Ovary cord structure and oogenesis in Hirudo medicinalis and Haemopis sanguisuga (Clitellata, Annelida): remarks on different ovaries organization in Hirudinea

In Hirudo medicinalis and Haemopis sanguisuga, two convoluted ovary cords are found within each ovary. Each ovary cord is a polarized structure composed of germ cells (oogonia, developing oocytes, nurse cells) and somatic cells (apical cell, follicular cells). One end of the ovary cord is club-shaped and comprises one huge apical cell, numerous oogonia, and small cysts (clusters) of interconnected germ cells. The main part of the cord contains fully developed cysts composed of numerous nurse cells connected via intercellular bridges with the cytophore, which in turn is connected by a cytoplasmic bridge with the growing oocyte. The opposite end of the cord degenerates. Cord integrity is ensured by flattened follicular cells enveloping the cord; moreover, inside the cord, some follicular cells (internal follicular cells) are distributed among germ cells. As oogenesis progresses, the growing oocytes gradually protrude into the ovary lumen; as a result, fully developed oocytes arrested in meiotic metaphase I float freely in the ovary lumen. This paper describes the successive stages of oogenesis of H. medicinalis in detail. Ovary organization in Hirudinea was classified within four different types: non-polarized ovary cords were found in glossiphoniids, egg follicles were described in piscicolids, ovarian bodies were found characteristic for erpobdellids, and polarized ovary cords in hirudiniforms. Ovaries with polarized structures equipped with apical cell (i.e. polarized ovary cords and ovarian bodies) (as found in arhynchobdellids) are considered as primary for Hirudinea while non-polarized ovary cords and the occurrence of egg follicles (rhynchobdellids) represent derived condition.     

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.     

Ovarian structure,folliculogenesis and oogenesis of the annual killifish Millerichthys robustus (Cyprinodontiformes: Cynolebiidae)

Cellular aspects of oocyte development of the Mexican rivulus Millerichthys robustus were morphologically described in order to analyze ovarian function and the cellular recruitment dynamics associating it with life history strategies of annual killifishes. Millerichthys is an iteroparous batch spawner with continuous oocyte recruitment and indeterminate fecundity with asynchronous development of the follicles. It has two ovaries of cystovarian type, with a central lumen, which communicates with the outside through the caudal region of the ovary, that is, the gonoduct. From the walls of the ovary, irregular lamellae composed of germinal epithelium and vascularized stroma project. Oogenesis starts with oogonial proliferation, found alone or in nests within the germinal epithelium. The oogonia come into meiosis becoming oocytes and advancing to the chromatin nucleolus stage and to early primary growth stage. Folliculogenesis is completed in the primary growth stage and cortical alveoli step. Follicles moves toward the stroma, but they continue to be attached to the germinal epithelium through the basement membrane until ovulation. The inclusion of fluid yolk in the follicles during the secondary growth stage was observed. During ovulation, the follicle collapsed, the oocyte was released into the lumen, and the constitutive elements of the post-ovulatory follicle complex remained in the stroma.     

Oogenesis: From Oogonia to Ovulation in the Flagfish,Jordanella floridae Goode and Bean, 1879 (Teleostei: Cyprinodontidae)

We provide histological details of the development of oocytes in the cyprinodontid flagfish, Jordanella floridae. There are six stages of oogenesis: Oogonial proliferation, chromatin nucleolus, primary growth (previtellogenesis [PG]), secondary growth (vitellogenesis), oocyte maturation and ovulation. The ovarian lamellae are lined by a germinal epithelium composed of epithelial cells and scattered oogonia. During primary growth, the development of cortical alveoli and oil droplets, are initiated simultaneously. During secondary growth, yolk globules coalesce into a fluid mass. The full‐grown oocyte contains a large globule of fluid yolk. The germinal vesicle is at the animal pole, and the cortical alveoli and oil droplets are located at the periphery. The disposition of oil droplets at the vegetal pole of the germinal vesicle during late secondary growth stage is a unique characteristic. The follicular cell layer is composed initially of a single layer of squamous cells during early PG which become columnar during early vitellogenesis. During primary and secondary growth stages, filaments develop among the follicular cells and also around the micropyle. The filaments are seen extending from the zona pellucida after ovulation. During ovulation, a space is evident between the oocyte and the zona pellucida. Asynchronous spawning activity is confirmed by the observation that, after ovulation, the ovarian lamellae contain follicles in both primary and secondary growth stages; in contrast, when the seasonal activity of oogenesis and spawning ends, after ovulation, the ovarian lamellae contain only follicles in the primary growth stage. J. Morphol. 277:1339–1354, 2016. © 2016 Wiley Periodicals, Inc.     

Proliferation of Oogonia and folliculogenesis in the viviparous teleost Ilyodon whitei (Goodeidae)

Oogonial proliferation in fishes is an essential reproductive strategy to generate new ovarian follicles and is the basis for unlimited oogenesis. The reproductive cycle in viviparous teleosts, besides oogenesis, involves development of embryos inside the ovary, that is, intraovarian gestation. Oogonia are located in the germinal epithelium of the ovary. The germinal epithelium is the surface of ovarian lamellae and, therefore, borders the ovarian lumen. However, activity and seasonality of the germinal epithelium have not been described in any viviparous teleost species regarding oogonial proliferation and folliculogenesis. The goal of this study is to identify the histological features of oogonial proliferation and folliculogenesis during the reproductive cycle of the viviparous goodeid Ilyodon whitei. Ovaries during nongestation and early and late gestation were analyzed. Oogonial proliferation and folliculogenesis in I. whitei, where intraovarian gestation follows the maturation and fertilization of oocytes, do not correspond to the late oogenesis, as was observed in oviparous species, but correspond to late gestation. This observation offers an example of ovarian physiology correlated with viviparous reproduction and provides elements for understanding the regulation of the initiation of processes that ultimately result in the origin of the next generation. These processes include oogonia proliferation and development of the next batch of germ cells into the complex process of intraovarian gestation. J. Morphol. 275:1004–1015, 2014. © 2014 Wiley Periodicals, Inc.     

Ovarian structure of the marsupial frog Gastrotheca riobambae (Fowler)

The mature ovary of Gastrotheca riobambae presents large oocytes (3 mm in diameter) of pale yellow color. After ovulation and the beginning of embryonic incubation, the empty postovulatory follicles can be recognized in the ovary for about 30 days. The granulosa of these follicles never fills the follicular lumen and this cavity becomes filled with fluid during the first five days of incubation. Later, at 18 days of incubation, the lumen is mostly empty and contains few cells of the granulosa. Shrinkage results in the disappearance of the follicular cavity by approximately the thirtieth day of incubation. The thecae are thick and become separated by a space. This space is filled progressively with cells, blood capillaries and fluid. After the thirtieth to fortieth day of incubation, these follicles become regressive and disappear. The postovulatory follicles of Gastrotheca may correspond to corpora lutea. The evidence suggests that pouch vascularization and the formation of embryonic chambers of pouch tissue may be under ovarian control. In addition, the process of vitellogenesis is influenced by incubation, as most growth of the ovarian oocyte occurs after birth of the tadpoles. Follicular atresia is common and is similar to that of other frogs.     

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.     

Ovarian structure and oogenesis of the oviparous goodeids Crenichthys baileyi (Gilbert, 1893) and Empetrichthys latos Miller, 1948 (teleostei,Cyprinodontiformes)

The cyprinodontiform family Goodeidae comprises two biogeographically disjunct subfamilies: the viviparous Goodeinae endemic to the Mexican Plateau, and the oviparous Empetrichthyinae, known only from relict taxa in Nevada and California. Ovarian characteristics of two oviparous species of goodeid, Crenichthys baileyi and Empetrichthys latos, studied using museum collections, are compared with those of viviparous species of goodeids. Both subfamilies have a single, cystovarian ovary. The ovary in the viviparous Goodeinae has an internal septum that divides the ovarian lumen into two compartments, and it may possess oogonia. There is no ovarian septum in the oviparous C. baileyi and E. latos. Oogenesis is similar in both subfamilies with regard to the proliferation of oogonia, initiation of meiosis, primary growth and development of an oocyte during secondary growth in which fluid yolk progressively fuses into a single globule. Notably, eggs of C. baileyi and E. latos are approximately double the size of those of the viviparous Goodeinae in which embryos develop inside the ovarian lumen and are nourished, in part, by nutrients transferred from the maternal tissues, a mode of embryo development called matrotrophy. Egg envelopes of the two subfamilies differ in that those of C. baileyi and E. latos have a relatively thick zona pellucida, attachment fibrils or filaments that develop between the follicle cells during oogenesis, and a micropyle observed only in E. latos. In contrast, viviparous goodeid eggs have a relatively thin zona pellucida, but lack adhesive fibrils, and a micropyle was not observed. These reproductive characters are compared with those of species of the eastern North American Fundulus, a representative oviparous cyprinodontiform. One newlyrecognized shared, derived character, a single, median ovoid ovary with no obvious external evidence of fusion, supports monophyly of the Goodeidae. Differences among the goodeid subfamilies and Fundulus are interpreted relative to the oviparous versus viviparous modes of reproduction. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Ovary of the pipefish,Syngnathus scovelli

Gross dissection, light microscopy, and transmission electron microscopy were used to generate a detailed understanding of the ovarian anatomy of the pipefish, Syngnathus scovelli. The ovary is a cylindrical tube bounded by an outer layer consisting of a smooth muscle wall and an inner layer of luminal epithelium, with follicles sandwiched between the two layers. A remarkable feature of this ovary is a sequential pattern of follicle development. This pattern begins at the germinal ridge with a gradient of follicles of increasing developmental age extending to the mature edge. The germinal ridge is an outpocketed region of the luminal epithelium containing early germinal cells and somatic prefollicular cells. Therefore, the germinal ridge and luminal epithelium share the same ovarian compartment and follicle formation occurs within this compartment. The mature edge is defined as the site of oocyte maturation and ovulation. The outer ovarian wall contains unmyelinated nerve fibers throughout. Longitudinally oriented unmyelinated nerves are also observed near the smooth muscle bundles associated with the mature edge. Oocytes near the mature edge are polarized such that the germinal vesicle (nucleus) is generally oriented toward the luminal epithelium. The sandwichlike organization of the ovary results in follicles that have a shared theca. An extensive lymphatic network is also interspersed among the follicles. Thus, the exceptional features of the pipefish ovary make it particularly well suited for the examination of early events in oogenesis. Specifically, we characterize pipefish folliculogenesis in detail.     

Ovary structure and early oogenesis in the remipede, Godzilliognomus frondosus (Crustacea, Remipedia): phylogenetic implications

Remipedia are enigmatic crustaceans of uncertain phylogenetic position with the general consensus that they are crucial for understanding the crustacean/arthropod evolution. It has been demonstrated previously that the features of the ovary organization and subcellular aspects of oogenesis are useful in resolving phylogenetic relationships in arthropods such as hexapods and onychophorans. The structure of the female gonads in Remipedia remains largely unknown; therefore, we examined the gross morphology and ultrastructural details of the ovary in a remipede, Godzilliognomus frondosus, with special emphasis on characters relevant to phylogenetic reconstructions. The ovaries of G. frondosus are located in the anterior part of the body and are composed of a single anterior proliferative zone (the germarium) and paired ovarian tubes (the vitellarium). The oocytes undergo subsequent stages of development within the lumen of the ovarian tubes, hence the remipede ovaries can be classified as endogenous. During oogenesis, each oocyte is enveloped by a set of characteristic somatic follicular cells, which results in the formation of distinct ovarian follicles. Here, we demonstrate that Remipedia share significant similarities in the ovary organization with Cephalocarida, including the anterior location of the ovary, the anterior-most position of the germarium and the endogenous type of oocyte development. Phylogenetic implications of our findings are discussed.     

Progressive redistribution of alcohol dehydrogenase during vitellogenesis in Drosophila melanogaster: characterization of ADH-positive bodies in mature oocytes

Summary The use of monoclonal antibodies against Drosophila alcohol dehydrogenase (ADH) provides a powerful tool in the analysis of the tissue and temporal patterns of Adh gene expression. Immunocytochemical techniques at the light- and electron-microscopic levels have been used to determine the distribution of ADH in the ovarian follicles of D. melanogaster during oogenesis. In the early stages of oogenesis, small amounts of ADH are detectable in the cystocytes. At the beginning of vitellogenesis (S₇), ADH appears to be located mainly in the nurse cells. From stage S₉ onwards, the ADH protein is evenly distributed in the ooplasm until the later stages of oogenesis (S_13–14), when multiple ADH-positive bodies of varying size appear in the ooplasm. This change in distribution is a result of the compartmentalization of the ADH protein within the glycogen yolk or -spheres. Yolk becomes enclosed within the lumen of the primitive gut during embryonic development, and thus our results suggest a mechanism for the transfer of maternally-inherited enzymes to the gut lumen via yolk spheres.     

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.     

Long noncoding RNA H19 in ovarian biology and placenta development

As the first long noncoding RNA to be discovered, H19 has gained substantial attention as a key regulator of several biological processes and its roles in female reproductive biology are gradually getting revealed. Herein, we have summarized the current evidence regarding H19 expression pattern and involvement in the developmental and pathological processes associated with the ovary and the placenta. The findings indicate that within the ovaries, H19 is expressed in the antral and cystic atretic follicles as well as in the corpora lutea but absent in the primordial, primary, and secondary follicles. Its normal expression promotes the maturation of antral follicles and prevents their premature selection for the ovulatory journey while its aberrant induction promotes polycystic ovary syndrome development and ovarian cancer metastasis. In the placenta, H19 is highly expressed in the cytotrophoblasts and extravillous trophoblasts but weakly expressed in the syncytiotrophoblast layer and potentially controls trophoblast cell fate decisions during placenta development. Abnormal expression of H19 is observed in the placental villi of pregnancies affected by pre-eclampsia and fetal growth restriction. Therefore, dysregulated H19 is a candidate biomarker and therapeutic target for the mitigation of ovarian and placenta-associated diseases.     

Morphological patterns of cell death in ovarian follicles of primary and secondary growth and postovulatory follicle complex of the annual killifish Millerichthys robustus (Cyprinodontiformes: Cynolebiidae)

The dynamics of cellular development and homeostasis of the ovary depend on the balance between proliferation and cell death throughout the reproductive cycle. Millerichthys robustus is an annual fish whose ovarian follicles develop asynchronously, allowing daily reproduction from sexual maturity until death. The objective of this research is to describe, histologically, the processes of follicular atresia and regression of postovulatory follicular complexes (POC) throughout a reproductive cycle of M. robustus. Patterns of cell death were documented by apoptosis in atretic follicles and POC, and necrosis in the POC after ovulation with an associated inflammatory response. Atretic follicles were seen from the onset of sexual maturity, during week three post-hatching (PH), both in primary growth (from the Cortical alveoli step, with folliculogenesis completed) and secondary growth Stages, with a higher prevalence in the latter. POCs were observed in different stages of regression from week four PH until the death of the fish. The apoptotic characteristics found were: (i) fragmentation of the nuclear membrane and zona pellucida, and liquefaction of the cortical alveoli and yolk; (ii) follicular cells becoming phagocytic, increasing their size, and migrating within the oocyte; and (iii) formation of an intrafollicular lumen, a product of phagocytosis of the oocyte constituents and dispersed pigments that remain after the digestion of yolk and cortical alveoli. The morphological changes of the follicular cells of the POC, from a squamous morphology after ovulation to columnar during its regression with PAS+ contents, was documented, suggesting a secretory activity.     

Ultrastructural analysis of the ovary and oogenesis in Spinicaudata and Laevicaudata (Branchiopoda) and its phylogenetic implications

Recent molecular studies have indicated a close relationship between Crustacea and Hexapoda and postulated their unification into the Pancrustacea/Tetraconata clade. Certain molecular analyses have also suggested that the crustacean lineage, which includes the Branchiopoda, might be the sister group of Hexapoda. We test this hypothesis by analyzing the structure of the ovary and the ultrastructural features of oogenesis in two branchiopod species, Cyzicus tetracerus and Lynceus brachyurus, representing two separate orders, Spinicaudata and Laevicaudata, respectively. The female gonads of these species have not been investigated before. Here, we demonstrate that in both studied species the ovarian follicles develop inside characteristic ovarian protrusions and comprise a germline cyst surrounded by a simple somatic (follicular) epithelium, supported by a thin basal lamina. Each germline cyst consists of one oocyte and three supporting nurse cells, and the oocyte differentiates relatively late during ovarian follicle development. The synthesis of oocyte reserve materials involves rough endoplasmic reticulum and Golgi complexes. The follicular cells are penetrated by a complex canal system and there is no external epithelial sheath covering the ovarian follicles. The structure of the ovary and the ultrastructural characteristics of oogenesis are not only remarkably similar in both Cyzicus and Lynceus, but also share morphological similarities with Notostraca as well as the basal hexapods Campodeina and Collembola. Possible phylogenetic implications of these findings are discussed.     

Effects of methoxychlor on IGF-I signaling pathway in rat ovary

Follicular development and other ovarian functions are regulated by growth factors that can be affected by exogenous agents. Methoxychlor (MXC) is an organochloride pesticide that causes female infertility. We investigated how MXC affects the distribution of developing ovarian follicles in adult rats after treatment between embryonic day (E) 18 and postnatal day (PND) 7. We also measured insulin-like growth factor-I (IGF-I) and its receptor, IGF-IR, expressions in ovarian follicles and investigated whether MXC changed the levels of IGF-I and IGF-IR in the ovary. Using immunohistochemical (IHC) staining, we detected IGF-I expression in oocytes and granulosa cells of the follicles, luteal cells, interstitial cells, theca externa and theca interna, and the smooth muscle of ovarian vessels. IGF-IR was co-localized with IGF-I in the ovary except for the theca externa. IGF-I expression was decreased in granulosa cells of preantral and antral follicles after treatment with MXC compared to granulosa cells of preantral and antral follicles of the control group. We also observed that oocytes of secondary follicles and granulosa cells of secondary and preantral follicles of the MXC treated groups showed increased IGF-IR expression compared to oocytes of secondary follicles and granulosa cells of secondary and preantral follicles of the control group. We also detected more secondary and preantral follicles, and fewer primordial and antral follicles after MXC administration compared to controls. Therefore, the IGF signaling pathway may participate in MXC induced ovary dysfunction and female infertility.     

Unlike in Drosophila Meroistic Ovaries,Hippo Represses Notch in Blattella germanica Panoistic Ovaries,Triggering the Mitosis-Endocycle Switch in the Follicular Cells

During insect oogenesis, the follicular epithelium undergoes both cell proliferation and apoptosis, thus modulating ovarian follicle growth. The Hippo pathway is key in these processes, and has been thoroughly studied in the meroistic ovaries of Drosophila melanogaster. However, nothing is known about the role of the Hippo pathway in primitive panoistic ovaries. This work examines the mRNA expression levels of the main components of the Hippo pathway in the panoistic ovary of the basal insect species Blattella germanica, and demonstrates the function of Hippo through RNAi. In Hippo-depleted specimens, the follicular cells of the basal ovarian follicles proliferate without arresting cytokinesis; the epithelium therefore becomes bilayered, impairing ovarian follicle growth. This phenotype is accompanied by long stalks between the ovarian follicles. In D. melanogaster loss of function of Notch determines that the stalk is not developed. With this in mind, we tested whether Hippo and Notch pathways are related in B. germanica. In Notch (only)-depleted females, no stalks were formed between the ovarian follicles. Simultaneous depletion of Hippo and Notch rescued partially the stalk to wild-type. Unlike in the meroistic ovaries of D. melanogaster, in panoistic ovaries the Hippo pathway appears to regulate follicular cell proliferation by acting as a repressor of Notch, triggering the switch from mitosis to the endocycle in the follicular cells. The phylogenetically basal position of B. germanica suggests that this might be the ancestral function of Hippo in insect ovaries.