The telotrophic ovary known from Neuropterida exists also in the myxophagan beetle <Emphasis Type="Italic">Hydroscapha natans</Emphasis>

The ovary structure of the myxophagan beetle, Hycdoscapha natans, was investigated by means of light and electron microscopy for the first time. Each of the two ovaries consists of three ovarioles, the functional units of insect oogenesis. The ovary type is telotrophic meroistic but differs strongly from the telotrophic ovary found among all polyphagous beetles investigated so far. All characters found here are typical of telotrophic ovaries of Sialidae and Raphidioptera. Both taxa belong to the Neuropterida. As in all telotrophic ovaries, all nurse cells are combined in an anterior chamber, the tropharium. The tropharium houses two subsets of germ cells: numerous nurse cell nuclei are combined in a central syncytium without any cell membranes in between, surrounded by a monolayer of single-germ cells, the tapetum cells. Each tapetum cell is connected to the central syncytium via an intercellular bridge. Tapetum cells of the posterior zone, which sufficiently contact prefollicular cells, are able to grow into the vitellarium and develop as oocytes. During previtellogenic and early vitellogenic growth, oocytes remain connected with the central syncytium of the tropharium via their anterior elongations, the nutritive cords. The morphological data are discussed in the light of those derived from ovaries of other Coleoptera and from the proposed sister group, the Neuropterida. The data strongly support a sister group relationship between Coleoptera and Neuropterida. Furthermore, several switches between polytrophic and telotrophic ovaries must have occurred during the radiation of ancient insect taxa.     

Formation and structure of nutritive cords in telotrophic ovarioles of snake flies (Insecta: Raphidioptera)

Telotrophic ovariole of Raphidia spp. is composed of the anteriorly located terminal filament, tube-shaped tropharium, the vitellarium and the ovariole stalk. The tropharium consists of a central syncytial core surrounded by one cell thick layer of tapetum cells. Early previtellogenic oocytes differentiate at the base of tropharium. Both the oocytes and the tapetum cells are connected with the central syncytium by delicate intercellular bridges. At the onset of previtellogenic growth, the anterior parts of the oocytes become extended and form long cytoplasmic projections--nutritive cords. Each nutritive cord contains numerous microtubules that show no preferential orientation within the cord but diminishing anterior-posterior gradient of distribution. Irregular arrangement of microtubules indicates that this cytoskeletal scaffold does not play any role in directed transport within the ovariole but instead constitutes one of the elements of the structural framework of the nutritive cord. Besides microtubules, the stability of the nutritive cords in Raphidia ovarioles is maintained by the rim-shaped membrane foldings lined with microfilaments.     

The larval development of the telotrophic meroistic ovary in the bug Dysdercus intermedius (Heteroptera, Pyrrhocoridae)

Bug ovaries are of the telotrophic meroistic type. Nurse cells are restricted to the anterior tropharium and are in syncytial connection with the oocytes via the acellular trophic core region into which cytoplasmic projections of oocytes and nurse cells open. The origin of intercellular connections in bug ovaries is not well understood. In order to elucidate the cellular processes underlying the emergence of the syncytium, we analysed the development of the ovary of Dysdercus intermedius throughout the five larval instars. Up to the third instar, the germ cell population of an ovariole anlage forms a single, tight rosette. In the center of the rosette, phosphotyrosine containing proteins and f-actin accumulate. This center is filled with fusomal cytoplasm and closely interdigitating cell membranes known as the membrane labyrinth. With the molt to the fourth instar germ cells enhance their mitotic activity considerably. As a rule, germ cells divide asynchronously. Simultaneously, the membrane labyrinth expands and establishes a central column within the growing tropharium. In the fifth instar the membrane labyrinth retracts to an apical position, where it is maintained even in ovarioles of adult females. The former membrane labyrinth in middle and posterior regions of the tropharium is replaced by the central core to which nurse cells and oocytes are syncytially connected. Germ cells in the most anterior part of the tropharium, i.e. those in close proximity to the membrane labyrinth remain proliferative. The posterior-most germ cells enter meiosis and become oocytes. The majority of the ovarioles' germ cells, located in between these two populations, endopolyploidize and function as nurse cells. We conclude that the extensive multiplication of germ cells and their syncytial assembly during larval development is achieved by incomplete cytokineses followed by massive membrane production. Membranes are degraded as soon as the trophic core develops. For comparative reasons, we also undertook a cursory examination of early germ cell development in Dysdercus intermedius males. All results were compatible with the known basic patterns of early insect spermatogenesis. Germ cells run through mitotic and meiotic divisions in synchronous clusters emerging from incomplete cytokineses. During the division phase, the germ cells of an individual cluster are connected by a polyfusome rich in f-actin.     

Structure of ovarioles in Adelges laricis, a representative of the primitive aphid family Adelgidae

The structure of ovaries has been analysed in advanced aphids only. In this paper we report the results of ultrastructural studies on the ovarioles of Adelges laricis, a representative of the primitive aphid family, Adelgidae. The ovaries of the studied species are composed of five telotrophic‐meroistic ovarioles that are subdivided into a terminal filament, tropharium (= trophic chamber) and vitellarium. The tropharium houses trophocytes (= nurse cells) and arrested oocytes. The vitellarium consists of one or two ovarian follicles. The total number of germ cells (trophocytes + oocytes) in the ovarioles analysed varies from 50 to 92 and is substantially higher than in previously studied aphids. The centre of the tropharium is occupied by a cell‐free region, termed a trophic core, which is connected both with trophocytes and oocytes. Trophocytes are connected to the core by means of cytoplasmic strands, whereas oocytes by nutritive cords. Both trophic core and nutritive cords are filled with parallel arranged microtubules. In the light of obtained results the anagenesis of hemipteran ovaries is discussed.     

Mayflies (ephemeroptera), the most “primitive” winged insects,have telotrophic meroistic ovaries

Germ line cell cluster formation in ovarioles of three different stages, each from a different mayfly species, was studied using ultra-thin serial sectioning. In the analysed ovariole of Cloeön sp., only one linear, zigzag germ line cell cluster was found, consisting of sibling cells connected by intercellular bridges which represent remnants of preceding synchronized mitotic cycles followed by incomplete cytokinesis. A polyfusome stretched through all sibling cells. At the tip of the ovariole, cytokinesis occurred without preceding division of nuclei; thus, intercellular bridges were lined up but the remaining cytoplasm between the bridges had no nuclei. The analysed Siphlonurus armatus vitellarium contained five oocytes at different stages of development. Each oocyte in the vitellarium was connected via a nutritive cord to the linear cluster of its sibling cells in the terminal trophic chamber. Each cluster had the same architecture as was found in Cloëon. The 3-dimensional arrangement and distribution of closed intercellular bridges strongly suggest that all five clusters are derived from a single primary clone. The position of oocytes within each cluster is random. However, each oocyte is embraced by follicular or prefollicular cells whilst all other sibling cells are enclosed by somatic inner sheath cells, clearly distinguishable from prefollicular cells. In the analysed ovariole of Ephemerella ignita, two small linear clusters were found in the tropharium beside two single cells, two isolated cytoplasmic bags with intercellular bridges but no nuclei, and some degenerating aggregates. One cluster was still connected to a growing oocyte via a nutritive cord. In all species the nurse cells remained small and no indications of polyploidization were found. We suggest that this ancient and previously unknown telotrophic meroistic ovary has evolved directly from panoistic ancestors.     

Unusual organization of the tropharium in the telotrophic ovarioles of an insect, Saldula saltatoria

The tropharium of the common shorebug Saldula saltatoria consists of 2 zones: the apical mitotic region and the distal one comprising numerous mononucleate nurse cells. Each individual nurse cell is connected to the centrally located trophic core by a thin cytoplasmic projection referred to as a trophic process. The accumulations of a dense material interpreted as the remnants of intercellular bridge rim are observed associated with the trophic process membrane. In the light of these results the establishment of telotrophic ovarioles in hemipterans is discussed.     

The Ovary of Tubifex tubifex (Clitellata,Naididae, Tubificinae) Is Composed of One,Huge Germ-Line Cyst that Is Enriched with Cytoskeletal Components

Recent studies on the ovary organization and oogenesis in Tubificinae have revealed that their ovaries are small polarized structures that are composed of germ cells in subsequent stages of oogenesis that are associated with somatic cells. In syncytial cysts, as a rule, each germ cell is connected to the central cytoplasmic mass, the cytophore, via only one stable intercellular bridge (ring canal). In this paper we present detailed data about the composition of germ-line cysts in Tubifex tubifex with special emphasis on the occurrence and distribution of the cytoskeletal elements. Using fixed material and live cell imaging techniques, we found that the entire ovary of T. tubifex is composed of only one, huge multicellular germ-line cyst, which may contain up to 2,600 cells. Its architecture is broadly similar to the cysts that are found in other clitellate annelids, i.e. a common, anuclear cytoplasmic mass in the center of the cyst and germ cells that are connected to it via intercellular bridges. The cytophore in the T. tubifex cyst extends along the long axis of the ovary in the form of elongated and branched cytoplasmic strands. Rhodamine-coupled phalloidin staining revealed that the prominent strands of actin filaments occur inside the cytophore. Similar to the cytophore, F-actin strands are branched and they are especially well developed in the middle and outermost parts of the ovary. Microfilaments are also present in the ring canals that connect the germ cells with the cytophore in the narrow end of the ovary. Using TubulinTracker, we found that the microtubules form a prominent network of loosely and evenly distributed tubules inside the cytophore as well as in every germ cell. The well-developed cytoskeletal elements in T. tubifex ovary seem to ensure the integrity of such a huge germ-line cyst of complex (germ cells - ring canals - cytophore) organization. A comparison between the cysts that are described here and other well-known female germ-line cysts is also made.     

Morphological Studies on the Testis of Adults of Moniliformis (Acanthocephala)

The morphology of the testis in young adult males of Moniliformis dubius developing in the rat has been studied with the aid of light and electron microscopes. In one-day-old male worms, the testis is organised as two zones. One zone consists of individual germ-line cells, while the other consists of a supporting syncytium which embeds the cells and forms the boundary of the testis. The surface of the testis is covered by a fibrous non-cytoplasmic coat. In seven-day-old male worms, the syncytium has lost its compact form, breaking down into multinucleate units connected by cytoplasmic processes and apparently forming a loose syncytial network throughout the testis. The germ cells are now randomly distributed and are surrounded by wide spaces among the segments of the supporting syncytium. The fibrous coat, lined internally by an irregular layer of the syncytium, forms the testis envelope. This basic structure is maintained in the testis of 14-day-old and sexually mature male worms.     

Structure of ovaries of scale insects: ii. margarodidae (INSECTA,hemiptera, COCCINEA)

The paired, spindle-shaped ovaries of the second instar of the Polish cochineal, Porphyrophora polonica (L.) (Hemiptera: Coccinea) are filled with cystocytes that are arranged into rosettes. In the centre of each rosette, there is a polyfusome. During the third instar, cystocytes differentiate into oocytes and trophocytes (nurse cells) and ovarioles are formed. Ovaries of adult females are composed of about 300 ovarioles of the telotrophic type. Each of them is subdivided into a tropharium (trophic chamber) and vitellarium. The tropharium consists of trophocytes and arrested oocytes that may develop. The number of germ cells in the trophic chambers varies from 11 to 18 even between the ovarioles of the same ovary. The obtained results seem to confirm the concept of a monophyletic origin of the primitive scale insects (Archaeococcoidea).     

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.     

Conversion of midbodies into germ cell intercellular bridges

Whereas somatic cell cytokinesis resolves with abscission of the midbody, resulting in independent daughter cells, germ cell cytokinesis concludes with the formation of a stable intercellular bridge interconnecting daughter cells in a syncytium. While many proteins essential for abscission have been discovered, until recently, no proteins essential for mammalian germ cell intercellular bridge formation have been identified. Using TEX14 as a marker for the germ cell intercellular bridge, we show that TEX14 co-localizes with the centralspindlin complex, mitotic kinesin-like protein 1 (MKLP1) and male germ cell Rac GTPase-activating protein (MgcRacGAP) and converts these midbody matrix proteins into stable intercellular bridge components. In contrast, septins (SEPT) 2, 7 and 9 are transitional proteins in the newly forming bridge. In cultured somatic cells, TEX14 can localize to the midbody in the absence of other germ cell-specific factors, suggesting that TEX14 serves to bridge the somatic cytokinesis machinery to other germ cell proteins to form a stable intercellular bridge essential for male reproduction.     

The flea ovary: ultrastructure and analysis of cell clusters

Panoistic ovarioles are found in the order of fleas (Siphonaptera). Only in some species of the Hystrichopsylloidea do polytrophic meroistic ovaries occur. No stem cells and no dividing cystocytes are found in female imagines of Hystrichopsylla talpae. However, each germ cell cluster consists of 32 cells which are generated by five mitotic cycles during the pupal stage. One of the cells containing five intercellular bridges becomes the oocyte, the others serve as nurse cells. Thus, germ cell cluster formation follows the 2(n)-rule. However, no polyfusome is found and nurse cells do not form a rosette. Furthermore, nurse cells remain small and show the same ultrastructural characters as the oocytes, which became distinguishable from nurse cells only by their enhanced growth during pre-vitellogenesis. The first phase of pre-vitellogenesis is dominated by the production of an unknown cytoplasmatic component, consisting of spherical particles, clearly distinguishable from ribosomes by diameter and contrast. The next phase is characterized by a tremendous increase in the production of ribosomes. During this second phase another cytoplasmic component consisting of ball-like structures becomes prominent. During pre-vitellogenesis, germ cell nuclei undergo a pronounced structural change in which, finally, numerous extranucleolar particles predominate. Thus, H. talpae has a polytrophic meroistic ovary, but its oocyte genomes behave panoistically.     

Formation of germ-line cysts with a central cytoplasmic core is accompanied by specific orientation of mitotic spindles and partitioning of existing intercellular bridges

Animal germ cells tend to form clonal groups known as clusters or cysts. Germ cells within the cyst (cystocytes) are interconnected by intercellular bridges and thus constitute a syncytium. Our knowledge of the mechanisms that control the formation of germ-cell clusters comes from extensive studies carried on model organisms (Drosophila, Xenopus). Germ-cell clusters have also been described in worms (annelids, flat worms and nematodes), although their architecture differs significantly from that known in arthropods or vertebrates. Their peculiar feature is the presence of a central anucleate cytoplasmic core (cytophore, rachis) around which the cystocytes are clustered. Each cystocyte in such a cluster always has one intercellular bridge connecting it to the central cytoplasmic core. The way that such clusters are formed has remained a riddle for decades. By means of light, fluorescence and electron microscopy, we have analysed the formation and architecture of cystocyte clusters during early stages of spermatogenesis and oogenesis in a few species belonging to clitellate (oligochaetous) annelids. Our data indicate that the appearance of germ cells connected via a central cytophore is accompanied by a specific orientation of the mitotic spindles during cystocyte divisions. Spindle long axes are always oriented tangentially to the surface of the cytophore. In consequence, cystocytes divide perpendicularly to the plane of the existing intercellular bridge. Towards the final stages of cytokinesis, the contractile ring of the cleavage furrow merges with the rim of the intercellular bridge that connects the dividing cystocyte with the cytophore and forces partition of the existing bridge into two new bridges. This work was supported by the following research grants: 2P04C004 28 from the Ministry of Science and Informatization (to P. Świątek and J. Klag) and DS/1018/IZ/2007 (to J. Kubrakiewicz).     

The occurrence of intercellular bridges in groups of cells exhibiting synchronous differentiation

A previous electron microscopic study of the cat testis revealed that spermatids derived from the same spermatogonium are joined together by intercellular bridges. The present paper records the observation of similar connections between spermatocytes and between spermatids in Hydra, fruit-fly, opossum, pigeon, rat, hamster, guinea pig, rabbit, monkey, and man. In view of these findings, it is considered likely that a syncytial relationship within groups of developing male germ cells is of general occurrence and is probably responsible for their synchronous differentiation. When clusters of spermatids, freshly isolated from the germinal epithelium are observed by phase contrast microscopy, the constrictions between the cellular units of the syncytium disappear and the whole group coalesces into a spherical multinucleate mass. The significance of this observation in relation to the occurrence of abnormal spermatozoa in semen and the prevalence of multinucleate giant cells in pathological testes is discussed. In the ectoderm of Hydra, the clusters of cnidoblasts that arise from proliferation of interstitial cells are also connected by intercellular bridges. The development of nematocysts within these groups of conjoined cells is precisely synchronized. Both in the testis of vertebrates and the ectoderm of Hydra, a syncytium results from incomplete cytokinesis in the proliferation of relatively undifferentiated cells. The intercellular bridges between daughter cells are formed when the cleavage furrow encounters the spindle remnant and is arrested by it. The subsequent dissolution of the spindle filaments establishes free communication between the cells. The discovery of intercellular bridges in the two unrelated tissues discussed here suggests that a similar syncytial relationship may be found elsewhere in nature where groups of cells of common origin differentiate synchronously.     

The Occurrence of Intercellular Bridges in Groups of Cells Exhibiting Synchronous Differentiation

A previous electron microscopic study of the cat testis revealed that spermatids derived from the same spermatogonium are joined together by intercellular bridges. The present paper records the observation of similar connections between spermatocytes and between spermatids in Hydra, fruit-fly, opossum, pigeon, rat, hamster, guinea pig, rabbit, monkey, and man. In view of these findings, it is considered likely that a syncytial relationship within groups of developing male germ cells is of general occurrence and is probably responsible for their synchronous differentiation. When clusters of spermatids, freshly isolated from the germinal epithelium are observed by phase contrast microscopy, the constrictions between the cellular units of the syncytium disappear and the whole group coalesces into a spherical multinucleate mass. The significance of this observation in relation to the occurrence of abnormal spermatozoa in semen and the prevalence of multinucleate giant cells in pathological testes is discussed. In the ectoderm of Hydra, the clusters of cnidoblasts that arise from proliferation of interstitial cells are also connected by intercellular bridges. The development of nematocysts within these groups of conjoined cells is precisely synchronized. Both in the testis of vertebrates and the ectoderm of Hydra, a syncytium results from incomplete cytokinesis in the proliferation of relatively undifferentiated cells. The intercellular bridges between daughter cells are formed when the cleavage furrow encounters the spindle remnant and is arrested by it. The subsequent dissolution of the spindle filaments establishes free communication between the cells. The discovery of intercellular bridges in the two unrelated tissues discussed here suggests that a similar syncytial relationship may be found elsewhere in nature where groups of cells of common origin differentiate synchronously.     

Sperm development in the teleost Oryzias latipes

Summary In Oryzias latipes the processes of spermatogenesis and spermiogenesis occur within testicular or germinal cysts which are delimited by a single layer of lobule boundary cells. These cells, in addition to comprising the structural component of the cyst wall, ingest residual bodies cast off by developing spermatids. Therefore, they are deemed to be the homologue of mammalian Sertoli cells. The germ cells within a cyst develop synchronously owing to the presence of intercellular bridges connecting adjacent cells. Since bridges also connect spermatogonia, it seems probable that all of the germ cells within a cyst may form a single syncytium and do not exist as individual cells until the completion of spermiogenesis when the residual bodies are cast off. Significant differences between spermiogenesis in O. latipes and in the related poeciliid teleosts are discussed.     

The ovaries of aphids (Hemiptera,Sternorrhyncha, Aphidoidea): morphology and phylogenetic implications

The ovaries of aphids belonging to the families Eriosomatidae, Anoeciidae, Drepanosiphidae, Thelaxidae, Aphididae, and Lachnidae were examined at the ultrastructural level. The ovaries of these aphids are composed of several telotrophic ovarioles. The individual ovariole is differentiated into a terminal filament, tropharium, vitellarium, and pedicel (ovariolar stalk). Terminal filaments of all ovarioles join together into the suspensory ligament, which attaches the ovary to the lobe of the fat body. The tropharium houses individual trophocytes and early previtellogenic oocytes termed arrested oocytes. Trophocytes are connected with the central part of the tropharium, the trophic core, by means of broad cytoplasmic processes. One or more oocytes develop in the vitellarium. Oocytes are surrounded by a single layer of follicular cells, which do not diversify into distinct subpopulations. The general organization of the ovaries in oviparous females is similar to that of the ovaries in viviparous females, but there are significant differences in their functioning: (1) in viviparous females, all ovarioles develop, whereas in oviparous females, some of them degenerate; (2) the number of germ cells per ovariole is usually greater in females of the oviparous generation than in females of viviparous generations; (3) in oviparous females, oocytes in the vitellarium develop through three stages (previtellogenesis, vitellogenesis, and choriogenesis), whereas in viviparous females, the development of oocytes stops after previtellogenesis; and (4) in the oocyte cytoplasm of oviparous females, lipid droplets and yolk granules accumulate, whereas in viviparous females, oocytes accrue only lipid droplets. Our results indicate that a large number of germ cells per ovariole represent the ancestral state within aphids. This trait may be helpful in inferring the phylogeny of Aphidoidea.