近端输卵管内具两瓣蘑菇状突起,上有不规则短指状分枝,突...     

Ovarian maturation and oogenesis in the blue swimmer crab,Portunus pelagicus (Decapoda: Portunidae)

The study was aimed at understanding the process of reproduction and the changes happening in the ovary of Portunus pelagicus during maturation, which would be useful for its broodstock development for hatchery purposes. For that, tissue samples from different regions of the ovary at various stages of maturation were subjected to light and electron microscopy, and based on the changes revealed and the differences in ovarian morphology, the ovary was divided into five stages such as immature (previtellogenic oocytes), early maturing (early vitellogenic oocytes), late maturing (late vitellogenic oocytes), mature (vitellogenic oocytes), and spent (resorbing oocytes). The ovarian wall comprised of an outermost thin pavement epithelium, a middle layer of connective tissue, and an innermost layer of germinal epithelium. The oocytes matured as they moved from the centrally placed germinal zone toward the ovarian wall. The peripheral arrangement of nucleolar materials and the high incidence of cell organelles during the initial stages indicated vitellogenesis I. Movement of follicle cells toward oocytes in the early maturing stage and low incidence of mitochondria and endoplasmic reticulum in the ooplasm during late vitellogenic stage marked the commencement and end of vitellogenesis II, respectively. Yolk granules at various stages of development were seen in the ooplasm from late vitellogenic stage onwards. The spent ovary had an area with resorbing oocytes and empty follicle cells denoting the end of one reproductive cycle and another area with oogonial cells and previtellogenic oocytes indicating the beginning of the next.     

Anatomy of the ovaries of the starfish Asterias rubens (Echinodermata)

Summary The ovaries of the starfish Asterias rubens were studied histologically and ultrastructurally. The reproductive system in female specimens consists of ten separate ovaries, two in each ray. Each ovary is made up of a rachis with lateral primary and secondary folds: the acini maiores and acini minores. The ovarian wall is composed of an outer and an inner part, separated by the genital coelomic sinus. The ovarian lumen contains oocytes in various phases of oogenesis, follicle cells, nurse cells, phagocytosing cells and steroid-synthesizing cells.Oogenesis is divided into four phases: (i) multiplication phase of oogonia, (ii) initial growth phase of oocytes I, (iii) growth phase proper of oocytes I, and (iv) post-growth phase of oocytes I. The granular endoplasmic reticulum and the Golgi complex of the oocytes appear to be involved in yolk formation, while the haemal system, haemal fluid and nurse cells may also be important for vitellogenesis. The haemal system is discussed as most likely being involved in synchronizing the development of the ovaries during the annual reproductive cycle and in inducing, stimulating and regulating the function of the ovaries.Steroid-synthesizing cells are present during vitellogenesis; a correlation between the presence of these cells and vitellogenesis is discussed.     

Interrelationships between developing oocytes and ovarian tissues in the chiton Sypharochiton septentriones (Ashby) (Mollusca, Polyplacophora)

Oogenesis and the relationships between oocytes and other ovarian tissues have been studied in Sypharochiton septentriones. The ovarian tissues were examined by electron microscopy and by histochemical methods. The sac-like ovary is dorsal, below the aorta, and opens to the exterior by two posterior oviducts. Ventrally, the ovarian epithelium is folded inwards to form a series of plates of tissue, which support the developing ova. Each ovum is attached to a tissue plate by a stalk, the plasma membrane of which is bathed by the blood in the tissue plate sinus. Dorsally, ciliated vessels from the aorta enter the ovary and open into blood sinuses in the top of the plates. After each germinal epithelial cell rounds up to become a primary oogonium, it undergoes four mitotic divisions to give rise to a cluster of 16 secondary oogonia. Of these, the outer ones become follicle cells and the inner ones become oocytes. As in other molluses, the increases in nuclear and nucleolar volume are relatively greatest towards the end of previtellogenesis, when chromosomal and nucleolar activity are most intense. This phase of activity is accompanied by a great increase in cytoplasmic basophilia. Subsequently this basophilia is decreased during vitellogenesis, when chromosomal and nucleolar activity diminish. Fluid filled interstices appear in the cytoplasm during early vitellogenesis. Protein yolk deposition is associated with these interstices, but the lipid yolk appears to arise de novo. The follicle cells do not appear to be directly involved in oocyte nutrition. At times during oogenesis, certain manifestations of polarity can be found in the oocyte. This polarity is based on an apical-basal axis and can be related to the nutritive source of the oocyte, namely the blood which bathes the plasma membrane of the oocyte in the stalk. Numerous granulated cells are present in the ovarian tissue plates and ventral epithelium as storage cells containing lysosomes, and they are capable of phagocytosis and micropinocytosis of extracellular material. A scheme is outlined whereby reserves in these cells may be incorporated into the oocyte cytoplasm. Lysosomal activity is responsible for autolysis of the cells as well as resorption of unspawned ova.     

Ovarian germinal epithelium and folliculogenesis in the common snook, Centropomus undecimalis (Teleostei: centropomidae)

The ovarian germinal epithelium in the common snook, Centropomus undecimalis, is described. It consists of epithelial and prefollicle cells that surround germ cells, either oogonia or oocytes, respectively. The germinal epithelium borders a body cavity, the ovarian lumen, and is supported by a basement membrane that also separates the epithelial compartment of the ovarian lamellae from the stromal compartment. During folliculogenesis, the epithelial cells, whose cytoplasmic processes encompass meiotic oocytes, transform into prefollicle cells, which become follicle cells at the completion of folliculogenesis. The follicle is a derivative of the germinal epithelium and is composed of the oocyte and surrounding follicle cells. It is separated from the encompassing theca by a basement membrane. The cells that form the theca interna are derived from prethecal cells within the extravascular space of the ovarian stroma. The theca externa differentiates from undifferentiated cells within the stromal compartment of the ovary, from within the extravascular space. The theca interna and the theca externa are not considered to be part of the follicle and are derived from a different ovarian compartment than the follicle. Meiosis commences while oocytes are still within the germinal epithelium and proceeds as far as arrested diplotene of the first meiotic prophase. The primary growth phase of oocyte development also begins while oocytes are still within the germinal epithelium or attached to it in a cell nest. The definitions used herein are consistent between sexes and with the mammalian literature.     

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 ultrastructure of oogenesis and yolk formation in Labidocera aestiva (Copepoda: Calanoida)

Yolk formation in the oocytes of the free-living, marine copepod, Labidocera aestiva (order Calanoida) involves both autosynthetic and heterosynthetic processes. Three morphologically distinct forms of endogenous yolk are produced in the early vitellogenic stages. Type 1 yolk spheres are formed by the accumulation and fusion of dense granules within vesicular and lamellar cisternae of endoplasmic reticulum. A granular form of type 1 yolk, in which the dense granules within the cisternae of endoplasmic reticulum do not fuse, appears to be synthesized by the combined activity of endoplasmic reticulum and Golgi complexes. Type 2 yolk bodies subsequently appear in the ooplasm but their formation could not be attributed to any particular oocytic organelle. In the advanced stages of vitellogenesis, a single narrow layer of follicle cells becomes more developed and forms extensive interdigitations with the oocytes. Extra-oocytic yolk precursors appear to pass from the hemolymph into the follicle cells and subsequently into the oocytes via micropinocytosis. Pinocytotic vesicles fuse in the cortical ooplasm to form heterosynthetically derived type 3 yolk bodies.     

Ultrastructure of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, 1959)

Ultrastructural features of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, '59) have been described. The ovaries are paired, sac-like follicles suspended by mesenteries in the ventral coelom throughout the midbody region of the mature worm. Oogenesis is unsynchronized and occurs entirely within the ovary, where developing gametogenic stages are segregated spatially within a germinal and a growth zone. Multiplication of oogonia and differentiation of oocytes into the late stages of vitellogenesis occur in the germinal region of the ovary, whereas late-stage vitellogenic oocytes and mature eggs are located in a growth zone. Follicle cells envelop the oocytes in the germinal zone of the ovary and undergo hypertrophy and ultrastructural changes that correlate with the onset of vitellogenesis. These changes include the development of extensive arrays of rough ER and numerous Golgi complexes, formation of microvilli along the surface of the ovary, and the initiation of extensive endocytotic activity. Oocytes undergo similar, concomitant changes such as the differentiation of surface microvilli, the formation of abundant endocytotic pits and vesicles along the oolemma, and the appearance of numerous Golgi complexes, cisternae of rough ER, and yolk bodies. Yolk synthesis appears to occur by both autosynthetic and heterosynthetic processes involving the conjoined efforts of the Golgi complex and rough ER of the oocyte and the probable addition of extraovarian (heterosynthetic) yolk precursors. Evidence is presented that implicates the follicle cells in the synthesis of yolk precursors for transport to the oocytes. At ovulation, mature oocytes are released from the overy after the overlying follicle cells apparently withdraw. Bundles of microfilaments within the follicle cells may play a role in this withdrawal process.     

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.     

Heterologous gap junctions between oocytes and follicle cells of an insect,Dysdercus intermedius,and their potential role as ion current pathways

Summary In telotrophic insect ovaries, the oocytes develop in association with two kinds of supporting cells. Each ovary contains five to seven ovarioles. An ovariole consists of a single strand of several oocytes. At the apex of each ovariole is a syncytium of nurse cells (the tropharium), which connects by strands of cytoplasm (the trophic cords) to four or more previtellogenic oocytes. In addition, each oocyte is surrounded by an epithelium of follicle cells, with which it may form gap junctions. To study the temporal and spatial patterns of these associations, Lucifer yellow was microinjected into ovaries of the red cotton bug, Dysdercus intermedius. Freeze-fracture replicas were examined to analyze the distribution of gap junctions between the oocyte and the follicle cells. Dye-coupling between oocytes and follicle cells was detectable early in previtellogenesis and was maintained through late vitellogenesis. It was restricted to the lateral follicle cells. The anterior and posterior follicle cells were not dye-coupled. Freeze-fracture analysis showed microvilli formed by the oocyte during mid-previtellogenesis, and the gap junctions became located at the tips of these. As the microvilli continued to elongate until late vitellogenesis, gap junction particles between them and follicle cell membranes became arranged in long arrays. The morphological findings raise questions about pathways for the intrafollicular phase of the ion currents known to surround the previtellogenic and vitellogenic growth zones of the ovariole.Supported by the Deutsche Forschungsgemeinschaft (Schwerpunkt Differenzierung)     

泥螺卵黄发生过程中线粒体的变化

利用透射电镜（TEM）技术研究了泥螺卵黄发生过程中线粒体的形态结构的变化特点,结果表明,从卵黄发生早期到晚期,卵母细胞内线粒体经历了从外部形态到内部结构的一系列变化。卵黄合成初期的卵母细胞内,线粒体多,结构典型,仅部分线粒体外膜破裂,嵴 和内膜逐渐消失,卵黄发生中期,线粒体基质空泡化,嵴和内膜消失,腔内充满颗粒状物质,最后演变成卵黄颗粒,随着卵母细胞的发育,卵黄颗粒的数量和直径逐渐增加,卵黄发生后期,卵质中胞器不发达,细胞质中充满卵黄颗粒,在卵黄颗粒之间仅有少量线粒体存在,提供细胞代谢所需的能量,此外,对线粒体在卵黄形成中的功能,去向及行为变化等 进行了讨论。     

Ultrastructural observations on possible sites of steroid biosynthesis in the ovarian follicular epithelium of two species of cichlid fish,Cichlasoma nigrofasciatum and Haplochromis multicolor

Summary The follicular epithelial layers of the developing ovary of two cichlid species were examined by electron microscopy for evidence of steroid secretion. As each oocyte grew, its follicular cell layers increased in height, eventually becoming somewhat columnar; no development could be detected in follicle cells of non-activated oocytes. Isolated cells close to capillaries in the thecal layer developed large amounts of smooth membrane indicative of steroidogenesis, appearing similar at maturity to testicular Leydig cells. In Cichlasoma nigrofasciatum the mitochondria of differentiated thecal elements contained microtubule-like inclusions. It is suggested that these cells may produce estrogens during vitellogenesis.In developing granulosa cells, active synthesis of granular endoplasmic reticulum occurred. This membrane appeared to arise from the nuclear envelope, and in the pre-ovulatory stage was always intermediate between smooth and granular forms, being only partly associated with ribosomes. Evidence for steroid biosynthesis in the granulosa at this time was therefore equivocal. Evidence was found of transfer of micropinocytotic vesicles from the granulosa cells into the ooplasm.The fate of the post-ovulatory follicle was investigated in Cichlasoma. Thecal elements remained separate from granulosa and unchanged in ultrastructure for up to ten days. The granulosa cells proliferated and differentiated within a few hours after ovulation into a cell type containing much smooth reticulum, characteristic of steroidogenesis. However, after approximately three days numerous signs of degenerative processes became visible. The significance of the observed ultrastructural changes in relation to endocrine function is discussed.     

The fine structure of the ovary of the feather star Nemaster rubiginosa (Echinodermata: Crinoidea)

The outer layer of the crinoid ovary consists of coelomic epithelium, smooth muscles, and nerve cell processes. The middle layer of the ovary contains non-germinal accessory cells, small germinal cells (either oogonia or pre-leptotene primary oocytes), and post-pachytene primary oocytes; all these cells are completely embedded in a haemal matrix of 200 A-diameter granules. The primary oocytes larger than 20mu in diameter have abundant invaginations in the plasma membrane, suggesting uptake of materials from the haemal matrix. The innermost layer of the ovary is a ciliated epithelium lining the cell-free ovarian lumen.     

Phylogenetic significance of the structure of adult ovary and oogenesis in a primitive chilognathan diplopod,Hyleoglomeris japonica Verhoeff (Glomerida,Diplopoda)

Some histological details of the adult ovary of Hyleoglomeris japonica are described for the first time in the glomerid diplopods. The ovary is a single, long sac-like organ extending from the 4th to the 12th body segment along the median body axis, lying between the alimentary canal and the ventral nerve cord. The ovarian wall consists of a layer of thin ovarian epithelium which surrounds a wide ovarian lumen. A pair of longitudinal “germ zones,” including female germ cells, runs in the lateral ovarian wall. Each germ zone consists of two types of oogenetic areas: 1) 8–12 narrow patch-shaped areas for oogonial proliferation, arranged metamerically in a row along each of the dorsal and ventral peripheries, and 2) the remaining wide area for oocyte growth. Oogonial proliferation areas include oogonia, very early previtellogenic oocytes, and young somatic interstitial cells, among the ovarian epithelial cells. The larger early previtellogenic oocytes in the oogonial proliferation areas are located nearer to the oocyte growth area, and migrate to the oocyte growth area. They are surrounded by a layer of follicle cells and are connected with the ovarian epithelium of the oocyte growth area by a portion of their follicles. They grow into the ovarian lumen, but their follicles are still connected with the oocyte growth area. Various sizes of the previtellogenic and vitellogenic oocytes in the ovarian lumen are connected with the oocyte growth area; the smaller oocytes are connected nearer to the dorsal and ventral oogonial proliferation areas, while the larger ones are connected nearer to the longitudinal middle line of the oocyte growth area. Following the completion of vitellogenesis and egg membrane formation in the largest primary oocytes, the germinal vesicles break down. Ripe oocytes are released from their follicles directly into the ovarian lumen to be transported into the oviducts. Ovarian structure and oogenesis of H. japonica are very similar to those of other chilognathan diplopods. At the same time, however, some characteristic features of the ovary of H. japonica are helpful for understanding the structure and evolution of the diplopod ovaries. Some aspects of the phylogenetic significance in the paired germ zones of H. japonica are discussed. J. Morphol 231:277–285, 1997. © 1997 Wiley-Liss, Inc.     

Ovary of the golden crab,Chaceon fenneri: I. Post-spawning and oosorption

Oosorption has been considered an important strategy in many invertebrate species which occurs in response to behavioral, ecological, or physiological factors. In crustaceans, the early light microscopic studies of the ovary attributed a role in oosorption to follicle cells, hemocytes, or phagocytes. In this study, ovaries were collected from female golden crabs following spawning and processed for examination by electron microscopy. Following spawning, several unspawned oocytes which had become dissociated from their follicle cells were found in the ovaries. They appeared to be lodged within the lumen. Such oocytes were observed undergoing various stages of autolysis. At no time were hemocytes or recognizable phagocytes found in the lumen of the ovaries or in contact with the degenerating oocytes. Follicle cells which had surrounded the oocytes prior to the time of spawning exhibited disrupted membranes. Resorption of unspawned eggs appears to occur by autolysis of the individual oocytes. Several of the females who had recently spawned had numerous sperm in their ovaries. Such sperm may have been pressed into the lumen at the time of spawning or during the fixation process.     

Fine Structure of the Ovarian Development in the Orb-web Spider, Nephila clavata

ABSTRACT Fine structural changes of the ovary and cellular composition of oocyte with respect to ovarian development in the orb-web spider, Nephila clavata were examined by scanning and transmission electron microscopy. Unlike the other arthropods, the ovary of this spider has only two kinds of cells-follicle cells and oocytes. During the ovarian maturation, each oocyte bulges into the body cavity and attaches to surface of the elongated ovarian epithelium through its peculiar short stalk attachments. In the cytoplasm of the developing oocyte two main types of yolk granules, electron-dense proteid yolk and electron-lucent lipid yolk granules, are compactly aggregated with numerous glycogen particles. The cytoplasm of the developing oocyte contains a lot of ribosomes, poorly developed rough endoplasmic reticulum, mitochondria and lipid droplets. These cell organelles, however, gradually degenerate by the later stage of vitellogenesis. During the active vitellogenesis stage, the proteid yolk is very rapidly formed and the oocyte increases in size. However, the micropinocytosis invagination or pinocytotic vesicles can scarcely be recognized, although the microvilli can be found in some space between the oocyte and ovarian epithelium. During the vitellogenesis, the lipid droplets in the cytoplasm of oocytes increase in number, and become abundant in the peripheral cytoplasm close to the stalks. On completion of the yolk formation the vitelline membrane, which is composed of an inner homogeneous electron-lucent component and an outer layer of electron-dense component is formed around the oocyte.