Structure of the ovary and the differentiation of follicular epithelium in the pig louse, Haematopinus suis (Insecta: Phthiraptera)

The female reproductive system of the pig louse, Haematopinus suis (Insecta: Phthiraptera) is composed of paired ovaries, lateral oviducts, and a common oviduct that leads into a vagina. Clusters of mycetocytes (= cells filled with symbiotic organisms) are associated with lateral oviducts. Each ovary is composed of five loosely arranged ovarioles of the polytrophic-meroistic type. An individual ovariole is covered by a basal lamina and is composed of a terminal filament, germarium, and vitellarium. The terminal filament is composed of large, disc-shaped cells that are orientated perpendicularly to the long axis ofthe ovariole. The basal part of the terminal filament is separated from the germarium by a well-developed transverse septum. The germarium is short and filled with clusters of oogonial cells. In each cluster the cells arejoined by intercellular bridges, filled with fusomal material. Within the cluster, only one cell, the future oocyte, enters the prophase of the first meiotic division; the other cells differentiate into nurse cells. The basal part ofthe germarium is filled with the somatic prefollicular cells. The boundary between the germarium and the vitellarium is not distinct. The vitellarium contains linearly arranged ovarian follicles in subsequent stages of oogenesis (previtellogenesis, vitellogenesis and choriogenesis). Each follicle consists of an oocyte and 7 nurse cells and is surrounded by follicular cells. During oogenesis the follicular cells diversify, so that ultimately, five morphologically distinct subpopulations of these cells can be distinguished: (1) cells in contact with the nurse cells, (2) anterior cells, (3) mainbody cells, (4) posterior cells, and (5) interfollicular cells. Interestingly, the follicular cells associated with the anterior part of the oocyte, i.e. located in space at the oocyte/nurse cell border (fold cells) are mitotically active throughout previtellogenesis. It might be suggested, in this context, that the separation of the oocyte from the nurse cell compartment is brought about by mitotic divisions, consequent multiplication and centripetal migration of these cells.     

Cyst geometry in the egg chambers of Calliphora erythrocephala Mg. (Diptera: Calliphoridae) ovaries

In the germarium of polytrophic ovarioles of Calliphora erythrocephala (Mg.) fly, four mitotic divisions of cystoblasts give rise to 16-cell germ-line cysts. One cell differentiates into an oocyte, while the remaining 15 cells become nurse cells. Concomitantly actin-rich ring canals are formed at the intercellular junctions. The present study considers a mutual arrangement of the ring canals formed after the second to fourth mitoses relative to the ring canal formed after the first mitotic division in different regions of the germarium and egg chambers. During the cyst formation and its movement to the posterior end of the germarium, the ring canals are displaced relative to one another, thereby giving different branching variants of the cyst. The pattern of cell interconnections becomes stable in germarium region 2b and does not change during the cyst movement along the ovariole despite the cyst polarizes and increases in size.     

An EM autoradiographic study on ovarian follicle cells of Drosophila melanogaster with special reference to the formation of egg coverings.

Ovarian follicle cells of Drosophila melanogaster have been studied by ultrastructural and autoradiographic analyses. During their migration through the germarium, follicle cells undergo several structural changes and, of these, the most conspicuous one occurs at the level of the nucleolus. By the time the first ovarian chamber is formed, follicle cells have formed a layer of uniform thickness all around a cluster or nurse cells and the oocyte. Following the initiation of vitellogenesis, the follicle cells overlaying the oocyte become columnar while those over the nurse cells become very thin. During stages 9-10, the columnar follicle cells are involved in the formation of the vitelline membrane, while from stages 11 to 13 these cells produce the endochorion. An EM autoradiographic analysis has shown that the rate of 3H-uridine incorporation in follicle cells nuclei is low in previtellogenic chambers, while it becomes very high in nuclei of stage 9-10 chambers. After short exposure to uridine, silver grains are located predominantly over nucleoli. Evidence from incorporation studies with 3H-lysine indicates that the columnar follicle cells and the region of the various egg coverings are highly labelled within an hour of incubation in the tracer. The observations confirm that columnar follicle cells are the only cells in the chamber involved in the formation of materials which make up the egg coverings.     

The origin and early differentiation of the egg chamber of Drosophila melanogaster

The egg chamber of Drosophila melanogaster consists of 16 interconnected cells surrounded by a monolayer of follicle cells. Each 16 cell cluster (from which the oocyte and 15 nurse cells differentiate) arises within the germarial region of an ovariole. To study the ultrastructure of the early stages in the formation and differentiation of egg chambers, a three dimensional reconstruction was made from serial thin sections through a germarium from a 24-hour old, virgin female. The germarium was found to be subdivided into three regions: (1) The mitotically active area where clusters of 16 cells originate from a series of cystocyte divisions, (2) the region where these cells interact with mesodermal cells, and (3) the region where the germarial cyst is transformed into the first egg chamber in the vitellarium. Since cystocytes were found to decrease in size with each division, the possibility exists that cell size may determine when the divisions cease. Models are presented which mimic with varying degrees of success the developmental changes the germarial cells undergo with time. Hypothesis are developed to explain why stem line oogonia are restricted to the anterior portion of the germarium, why mesodermal cells first interact with cystocytes in region 2, and why the oocyte is oriented posteriorly. The nuclear differentiations of the component cells of the chamber are described and correlated with observed differences in radiosensitivity. Symbionts were observed in the germaria of several strains of Drosophila, and the bearing of these findings upon nutritional studies is discussed.     

A very simple mode of follicular cell diversification in Euborellia fulviceps (Dermaptera, Anisolabididae) involves actively migrating cells

The ovaries of Euborellia fulviceps are composed of five elongated ovarioles of meroistic-polytrophic type. The individual ovariole has three discernible regions: the terminal filament, germarium, and vitellarium. The terminal filament is a stalk of flattened, disc-shaped somatic cells. In the germarium, germline cells in subsequent stages of differentiation are located, and the vitellarium comprises numerous ovarian follicles arranged linearly. The individual ovarian follicles within the vitellarium are separated by prominent interfollicular stalks. The follicles are composed by two germline cells only: an oocyte and a single, polyploid nurse cell, which are surrounded by a monolayer of somatic follicular cells (FCs). During subsequent stages of oogenesis, initially uniform follicular epithelium begins to diversify into morphologically and physiologically distinct subpopulations. In E. fulviceps, the FC diversification mode is rather simple and leads to the formation of only three different FC subpopulations: (1) cuboidal FCs covering the oocyte, (2) stretched FCs surrounding the nurse cell and (3) FCs actively migrating between oocyte and a nurse cell. We found that FCs from the latter subpopulation send long and thin filopodium-like and microtubule-rich processes penetrating between the oocyte and nurse cell membranes. This suggests that, in E. fulviceps, cells from at least one FCs subpopulation show the ability to change position within an ovarian follicle by means of active migration.     

An EM autoradiographic study on ovarian follicle cells of Drosophila melanogaster with special reference to the formation of egg coverings

Summary Ovarian follicle cells of Drosophila melanogaster have been studied by ultrastructural and autoradiographic analyses.During their migration through the germarium, follicle cells undergo several structural changes and, of these, the most conspicuous one occurs at the level of the nucleolus. By the time the first ovarian chamber is formed, follicle cells have formed a layer of uniform thickness all around a cluster or nurse cells and the oocyte. Following the initiation of vitellogenesis, the follicle cells overlying the oocyte become columnar while those over the nurse cells become very thin. During stages 9–10, the columnar follicle cells are involved in the formation of the vitelline membrane, while from stages 11 to 13 these cells produce the endochorion.An EM autoradiographic analysis has shown that the rate of ³H-uridine incroporation in follicle cell nuclei is low in previtellogenic chambers, while it becomes very high in nuclei of stage 9–10 chambers. After short exposure to uridine, silver grains are located predominantly over nucleoli.Evidence from incorporation studies with ³H-lysine indicates that the columnar follicle cells and the region of the various egg coverings are highly labelled within an hour of incubation in the tracer.The observations confirm that columnar follicle cells are the only cells in the chamber involved in the formation of materials which make up the egg coverings.This work was partly supported by C.N.R. (Italy)I am indebted to Dr. J. Jacob from the Institute of Animal Genetics (Edinburgh) for introducing me to the use of EM autoradiography     

刘氏蝎蛉卵巢管结构和卵子发生（英文）

卵巢管结构及卵子发生过程在探讨昆虫系统发育关系中有重要意义, 深入研究长翅目昆虫卵巢管结构及卵子发生可为确定其在全变态类昆虫中的系统发育地位提供依据。本文利用光学显微镜和扫描、透射电子显微镜技术研究了刘氏蝎蛉Panorpa liui Hua卵巢管超微结构及卵子发生过程。结果表明：蝎蛉卵巢由12根多滋式卵巢小管组成, 每个卵巢小管分为端丝、生殖区和生长区。根据滋养细胞、卵母细胞及滤泡细胞的变化, 卵子发生过程可分为5个阶段：卵黄发生前早期、卵黄发生前中期、卵黄发生前后期、卵黄发生期及卵壳形成期。在卵黄发生期, 滋养细胞为卵母细胞提供养分后逐渐消亡, 而此时的卵母细胞可通过滤泡之间的细胞间隙从血淋巴中获取营养。在卵壳形成期间, 3种不同类型的滤泡细胞参与形成不同区域的卵壳, 从而形成不同花饰的卵壳表面。据此推测, 与其他目的滋养细胞数目相比, 每个卵室中2次有丝分裂形成3个滋养细胞可能是比较原始的特征, 表明长翅目昆虫可能是全变态类群中近基部的分支。     

黄胫小车蝗卵子发生及卵母细胞凋亡的显微观察

对黄胫小车蝗(Oedaleus infernalis)卵子发生过程和卵母细胞凋亡进行显微观察。结果表明,黄胫小车蝗卵子发生可明显分为3个时期10个阶段,即卵黄发生前期、卵黄发生期和卵壳形成期。第1阶段,卵母细胞位于卵原区,经历减数第一次分裂;第2阶段,卵母细胞核内染色体解体成网状,滤泡细胞稀疏地排列在卵母细胞周围;第3阶段,滤泡细胞扁平状,在卵母细胞周围排成一层;第4阶段,滤泡细胞呈立方形排在卵母细胞周围;第5阶段,滤泡细胞呈长柱形排在卵母细胞周围,滤泡细胞之间、滤泡细胞与卵母细胞之间出现空隙;第6阶段,卵母细胞边缘开始出现卵黄颗粒;第7阶段,卵母细胞中沉积大量卵黄,胚泡破裂;第8阶段,滤泡细胞分泌卵黄膜包围卵黄物质;第9阶段,滤泡细胞分泌卵壳;第10阶段,卵壳分泌结束,卵子发育成熟。卵母细胞发育过程中的凋亡发生在卵黄发生前期,主要表现为滤泡细胞向卵母细胞内折叠,胞质呈团块状等特征。     

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.     

Structure of the ovary and "nurse cells" in a freshwater ostracod, Cyprinotus uenoi Brehm (Podocopida: Cypridoidea)

The adult female of the freshwater ostracod Cyprinotus uenoi Brehm, 1936 (Podocopida: Cypridoidea) has a pair of long, sac-like ovaries separately lying in the posterior part of the left and the right carapace valves. Oogonia and very early previtellogenic oocytes are located in the terminal germarium of each ovary. In the germarium, the oogonia occur in the most terminal region, and the very early previtellogenic oocytes are located in the remainder, arranged in order of size, the larger ones nearer the ovarian lumen. Most of the growing oocytes, previtellogenic and vitellogenic, are found in the ovarian lumen, the larger ones farther from the germarium. In the germarium, a cytoplasmic bridge connects a pair of adjoining germ cells, resulting from an incomplete cytokinesis of oogonial division. Among the previtellogenic and early vitellogenic oocytes in the ovarian lumen, "nurse cells" are found as small, spherical cells in mostly the same number as these oocytes. A cytoplasmic bridge connects each "nurse cell" to an adjoining oocyte. Based on the manner of connection and some morphological features, we consider that each "nurse cell" originates from one of each pair of adjoining germ cells connected by a cytoplasmic bridge in the germarium, as in the true nurse cells of several branchiopod crustaceans and insects with meroistic ovarioles.     

The spatial arrangement of germ line cells in ovarian follicles of the mutantdicephalic inDrosophila melanogaster

Summary The pattern of intercellular connections between germ line cells has been studied in follicles of the mutantdicephalic (dic), which possess nurse cell clusters at both poles. Staining of follicles with a fluorescent rhodamine conjugate of phalloidin reveals ring canals and cell membranes and thus allows us to reconstruct the spatial organization of the follicle. Each germ line cell can be identified by the pattern of cell-cell connections which reflect the mitotic history of individual cells in the 16-cell cluster. The results indicate that in both wild-type anddicephalic cystocyte clusters one of the two cells with four ring canals normally becomes the pro-oocyte. However, in some follicles (dicephalic and wild-type) oocytes were found with fewer or more than four ring canals. Indic follicles, one or several nurse cells may become disconnected from the other cells during oocyte growth at stage 9–10. Such disconnected cells cannot later on empty their cytoplasm into the oocyte. This, in turn, might be of consequence for the determination of axial polarity of the embryo.     

F-actin distribution in the ovaries of pre-vitellogenic and vitellogenic black blowflies,Phormia regina (Meigen) (Diptera : Calliphoridae)

The spatial distribution of F-actin microfilaments in the ovaries of previtellogenic and vitellogenic female black blowflies, Phormia regina (Diptera : Calliphoridae), as the females shift from a sugar to a liver diet, is determined using rhodamine-labelled phalloidin (rh-phalloidin). During the pre-vitellogenic stages of ovarian development (i.e. corresponding to a sugar diet) a single bright fluorescent layer marks the interface between follicle cells and the oocyte. Fluorescence is also most evident at the inner surface of the ring canals of the nurse cells. This is observed in the nurse cells both in the distal part of the germarium, and in the vitellogenic growing oocyte. However, when liver-fed (i.e. necessary for vitellogenesis), 2 bright fluorescent layers are observed at the follicle cell-oocyte interface. In addition, the cytoplasm of the nurse cells during vitellogenesis appears full of fluorescent microfilaments and the actin rings are found to increase in size and thickness. The changing organization of the F-actin microfilaments in the follicles during the process of both egg chamber and oocyte formation is discussed and possible functions considered.     

Collective oscillations of coupled cell cycles

Problems with networks of coupled oscillators arise in multiple contexts, commonly leading to the question about the dependence of network dynamics on network structure. Previous work has addressed this question in Drosophila oogenesis, in which stable cytoplasmic bridges connect the future oocyte to the supporting nurse cells that supply the oocyte with molecules and organelles needed for its development. To increase their biosynthetic capacity, nurse cells enter the endoreplication program, a special form of the cell cycle formed by the iterated repetition of growth and synthesis phases without mitosis. Recent studies have revealed that the oocyte orchestrates nurse cell endoreplication cycles, based on retrograde (oocyte to nurse cells) transport of a cell cycle inhibitor produced by the nurse cells and localized to the oocyte. Furthermore, the joint dynamics of endocycles has been proposed to depend on the intercellular connectivity within the oocyte-nurse cell cluster. We use a computational model to argue that this connectivity guides, but does not uniquely determine the collective dynamics and identify several oscillatory regimes, depending on the timescale of intercellular transport. Our results provide insights into collective dynamics of coupled cell cycles and motivate future quantitative studies of intercellular communication in the germline cell clusters.     

Oogenesis in four species of Piscicola (Hirudinea, Rhynchobdellida)

Piscicola has a pair of elongated sac-shaped ovaries. Inside the ovaries are numerous small somatic cells and regularly spherical egg follicles. Each follicle is composed of three types of cells: many (average 30) germ cells (cystocytes) interconnected by intercellular bridges in clones (cysts), one intermediate cell, and three to five outer follicle cells (envelope cells). Each germ cell in a clone has one intercellular bridge connecting it to the central anucleate cytoplasmic mass, the cytophore. Each cluster of germ cells is completely embedded inside a single huge somatic follicle cell, the intermediate (interstitial) cell. The most spectacular feature of the intermediate cell is its development of a system of intracytoplasmic canals apparently formed of invaginations of its cell membrane. Initially the complex of germ cell cluster + intermediate cell is enclosed within an envelope composed of squamous cells. As oogenesis progresses the envelope cells gradually degenerate. All the germ cells that have terminated their mitotic divisions are of similar size and enter meiotic prophase, but one of the cystocytes promptly starts to grow faster and differentiates into the oocyte, whereas the remaining cystocytes withdraw from meiosis and become nurse cells (trophocytes). Numerous mitochondria, ER, and a vast amount of ribosomes are transferred from the trophocytes via the cytophore toward the oocyte. Eventually the oocyte ingests all the content of the cytophore, and the trophocytes degenerate. Little vitellogenesis takes place; the oocyte gathers nutrients in the form of small lipid droplets. At the end of oogenesis, an electron-dense fibrous vitelline envelope appears around the oocyte, among short microvilli. At the same time, electron-dense cortical granules occur in the oocyte cortical cytoplasm; at the end of oogenesis they are numerous, but after fertilization they disappear from the ooplasm. In the present article we point out many differences in the course of oogenesis in two related families of rhynchobdellids: piscicolids and glossiphoniids.     

The histology and histochemistry of oogenesis in the water strider, Gerris remigis Say

In each ovariole of Gerris remigis, nurse cells arise by mitotic divisions at the anterior end of the germarium. These cells enlarge as they move posteriorly. This size increase is possibly caused by fusion of cells, but probably by endopolyploidy as well. The nurse cells then establish connections with a central trophic core, which receives the products of subsequent nurse cell degradation. Two possible pathways of nuclear degradation are suggested: one involves the condensation of chromatin within the nucleus; the other, the release of DNA as fine granules into the cytoplasm. Cytoplasmic areas containing such DNA are also rich in proteinaceous granules, but have a meager content of RNA. The remainder of the cytoplasm of the mature nurse cells contains a high concentration of RNA, as do the nucleoli. Posteriorly the trophic core connects via nutritive cords with each developing oocyte in the prefollicular region and in the anterior vitellarium. RNA is apparently contributed to the ooplasm via the trophic stream. Patches of cytoplasmic DNA are present in the young oocytes; the origin and fate of this DNA is uncertain. During early oocyte maturation chromosomal stainability decreases, and the nucleolus enlarges. In previtellogenic stages, numerous proteinaceous bodies appear in association with the nucleolus-chromosome complex. These bodies, like the nucleolus, have only a low RNA content. They may pass to the cytoplasm, but cannot be traced with certainty. During the latter part of this period a complex population of small proteinaceous and lipid preyolk bodies accumulates peripherally in the oocyte. Definitive protein and lipid yolk are probably derived by the enlargement and inward migration of these bodies. The oocytes are each surrounded by a layer of follicle cells proliferated in the prefollicular region. These become binucleate and enlarge as the enclosed oocytes grow and elongate. RNA also increases in the nucleoli and cytoplasm of the follicle cells as they move posteriorly in the vitellarium. There is no evidence of transfer of nucleic acids or protein from the follicle cells to the oocyte. The nurse cells are therefore implicated as the major source of nucleic acids for the maturing oocyte.     

Ultrastructural observations on the oogenesis of Triops cancriformis (Crustacea,Notostraca)

Summary Each ovarian follicle of Triops cancriformis is four-celled; these cells (one oocyte and three nurse cells) are interconnected by cytoplasmic bridges. In the course of differentiation, the nurse cells are early recognizable; they increase in size more than the oocyte and their nuclei contain many nucleoli. For the first time in Arthropoda, yolk globules are reported to be present in nurse cell cytoplasm; these globules arise from the smooth endoplasmic reticulum. The functional significance of the intercellular bridges and the trophic role of the nurse cells are discussed.The authors are grateful to Dr. Bruno Sabelli for his support and to Mr. Francesco Monte for his technical assistance     

The follicle cell-oocyte interaction in ovarian follicles of the stick insect Bacillus rossius (Rossi): (Insecta: Phasmatodea)

Developing ovarian follicles of Bacillus rossius have been examined ultrastructurally in an attempt to understand how inception of vitel-logenesis is controlled. Early vitellogenic follicles are characterized by a thick cuboidal epithelium that is highly interlocked with the oocyte plasma membrane. Gap junctional contacts are present both at the follicle cell/oocyte interface and in between adjacent follicle cells. In addition, microvilli of follicle cells protrude deeply into the cortical ooplasm of these early vitellogenic oocytes. With the onset of vitellogenesis, wide intercellular spaces appear in the follicle cell epithelium and at the follicle cell/oocyte interface. Gap junctions become progressively reduced both on the follicle cell surface and on the oocyte plasma membrane. Microvilli from the two cell types no longer interlock. From a theoretical standpoint each of the two structural differentiations present at the follicle cell/oocyte interface—gap junctions and follicle cell microvilli—could potentially trigger inception of vitellogenesis. Gap junctions might permit the passage of a regulatory molecule, transferring from follicle cells to oocyte, which would control the assembly of coated pits on the oocyte plasma membrane. Alternatively cell interaction via microvilli might induce the appearance of coated pits, thus creating a membrane focus for vitellogenin receptors. Both possibilities are discussed in relation to current literature.