Formation of mammalian oocytes and their growth, differentiation and maturation within ovarian follicles

Does the bone morphogenetic protein 7 inhibit oocyte maturation by autocrine/paracrine in mud crab?

A bone morphogenetic protein ligand (BMP7) and its two receptors (BMPRIB and BMPRII) were recently cloned and characterized in the mud crab, Scylla paramamosain. However specific functions of BMP7 and the mechanistic pathways regulating its function are largely unidentified. In the present study, we separated oocytes and follicle cells from the ovarian explants of S. paramamosain. Subsequent analysis using semi-quantitative PCR demonstrated that the mRNA of Sp-BMP7 was exclusively expressed in follicle cells while Sp-BMPRs were expressed in both oocytes and follicle cells. In vitro experiments further showed that the mRNA and protein levels of Cyclin B increased but Sp-BMP7 declined in 17α, 20β-Dihydroxyprogesterone (DHP)-induced oocytes. Furthermore, the inhibitory effects of Sp-BMP7 were not affected by the elimination of the contact/gap junction-mediated communication between oocytes and follicle cells. Our data indicate that BMP7 may play a role in the suppression of DHP-induced oocyte maturation by affecting autocrine/paracrine pathways in S. paramamosain.

Oral propylene glycol modifies follicular fluid and gene expression profiles in cumulus-oocyte complexes and embryos in feed-restricted heifers

Dietary supplementation with propylene glycol (PG) increases in vitro production of high-quality embryos in feed-restricted heifers. The aim of the present study was to evaluate the effects of PG in feed-restricted heifers on follicular fluid insulin and insulin-like growth factor (IGF) 1 concentrations, expression of IGF system genes in oocytes and cumulus cells and the expression of selected genes in blastocysts. Feed-restricted (R) heifers were drenched with water or PG during induced oestrous cycles (400mL of PG or water/drench, daily drenching at 1600 hours for the first 9 days of the oestrous cycle). Ovum pick-up (OPU) was performed after superovulation to produce in vitro embryos and without superovulation to recover oocytes, cumulus cells and follicular fluid. OPU was also performed in a control group (not feed restricted and no drenching). Follicular fluid IGF1 concentrations were reduced by R, and PG restored IGF1 concentrations to those seen in the control group. In cumulus cells, expression of IGF1, IGF1 receptor (IGF1R) and IGF binding protein 4 (IGFBP4) was decreased in the R group, and fully (IGF1 and IGF1R) or partially (IGFBP4) restored to control levels by PG. Blastocyst perilipin 2 (PLIN2; also known as adipophilin), Bcl-2-associated X protein (BAX), SCL2A1 (facilitated glucose/fructose transporter GLUT1), aquaporin 3 (AQP3), DNA (cytosine-5)-methyltransferase 3A (DNMT3A) and heat shock 70-kDa protein 9 (HSPA9B) expression were decreased in R heifers; PG restored the expression of the last four genes to control levels. In conclusion, these results suggest that, during follicular growth, PG exerts epigenetic regulatory effects on gene expression in blastocyst stage embryos.

Synergy of Paracrine Signaling During Early-Stage Mouse Ovarian Follicle Development In Vitro

Introduction

Paracrine signals, such as soluble cytokines and extracellular matrix cues, are essential for the survival and development of multicellular ovarian follicles. While it is well established that hydrogel-based culture systems successfully support the growth of late-stage follicles for fertility preservation, growing small, early-stage ovarian follicles still proves to be challenging. We hypothesized that paracrine factors secreted from neighboring follicles may be crucial for improving the survival of early-stage follicles in vitro.

Methods

To test our hypothesis, we investigated the bi-directional crosstalk of the paracrine signals, such as cell-secreted cytokines, sex hormones and transcription factors (TFs), in follicles encapsulated and cultured for 12 days in alginate in groups of five (5×) and ten (10×).

Results

The differential profiles of TF activity and secretome during folliculogenesis were analyzed using TRanscriptional Activity CEllular aRray (TRACER) and data-driven multivariate modeling approach. The mechano- and oxygen-responsive TFs, NF-κB and HIF1, exhibited a unique upregulation signature in 10× follicles. Consistently, levels of proangiogenic factors, such as VEGF-A and angiopoietin-2, were significantly higher in 10× follicles than those in 5× follicles, reaching 269.77 and 242.82 pg/mL on the last day of culture. The analysis of TRACER and secreted cytokines also revealed critical early interactions between cytokines and TFs, correlating with the observed phenotypical and functional differences between conditions.

Conclusions

We identified unique signatures of synergism during successful early-stage ovarian follicle development. These findings bring us closer to understanding of mechanisms underlying the downstream effects of interactions between the extracellular microenvironment and early-stage folliculogenesis in vitro.

In vitro culture of isolated preantral and antral follicles of goats using human recombinant FSH: Concentration-dependent and stage-specific effect

The present study aimed to investigate a concentration-response curve of human recombinant FSH (hrFSH) for in vitro culture of isolated preantral and early antral follicles of goats. Isolated follicles were cultured for 18 days using the following treatments: basic culture medium (control); or control medium supplemented with 10, 50, and 100 mIU/mL of hrFSH. At the end of the culture, cumulus-oocyte complexes were recovered and subjected to in vitro maturation. The following endpoints were evaluated: follicle morphology, growth rate and antrum formation, oocyte viability and meiotic stage, and estradiol production, as well as relative expression of FSH receptor (FSHR), and steroidogenic enzyme (3β-HSD, CYP17, and CYP19A1) genes. In antral follicles, the FSH addition at 50 mIU/mL increased follicular diameter and growth rate, percentage of fully developed oocytes, and oocyte diameter (P <  0.05), and tended to increase the percentage of MII oocytes when compared to the control (P =  0.07). With preantral follicles, FSH addition at 100 mIU/mL increased relative abundance of mRNA for CYP19A1 when compared to the control (P <  0.05). At the same FSH concentrations of 100 and 50 mIU/mL, there was a greater relatively abundance of mRNA for 3β-HSD and CYP17 in preantral than in antral follicles (P <  0.05). For preantral and antral follicle comparisons when the same treatments were imposed, there were greater concentrations of estradiol for antral follicles (P <  0.05). In conclusion, hrFSH enhanced in a concentration-dependent manner the in vitro development of caprine antral follicles; however, there was no positive effect in the culture of preantral follicles.

Spatial distribution of preantral follicles in the equine ovary

Comprehensive studies on spatial distribution of preantral follicles in the ovary are scarce. Considering that preantral follicles represent the main ovarian reserve, harvesting of these follicles is crucial for the development/use of assisted reproductive techniques. Therefore, knowledge on follicle spatial distribution can be helpful for targeting areas with richer number of preantral follicles through biopsy procedures. The aim of this study was to assess the distribution and localization of equine preantral follicles according to: (i) age, (ii) ovarian portion (lateral and intermediary) and region (dorsal and ventral), (iii) distance from the geometric center, and (iv) follicular class. Ovaries from young and old mares (n = 8) were harvested in a slaughterhouse and submitted to histological processing for further evaluation. For data analyses, a novel methodology was developed according to the geometric center of each histological section for a precise determination of preantral follicle distribution. Results indicated that (i) equine preantral follicles are clustered and located near to the ovarian geometric center, and that aging induced their dispersion through the ovarian cortex; (ii) the distance from the geometric center was shorter for developing follicles than primordial; and (iii) secondary follicles were more distant from the geometric center but closer to the ovulation fossa. In conclusion, the spatial distribution of preantral follicles was successfully determined in the equine ovary and was affected by age, region, and portion.

SIRT1 induces resistance to apoptosis in human granulosa cells by activating the ERK pathway and inhibiting NF-κB signaling with anti-inflammatory functions

SIRT1, a member of the sirtuin family, has recently emerged as a vital molecule in controlling ovarian function. The aims of the present study were to investigate SIRT1 expression and analyze SIRT1-mediated apoptosis in human granulosa cells (GCs). Human ovarian tissues were subjected to immunohistochemistry for localization of SIRT1 expression. SIRT1 knockdown in a human ovarian GC tumor line (COV434) was achieved by small interfering RNA, and the relationship between apoptosis and SIRT1 was assessed by quantitative reverse transcription polymerase chain reaction and western blotting. We further detected SIRT1 expression in human luteinized GCs. Associations among SIRT1 knockdown, SIRT1 stimulation (resveratrol) and expression of ERK1/2 and apoptotic regulatory proteins were analyzed in cell lines and luteinized GCs. Resveratrol downregulated the levels of nuclear factor (NF)-κB/p65, but this inhibitory effect was attenuated by suppressing SIRT1 activity. The NF-κB/p65 inhibitor pyrrolidine dithiocarbamate achieved similar anti-apoptosis effects. These results suggest that SIRT1 might play an anti-apoptotic role in apoptosis processes in GCs, possibly by sensing and regulating the ERK1/2 pathway, which has important clinical implications. Thus, our study provides a mechanistic link, whereby activation of SIRT1 function might help to sustain human reproduction by maintaining GCs as well as oocytes, offering a novel approach for developing a new class of therapeutic anti-inflammatory agents.

Research progress on the role of melatonin and its receptors in animal reproduction: A comprehensive review

Melatonin and its receptors play a crucial role in the regulation of the animal reproductive process, primarily in follicular development. However, the role that melatonin performs in regulating hormones related with reproduction remains unclear. Melatonin and its receptors are present both in female and male animals' organs, such as ovaries, heart, brain and liver. Melatonin regulates ovarian actions and is a key mediator of reproductive actions. Melatonin has numerous effects on animal reproduction, such as protection of gametes and embryos, response to clock genes, immune-neuroendocrine, reconciliation of seasonal variations in immune function, and silence or blockage of genes. The growth ratio of reproductive illnesses in animals has raised a remarkable concern for the government, animal caretakers and farm managers. In order to resolve this challenging issue, it is very necessary to conduct state-of-the-art research on melatonin and its receptors because melatonin has considerable physiognomies. This review article presents a current contemporary research conducted by numerous researchers from the entire world on the role of melatonin and its receptors in animal reproduction, from the year 1985 to the year 2017. Furthermore, this review shows scientific research challenges related to melatonin receptors and their explanations based on the findings of 172 numerous research articles, and also represents significant proficiencies of melatonin in order to show enthusiastic study direction for animal reproduction researchers.

Effect of co-culture human endothelial progenitor cells with porcine oocytes during maturation and subsequent embryo development of parthenotes in vitro

Human endothelial progenitor cells (EPCs) have been applied to regenerative medicine for their roles in angiogenesis as well as neovascularization, and these angiogenetic functions have beneficial effects on maturation of ovarian follicles. However, little information is available on whether EPCs on culture systems affect oocyte maturation and subsequent embryo development. Therefore, the objective of this study was to investigate the effect of EPC co-culture on porcine oocytes during in vitro maturation (IVM) and subsequent embryo development, and to examine gene expression in cumulus cells, oocytes and blastocysts. The effect of co-culture using EPC on porcine oocyte IVM was investigated. Oocytes were activated using electrical stimulation and embryo developmental competence was estimated. The expression of the genes related to cumulus expansion, oocyte maturation, embryo development, and apoptosis were analyzed. In result, there was a significantly increased maturation rate in EPC group compared with control (p < 0.05). Also, oocytes co-cultured with EPCs exhibited significantly improved blastocyst formation rates (p < 0.05). The expression of mRNAs associated with cumulus expansion and apoptosis in cumulus cells was significantly up-regulated in EPC group. Also, markedly increased levels of GDF9, BMP15, and BCL2 were observed in oocytes from the EPC group. Blastocysts in the co-culture group showed significantly higher SOX2, OCT4, and NANOG levels. In conclusion, co-culturing porcine oocytes with EPCs improves their maturation by regulating genes involved in cumulus cell expansion, oocyte maturation, and apoptosis. Moreover, EPC co-culture during IVM enhanced embryo development as shown by increased blastocyst formation rate and pluripotency-related gene expression.

Genome editing in fishes and their applications

There have been revolutionary progresses in genome engineering in the past few years. The newly-emerged genome editing technologies including zinc-finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN) and clustered regularly interspaced short palindromic repeats associated with Cas9 (CRISPR/Cas9) have enabled biological scientists to perform efficient and precise targeted genome editing in different species. Fish represent the largest group of vertebrates with many species having values for both scientific research and aquaculture industry. Genome editing technologies have found extensive applications in different fish species for basic functional studies as well asapplied research in such fields as disease modeling and aquaculture. This mini-review focuses on recent advancements and applications of the new generation of genome editing technologies in fish species, with particular emphasis on their applications in understanding reproductive functions because the reproductive axis has been most systematically and best studied among others and its function has been difficult to address with reverse genetics approach.

Cell-secreted vesicles containing microRNAs as regulators of gamete maturation

Mammalian gamete maturation requires extensive signaling between germ cells and their surrounding somatic cells. In the ovary, theca cells, mural granulosa cells, cumulus cells and the oocyte all secrete factors throughout follicle growth and maturation that are critical for ovulation of a high-quality oocyte with the competence to develop into an embryo. Similarly, maturation of sperm occurs as it transits the epididymis during which epididymal epithelium and sperm exchange secretory factors that are required for sperm to gain motility and fertility. Recent studies in a variety of species have uncovered the presence of cell-secreted vesicles in follicular fluid (microvesicles and exosomes) and epididymal fluid (epididymosomes). Moreover, these cell-secreted vesicles contain small non-coding regulatory RNAs called microRNAs, which can be shuttled between maturing gametes and surrounding somatic cells. Although little is known about the exact mechanism of how microRNAs are loaded into these cell-secreted vesicles or are transferred and modulate gene expression and function in gametes, recent studies clearly suggest that cell-secreted vesicle microRNAs play a role in oocyte and sperm maturation. Moreover, a role for cell-secreted vesicular microRNAs in gamete maturation provides for novel opportunities to modulate and discover new diagnostic markers associated with male or female fertility. This manuscript provides an overview of cell-secreted vesicles in ovarian follicular fluid and epididymal fluid and microRNAs and discusses recent discoveries on the potential function of cell-secreted vesicles as carriers of microRNAs in oocyte and sperm maturation.

Simulation of Morphogen and Tissue Dynamics

Morphogenesis, the process by which an adult organism emerges from a single cell, has fascinated humans for a long time. Modeling this process can provide novel insights into development and the principles that orchestrate the developmental processes. This chapter focuses on the mathematical description and numerical simulation of developmental processes. In particular, we discuss the mathematical representation of morphogen and tissue dynamics on static and growing domains, as well as the corresponding tissue mechanics. In addition, we give an overview of numerical methods that are routinely used to solve the resulting systems of partial differential equations. These include the finite element method and the Lattice Boltzmann method for the discretization as well as the arbitrary Lagrangian-Eulerian method and the Diffuse-Domain method to numerically treat deforming domains.

Delaying meiotic resumption during transportation of bovine cumulus–oocyte complexes: effects on development, apoptosis and caspases activity of in vitro-produced embryos

This study examined the effects of meiosis inhibition during bovine oocyte transportation on developmental competence and quality of produced embryos. The transportation medium was supplemented with: 100 μM butyrolactone I (BL), 500 μM IBMX + 100 μM forskolin (mSPOM), 100 μM milrinone (MR) or follicular fluid (bFF), and was carried out in a portable incubator for 6 h. Next, oocytes were in vitro matured (IVM) for 18 h, without the meiotic inhibitors, with the exception of mSPOM group, in which was added 20 μM cilostamide. The three control groups were IVM with 10% fetal calf serum (FCS) (Control Lab FCS) or 0.6% bovine serum albumin (BSA) (Control Lab BSA) in a CO2 in air incubator or in the portable incubator with 0.6% BSA (Control Transp BSA). Higher cleavage rates (P < 0.05) were obtained in the Control Lab FCS group (84.5 ± 5.3%) compared with the other groups (59.6 ± 3.4% to 70.9 ± 2.3%). Embryonic development was higher (P < 0.05) in the Control Lab FCS group (39.8 ± 4.7%) than in the Control Transp BSA (22.7 ± 3.4%) and MR (21.6 ± 2.3%) groups. However, they were similar (P > 0.05) to the other groups (23.6 ± 3.3% to 28.8 ± 2.7%). The total number of blastomeres was higher (P < 0.05) in the Control Lab FCS group (85.2 ± 5.6) than in Control Lab BSA (53.6 ± 2.9), Control Transp BSA (55.5 ± 4.4), BL (58.2 ± 3.0), mSPOM (57.9 ± 4.9) and MR (59.2 ± 3.9), but all these treatments did not differ (P > 0.05) from bFF (67.7 ± 4.2). No differences (P > 0.05) were found in apoptosis by the activity of caspases (139.0 ± 3.2 to 152.4 ± 6.5, expressed in fluorescence intensity) as well as the percentage of TUNEL-positive cells (12.3 ± 2.0% to 15.7 ± 1.7%). In conclusion, the transportation of oocytes over 6 h with BL, mSPOM or bFF enabled the acquisition of developmental competence at similar rates to the Control Lab FCS group.

MicroRNAs: tiny molecules with a significant role in mammalian follicular and oocyte development

The genetic regulation of female fertility (follicular development, oocyte maturation and early preimplantation embryo development) involves the spatio-temporal regulation of those genes that play key roles in various stages of the female reproductive axis. MicroRNAs (miRNAs), a class of small non-coding RNAs, are known to regulate the expression of a large proportion of such genes. In recent decades, multiple studies have aimed to determine the roles of these non-coding RNAs in mammalian follicular development, oocyte growth and embryo development. These studies have applied a variety of approaches, including conditional knockout of miRNA biogenesis genes, high-throughput sequencing technologies for pattern recognition in miRNA expression and loss- and gain-of-function of miRNAs in various animal models. In addition to the cellular miRNAs, a large variety of RNAs are found in circulation, being coupled with extracellular vesicles, proteins and lipids. Because of their potential as diagnostic markers for abnormal physiologies, there is increasing interest in the identification of extracellular miRNAs in various biological fluids and spent in vitro culture media. This review focuses on studies addressing the expression and potential role of cellular and extracellular miRNAs in mammalian follicular cell physiology and subsequent ovarian functionality and oocyte maturation.

Stimulation of primordial follicle assembly by estradiol-17β requires the action of bone morphogenetic protein-2 (BMP2)

Primordial follicle (PF) pool determines the availability of follicles for ovulation in all mammals. Premature depletion of the PF reserve leads to subfertility or infertility. Bone morphogenetic protein 2 (BMP2) promotes PF formation by facilitating oocyte and granulosa cell development. Estradiol-17β (E2) upregulates PF formation in developing hamster ovaries. However, if BMP2 mediates E2 effect is not known. We hypothesize that E2 facilitates the effect of BMP2 on somatic to granulosa cell transition. BMP2 and E2 together significantly upregulated the percentage of PFs in hamster fetal ovaries in vitro compared with either of the treatments alone. E2 also promoted BMP2 expression in vivo. Inhibition of BMP2 receptors suppressed E2-stimulation of PF formation while knockdown of BMP2 in vitro significantly suppressed the E2 effect. In contrast, estrogen receptor blocker did not affect BMP2 action. Inhibition of the activity of E2 or BMP2 receptors, either alone or combined during the last two days of the culture (C6-C8) resulted in a significant decrease in PF formation by C8, suggesting that both BMP2 and E2 action is essential for somatic cell differentiation for PF formation. Together, the results suggest that E2 activates BMP2-BMPR system leading to the formation of primordial follicles.

The development and integrity of equine pre-antral follicles cultured in vitro with follicle-stimulating hormone (FSH) supplementation

This study investigated the effects of different concentrations of FSH (10, 50, 100 and 200 ng/ml) in supplemented MEM+ on the development of equine pre-antral follicles that were cultured in vitro for 2 or 6 days. The ovaries (n = 5) from mares in seasonal anoestrus were collected from a local abattoir. Ten ovarian tissue fragments of approximately 3 × 3 × 1 mm were obtained from each animal. The fragments were cultured in situ for 2 days (D2) or 6 days (D6) in MEM+ or MEM+ supplemented with FSH at four different concentrations, establishing the following 11 groups: control (D0); MEM + (D2); MEM + (D6); MEM + 10 ng/ml of FSH (D2); MEM + 10 ng/ml of FSH (D6); MEM + 50 ng/ml of FSH (D2); MEM + 50 ng/ml of FSH (D6); MEM + 100 ng/ml of FSH (D2); MEM + 100 ng/ml of FSH (D6); MEM + 200 ng/ml of FSH (D2); and MEM + 200 ng/ml of FSH (D6). Follicles were observed in only 9.65% (388 of 4,018) of the histological sections. Of the 861 follicles evaluated, 488 were in the primordial stage, and 373 were in various developmental stages; 59.7% were morphologically normal. Regarding the integrity of the pre-antral follicles, the groups with 100 ng/ml FSH of 2-days culture as well as 50, 100 and 200 ng/ml FSH of 6-days culture provided the best results. In conclusion, the in vitro culture of abattoir-derived equine ovarian fragments presented better morphological integrity when supplemented with FSH for 6 days, in comparison with the MEM culture group. However, no clear effects were observed with FSH regarding the promotion of activation from a primordial to a developing follicle.

Histone acetyltransferase KAT8 is essential for mouse oocyte development by regulating reactive oxygen species levels

Proper oocyte development is crucial for female fertility and requires timely and accurate control of gene expression. K (lysine) acetyltransferase 8 (KAT8), an important component of the X chromosome dosage compensation system in Drosophila, regulates gene activity by acetylating histone H4 preferentially at lysine 16. To explore the function of KAT8 during mouse oocyte development, we crossed Kat8^flox/flox mice with Gdf9-Cre mice to specifically delete Kat8 in oocytes. Oocyte Kat8 deletion resulted in female infertility, with follicle development failure in the secondary and preantral follicle stages. RNA-seq analysis revealed that Kat8 deficiency in oocytes results in significant downregulation of antioxidant genes, with a consequent increase in reactive oxygen species. Intraperitoneal injection of the antioxidant N-acetylcysteine rescued defective follicle and oocyte development resulting from Kat8 deficiency. Chromatin immunoprecipitation assays indicated that KAT8 regulates antioxidant gene expression by direct binding to promoter regions. Taken together, our findings demonstrate that KAT8 is essential for female fertility by regulating antioxidant gene expression and identify KAT8 as the first histone acetyltransferase with an essential function in oogenesis.

Expressão do mRNA para IGF-2 em oócitos e células do cumulus extraídos de folículos antrais e pré-antrais de ovelhas nativas do Estado de Pernambuco

RESUMO: Objetivou-se avaliar a expressão do mRNA para o gene do fator de crescimento IGF-2 em oócitos e células do cumulus de ovelhas em diferentes estágios do desenvolvimento folicular. Os folículos classificados morfologicamente como antrais (terciários e pré-ovulatórios) foram aspirados manualmente para obtenção dos oócitos e células do cumulus. Os folículos pré-antrais (secundários) foram extraídos do córtex ovariano, por microdissecção, e os oócitos retirados. Nos dois grupos, os oócitos foram desnudados e agrupados em “pools” de dez células cada (Grupo A, n=10; Grupo B, n=10) e dez amostras com grupos de células do cumulus (Grupo A1, n=10, B1, n=10). O mRNA foi extraído e convertido em cDNA utilizando a técnica da RT-PCR, utilizando Oligo DT randômico para o mRNA. A análise da expressão confirmou a expressão gênica para IGF-2 nos grupos de oócitos e células do cumulus. Houve um aumento da expressão relativa do mRNA para IGF-2 nos grupos de oócitos durante a fase mais tardia do desenvolvimento folicular e as diferenças foram consideradas significantes (p<0,05). Não houve variação significante da expressão de IGF2 entre os grupos de células do cumulus. Conclui-se que o fator de crescimento IGF-2 tem níveis mais elevados de expressão em oócitos ovinos, na segunda fase do desenvolvimento folicular, mas expressão semelhante em células do cumulus durante as fases estudadas do desenvolvimento folicular.

A curated transcriptomic dataset collection relevant to embryonic development associated with in vitro fertilization in healthy individuals and patients with polycystic ovary syndrome

The collection of large-scale datasets available in public repositories is rapidly growing and providing opportunities to identify and fill gaps in different fields of biomedical research. However, users of these datasets should be able to selectively browse datasets related to their field of interest. Here we made available a collection of transcriptome datasets related to human follicular cells from normal individuals or patients with polycystic ovary syndrome, in the process of their development, during in vitro fertilization. After RNA-seq dataset exclusion and careful selection based on study description and sample information, 12 datasets, encompassing a total of 85 unique transcriptome profiles, were identified in NCBI Gene Expression Omnibus and uploaded to the Gene Expression Browser (GXB), a web application specifically designed for interactive query and visualization of integrated large-scale data. Once annotated in GXB, multiple sample grouping has been made in order to create rank lists to allow easy data interpretation and comparison. The GXB tool also allows the users to browse a single gene across multiple projects to evaluate its expression profiles in multiple biological systems/conditions in a web-based customized graphical views. The curated dataset is accessible at the following link: http://ivf.gxbsidra.org/dm3/landing.gsp.

Genome-scale identification of nucleosome organization by using 1000 porcine oocytes at different developmental stages

The nucleosome is the basic structural unit of chromosomes, and its occupancy and distribution in promoters are crucial for the regulation of gene expression. During the growth process of porcine oocytes, the "growing" oocytes (SF) have a much higher transcriptional activity than the "fully grown" oocytes (BF). However, the chromosome status of the two kinds of oocytes remains poorly understood. In this study, we profiled the nucleosome distributions of SF and BF with as few as 1000 oocytes. By comparing the altered regions, we found that SF tended toward nucleosome loss and more open chromosome architecture than BF did. BF had decreased nucleosome occupancy in the coding region and increased nucleosome occupancy in the promoter compared to SF. The nucleosome occupancy of SF was higher than that of BF in the GC-poor regions, but lower than that of BF in the GC-rich regions. The nucleosome distribution around the transcriptional start site (TSS) of all the genes of the two samples was basically the same, but the nucleosome occupancy around the TSS of SF was lower than that of BF. GO functional annotation of genes with different nucleosome occupancy in promoter showed the genes were mainly involved in cell, cellular process, and metabolic process biological process. The results of this study revealed the dynamic reorganization of porcine oocytes in different developmental stages and the critical role of nucleosome arrangement during the oocyte growth process.

Effects of bone morphogenetic protein 4 (BMP4) on in vitro development and survival of bovine preantral follicles enclosed in fragments ovarian tissue

The aim of this study was to evaluate the effects of different concentrations of BMP4 on activation, development and mRNA expression of GDF9, BMP15, PCNA, Bax and Bcl2 in cultured bovine follicles enclosed in ovarian tissues. Ovarian tissue fragments were cultured for 6 days in α-MEM+ alone or supplemented with different concentrations of BMP4 (10, 50 or 100 ng/ml). Classical histology was performed to analyze follicle growth and morphology, while real-time PCR was used to analyze mRNA levels in fresh and cultured tissues. After 6 days, the culture of ovarian tissue in α-MEM+ alone or supplemented with 10, 50 or 100 ng/ml BMP4 promoted follicular activation. The different concentrations of BMP4 maintained the percentage of normal follicles similar to results of the control. The presence of 100 ng/ml BMP-4 in culture medium increased oocyte and follicular diameters of primary and secondary follicles when compared with those follicles from uncultured control or cultured in α-MEM+ alone (P < 0.05). The tissues cultured in the presence of increasing concentrations of BMP4 had an increase in mRNA expression of the tested genes, but despite this the differences were not statistically significant. In conclusion, 100 ng/ml BMP4 promotes an increase in diameters of follicles and oocytes of primary and secondary follicles after 6 days of in vitro culture.

New approaches regarding the in vitro maturation of oocytes: manipulating cyclic nucleotides and their partners in crime

Several discoveries have been described recently (5-10 years) about the biology of ovarian follicles (oocyte, cumulus cells and granulosa cells), including new aspects of cellular communication, the control of oocyte maturation and the acquisition of oocyte competence for fertilization and further embryo development. These advances are nourishing assisted reproduction techniques (ART) with new possibilities, in which novel culture systems are being developed and tested to improve embryo yield and quality. This mini-review aims to describe how the recent knowledge on the physiological aspects of mammalian oocyte is reflecting as original or revisited approaches into the context of embryo production. These new insights include recent findings on the mechanisms that control oocyte maturation, especially modulating intraoocyte levels of cyclic nucleotides during in vitro maturation using endogenous or exogenous agents. In this mini-review we also discuss the positive and negative effects of these manipulations on the outcoming embryo.

Stem cell factor promotes in vitro ovarian follicle development in the domestic cat by upregulating c-kit mRNA expression and stimulating the phosphatidylinositol 3-kinase/AKT pathway

In the present study we examined the effects of stem cell factor (SCF; 50 vs 100 ng mL–1) alone or in combination with epidermal growth factor (EGF; 100 ng mL–1) on: (1) the in vitro viability and growth of cat follicles within ovarian cortices; (2) phosphatidylinositol 3-kinase (PI3K)/AKT and mitogen-activated protein kinase (MAPK) phosphorylation; and (3) c-kit and FSH receptor (FSHr) mRNA expression. At 100 ng mL–1, SCF increased (P ≤ 0.05) the percentage and size of secondary follicles after 14 days of in vitro culture and sustained AKT phosphorylation after 3 days incubation. EGF suppressed this beneficial effect and reduced (P ≤ 0.05) the percentage of structurally normal follicles and FSHr expression when combined with 100 ng mL–1 SCF. Expression of c-kit mRNA was higher (P ≤ 0.05) in the presence of 100 ng mL–1 SCF compared with fresh follicles and cohorts cultured under other conditions. A c-kit inhibitor suppressed follicle growth and reduced AKT phosphorylation. Collectively, the results demonstrate that SCF promotes cat follicle development by upregulating c-kit mRNA expression and AKT phosphorylation. EGF suppresses the stimulating effect of SCF, leading to downregulation of FSHr expression.

Preantral follicle density in ovarian biopsy fragments and effects of mare age

The aims of the present study were to: (1) evaluate preantral follicle density in ovarian biopsy fragments within and among mares; (2) assess the effects of mare age on the density and quality of preantral follicles; and (3) determine the minimum number of ovarian fragments and histological sections needed to estimate equine follicle density using a mathematical model. The ovarian biopsy pick-up method was used in three groups of mares separated according to age (5–6, 7–10 and 11–16 years). Overall, 336 preantral follicles were recorded with a mean follicle density of 3.7 follicles per cm2. Follicle density differed (P < 0.05) among animals, ovarian fragments from the same animal, histological sections and age groups. More (P < 0.05) normal follicles were observed in the 5–6 years (97%) than the 11–16 years (84%) age group. Monte Carlo simulations showed a higher probability (90%; P < 0.05) of detecting follicle density using two experimental designs with 65 histological sections and three to four ovarian fragments. In summary, equine follicle density differed among animals and within ovarian fragments from the same animal, and follicle density and morphology were negatively affected by aging. Moreover, three to four ovarian fragments with 65 histological sections were required to accurately estimate follicle density in equine ovarian biopsy fragments.

Maternal Sall4 Is Indispensable for Epigenetic Maturation of Mouse Oocytes

Sall4 (Splat-like 4) plays important roles in maintaining pluripotency of embryonic stem cells and in various developmental processes. Here, we find that Sall4 is highly expressed in oocytes and early embryos. To investigate the roles of SALL4 in oogenesis, we generated Sall4 maternal specific knock-out mice by using CRISPR/Cas9 system, and we find that the maternal deletion of Sall4 causes developmental arrest of oocytes at germinal vesicle stage with non-surrounded nucleus, and the subsequent meiosis resumption is prohibited. We further discover that the loss of maternal Sall4 causes failure in establishment of DNA methylation in oocytes. Furthermore, we find that Sall4 modulates H3K4me3 and H3K27me3 modifications by regulating the expression of key histone demethylases coding genes Kdm5b, Kdm6a, and Kdm6b in oocytes. Moreover, we demonstrate that the aberrant H3K4me3 and H3K27me3 cause mis-expression of genes that are critical for oocytes maturation and meiosis resumption. Taken together, our study explores a pivotal role of Sall4 in regulating epigenetic maturation of mouse oocytes.

Candidate Gene Expression in Bos indicus Ovarian Tissues: Prepubertal and Postpubertal Heifers in Diestrus

Growth factors such as bone morphogenetic proteins 6, 7, 15, and two isoforms of transforming growth factor-beta (BMP6, BMP7, BMP15, TGFB1, and TGFB2), and insulin-like growth factor system act as local regulators of ovarian follicular development. To elucidate if these factors as well as others candidate genes, such as estrogen receptor 1 (ESR1), growth differentiation factor 9 (GDF9), follicle-stimulating hormone receptor (FSHR), luteinizing hormone receptor (LHR), bone morphogenetic protein receptor, type 2 (BMPR2), type 1 insulin-like growth factor receptor (IGFR1), and key steroidogenic enzymes cytochrome P450 aromatase and 3-β-hydroxysteroid dehydrogenase (CYP19A1 and HSD3B1) could modulate or influence diestrus on the onset of puberty in Brahman heifers, their ovarian mRNA expression was measured before and after puberty (luteal phase). Six postpubertal (POST) heifers were euthanized on the luteal phase of their second cycle, confirmed by corpus luteum observation, and six prepubertal (PRE) heifers were euthanized in the same day. Quantitative real-time PCR analysis showed that the expression of FSHR, BMP7, CYP19A1, IGF1, and IGFR1 mRNA was greater in PRE heifers, when contrasted to POST heifers. The expression of LHR and HSD3B1 was lower in PRE heifers. Differential expression of ovarian genes could be associated with changes in follicular dynamics and different cell populations that have emerged as consequence of puberty and the luteal phase. The emerging hypothesis is that BMP7 and IGF1 are co-expressed and may modulate the expression of FSHR, LHR and IGFR1, and CYP19A1. BMP7 could influence the downregulation of LHR and upregulation of FSHR and CYP19A1, which mediates the follicular dynamics in heifer ovaries. Upregulation of IGF1 expression prepuberty, compared to postpuberty diestrus, correlates with increased levels FSHR and CYP19A1. Thus, BMP7 and IGF1 may play synergic roles and were predicted to interact, from the expression data (P = 0.07, r = 0.84). The role of these co-expressed genes in puberty and heifers luteal phase merits further research.

Identification of molecular markers for oocyte competence in bovine cumulus cells

Cumulus cells (CCs) have an important role during oocyte growth, competence acquisition, maturation, ovulation and fertilization. In an attempt to isolate potential biomarkers for bovine in vitro fertilization, we identified genes differentially expressed in bovine CCs from oocytes with different competence statuses, through microarray analysis. The model of follicle size, in which competent cumulus-oocyte complexes (COCs) were recovered from bigger follicles (≥8.0 mm in diameter) and less competent ones from smaller follicles (1-3 mm), was used. We identified 4178 genes that were differentially expressed (P < 0.05) in the two categories of CCs. The list was further enriched, through the use of a 2.5-fold change in gene expression as a cutoff value, to include 143 up-regulated and 80 down-regulated genes in CCs of competent COCs compared to incompetent COCs. These genes were screened according to their cellular roles, most of which were related to cell cycle, DNA repair, energy metabolism, metabolism of amino acids, cell signaling, meiosis, ovulation and inflammation. Three candidate genes up-regulated (FGF11, IGFBP4, SPRY1) and three down-regulated (ARHGAP22, COL18A1 and GPC4) in CCs from COCs of big follicles (≥8.1 mm) were selected for qPCR analysis. The selected genes showed the same expression patterns by qPCR and microarray analysis. These genes may be potential genetic markers that predict oocyte competence in in vitro fertilization routines.

The efficacy of an inhibin DNA vaccine delivered by attenuated Salmonella choleraesuis on follicular development and ovulation responses in crossbred buffaloes

The aim of this study was to evaluate the efficacy of an inhibin DNA vaccine delivered by attenuated Salmonella choleraesuis on follicular development and ovulation responses in crossbred buffaloes. A total of 158 crossbred buffaloes divided into four groups and were intramuscularly injected with 1×10(10) (T1, n=41), 1×10(9) (T2, n=37), 1×10(8) (T3, n=37) or 0 (C, n=43) CFU/ml bacteria delivered inhibin vaccine in 10ml PBS on day 0 and 14, respectively. All animals were administered with 1000 IU PMSG on day 28, 0.5mg PGF2α on day 30 and 200μg GnRH on day 32. The results showed buffaloes immunized with the bacteria delivered inhibin vaccine had significantly higher titers of anti-inhibin IgG antibody than control group (P<0.01). The number and diameter of large follicles (≥10mm) as well as ovulatory follicles in group T1 was significantly greater than group C (P<0.05). The growth speed of dominant follicles in group T1 was significantly faster than groups T3 and C (P<0.05), resulting in a greater conception rate in buffaloes with positive antibodies. These results demonstrate that immunization with the bacterial delivered inhibin vaccine, coupled with the estrus synchronization protocol, could be used as an alternative approach to improve fertility in crossbred buffaloes.

Developmental programming: rescuing disruptions in preovulatory follicle growth and steroidogenesis from prenatal testosterone disruption

BACKGROUND

Prenatal testosterone (T) excess from days 30-90 of gestation disrupts gonadotropin surge and ovarian follicular dynamics and induces insulin resistance and functional hyperandrogenism in sheep. T treatment from days 60-90 of gestation produces a milder phenotype, albeit with reduced fecundity. Using this milder phenotype, the aim of this study was to understand the relative postnatal contributions of androgen and insulin in mediating the prenatal T induced disruptions in ovarian follicular dynamics.

METHODS

Four experimental groups were generated: 1) control (vehicle treatment), 2) prenatal T-treated (100 mg i.m. administration of T propionate twice weekly from days 60-90 of gestation), 3) prenatal T plus postnatal anti-androgen treated (daily oral dose of 15 mg/kg/day of flutamide beginning at 8 weeks of age) and 4) prenatal T and postnatal insulin sensitizer-treated (daily oral dose of 8 mg/day rosiglitazone beginning at 8 weeks of age). Follicular response to a controlled ovarian stimulation protocol was tested during their third breeding season. Main outcome measures included the determination of number and size of ovarian follicles and intrafollicular concentrations of steroids.

RESULTS

At the end of the controlled ovarian stimulation, the number of follicles approaching ovulatory size (≥6 mm) were ~35 % lower in prenatal T-treated (6.5 ± 1.8) compared to controls (9.8 ± 2.0). Postnatal anti-androgen (10.3 ± 1.9), but not insulin sensitizer (5.0 ± 0.9), treatment prevented this decrease. Preovulatory sized follicles in the T group had lower intrafollicular T, androstenedione, and progesterone compared to that of the control group. Intrafollicular steroid disruption was partially reversed solely by postnatal insulin sensitizer treatment.

CONCLUSIONS

These results demonstrate that the final preovulatory follicular growth and intrafollicular steroid milieu is impaired in prenatal T-treated females. The findings are consistent with the lower fertility rate reported earlier in these females. The finding that final follicle growth was fully rescued by postnatal anti-androgen treatment and intrafollicular steroid milieu partially by insulin sensitizer treatment suggest that both androgenic and insulin pathway disruptions contribute to the compromised follicular phenotype of prenatal T-treated females.

Ovarian follicle development in vitro and oocyte competence: advances and challenges for farm animals

During the last 2 decades, research on in vitro preantral follicle growth and oocyte maturation has delivered fascinating advances concerning the knowledge of processes regulating follicle growth and the developmental competence of oocytes. These advances include (1) information about the role of several hormones and growth factors on in vitro activation of primordial follicles; (2) increased understanding of the intracellular pathway involved in the initiation of primordial follicle growth; (3) the growth of primary and secondary follicles up to antral stages; and (4) production of embryos from oocytes from in vitro grown preantral follicles. This review article describes these advances, especially in regard farm animals, and discusses the reasons that limit embryo production from oocytes derived from preantral follicles cultured in vitro.

Molecular characterization and quantification of the follicle-stimulating hormone receptor in turbot (Scophthalmus maximus)

Molecular cloning, characterization, and functional analysis of follicle-stimulating hormone receptor (FSHR) in female turbot (Scophthalmus maximus) were evaluated. Results showed that the full-length FSHR cDNA was 3824 bp long and contained a 2202 bp open reading frame that encoded a mature protein of 733 amino acids (aa) and a signal peptide of 18 aa. Multiple sequence analyses showed that turbot FSHR has high homology with the corresponding genes of other teleosts and significant homology with that of Hippoglossus hippoglossus. Turbot FSHR has the typical structural architecture of glycoprotein hormone receptors consisting of a large N-terminal extracellular domain, seven transmembrane domains and short C-terminal intracellular domain. FSHR mRNA was found to be abundant in the ovaries, but deficient in eyes, intestine, brain, muscle, gills, spleen, stomach, heart and kidney. Furthermore, FSHR mRNA was found to increase gradually from pre-vitellogenesis to migratory nucleus stages, with the highest values observed during the late vitellogenesis stage of the reproductive cycle. However, FSHR mRNA was found to decrease dramatically during the atresia stage. Meanwhile, functional analysis with HEK293T cells continual expressing FSHR demonstrated that FSHR was specifically stimulated by ovine FSH, but not ovine LH. These results indicate that turbot FSHR is mainly involved in the stimulation of vitellogenesis, regulation of oocyte maturation as well as promotion of ovarian development via specific ligand binding. These findings open doors to further investigation of physiological functions of FSHR, which will be valuable for fish reproduction and broodstock management.

Optimized Protocols for In Vitro Maturation of Rat Oocytes Dramatically Improve Their Developmental Competence to a Level Similar to That of Ovulated Oocytes

The developmental capacity of in vitro-matured (IVM) oocytes is markedly lower than that of their in vivo-matured (IVO) counterparts, suggesting the need for optimization of IVM protocols in different species. There are few studies on IVM of rat oocytes, and there are even fewer attempts to improve ooplasmic maturation compared to those reported in other species. Furthermore, rat oocytes are well known to undergo spontaneous activation (SA) after leaving the oviduct; however, whether IVM rat oocytes have lower SA rates than IVO oocytes and can potentially be used for nuclear transfer is unknown. In this study, we investigated the effects of maturation protocols on cytoplasmic maturation of IVM rat oocytes and observed the possibility to reduce SA by using IVM rat oocytes. Ooplasmic maturation was assessed using multiple markers, including pre- and postimplantation development, meiotic progression, CG redistribution, redox state, and the expression of developmental potential- and apoptosis-related genes. The results showed that the best protocol consisting of modified Tissue Culture Medium-199 (TCM-199) supplemented with cysteamine/cystine and the cumulus cell monolayer dramatically improved the developmental competence of rat oocytes and supported both pre- and postimplantation development and other ooplasmic maturation makers to levels similar to that observed in ovulated oocytes. Rates of SA were significantly lower in IVM oocytes than in IVO oocytes when observed at the same intervals after nuclear maturation. In conclusion, we have optimized protocols for IVM of rat oocytes that sustain ooplasmic maturation to a level similar to ovulated oocytes. The results suggest that IVM rat oocytes might be used to reduce SA for rat cloning.

Three dimensional in vitro culture of preantral follicles following slow-freezing and vitrification of mouse ovarian tissue

To evaluate the effects slow-freezing and vitrification on three dimensional in vitro culture of preantral follicles, ovaries of 12-14 days old female NMRI mice were isolated and randomly assigned to fresh control, slow-freezing and vitrification groups. Slow-freezing was performed using programmable freezer. Vitrification was carried out in a medium consisting of ethylene glycol (EG) and dimethyl sulphoxide (Me2SO) by needle immersion method. middle sized preantral follicles were mechanically isolated and cultured for 12 days in 0.7% sodium alginate gel. The follicles development and quantitative expression of oocyte specific genes (Bmp15, Gdf9, Fgf8) and the growth related genes (Igf1, Kit, Kit-l) were assessed after 1, 8 and 12 days of culture. Both cryopreserved groups showed reduction of follicular survival rates compared to the control group on days 8 and 12 of culture (P < 0.05). Antrum formation rates reduced in slow-freezing after 12 days of culture (P < 0.05). Evaluation of gene expression showed reduction of Bmp15, Gdf9, Fgf8, Kit and Kit-l during 12 days of culture (P < 0.05). Kit and Kit-l expression in slow-freezing group significantly reduced on day 8 of culture (p < 0.05). Igf1 expression was lower in slow-freezing group on 1st day of culture than vitrification and control groups (P < 0.05). Finally, intergroup comparison showed same expression pattern of genes after 12 days of culture. Thus, cryopreservation of mouse ovaries by both methods can preserve most developmental parameters and expression of maturation genes. However, vitrification is a better method for cryopreservation of mouse ovaries due to greater antrum formation and expression of growth related markers.

MicroRNAs in ovarian function and disorders

MicroRNAs (miRNAs) are endogenous, small, noncoding single-stranded RNA molecules approximately 22 nucleotides in length. miRNAs are involved in the post-transcriptional regulation of various important cellular physiological and pathological processes, including cell proliferation, differentiation, apoptosis, and hormone biosynthesis and secretion. Ovarian follicles are the key functional units of female reproduction, and the development of these follicles is a complex and precise process accompanied by oocyte maturation as well as surrounding granulosa cell proliferation and differentiation. Numerous miRNAs expressed in the ovary regulate ovarian follicle growth, atresia, ovulation and steroidogenesis and play an important role in ovarian disorders. This review considers recent advances in the identification of miRNAs involved in the regulation of ovarian function as well as the possible influence of miRNAs on ovarian-derived disorders, such as ovarian cancer, polycystic ovarian syndrome and premature ovarian failure. An improved understanding of the regulation of ovarian function by miRNAs may shed light on new strategies for ovarian biology and ovarian disorders.

Molecular cloning, characterization, and expression analysis of luteinizing hormone receptor gene in turbot (Scophthalmus maximus)

The luteinizing hormone receptor (LHR) plays a crucial role in female reproduction. In the present study, full-length sequence coding for the LHR was obtained from female turbot (Scophthalmus maximus) by homology cloning and a strategy based on rapid amplification of cDNA end-polymerase chain reaction. The full-length LHR cDNA was 3,184 bp long and contained a 2,058-bp open reading frame which encoded a protein of 685 amino acids. Multiple sequence alignments of the turbot LHR manifested high homologies with the corresponding sequences of available teleosts and representative vertebrates, and significant homology with that of Hippoglossus hippoglossus. In addition, the turbot LHR showed typical characteristics of glycoprotein receptors, including a long N-terminal extracellular domain, seven transmembrane domains, and a short C-terminal intracellular domain. LHR mRNA was abundant in the ovary, but was deficient in extra-ovarian tissues. Furthermore, LHR mRNA gradually developed from previtellogenesis to migratory nucleus stage, with the highest values observed in migratory nucleus stage during reproductive cycle. However, LHR mRNA sharply decreased in atresia stage. These results suggested that LHR is a typical G protein-coupled receptor that is involved in the promotion of turbot ovarian development and may be related to the final maturation and ovulation of oocyte. These findings contribute to the understanding of the potential roles of LHR in controlling the fish reproductive cycle.

Three-dimensional systems for in vitro follicular culture: overview of alginate-based matrices

The in vitro culture of ovarian follicles has provided critical insight into the biology of the follicle and its enclosed oocyte and the physical interaction and communication between the theca and granulosa cells and the oocyte that is necessary to produce meiotically competent oocytes. Various two-dimensional (2D) and three-dimensional (3D) culture systems have been developed to evaluate the effect of growth factors, hormones, extracellular matrix components and culture conditions on follicle development and oocyte growth and maturation. Among these culture systems, 3D systems make it possible to maintain follicle structure and support communication between the various cell compartments within the follicle. In this review article, we will discuss the three main approaches to ovarian follicle culture: 2D attachment systems, 3D floating systems and 3D encapsulated systems. We will specifically emphasise the development of and advances in alginate-based encapsulated systems for in vitro follicle culture.

Roles of thyroid hormones in follicular development in the ovary of neonatal and immature rats

Thyroid hormones (TH) play a critical role in ovarian follicular development, maturation and the maintenance of various endocrine functions. However, whether TH can affect ovarian follicular development in neonatal and immature rats remains unclear. Therefore, the aim of the present study was to elucidate the effect of TH on ovarian follicular development in neonatal and immature rats. Thirty female post-lactation mothers of Sprague-Dawley rat pups were randomly divided into three groups: control, hyperthyroid (hyper), and hypothyroid (hypo). On postnatal days (PND) 10 and 21, body weights, serum hormones, ovarian histologic changes, and immunohistochemistry of thyroid hormone receptor alpha 1 (TRα1) and nitric oxide synthase types (NOS), and NOS activities, were determined. The data showed that body weights significantly decreased in both hyper and hypo groups compared with the control group (P < 0.05). In addition, the hyper group had increased serum concentrations of T3, T4, and E2; whereas the hypo group manifested reduced serum concentrations of T3, T4, and E2 on PND 10 and 21. The hyper and hypo groups showed significantly reduced total number of primordial, primary and secondary follicles on PND 10 and 21 compared with the control group (P < 0.05). Similarly, antral follicle numbers in the hyper and hypo groups were significantly decreased on PND 21 compared with the control group (P < 0.05). Immunostaining indicated that TRα1 and NOS were expressed in ovarian surface epithelium and oocytes of growing and antral follicles, with strong staining of the granulosa and theca cells of follicles. NOS activities were significantly augmented in the hyper, but diminished in the hypo groups on PND 10 and 21. In summary, our findings suggest that TH play important roles in ovarian functions and in the regulation of NOS activity. Our results also indicate that a relationship exists between the TH and NO signaling pathways during the process of ovarian follicular development in neonatal and immature rats.