Destruction of the germinal disc region of an immature preovulatory follicle suppresses follicular maturation and ovulation

Androgen and mineralocorticoid receptors are present on the germinal disc region in laying hens: Potential mediators of sex ratio adjustment in birds?

Female birds skew offspring sex ratios based on environmental and social stimuli; however, the mechanism mediating this phenomenon remains unknown. Growing evidence suggests that testosterone and corticosterone may influence meiosis, as they skew sex ratios when given immediately before chromosomal segregation. It is unclear if these hormones act on the germinal disc (GD) or through a downstream mediator. It is also unknown whether the GD contains receptors for these hormones. If testosterone and/or corticosterone act on the GD to skew sex ratios, then the GD should have receptors for them and that receptor levels should be higher in the GD regions compared to other follicular regions. Furthermore, fluctuations of receptor levels should occur near meiotic segregation. We collected ovarian follicles at 5 h pre-ovulation (just before meiotic segregation) and 20 h pre-ovulation (when sex chromosomes are arrested), and measured androgen receptor (AR) and mineralocorticoid receptor (MR) protein levels via Western blot. ARs and MRs were on the follicle in the GD and non-GD regions, and at 5 h and 20 h pre-ovulation. Both AR and MR protein levels were higher in the GD region than the non-GD region at both time points, but did not differ between time points. These results suggest that hen ovarian follicles have receptors for testosterone and corticosterone, and that the ability for testosterone to respond may be specifically higher in the GD-region, providing further support for the role of testosterone in the alteration of meiotic segregation.

Control of Oocyte Growth and Development by Intercellular Communication Within the Follicular Niche

In the mammalian ovary, each oocyte grows and develops within its own structural and developmental niche-the follicle. Together with the female germ cell in the follicle are somatic granulosa cells, specialized companion cells that surround the oocyte and provide support to it, and an outer layer of thecal cells that serve crucial roles including steroid synthesis. These follicular compartments function as a single physiological unit whose purpose is to produce a healthy egg, which upon ovulation can be fertilized and give rise to a healthy embryo, thus enabling the female germ cell to fulfill its reproductive potential. Beginning from the initial stage of follicle formation and until terminal differentiation at ovulation, oocyte and follicle growth depend absolutely on cooperation between the different cellular compartments. This cooperation synchronizes the initiation of oocyte growth with follicle activation. During growth, it enables metabolic support for the follicle-enclosed oocyte and allows the follicle to fulfill its steroidogenic potential. Near the end of the growth period, intra-follicular interactions prevent the precocious meiotic resumption of the oocyte and ensure its nuclear differentiation. Finally, cooperation enables the events of ovulation, including meiotic maturation of the oocyte and expansion of the cumulus granulosa cells. In this chapter, we discuss the cellular interactions that enable the growing follicle to produce a healthy oocyte, focusing on the communication between the germ cell and the surrounding granulosa cells.

The role of steroid hormones in the adjustment of primary sex ratio in birds: compiling the pieces of the puzzle

There is ample evidence that birds have the ability to adjust their offsprings' sex ratios before fertilization occurs. Recent work has focused on pinpointing when during the process of oocyte maturation adjustment of sex ratio takes place. Additionally, there is growing support for the idea that there is hormonal control over the process of adjustment of sex ratio in birds. Whether steroid hormones represent direct mediators of the process, however, remains unclear. This review outlines the precise points during maturation of ovarian follicles during which adjustment of primary sex ratios could potentially occur, compiles the evidence for hormonal involvement in the process of primary adjustment of sex ratio, and discusses potential hormonal targets during maturation and fertilization of oocytes where hormones may trigger adjustment of sex ratio in birds.

Review. Meiotic drive and sex determination: molecular and cytological mechanisms of sex ratio adjustment in birds

Differences in relative fitness of male and female offspring across ecological and social environments should favour the evolution of sex-determining mechanisms that enable adjustment of brood sex ratio to the context of breeding. Despite the expectation that genetic sex determination should not produce consistent bias in primary sex ratios, extensive and adaptive modifications of offspring sex ratio in relation to social and physiological conditions during reproduction are often documented. Such discordance emphasizes the need for empirical investigation of the proximate mechanisms for modifying primary sex ratios, and suggests epigenetic effects on sex-determining mechanisms as the most likely candidates. Birds, in particular, are thought to have an unusually direct opportunity to modify offspring sex ratio because avian females are heterogametic and because the sex-determining division in avian meiosis occurs prior to ovulation and fertilization. However, despite evidence of strong epigenetic effects on sex determination in pre-ovulatory avian oocytes, the mechanisms behind such effects remain elusive. Our review of molecular and cytological mechanisms of avian meiosis uncovers a multitude of potential targets for selection on biased segregation of sex chromosomes, which may reflect the diversity of mechanisms and levels on which such selection operates in birds. Our findings indicate that pronounced differences between sex chromosomes in size, shape, size of protein bodies, alignment at the meiotic plate, microtubule attachment and epigenetic markings should commonly produce biased segregation of sex chromosomes as the default state, with secondary evolution of compensatory mechanisms necessary to maintain unbiased meiosis. We suggest that it is the epigenetic effects that modify such compensatory mechanisms that enable context-dependent and precise adjustment of primary sex ratio in birds. Furthermore, we highlight the features of avian meiosis that can be influenced by maternal hormones in response to environmental stimuli and may account for the precise and adaptive patterns of offspring sex ratio adjustment observed in some species.

Role of nitric oxide in ovarian follicular development and egg production in Japanese quail (Coturnix coturnix japonica)

Role of nitric oxide (NO) in regulating the reproductive functions at hypothalamo-hypophysealovarian axis in Japanese quail was studied. In first experiment, metabolites of NO, i.e. nitrite and nitrate (NO2 and NO3) were estimated together in hypothalamus, serum and ovarian follicles of good and poor layers. In the second experiment, different NO modulators such as L-arginine (L-Arg), sodium nitroprusside (SNP) and N(G)-nitro-L-arginine methyl ester, HCl (L-NAME) were administered to the birds. In the first experiment, significantly higher (P < 0.01) NO2 and NO3 levels in serum, hypothalamus and largest (F1) ovarian follicles were observed in good layers as compared to poor layers. Higher (P < 0.05) NO2 and NO3 concentration was observed in F1 follicles than smaller follicles (F2) only in good layers. The NO2 and NO3 concentration was significantly reduced (P < 0.05) in post ovulatory follicles (POFs) in comparison to F1 and F2 follicles. In the second experiment, the serum NO2 and NO3 concentrations were higher (P < 0.05) in the SNP, lower (P < 0.05) in the L-Name group and unchanged in the L-Arg treated group in comparison to control group. compared to control, L-Arg and SNP increased (P < 0.05) the hypothalamic NO2 and NO3 concentration where as L-NAME reduced (P < 0.05) these levels. The NO2 and NO3 concentration was increased (P < 0.05) as the follicle size increased and it was significantly reduced (P < 0.05) in POFs. The higher (P < 0.05) follicular NO2 and NO3 concentration was observed in L-Arg group in comparison to control group. Egg production was also found to be higher (P < 0.05) in L-Arg group whereas it was not different (P > 0.05) in SNP and L-NAME treated groups. The yolk weight and yolk to albumin ratio was reduced (P < 0.05) in L-NAME group in comparison to control group. It may be concluded from the present study that NO plays a key role in regulating follicular development, ovulatory mechanisms and egg production in Japanese quail.

Changes in the populations of mitotic and apoptotic cells in white follicles during atresia in hens

The aim of this study was to determine whether the population of mitotic cells changes in correlation with apoptotic cell population in follicular tissues during atresia of white follicles in hens. Hens were injected with 5-bromo-2'-deoxyuridine (BrdU) 1 h before tissue collection. The small white follicles were classified as healthy follicles and as early or late atretic follicles by histological observation. Mitotic and apoptotic cells were determined by immunocytochemistry for BrdU and terminal deoxytranceferase-mediated dUTP nick end labeling (TUNEL), respectively. The BrdU labeling was observed in some of the granulosa cells and thecal fibroblast-like cells in healthy follicles, whereas the population of the labeled cells was reduced in the granulosa and theca layers of atretic follicles. The image analysis confirmed that the frequency of BrdU-positive cells declined significantly in the granulosa and theca layers of early atretic follicles compared with those of healthy follicles. In contrast, the TUNEL-positive cells were negligible in healthy follicles. However, they were localized in the granulosa and theca layers of early and late atretic follicles, and those in the theca layer were more inside than outside. The frequency of TUNEL-positive cells was significantly increased with the progress of atretic changes. These results suggest that the population of mitotic cells decreases in association with increase of apoptotic cells during the atretic process of white follicles.

Germinal disc-derived epidermal growth factor: a paracrine factor to stimulate proliferation of granulosa cells

The germinal disc (GD) of the chicken oocyte produces factors that influence proliferation and differentiation of granulosa cells. Granulosa cells proximal to the GD are more proliferative, whereas granulosa cells distal to the GD are more differentiated. Previously, we had found epidermal growth factor (EGF) was present in the GD. In this study, we tested the hypothesis that EGF is the GD-derived paracrine factor that stimulates proliferation of granulosa cells. Northern analysis, reverse transcription-polymerase chain reaction, and radioimmunoassay indicated that the GD and granulosa cells but not theca cells are the sources of EGF in chicken preovulatory follicles. However, only the conditioned medium from the GD region (GDR = GD + overlying granulosa cells) but not the granulosa cell-conditioned medium stimulated proliferation of granulosa cells. Pretreatment of conditioned media with EGF antibody abolished the proliferation-stimulating effect of the GDR-conditioned medium. We conclude that EGF is one of the paracrine factors produced by the GD to stimulate proliferation of granulosa cells. Granulosa cells proximal to the GD express a proliferative phenotype possibly because they are exposed to a greater amount of EGF derived from the GD.

Sphingolipid regulation of female gonadal cell apoptosis

Ceramide and sphingosine-1-phosphate (S1P) are sphingosine-based lipid signaling molecules that have been implicated as key mediators of cellular growth, differentiation, and apoptosis. The cellular response depends on cell type, on the absence or presence of other signals initiated by the same or another stimulus, and on the subcellular location of sphingomyelin hydrolysis leading to ceramide generation. Consistent with mounting evidence implicating components of the sphingomyelin pathway as mediators of cellular life and death in nonreproductive tissues, recent data have indicated that sphingolipid-based signaling events are also prominent features of cellular development and apoptosis in the fetal and postnatal female gonads. This area of investigation represents a new research avenue of considerable significance for both basic biology and clinical medicine because of the massive levels of developmental death that occur normally in the female germ line, especially during gametogenesis, as well as of the central role of oocyte apoptosis in female gonadal failure resulting from pathologic insults.

Localization of nitric oxide-related substances in the quail ovary during folliculogenesis

In the present study, nitric oxide (NO)-related substances, namely NO synthase (NOS), L-citrulline, cGMP and nitrotyrosine, have been localized in the quail ovary, using NADPH-diaphorase staining and immunohistochemical methods. The results indicate the presence of the NOS isoforms, showing distinct cell-specific distribution patterns in the quail ovary. Inducible NOS is primarily present in leukocytes, endothelial NOS in granulosa cells, and neuronal NOS in nerve cells, oocytes, interstitial cells and granulosa cells of pre-hierarchal follicles and of the germinal disc region of pre-ovulatory follicles. NOS activity, indicated by the presence of L-citrulline, is observed in oocytes, nerve cells, interstitial cells and a few granulosa cells of pre-hierarchal follicles. Detection of accumulated cGMP indicates that granulosa cells of pre-hierarchal and of pre- and post-ovulatory follicles, the theca interna of pre-ovulatory follicles, and oocytes are main targets of NO. Nitrotyrosine, a marker of peroxynitrite activity, is mainly localized in atretic follicles and in post-ovulatory follicles. It is concluded that the quail ovary possesses a NO/NOS system, and that NO may be considered as a mediator involved in various ovarian processes, including atresia.

Mouse oocytes regulate granulosa cell steroidogenesis throughout follicular development

Mouse oocytes secrete a factor(s) that inhibits progesterone and enhances estradiol production by granulosa cells. This study determined the ability of mouse oocytes to secrete this steroid-regulating factor during oocyte growth and the ability of granulosa cells to respond to the factor during follicular development. Oocyte-granulosa cell complexes from preantral and antral follicles were oocytectomized (OOX; oocytes microsurgically removed) and cultured for up to 48 h with FSH (150 ng/ml) and testosterone (500 nM). At all stages of development examined, OOX complexes produced more progesterone than did intact complexes, from 1.45-fold for early growing follicles to 23-fold for complexes from antral follicles. Significant estradiol production was restricted to intact complexes from late antral follicles. Progesterone accumulation by OOX complexes cocultured with oocytes was inhibited by all stages of oocytes examined, with maximal inhibition by fully grown oocytes. Ovulated complexes produced large quantities of progesterone, even though oocytes secreted progesterone-inhibitory factor, because of a desensitization of cumulus cells to the factor during their terminal differentiation. Even in the presence of abundant pregnenolone, OOX complexes showed reduced ability to produce and/or accumulate progesterone in the presence of oocytes, suggesting that the oocyte-secreted factor, either directly or indirectly, regulates the activity of 3beta-hydroxysteroid dehydrogenase and/or progesterone metabolism. These results demonstrate that oocytes secrete a factor with steroid-regulating activity in increasing amounts and/or potency during follicular development, but responsiveness of cumulus cells to this factor declines during luteinization.

Destruction of the germinal disc region of an immature preovulatory chicken follicle induces atresia and apoptosis

The germinal disc region (GDR), which contains the germinal disc and overlying granulosa cells, is essential for completion of maturation of the preovulatory chicken follicle. The current study was conducted to test the hypothesis that destruction of the GDR (GDRX) of an immature preovulatory chicken follicle blocks ovulation, induces apoptosis, and causes atresia. The GDR of immature preovulatory follicles (F2) were destroyed by freezing with dry ice (3 mm in diameter) 48-50 h before ovulation. As a control for the effect of freezing, a nonGDR portion (a portion of the follicular wall opposite to the GDR relative to the follicular stalk) of other F2 follicles were destroyed (nonGDRX). Treatment of F2 follicles by GDRX caused atresia and blocked ovulation of all treated follicles (6 of 6), whereas none of the nonGDRX follicles (0 of 5) underwent atresia. Treatment of follicles by GDRX induced apoptotic DNA fragmentation (laddering) in theca and granulosa layers obtained from the frozen area and in the theca layer obtained from the follicular wall distal to the frozen area. In contrast, apoptosis was only present in theca and granulosa layers in the frozen area of the nonGDRX follicle. Furthermore, the in situ DNA end-labeling technique demonstrated that in the GDRX follicle 24 h after treatment, cells in the theca interna, endothelial cells in blood vessels of the theca externa, and a few granulosa cells underwent apoptosis. These results indicate that destruction of the GDR of an immature preovulatory follicle causes atresia and apoptosis and blocks ovulation. These novel findings suggest that the GDR maintains development of the chicken preovulatory follicle by producing one or more survival factors. Without the GDR, chicken follicles cannot develop further and they eventually die.

Epidermal growth factor in the germinal disc and its potential role in follicular development in the chicken

The germinal disc region (GDR; germinal disc + overlying granulosa cells) of the hen's ovarian follicle secretes one or more factors that stimulate proliferation of, and decrease progesterone (P4) production by, granulosa cells. Destruction of the GDR results in apoptosis and atresia of the follicle. These data suggested that the GDR produces a growth factor(s) to sustain the development of the follicle. These findings prompted us to investigate two questions: 1) Is epidermal growth factor (EGF) or transforming growth factor alpha (TGFalpha), which binds to the EGF receptor, present in the GDR? 2) Does EGF regulate granulosa cell functions in the hen? Immunocytochemistry revealed that EGF, but not TGFalpha, was present in the germinal disc of the four largest preovulatory follicles of the hen. TGFalpha was found only in the theca interna. To determine whether EGF regulates granulosa cell functions, granulosa layer explants (13 mm in diameter) from the second-largest preovulatory follicle were cultured for 36 h with 0,0.017, or 0.17 microM EGF. Proliferation, apoptosis, and P4 production of granulosa layer explants were then measured by using a colorimetric method for determining viable cell number, gel electrophoresis, and RIA, respectively. EGF regulates several functions of granulosa layer explants by stimulating proliferation, inhibiting apoptosis, and decreasing basal P4 production. These data indicate that EGF is present in the germinal disc and may be one of the factors that regulate follicular development in the hen.

Maturation and ovulation of Japanese quail oocytes under in vitro conditions

1. An in vitro system for ovulation and maturation of Japanese quail oocytes is described. 2. Ovarian follicles removed from the ovary at 2, 4 or 6 h before the estimated time of ovulation may ovulate under in vitro conditions. 3. The presence of progesterone in the medium had a stimulatory effect on the process of maturation, as has been shown for Xenopus oocytes.

Oocyte-secreted factors regulate granulosa cell steroidogenesis

Investigations of strains of mice defective in germ cell development have revealed the importance of oocytes for the initial stages of folliculogenesis (Pellaset al., 1991; Huanget al., 1993). Various aspects of follicular development are dependent upon and/or influenced by the presence of oocytes, including granulosa cell proliferation (Vanderhydenet al., 1990, 1992) and cumulus expansion (Buccioneet al., 1990; Salustriet al., 1990; Vanderhydenet al., 1990; Vanderhyden, 1993). We are investigating the possibility that oocytes influence one of the primary functions of granulosa cells: steroidogenesis. In many species, granulosa cells removed from preovulatory follicles luteinisein vitro(Channinget al., 1982), presumably due to loss of contact with follicular luteinisation inhibitory factor(s). Indeed, follicular fluid can prevent granulosa cell luteinisationin vitro(Ledwitz-Rigbyet al., 1977). Follicular fluid, however, may simply be the medium for transport of factors secreted by oocytes to regulate granulosa cell activities.