Oocytes develop from interconnected cystocytes in the panoistic ovary of Nemoura sp. (Pictet) (Plecoptera : Nemouridae)

The 2 ovaries of Nemoura sp. (Plecoptera : Nemouridae) are comb-like and house about 60–70 ovarioles each. By ultrathin serial sections through a whole ovariole of a last-larval instar, we gathered information on its ultrastructure and 3-dimensional architecture. The germarial region contains several clusters of interconnected oogonia or oocytes. The intercellular bridges (ring canals) are filled with fusomes. Most of the fusomes assemble to polyfusomes and some of the intercellular bridges move together and their cells assemble to rosettes. Results indicate that existence of polyfusomes is not sufficient for rosette formation. The oogonia or oocytes of each cluster develop synchronously. Oocytes detach from clusters next to intercellular bridges. A transdetermination of oogonia to nurse cells does not occur. Thus, the stone flies remain true panoists.     

Activity of the ovarian germinal epithelium in the freshwater catfish,Pimelodus maculatus (Teleostei: Ostariophysi: Siluriformes): Germline cysts,follicle formation and oocyte development

Distinct types of oogonia are found in the germinal epithelium that borders the ovarian lamellae of Pimelodus maculatus: A‐undifferentiated, A‐differentiated and B‐oogonia. This is similar to the situation observed for spermatogonia in the vertebrate testis. The single A‐undifferentiated oogonia divide by mitosis giving rise to A‐groups of single differentiated oogonia, each enclosed by epithelial cells that are prefollicle cells. Subsequently, the single A‐differentiated oogonia proliferate to generate B‐oogonia that are interconnected by cytoplasmic bridges, hence, forming germline cysts. The prefollicle cells associated with them also divide. Within the germline cysts, B‐oogonia enter meiosis becoming oocytes. Meiotic prophase and early folliculogenesis occur within the germline cysts. During folliculogenesis, prefollicle cells grow between the oocytes, encompassing and individualizing each of them. The intercellular bridges disappear, and the germline cysts are broken down. Next, a basement membrane begins to form around the nascent follicle, separating an oocyte and its associated prefollicle cells from the cell nest. Folliculogenesis is completed when the oocyte and the now follicle cells are totally encompassed by a basement membrane. Cells derived from the ovarian stroma encompass the newly‐formed ovarian follicle, and become the theca, thereby completing the formation of the follicle complex. Follicle complexes remain attached to the germinal epithelium as they share a portion of basement membrane. This attachment site is where the oocyte is released during ovulation. The postovulatory follicle complex is continuous with the germinal epithelium as both are supported by a continuous basement membrane. The findings in P. maculatus reinforce the hypothesis that ovarian follicle formation represents a conserved process throughout vertebrate evolution. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Intercellular bridges in ovaries of the newborn gerbil

This study describes intercellular bridges in the ovaries of neonatal gerbils. Electron microscopy has revealed the presence of true intercellular bridges, connecting oogonia or oocytes, in ovaries of newborn gerbils. The cytoplasm of the intercellular channels is similar to that of the connected cells, with mitochondria, smooth and rough endoplasmic reticulum, and free ribosomes present. Lysosomes are also occasionally present in the intercellular bridges and they may be involved in early waves of oocyte atresia. An electron-dense substance, 350-500 A thick, is located immediately beneath the unit membrane of the intercellular bridges. Accumulation of electron-dense material increases the thickness of the walls of the intercellular bridges, supporting and maintaining the patency of the channels. It is suggested that the intercellular channels probably allow the interchange of nutrients, organelles, and possibly regulatory materials as well.     

Gonads in Histiostoma mites (Acariformes: Astigmata): Structure and development

The development of male and female gonads in arrhenotokous and thelytokous species of Histiostoma was studied using transmission electron microscopy (TEM). All instars were examined: larvae, protonymphs, facultative heteromorphic deutonymphs (=hypopi), tritonymphs, and adults. In testis primordium, spermatogonia surrounding a testicular central cell (TCC) with a gradually enlarging, branched nucleus are present already at the larval stage. Spermatogonia and the TCC are connected via narrow, tubular intercellular bridges revealing that the TCC is a germline cell. Spermatocytes appear at the protonymphal stage. At the heteromorphic deutonymph stage, the testis primordium is similar to that of the protonymph, but in the tritonymph it is much larger and composed as in the adult: spermatids as well as sperm cells are present. The latter are congregated ventrally in the testis at the entrance of the deferent duct.In the larval ovary, an eccentrically located ovarian nutritive cell (ONC) is surrounded by oogonia which are connected with the ONC via tubular intercellular bridges. In later stages, the ovary grows and oocytes appear in the protonymph. Meiotic synaptonemal complexes in oocytes occur from the tritonymph stage. At about the time of the final molting, tubular intercellular bridges transform into peculiar diaphragm-crossed bridges known only in Histiostoma mites. In the adult female, growing oocytes at the end of previtellogenesis lose intercellular bridges and move ventro-laterally to the ovarian periphery towards the oviduct entrance. Vitellogenesis occurs in oviducts.Germinal cells in both the testis and ovary are embedded in a few somatic stroma cells which may be well discernible already in the larval ovary; in the testis, somatic stroma cells are evident not earlier than the end of the tritonymphal stage. The ovary has a thin wall of flat somatic cells, whereas the testis is covered by a basal lamina only.The obtained results suggest that gonads in Histiostoma and other Astigmata originate from two primordial cells only.     

Roles of cell-to-cell communication in development

Possible roles of cell-to-cell communication mediated by intercellular bridges and gap junctions in development of the female gamete and embryo are discussed. Synchronization of cell cycle events is presumably a role for intercellular bridges between germ cells. The follicle of the Cecropia moth reveals that an electrical polarity exists between nurse cells and oocytes which are connected by intercellular bridges and this polarity may generate differences that result in differentiation of the oogonia to become either the oocyte or nurse cells. Gap junction-mediated transfer of cyclic AMP, made in response to gonadotropin stimulation, between granulosa cells is discussed as a mechanism that allows cells within a tissue to respond to an external stimulus even though all cells in that tissue may not be exposed to the stimulus. A nutritional role for heterologous cell communication between follicle cells and the oocyte in oocyte growth is presented as an example of how gap junction-mediated communication can allow one cell type to influence the behavior of another cell type. During development, a restriction in communication between differentiating cells is frequently observed. Examples of this phenomenon in a mammal and an insect are presented.     

Intercellular bridges between oocytes in the chicken ovary

Summary Fine structural analysis of the functional (left) ovary of the newly-hatched chick reveals the presence of true intercellular bridges between developing oocytes in the early stages of the meiotic prophase. These structures are characterized by: 1) cytoplasmic continuity between the participating oocytes, 2) a dense, fibrillar material beneath the lateral limiting membrane and 3) numerous cellular organelles within their confines. In addition, microtubular elements, parallel to the long axis of the bridge, are routinely observed. This latter finding suggests that intercellular bridges originate through incomplete cytokinesis of mitotically active oogonia and that the dense material beneath the limiting membrane may represent the cortical microfilaments associated with the contractile ring. Functionally, these structures may serve as channels for transfer of nutrients and organelles between oocytes although the possibility that certain oocytes function as nurse cells, in the sense that these cells exist in invertebrate ovaries, seems unlikely. In addition, intercellular bridges may be responsible for both restriction of oogonial mitoses and meiotic synchrony.Partial support for this study was provided by grant DE-00241 from the National Institute for Dental Research administered by Dr. Melvin Hess. We gratefully acknowledge his support of the initial aspects of this study. We are pleased to express our appreciation to Mrs. Cindy G. Wilcox, Mr. Lloyd Thibodeau and Mr. Don Driscoll for their expert technical assistance.     

Vasa protein is localized in the germ cells and in the oocyte-associated pyriform follicle cells during early oogenesis in the lizard <Emphasis Type="Italic">Podarcis sicula</Emphasis>

The vasa gene, first identified in Drosophila, is a key determinant for germline formation in eukaryotes. Homologs of vasa have been identified and linked to germline development, in many invertebrates and vertebrates. Here, we analyze the distribution of Vasa in early germ cells (oogonia and oocytes) and previtellogenic ovarian follicles of the lizard Podarcis sicula. During most of its previtellogenic growth, the oocyte in this lizard species is structurally and functionally integrated through intercellular bridges with special follicle cells called pyriform cells. The pyriform cells function similarly to Drosophila nurse cells, but are somatic in origin. In the oogenesis of P. sicula, Vasa is initially highly detected in the oogonia, but its levels decrease in early stage oocytes before the onset of pyriform cell differentiation. In the later stages of oogenesis, the high level of Vasa is related with the nurse function of the pyriform follicle cells. These observations suggest that cells of somatic origin are engaged in the synthesis of Vasa in the oogenesis of this lizard.     

Oogenesis in the viviparous phoronid,Phoronis embryolabi

The study of gametogenesis is useful for phylogenetic analysis and can also provide insight into the physiology and biology of species. This report describes oogenesis in the Phoronis embryolabi, a newly described species, which has an unusual type of development, that is, a viviparity of larvae. Phoronid oogonia are described here for the first time. Yolk formation is autoheterosynthetic. Heterosynthesis occurs in the peripheral cytoplasm via fusion of endocytosic vesicles. Simultaneously, the yolk is formed autosynthetically by rough endoplasmic reticulum in the central cytoplasm. Each developing oocyte is surrounded by the follicle of vasoperitoneal cells, whose cytoplasm is filled with glycogen particles and various inclusions. Cytoplasmic bridges connect developing oocytes and vasoperitoneal cells. These bridges and the presence of the numerous glycogen particles in the vasoperitoneal cells suggest that nutrients are transported from the follicle to oocytes. Phoronis embryolabi is just the second phoronid species in which the ultrastructure of oogenesis has been studied, and I discuss the data obtained comparing them with those in Phoronopsis harmeri. Finally, I discuss the distribution of reproductive patterns across both, molecular and morphological phylogenetic trees in Phoronida proving that parental care has evolved independently several times in this phylum.     

Development of germ cells in the ovaries of human fetuses in the early periods]

Histological methods were used for studying 30 ovaries of early human embryos from 7 to 11 weeks of development. It was shown that in the process of development of the ovary the number of mitotically dividing oogonia decreased from 76% in 7-8 weeks to 41% in the period of 10-11 weeks. The mototic division of oogonia was characterized by high activity from 3,6% to 6,8%. However, as early as in the ovaries of 7-8 week embryos there occurred transition of a part of oogonia into oocytes of preleptotene stages which were characterized by processes of spiralization and despiralization of chromatin in the nuclei. The amount of such oocytes increases in the process of development of the embryo. The amount of oocytes at the stage of condensation of chromatin "prochromosomes" increases from 7,6% to 14,4%, the amount of oocytes at the stage of the following despiralization increased from 2,1% to 21% when comparing the ovaries of embryos of 7-8 and 10=11 weeks of development. The size of nucleoli was found to change in the period of preleptotene transformations in the oocyte nuclei. Photographs of the stages in question are presented made from histological and total preparations.     

Expression of activin subunits and receptors in the developing human ovary: activin A promotes germ cell survival and proliferation before primordial follicle formation

The formation of the essential functional unit of the ovary, the primordial follicle, occurs during fetal life in humans. Factors regulating oogonial proliferation and interaction with somatic cells before primordial follicle formation are largely unknown. We have investigated the expression, localisation and functional effects of activin and its receptors in the human fetal ovary at 14-21 weeks gestation. Expression of mRNA for the activin betaA and betaB subunits and the activin receptors ActRIIA and ActRIIB was demonstrated by RT-PCR. Expression of betaA mRNA increased 2-fold across the gestational range examined. Activin subunits and receptors were localised by immunohistochemistry. The betaA subunit was expressed by oogonia, and the betaB subunit and activin receptors were expressed by both oogonia and somatic cells. BetaA expression was increased in larger oogonia at later gestations, but was low in oocytes within newly formed primordial follicles. Treatment of ovary fragments with activin A in vitro increased both the number of oogonia present and oogonial proliferation, as detected by bromodeoxyuridine (BrdU) incorporation. These data indicate that activin may be involved in the autocrine and paracrine regulation of germ cell proliferation in the human ovary during the crucial period of development leading up to primordial follicle formation.     

Oogenesis in pig ovaries during the prenatal period: ultrastructure and morphometry

Oogenesis in fetal pig ovaries comprises the successive changes from the primordial germ cells to the dictyotene oocytes in primordial ovarian follicles. In this study the observations were carried out with an electron microscope and stereological analysis was performed. At the ultrastructural level there are no differences between the primordial germ cells and oogonia, but oogonia are connected with the intercellular bridges. The onset of the dictyotene phase was accompanied by the changes in the cytoplasm of oocytes. Near the nucleus, the yolk nucleus is formed containing numerous Golgi bodies, endoplasmic reticulum (ER), mitochondria and granules. ER proliferates in contact with the external leaflet of the nuclear envelope forming the narrow ER cisterns. Between the nuclear envelope and ER cisterns, the vesicles with grey content are visible. The proliferating ER forms numerous concentric cisterns around the nucleus. Next, the most external cisterns fragment, detach, and then form the cup-like structures. These structures separate the distinct areas of cytoplasm-compartments, which contain mitochondria, ribosomes and lipid droplets. The cells of cortical sex cords of the ovary, which encloses the oocyte, form the follicles. The volume of oocytes in forming follicle increases due to the increase in the number of the cell inclusions: lipid droplets, vacuoles and yolk globules. In the oocytes of primordial ovarian follicles, the compartments are transformed into the yolk globules, which are encountered by a sheath of ER cisterns and the grey vesicles; they contain the mitochondria, lipid droplets and light vacuoles. The role of the compartments and yolk globules as metabolic units is discussed in comparison with similar structures of the mature eggs of pigs and other mammal species.     

Oogenesis and egg shell formation in Heterakis gallinarum (Nematoda)

The structure of the developing ova and egg shell formation of Heterakis gallinarum has been described. The oogonia are small, undifferentiated cells which are arranged around a central cytoplasmic rachis. The oogonia and young oocytes are in cytoplasmic continuity with the rachis and it is suggested that the rachis may influence synchronous development of the oocytes. The oocytes contain two types of granule; refringent, which give rise to the ascaroside layer of the egg shell, and another kind which appear to be a type of yolk for the developing egg. After fertilization the spermatozoon produces numerous ribosomes; a second unit membrane appears beneath the oolemma, and the chitinous layer of the shell forms between the oolemma and this inner membrane. The refringent granules later produce the ascaroside layer of the shell between the chitinous layer and the inner membrane. The outermost layer of the shell is produced from material secreted by the cells of the uterus.     

Oogenesis of Tilapia mossambica. I. Oogonia and meiotic prophase oocytes

Using light and electron microscopy and autoradiography, the morphology and synthesis of DNA, RNA and proteins in oogonia and early meiotic prophase oocytes in Tilaria mossabique were studied. According to dimensions and morphological features observed it is possible to distinguish between two groups of oogonia: large oogonia corresponding to type A spermatogonia of mammals, and small actively dividing oogonia, located in groups and identical to type B spermatogonia. The morphology of oogonia and of the early meiotic prophase oocytes well compares with the pattern described for other species of bony fishes. In the cytoplasm of these cells dense bodies, nuage-material, free ribosomes, large mitochondria with lamellar cristae and Golgi cisterns are available. In the oocyte nuclei at zygotene and pahytene stages 3H-thymidine incorporation was seen mainly into the nucleolus-associated chromatin. Besides, the formation of a heterochromatin cape and the synaptonemal complex was observed. Incorporation of 3H-uridine and 3H-leucine in the nuclei of these cells was very poor.     

From primordial germ cell to primordial follicle: mammalian female germ cell development

In mammals, the final number of oocytes available for reproduction of the next generation is defined at birth. Establishment of this oocyte pool is essential for fertility. Mammalian primordial germ cells form and migrate to the gonad during embryonic development. After arriving at the gonad, the germ cells are called oogonia and develop in clusters of cells called germ line cysts or oocyte nests. Subsequently, the oogonia enter meiosis and become oocytes. The oocyte nests break apart into individual cells and become packaged into primordial follicles. During this time, only a subset of oocytes ultimately survive and the remaining immature eggs die by programmed cell death. This phase of oocyte differentiation is poorly understood but molecules and mechanisms that regulate oocyte development are beginning to be identified. This review focuses on these early stages of female germ cell development.     

Yolk formation in an annelid (Ophryotrocha puerilis, polychaeta)

The polychaete Ophryotrocha does not show a distinct breeding season. Egg masses are produced throughout the year (continuous breeder sensu Olive and Clark, 1978). A female specimen may contain up to three different generations of oocytes with oocyte growth and maturation in each batch being well synchronized. Oogenesis takes about 18 days from proliferation of the oogonia to mature eggs. In each segment pairs of sister cells interconnected by cytoplasmic bridges are located in outpocketings of the ventral mesentery which form the gonad wall. Presumptive oocytes and nurse cells are not easily distinguished at that time. Vitellogenesis is initiated while both oocytes and nurse cells are still in the ovary. Mitochondria, multivesicular bodies (transformed mitochondria ?), dense bodies, preformed yolk bodies of smaller size and lipid droplets are probably passed through the cytoplasmic bridge from the nurse cell to the oocyte. Yolk formation includes different mechanisms and materials of different origin. Autosynthetic yolk formation predominates during the first intraovarial growth phase. After detachment of oocyte-nurse cell-complexes from the gonad pinocytotic activity of nurse cells and particularly oocytes, increases considerably. The existence of coated vesicles suggests that external sources of yolk precursors contribute to yolk formation. Prior to oocyte maturation the remnants of the nurse cell are incorporated by oocytes.     

Electron microscopic study of the differentiation and development of trophocytes and oocytes in Gerris najas (Heteroptera).

The differentiation of oogonia and oocytes, and of trophocytes, from undifferentiated germ line cells has been studied in Gerris najas, a pond skater, from the fourth instar to the adult animal. For the first time criteria have been obtained which allow the distinction between poorly differentiated early oogonia and nurse cells. The most important criteria are the size, shape, and structure of nuclei and mucleoli. This is consistent with the different function of these cell types, which is primarily a different nuclear function: meiosis in the oocytes, and RNA synthesis to support the trophic core and the oocytes in the trophocytes.     

Chronology and dynamics of the development of female genital organs of cattle fetuses

The chronology and dynamics of the female germ cell development, of the mitotic activity of oogonia, and of the chromosome rearrangements at prophase I of meiosis have been quantitatively estimated in 30 cow embryos and foetuses at the age of 1.5 to 9 months. The sexual differentiation of the gonads was shown in a 1.5 month old embryo. The oocytes at the stages of preleptotene chromosome condensation and decondensation occurred in the 1.5 month old embryos and their maximum number was observed in the 2-5 month old foetuses. The leptotene oocytes were found in the 2-2.5 month old foetuses. The transition to zygotene and pachytene was also recorded in the 2-2.5 month old foetuses but their maximum number was observed in the 4-6 month old foetuses; their number was reduced to single oocytes thereafter. The first diplotene oocytes appeared in the 3 month old foetuses but the active transition of the oocytes to diplotene was observed after four months of development. The formation of a layer of follicle cells takes place around the diplotene oocytes. The vast majority of degenerating germ cells are the oocytes in zygotene-pachytene and in diplotene. The population of germ cells is formed by the mitotic division of oogonia in the cow foetuses, mainly at the age of 1.5 to 4 months of development.     

Microscopic structures of the ovary and female genital ducts of Supachai's caecilian,Ichthyophis supachaii Taylor, 1960 (Amphibia: Gymnophiona)

The structures of the female reproductive system (ovary, oviduct and cloaca) of Ichthyophis supachaii were investigated by dissection, histology and light microscopy. Paired, elongated, sac‐like ovaries are parallel to the gut and fat bodies. Follicle stages include germinal nests of oogonia and primary oocytes, early and late previtellogenic follicles, early and late vitellogenic follicles and atretic follicles. Germinal nests of oogonia comprise oogonia and prefollicular cells. Nests of primary oocytes contain clusters of synchronously developing primary oocytes enclosed by connective tissue. Primary oocytes are associated with follicular cells. Previtellogenic follicles initially form the vitelline envelope, theca cell layers and patches of ooplasmic glycoproteins. Vitellogenic follicles contain heterogeneously sized spherical yolk granules. Atresia is present in several stages of developing follicles. The oviduct is divided into the anterior, middle and posterior parts. All oviductal parts are lined by non‐ciliated epithelium. A small number of mucous cells are present in the middle part. The cloaca of female I. supachaii is divided into the anterior and posterior chambers. The anterior chamber is lined by glandular stratified columnar epithelium, while the posterior chamber has stratified cuboidal epithelium with less mucus production. Our results contribute to useful information on the reproductive biology of caecilians.     

Immunolocalization of estrogen receptor α in Neomysis japonica oocytes and follicle cells during ovarian development

Estrogen induces oocytes development and vitellogenesis in crustacean by interacting with estrogen receptor (ER) subtypes. In the present study, we detect for the first time the ERα in oocytes and follicle cells and hepatopancreas cells of mysis by immunohistochemistry using a specific ERα antibody. ERα was mainly localized in the nuclei of oocytes and follicle cells, while mainly detected in nuclei of oogonia (OG), previtellogenic oocyte (PR) and endogenous vitellogenic oocyte (EN) at previtellogenic and early vitellogenic stage (I-early III). Follicle cells in all stages of ovary (all vitellogenic stages) showed strong ERα positive reaction, and they were able to gradually move to oocytes during the development of oocytes. In addition, ERα was also localized in the nuclei and cytoplasm of four hepatopancreas cells (including E-, R-, F- and B-cell) in all ovary stages. These findings suggest, for the first time to our knowledge, that there could be a close link between oogenesis, follicle cells, hepatopancreas cells and endocrine regulation, and estrogens might be involved in the regulation of oocytes at early ovarian stage in mysis.     

Sex differentiation and aspects of gametogenesis in shortnose sturgeon Acipenser brevirostrum Lesueur

Shortnose sturgeon Acipenser brevirostrum gonad samples were collected from industry-reared fish and wild broodstock at various developmental stages to elucidate patterns of gonadal differentiation and maturation. Genital ridges, containing germ cells, were present in 26 day-old fish and distinct gonads were present by day 54. Sturgeon gonads are known to consist of two tissue types (adipose and gametogenic) and both were present at 72 day. Anatomical differentiation of gonads occurred by 6 months and was advanced by 15 months. Ovaries had distinct lamellae while testes remained non-lamellate. Gonial proliferation had occurred by 15 months, but the cells were not identifiable as spermatogonia or oogonia. Small white 'pinhead' oocytes were macroscopically visible in ovaries as early as 36 months. At 43 months ovaries were clearly organized, with some areas containing only immature oocytes and other containing oocytes apparently developing as cohorts. Individual fish showed considerable variation: the level of development remained unchanged at 84 months in some females, while others showed clear progression towards sexual maturation at 48 months. Sperm cells were present in males as early as 52 months. Advanced development of ovarian follicles was observed only in biopsies of re-conditioned broodstock of wild origin. In the year before spawning, the most advanced oocytes became pigmented, the chorion thickened, the nucleus (germinal vesicle) migrated towards the micropyle complex at the animal pole, and ovulation occurred in May under appropriate environmental conditions.