In vitro and in vivo regulation of follicular formation and activation in cattle

A New Understanding, Guided by Single-Cell Sequencing, of the Establishment and Maintenance of the Ovarian Reserve in Mammals

BACKGROUND

Oocytes are a finite and non-renewable resource that are maintained in primordial follicle structures. The ovarian reserve is the totality of primordial follicles, present from birth, within the ovary and its establishment, size, and maintenance dictates the duration of the female reproductive lifespan. Understanding the cellular and molecular dynamics relevant to the establishment and maintenance of the reserve provides the first steps necessary for modulating both individual human and animal reproductive health as well as population dynamics.

SUMMARY

This review details the key stages of establishment and maintenance of the ovarian reserve, encompassing germ cell nest formation, germ cell nest breakdown, and primordial follicle formation and activation. Furthermore, we spotlight several formative single-cell sequencing studies that have significantly advanced our knowledge of novel molecular regulators of the ovarian reserve, which may improve our ability to modulate female reproductive lifespans.

KEY MESSAGES

The application of single-cell sequencing to studies of ovarian development in mammals, especially when leveraging genetic and environmental models, offers significant insights into fertility and its regulation. Moreover, comparative studies looking at key stages in the development of the ovarian reserve across species has the potential to impact not just human fertility, but also conservation biology, invasive species management, and agriculture.

The putative roles of FSH and AMH in the regulation of oocyte developmental competence: from fertility prognosis to mechanisms underlying age-related subfertility

BACKGROUND

Fertility loss during female ageing is associated with increasing basal FSH and decreasing anti-Müllerian hormone (AMH) concentrations, together with compromised oocyte quality, presumably due to increased oxidative stress (OS) and DNA damage, as well as reduced metabolic and meiotic competences. Basal FSH and AMH circulatory concentrations have been broadly utilized as IVF success predictors, regardless of fluctuations in prognostic accuracy; basal FSH and AMH perform better in pre-advanced maternal age (AMA: >35 years) and AMA patients, respectively. The relationships between FSH and AMH intrafollicular levels and IVF outcomes suggest, nevertheless, that both hormones regulate oocyte competence, supporting the hypothesis that changes in FSH/AMH levels cause, at least in part, oocyte quality degradation during ageing. To understand the reasons behind the fluctuations in FSH and AMH prognostic accuracies and to clarify their participation in mechanisms determining oocyte competence and age-related subfertility, a deeper knowledge of the regulation of FSH and AMH intrafollicular signalling during the female reproductive lifespan, and of their effects on the cumulus-oocyte complex, is required.

OBJECTIVE AND RATIONALE

An extensive body of information on the regulation of FSH and AMH intrafollicular availability and signalling, as well as on the control of folliculogenesis and oocyte metabolism, has been accumulated. However, these datasets have been explored within the relatively narrow boundaries of their specific subjects. Given the aforementioned gaps in knowledge and their clinical relevance, herein we integrate clinical and basic data, within a wide biological perspective, aiming to shed light on (i) the reasons for the variability in the accuracy of serum FSH and AMH as fertility markers, and on (ii) the potential roles of these hormones in mechanisms regulating oocyte quality, particularly those associated with ageing.

SEARCH METHODS

The PubMed database encompassing the period between 1960 and 2021 was searched. Principal search terms were FSH, FSH receptor, AMH, oocyte, maternal age, cumulus, transzonal projections (TZPs), actin, OS, redox, reactive oxygen species, mitochondria, DNA damage, DNA repair, aneuploidy, spindle, meiosis, gene expression, transcription, translation, oocyte secreted factors (OSFs), cAMP, cyclic guanosine monophosphate, natriuretic peptide C, growth differentiation factor 9, bone morphogenetic protein 15 and fibroblast growth factor.

OUTCOMES

Our analysis suggests that variations in the accuracy of fertility prognosis reflect a modest association between circulatory AMH levels and oocyte quality as well as increasing basal FSH inter-cycle variability with age. In addition, the basic and clinical data articulated herein support the hypothesis that increased intrafollicular FSH levels, as maternal age advances, may override the physiological protective influences of AMH and OSFs against excessive FSH signalling in cumulus cells. This would result in the disruption of oocyte homeostasis via reduced TZP-mediated transfer of cumulus-derived molecules essential for meiotic competence, gene expression, redox activity and DNA repair.

WIDER IMPLICATIONS

In-depth data analysis, encompassing a wide biological perspective has revealed potential causative mechanisms of age-related subfertility triggered by alterations in FSH/AMH signalling during the female reproductive life. Insights from new mechanistic models arising from this analysis should contribute to advancing our comprehension of oocyte biology in humans and serve as a valuable reference for novel AMA subfertility treatments aimed at improving oocyte quality through the modulation of AMH/FSH action.

Supplementation of granulosa cells conditioned medium with pyruvate and testosterone could improve early follicular development in cultured 1-day-old mouse ovaries

In vitro activation of primordial follicles could serve as a safe method to preserve fertility in patients with cancer subjected to ovarian tissue cryopreservation during oncotherapy, however the culture medium for this purpose requires to be optimized. Granulosa cell conditioned medium (GCCM) has been recognized to enhance primordial follicle activation and the present study was conducted to understand whether addition of pyruvate, a combination of insulin, transferrin and selenium (ITS) or testosterone to GCCM could improve its efficiency in this regard. To this end, 1-day-old mouse ovaries were cultured in four different media including CON (control; containing GGCM only), PYR (containing GCCM plus pyruvate), ITS (containing GCCM plus ITS) or TES (containing GCCM plus testosterone) for 11 days. Furthermore, follicular dynamics and gene expression of factors involved in follicular development were assessed using histological examination and RT-PCR, respectively, on days 5 and 11 of culture. Pyruvate decreased follicular activation, but it enhanced the progression of follicles to the primary stage. Moreover, it upregulated Bmp15 and Cx37 (P < 0.05). In the ITS group, activation of follicles was not affected and total number of follicles was reduced by day 11 of culture. Additionally, ITS downregulated Pi3k, Gdf9, Bmp15 and Cx37 (P < 0.05). Although testosterone did not affect primordial follicle activation, it enhanced the development of follicles up to the preantral stage (P < 0.05). Furthermore, testosterone inhibited the expression of Pten but stimulated the expression of Gdf9 and Cx37 (P < 0.05). In conclusion, the present study revealed that inclusion of pyruvate and testosterone into GCCM could enhance the early development of follicles in cultured 1-day-old mouse ovaries.

Relationship between Anti-Müllerian hormone, antral follicular population and resumption of cyclicity in postpartum buffaloes

The present study was designed to determine the relationship between Anti-Müllerian hormone (AMH) and antral follicular population (AFP), milk yield (MY), body weight (BW), body condition score (BCS) and resumption of cyclicity (RC) in postpartum buffaloes. For the present study, 20 buffaloes divided into 2 groups: Group I (n=10): buffaloes resuming cyclicity <30 days of calving; Group II (n=10): anestrous buffaloes >90 days postpartum. Blood sampling was carried out in all buffaloes in both groups at day 30 postpartum (day 0 considered as calving day). BW and BCS were monitored fortnightly and MY was recorded every week during first month of lactation. In this study, we found that none of the parameters (AMH, MY, BW and BCS) differed significantly, though RC differed between the two groups based on cyclicity, monitored using transrectal ultrasonography. In addition, AFP between the two groups (A: 19.7±4.95 vs 15.7±5.08) showed a trend in difference, though non-significant. Correlation study between the parameters, i.e. AMH, AFP, MY, BW, BCS and RC in postpartum buffaloes revealed nonsignificant correlation between AMH with AFP, MY, BW, BCS and RC in postpartum buffaloes. In summary, this study failed to deduce any relationship between AMH with AFP, MY, BW, BCS and RC in postpartum buffaloes.

Insulin promotes preantral follicle growth and antrum formation through temporal expression of genes regulating steroidogenesis and water transport in the cat

The aims of the present study were to determine the effects of insulin, invitro, on: (1) the viability and growth of domestic cat ovarian follicles; (2) mRNA expression of genes regulating steroidogenesis (cytochrome P450 family 17 subfamily, A polypeptide 1 (Cyp17a1), cytochrome P450 family 19 subfamily, A polypeptide 1 (Cyp19a1) and steroidogenic acute regulatory protein (Star)) and water transport (aquaporins (AQPs) Aqp1, Aqp3, Aqp7, Aqp9); and (3) steroid production (17β-oestradiol (E2), progesterone (P4), androstenedione (A4)). Cat secondary follicles were isolated from ovarian cortices and cultured in 0 (Control), 1 or 10µgmL-1 insulin for 14 days (Day 0=culture onset). Follicle and oocyte viability (based on neutral red staining), diameter and antrum formation were assessed every 72h and at the end of incubation (Day 14). Expression of steroidogenic and water transport genes was evaluated on Days 0, 6 and 12, and E2, P4 and A4 concentrations in the culture medium were determined on Day 12. By Day 14, 1 and 10µgmL-1 insulin had significantly promoted (P<0.05) both antrum formation in a mean (±s.e.m.) 26.9±9.0% and 78.0±10.0% of follicles respectively, and follicle growth (diameter 151.4±4.5 and 169.9±10.5µm respectively) compared with Control (antrum formation in 3.3±3.3% of follicles and follicle diameter 129.1±6.6µm). High insulin (10µgmL-1) treatment increased follicle viability compared with Control (86.0±9.8% vs 38.1±10.9% respectively; P<0.05). However, insulin had no beneficial effect (P>0.05) on oocyte diameter. Cyp17a1 expression on Days 6 and 12 was higher (P<0.05) in follicles cultured in the low (1µgmL-1) compared with high (10µgmL-1) insulin treatment, with no significant difference between low or high insulin vs Control groups. Star expression was higher (P<0.01) in the low insulin compared with Control group on Day 6, but Star was undetectable in the high insulin group by Day 12. Compared with high insulin, low insulin increased (P<0.05) Aqp1 expression on Day 6, but there were no significant differences between these two groups on Day 12. In contrast, high insulin decreased (P<0.05) Aqp9 transcript levels compared with Control. Only P4 production was affected by insulin, with P4 concentrations in the medium being higher (P<0.05) in the low compared with high insulin and Control groups. In summary, the findings indicate that insulin promotes cat ovarian follicle growth and survival invitro, including enhanced antrum formation, with the likely mechanism involving temporal expression of Cyp17a1, Star and Aqp9 genes.

Early ovine preantral follicles have a potential to grow until antral stage in two-step culture system in the presence of aqueous extract of Justicia insularis

The objective of this study was to determine whether preantral follicles cultured in vitro for 7 days within ovine ovarian cortical strips could be isolated at the secondary follicles (SF) and grown until antral stage during an additional 6 days period of in vitro culture in the presence of aqueous extract of Justicia insularis. Fresh ovarian fragments from 16 adult sheep were fixed for histological analysis (Control 1) or in vitro cultured individually in α-MEM⁺ supplemented with 0.3 mg/ml J. insularis (Step 1) for 7 days. Part of the fragments then were fixed for histological analysis (in vitro culture group). Remaining fragments were exposed stepwise to increasing trehalose concentrations before immediate isolation of SF and viability assessment (Control 2) or after 6 days of culture in α-MEM⁺⁺ supplemented with 0.3 mg/ml J. insularis (Step 2). In Step 1, percentage of follicular activation was 80%. In Step 2, a significant increase (p < 0.05) in follicular diameter and antrum formation within 6 days in vitro culture of isolated follicles was achieved. The total antioxidant capacity from both steps significantly increase (p < 0.05) from day 2 to day 6. Confocal analysis of oocytes showed 57.14% oocytes with homogeneous distribution and 42.86% with peri-cortical distribution. In conclusion, SF can be successfully isolated from sheep ovarian cortex after 7 days of culture and are capable of surviving and forming an antral cavity if cultured in vitro for an additional 6 days in the presence of 0.3 mg/ml J. insularis.

Update on oogenesis in vitro

INTRODUCTION

Ovarian tissue is increasingly being collected from cancer patients and cryopreserved for fertility preservation. Alternately to the autologous transplantation, the development of culture systems that support oocyte development from the primordial follicle stage represent a valid strategy to restore fertility. The aim of this study is to review the most recent data regarding oogenesis in vitro and to provide an up-to-date on the contemporary knowledge of follicle growth and development in vitro.

EVIDENCE ACQUISITION

A comprehensive systematic MEDLINE search was performed since February 2018 for English-language reports by using the following terms: "ovary," "animal and human follicle," "in vitro growth and development," "ovarian tissue culture," "fertility preservation," "IVM," "oocyte." Previous published reviews and recent published original articles were preferred in order to meet our study scope.

EVIDENCE SYNTHESIS

Over time, many studies have been conducted with the aim to optimize the characteristics of ovarian tissue culture systems and to better support the three main phases: 1) activation of primordial follicles; 2) isolation and culture of growing preantral follicles; 3) removal from the follicle environment and maturation of oocyte cumulus complexes. While complete oocyte in vitro development has been achieved in mouse, with the production of live offspring, the goal of obtaining oocytes of sufficient quality to support embryo development has not been completely reached into higher mammals despite decades of effort.

CONCLUSIONS

Over the years, many improvements have been made on ovarian tissue cultures with the future purpose that patients will be provided with a greater number of developmentally competent oocytes for fertility preservation.

The Effects of Lysophosphatidic Acid on The Incidence of Cell Death in Cultured Vitrified and Non-Vitrified Mouse Ovarian Tissue: Separation of Necrosis and Apoptosis Border

Objective

The aim of the present study was to examine whether lysophosphatidic acid (LPA) could decrease cell death and improve in vitro culture (IVC) conditions in cultured vitrified mouse ovarian tissue.

Materials and Methods

In this experimental study, we collected and randomly divided 7-day-old mouse ovarian tissues into vitrified and non-vitrified groups. The ovaries were cultured in the presence and absence of LPA for one week. Morphology and follicular development were evaluated by hematoxylin and eosin (H&E) and Masson's trichrome (MTC) staining. The incidence of cell death was assessed by flow cytometry using annexin V/propidium iodide (PI) and a caspase-3/7 assay in all studied groups.

Results

The vitrified groups had a significantly decreased follicle developmental rate compared to the non-vitrified groups (P<0.05). Overall, qualitative and quantitative results showed prominent follicular degeneration in the vitrified groups compared with the respective non-vitrified groups. Both LPA treated groups had a significantly higher proportion of preantral follicles compared to the non-LPA treated groups (P<0.05). Flow cytometry analysis results showed significantly greater early and late apoptotic cells in all groups (17.83 ± 8.80%) compared to necrotic cells (7.97 ± 0.92%, P<0.05). The percentage of these cells significantly increased in the vitrified groups compared with non-vitrified groups. LPA treated groups had a lower percentage of these cells compared to non-LPA treated groups (P<0.05). The lower enzyme activity was observed in non-vitrified (especially in the LPA+ groups) cultured ovaries compared to the vitrified group (P<0.05).

Conclusion

Both vitrification and IVC adversely affected cell survival and caused cell death. We postulated that LPA supplementation of culture medium could improve the developmental rate of follicles and act as an anti-cell death factor in non-vitrified and vitrified ovarian tissues. It could be used for in vitro maturation of ovarian tissue.

Anti-Müllerian hormone inhibits activation and growth of bovine ovarian follicles in vitro and is localized to growing follicles

STUDY QUESTION

Does anti-Müllerian hormone (AMH) inhibit activation (initiation of growth) of primordial follicles and attenuate the growth of primary follicles in cattle, an excellent animal model for human ovarian follicular development?

SUMMARY ANSWER

AMH inhibited activation of bovine primordial follicles and attenuated the growth of activated follicles in vitro.

WHAT IS KNOWN ALREADY

In mice null mutant for AMH, the pool of primordial follicles is depleted prematurely and AMH inhibits follicle activation in vitro. Results of studies with human ovarian tissue in vitro were inconsistent. Our previous work provided indirect evidence that AMH inhibits follicle activation in bovine ovaries.

STUDY DESIGN, SIZE, DURATION

Pieces of fetal bovine ovarian cortex (2 pieces/culture well), obtained during mid or late pregnancy, were cultured in control medium or with graded doses of AMH for 2, 10 or 12 days. Effects of treatment on follicle activation and growth were determined by histological morphometry; follicles in every 20th histological section were staged (primordial or primary), counted, and measured. In addition, AMH was immunolocalized in bovine ovaries obtained at various times during pregnancy (n = 20 ovaries).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Bovine fetal ovaries at mid or late gestation were obtained at a commercial abattoir. Pieces of ovarian cortex were cultured without or with AMH and fixed for histological morphometry on Day 0 and at the end of culture. Treatments were applied to duplicate cultures from each of two or three fetuses. In 12-day cultures, addition of AMH was delayed until the third day. Histological analysis provided information about the types, numbers and sizes of follicles in cortical pieces before and after treatments. Ovaries obtained during the second and third trimesters were assessed for the presence of AMH by immunohistochemistry.

MAIN RESULTS AND THE ROLE OF CHANCE

AMH (100-500 ng/ml) inhibited follicle activation in response to an activator (insulin) in ovarian cortical pieces from fetal ovaries in late gestation. Dose-dependent inhibitory effects on the diameters of primary follicles and their oocytes were also observed. These results were obtained only when AMH was added to cultures in advance of insulin (presumably because it penetrates tissue more slowly than insulin). Results of experiments with cortical pieces from fetal ovaries at mid-gestation, when follicles are forming, showed that AMH did not inhibit the formation of follicles. Immunohistochemical localization of AMH showed that it is not present in fetal ovaries until the third trimester, when it was localized to the granulosa cells of secondary and small antral follicles.

LIMITATIONS REASONS FOR CAUTION

The experiments were performed with fetal ovaries because follicles form and follicle activation begins during fetal life in cattle (as it does in humans), so fetal ovarian cortex of later gestation provides tissue rich in primordial follicles. We assume, but have no experimental evidence, that our findings also apply to post-natal ovaries.

WIDER IMPLICATIONS OF THE FINDINGS

Although circulating AMH is used as an indication of the follicular reserve in women, little is known about AMH in human ovaries. Cattle are an excellent non-primate model for human ovarian follicular development and, hence, the findings suggest similar roles for AMH in human follicular development.

LARGE SCALE DATA

Not applicable.

STUDY FUNDNG/COMPETING INTEREST(S)

This research was supported by National Research Initiative Competitive Grants no. 00-35203-9151, 2003-35203-13532, and 2008-35203-05989) from the U.S. Dept. of Agriculture's National Institute of Food and Agriculture to JEF and by an NIH National Research Service Award (F32 HD08264) to RAC. There are no conflicts of interest or competing interests.

In-vitro regulation of primordial follicle activation: challenges for fertility preservation strategies

Ovarian tissue is increasingly being collected from cancer patients and cryopreserved for fertility preservation. While the only available option to restore fertility is autologous transplantation, this treatment is not appropriate for all patients due to the risk of reintroducing cancer cells and causing disease recurrence. Harnessing the full reproductive potential of this tissue to restore fertility requires the development of culture systems that support oocyte development from the primordial follicle stage. While this has been achieved in the mouse, the goal of obtaining oocytes of sufficient quality to support embryo development has not been reached in higher mammals despite decades of effort. In vivo, primordial follicles gradually exit the resting pool, whereas when primordial follicles are placed into culture, global activation of these follicles occurs. Therefore, the addition of a factor(s) that can regulate primordial follicle activation in vitro may be beneficial to the development of culture systems for ovarian tissue from cancer patients. Several factors have been observed to inhibit follicle activation, including anti-Müllerian hormone, stromal-derived factor 1 and members of the c-Jun-N-terminal kinase pathway. This review summarizes the findings from studies of these factors and discusses their potential integration into ovarian tissue culture strategies for fertility preservation.

Effect of lysophosphatidic acid on the follicular development and the expression of lysophosphatidic acid receptor genes during in vitro culture of mouse ovary

Lysophosphatidic acid (LPA) known as a serum-derived growth factor, is involved in several cell physiological functions in the female reproductive system including: oocyte maturation, in vitro fertilization and embryo implantation by its transmembrane G protein-coupled receptors. The aim of the present study was to examine the effect of LPA on in vitro follicular development of mouse ovarian tissue. Neonatal mouse ovarian tissues were cultured in five different concentrations of LPA (0, 5, 10, 20 and 40 µM). The developmental competence and the function of cultured ovarian tissue were assessed by morphological study using hematoxylin and eosin staining and hormonal analysis. The expression of LPA receptor (LPAR 1-4) genes were analyzed by real-time RT-PCR. The proportion of preantral follicles and the level of E₂ hormone were significantly higher in the 20 µM LPA-treated group than those in the other treatment groups. There was a significant difference in the expression of LPAR 1-4 genes in 20 µM LPA treated group in comparison with 0 µM LPA (control group) treated and non-cultured groups. In addition, the expression of LPAR1 gene was higher than other receptor genes in all studied groups. In conclusion supplementation of the media with 20 µM LPA, could improve the survival and developmental potential of follicles and it had positive effects on cell function and stimulation of E₂ synthesis in mouse whole ovarian tissues.

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.

Inhibitors of c-Jun phosphorylation impede ovine primordial follicle activation

STUDY HYPOTHESIS

Is the c-Jun-N-terminal kinase (JNK) pathway implicated in primordial follicle activation?

STUDY FINDING

Culture of ovine ovarian cortex in the presence of two different c-Jun phosphorylation inhibitors impeded pre-antral follicle activation.

WHAT IS KNOWN ALREADY

Despite its importance for fertility preservation therapies, the mechanisms of primordial follicle activation are poorly understood. Amongst different signalling pathways potentially involved, the JNK pathway has been previously shown to be essential for cell cycle progression and pre-antral follicle development in mice.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Ovine ovarian cortex pieces were cultured with varying concentrations of SP600125, JNK inhibitor VIII or anti-Mullerian hormone (AMH) in the presence of FSH for 9 days. Follicular morphometry and immunohistochemistry for proliferating cell nuclear antigen (PCNA), apoptosis and follicle activation (Foxo3a) were assessed.

MAIN RESULTS AND THE ROLE OF CHANCE

Inhibition of primordial follicle activation occurred in the presence of SP600125, JNK inhibitor VIII and AMH when compared with controls (all P < 0.05) after 2 days of culture. However, only in the highest concentrations used was the inhibition of activation associated with induction of follicular apoptosis (P < 0.05). In growing follicles, PCNA antigen expression was reduced when the JNK inhibitors or AMH were used (P < 0.05 versus control), indicating reduced proliferation of the somatic compartment.

LIMITATIONS, REASONS FOR CAUTION

Although we evaluated the effects of inhibition of c-Jun phosphorylation on primordial follicle development, we did not determine the cellular targets and mechanism of action of the inhibitors.

WIDER IMPLICATIONS OF THE FINDINGS

These results are the first to implicate the JNK pathway in primordial follicle activation and could have significant consequences for the successful development of fertility preservation strategies and our understanding of primordial follicle activation.

LARGE SCALE DATA

n/a.

STUDY FUNDING AND COMPETING INTERESTS

Dr Michael J. Bertoldo and the laboratories involved in the present study were supported by a grant from 'Région Centre' (CRYOVAIRE, Grant number #320000268). There are no conflicts of interest to declare.

Ovarian follicular growth suppression by long-term treatment with a GnRH agonist and impact on small follicle number, oocyte yield, and in vitro embryo production in Zebu beef cows

The aim of the present study was to evaluate small follicle number, oocyte yield, and in vitro embryo production (IVEP) in Zebu beef cows treated long term with a GnRH agonist to suppress ovarian follicular growth. Nelore (Bos indicus) cows (n = 20) showing regular estrous cycles were randomly assigned to one of two groups: control (n = 10, placebo ear implant without a GnRH agonist); GnRH agonist (n = 10, GnRH agonist ear implant containing 9.4-mg deslorelin). All cows underwent an ovum pick-up (OPU) session 14 days (Day 14) before the start of treatments (Day 0) followed by seven OPU-IVEP procedures at 30-day intervals (Days 0, 30, 60, 90, 120, 150, and 180). Semen from a single batch of a previously tested bull was used for all the IVEP. Cows treated with agonist reported a decrease over time in the proportion of animals with a (CL; P ≤ 0.05) and large follicles (>10 mm, P ≤ 0.05). These cows had a lesser number of medium + large follicles (>5 mm; 1.74 ± 0.5 vs. 4.13 ± 0.5; P ≤ 0.05), greater number of small follicles (2-5 mm; 44.3 ± 2.8 vs. 30.8 ± 1.8; P ≤ 0.05), greater yield of cumulus-oocyte complexes (COCs; 21.0 ± 2.3 vs. 15.6 ± 1.9; P ≤ 0.05), greater proportion of COCs cultured (79.2 vs. 73.9%; P ≤ 0.05), COCs cleaved (10.6 ± 1.5 vs. 6.8 ± 1.1, P ≤ 0.05), and cleaved rate (52.8 vs. 44.3%; P ≤ 0.05) compared with control cows. The number (3.4 ± 0.7 vs. 3.0 ± 0.6; P > 0.05) and proportion (16.5 vs. 19.1%; P > 0.05) of blastocysts produced were similar between agonist and control cows, respectively. The study has shown that Zebu beef cows treated long term with a GnRH agonist had follicular growth restricted to small follicles. This did not compromise the ability of oocytes to undergo IVF and embryonic development.

Regulation of Anti-Müllerian Hormone and Its Receptor Expression around Follicle Deviation in Cattle

The anti-Müllerian hormone (AMH) is an important marker of ovarian reserve and for predicting the response to superovulatory treatments in several species. The objective of this study was to investigate whether AMH and its receptor (AMHR2) are regulated in bovine granulosa cells during follicular development. In the first experiment, granulosa cells were retrieved from the two largest follicles on days 2 (before), 3 (at the expected time) or 4 (after deviation) of follicular wave. In the second experiment, four doses of FSH (30, 30, 20 and 20 mg) or saline were administered twice a day starting on Day 2 of the first follicular wave of the cycle. Granulosa cells and follicular fluid were collected from the two largest follicles 12 h after the last injection of FSH or saline. AMH mRNA abundance was similar in granulosa cells of the two largest follicles (F1 and F2) before deviation (Day 2), but greater in dominant (DF) than subordinate follicles (SF) at the expected time (Day 3) and after (Day 4) deviation (p < 0.05). In experiment 1, AMH mRNA levels declined in both DF and SF near the expected time and after deviation when compared to before deviation. There was no difference in AMHR2 mRNA levels before and during follicular deviation (p > 0.05), but they tended to be greater in DFs than SFs (p < 0.1) after deviation. Experiment 2 showed that AMH and AMHR2 mRNA in granulosa cells and AMH protein abundance in follicular fluid were similar (p > 0.05) between both co-dominant follicles collected from the FSH-treated cows. These findings indicate the followings: AMH mRNA levels decrease in both DFs and SFs during follicular deviation; granulosa cells from heathy follicles express more AMH mRNA compared to subordinate follicles undergoing atresia and FSH stimulates AMH and AMHR2 mRNA expression in granulosa cells of co-dominant follicles.

Changes in the transcriptome of bovine ovarian cortex during follicle activation in vitro

The signals that regulate activation, a key transition in ovarian follicular development, are still not well understood, especially in nonrodent species. To gain insight into the regulation of this transition in cattle, we combined a microarray approach with an in vitro system in which ovarian cortical pieces cultured in control medium are enriched for primordial follicles, whereas pieces cultured with insulin are enriched for primary follicles. Total RNA was extracted from cultured cortical pieces, and then transcripts were identified and analyzed using the Affymetrix Bovine Genome GeneChip array. Around 65% of the transcripts in the bovine GeneChip were detected in cultured cortical pieces. Comparison between pieces cultured with or without insulin generated 158 differentially expressed transcripts. Compared with controls, 90 transcripts were upregulated and 68 were downregulated by insulin. These transcripts are involved in many biological processes and functions, but most are associated with cellular growth or cell cycle/cell death. The transcript encoding ubiquitin-conjugating enzyme E2C (UBE2C) was significantly upregulated during follicle activation, and Ingenuity Pathways Analysis revealed that UBE2C can interact with the tumor suppressor phosphatase and tensin homolog (PTEN). Both PTEN mRNA and protein were lower in cortical pieces cultured with insulin than in controls. In addition, FOXO3a, a downstream effector of PTEN signaling, underwent nuclear-cytoplasmic shuttling during primordial to primary follicle development in bovine fetal ovaries, further suggesting the involvement of the PTEN pathway in follicle activation in cattle. Genes and pathways identified in this study provide interesting candidates for further investigation of mechanisms underlying follicle activation.

Damage to fetal bovine ovarian tissue caused by cryoprotectant exposure and vitrification is mitigated during tissue culture

PURPOSE

The objective of this study is to characterize the impact of exposure to cryoprotectants followed by vitrification on primordial follicle survival and activation using a fetal bovine model.

METHODS

In the first study, fetal bovine cortical pieces were exposed to cryoprotectants with or without sucrose and cultured up to 7 days in the presence or absence of insulin. In the second study, cortical pieces were exposed to cryoprotectants with or without sucrose, vitrified, and cultured up to 7 days after warming in the presence or absence of insulin. Viability and morphology of follicles, as well as proliferation and/or DNA repair in ovarian tissue were analyzed.

RESULTS

When compared to non-exposed controls, normal follicular morphology was affected in groups exposed to cryoprotectants only immediately post-exposure and after 1 day of culture, but improved by day 3 and did not significantly differ by day 7. Similarly, normal follicular morphology was compromised in vitrified groups after warming and on day 1 compared to controls, but improved by days 3 and 7. Proliferation and/or DNA repair in ovarian tissue was not affected by vitrification in this model. Cryoprotectant exposure and vitrification of ovarian tissue did not impair the activation of primordial follicles in response to insulin, although activation was delayed relative to non-exposed controls. Interestingly, sucrose had no noticeable protective effect.

CONCLUSION

Vitrified fetal bovine ovarian tissue has the intrinsic capacity to mitigate the immediate damage to primordial follicles' morphology and retains the capacity to activate. These findings provide a basis for a successful cryopreservation protocol for ovarian cortical tissue in other species including humans.

Plasma anti-mullerian hormone: an endocrine marker for in vitro embryo production from Bos taurus and Bos indicus donors

The aim of this study was to evaluate the association between plasma anti-mullerian hormone (AMH) concentration and in vitro embryo production (IVP) from Bos taurus (Holstein) and Bos indicus (Nelore) donors. A total of 59 Holstein (15 prepubertal heifers aged 8-10 mo, 15 cyclic heifers aged 12-14 mo, 14 lactating cows, and 15 nonlactating cows) and 34 Nelore (12 prepubertal heifers aged 10-11 mo, 10 prepubertal heifers aged 21-23 mo, and 12 cyclic heifers aged 24-26 mo) females were enrolled. All females underwent an ovum pick-up (OPU), without previous synchronization of the follicular wave, and IVP procedure. Immediately before the OPU procedure, blood samples were collected for subsequent AMH determination. A positive correlation was observed between the plasma AMH and number of in vitro embryos produced from Holstein (r = 0.36, P < 0.001) and Nelore (r = 0.50, P = 0.003) donors. For additional analyses, donors within each genotype were classified into 1 of 2 AMH categories (low or high) according to the average AMH concentration for each genotype. The results revealed that females classified as having high AMH presented a greater number of visible aspirated follicles (Holstein: 20.9 ± 1.5 vs 13.6 ± 0.9, P < 0.0001; Nelore: 54.3 ± 6.1 vs 18.6 ± 2.1, P < 0.0001) and a greater number of recovered cumulus-oocyte complexes (Holstein: 17.3 ± 1.5 vs 9.0 ± 0.9, P < 0.0001; Nelore: 45.3 ± 6.4 vs 13.4 ± 1.7, P < 0.0001). However, there was no difference in the blastocyst production rate (Holstein: 20.6% ± 4.0% vs 19.8% ± 4.2%, P = 0.60; Nelore: 33.7% ± 6.5% vs 27.4% ± 5.5%, P = 0.41, high and low AMH, respectively). Moreover, donors classified as having high AMH yielded a greater number of embryos produced per OPU (Holstein: 3.0 ± 0.7; Nelore: 7.0 ± 1.7) compared with those classified as having low AMH (Holstein: 1.2 ± 0.3, P = 0.04; Nelore: 2.2 ± 0.5, P = 0.007). In conclusion, although the plasma AMH concentration did not alter the ability of the cumulus-oocyte complex to reach the blastocyst stage, the AMH concentration in plasma can be an accurate endocrine marker for the in vitro embryo yield from either B. taurus (Holstein) or B. indicus (Nelore) donors. Therefore, AMH is a promising tool to enhance the overall efficiency of OPU-IVP programs in the field as a selective criterion for high embryo producing donors.

Monitoring preantral follicle survival and growth in bovine ovarian biopsies by repeated use of neutral red and cultured in vitro under low and high oxygen tension

The development and optimization of preantral follicle culture methods are crucial in fertility preservation strategies. As preantral follicle dynamics are usually assessed by various invasive techniques, the need for alternative noninvasive evaluation tools exists. Recently, neutral red (NR) was put forward to visualize preantral follicles in situ within ovarian cortical fragments. However, intense light exposure of NR-stained tissues can lead to cell death because of increased reactive oxygen species production, which is also associated with elevated oxygen tension. Therefore, we hypothesize that after repeated NR staining, follicle viability and dynamics can be altered by changes in oxygen tension. In the present study, we aim (1) to determine whether NR can be used to repeatedly assess follicular growth, activation, and viability and (2) to assess the effect of a low (5% O2) or high (20% O2) oxygen tension on the viability, growth, and stage transition of preantral follicles cultured in vitro by means of repeated NR staining. Cortical slices (n = 132; six replicates) from bovine ovaries were incubated for 3 hours at 37 °C in a Leibovitz medium with 50 μg/mL NR. NR-stained follicles were evaluated in situ for follicle diameter and morphology. Next, cortical fragments were individually cultured in McCoy's 5A medium for 6 days at 37 °C, 5% CO2, and 5% or 20% O2. On Days 4 and 6, the fragments were restained by adding NR to the McCoy's medium and follicles were reassessed. In both low and high oxygen tension treatment groups, approximately 70% of the initial follicles survived a 6-day in vitro culture, but no significant difference in follicle survival on Day 4 or 6 could be observed compared with Day 0 (P > 0.05). A significant decrease in the number of primordial and increase in primary and secondary follicles was observed within 4 days of culture (P < 0.001). In addition, a significant increase of the mean follicle diameter in NR-stained follicles was observed (P < 0.001), resulting in an average growth of 11.82 ± 0.81 μm (5% O2) and 11.78 ± 1.06 μm (20% O2) on Day 4 and 20.94 ± 1.24 μm (5% O2) and 19.12 ± 1.36 μm (20% O2) on Day 6 compared with Day 0. No significant differences in follicle growth rate or stage transition could be observed between 5% and 20% O2 (P > 0.05). In conclusion, after repeated NR staining, we could not find a difference between low and high oxygen tension in terms of follicle viability, stage transition, or growth. Therefore, under our culture conditions follicle dynamics are not determined by the oxygen tension in combination with quality assessment protocols using repeated NR staining.

Expression of fibroblast growth factor 10 and cognate receptors in the developing bovine ovary

In the mammalian ovary, FGF10 is expressed in oocytes and theca cells and is a candidate for paracrine signaling to the developing granulosa cells. To gain insight into the participation of FGF10 in the regulation of fetal folliculogenesis, we assessed mRNA expression patterns of FGF10 and its receptors, FGFR1B and FGFR2B, in relation to fetal follicle dynamics and localized FGF10 protein in bovine fetal ovaries at different ages. Primordial, primary, secondary, and antral follicles were first observed on Days 75, 90, 150, and 210 of gestation, respectively. The levels of GDF9 and BMP15 mRNA, markers for primordial and primary follicles, respectively, increased during fetal ovary development in a consistent manner with fetal follicle dynamics. CYP17A1 mRNA abundance increased from Day 60 to Day 75 and then from Day 120 to Day 150, coinciding with the appearance of secondary follicles. FGF10 mRNA abundance increased from Day 90, and this increase was temporally associated with increases in FGFR1B mRNA abundance and in the population of primary follicles. In contrast, FGFR2B mRNA expression was highest on Day 60 and decreased thereafter. FGF10 protein was localized to oogonia and oocytes and surrounding granulosa cells at all fetal ages. The present data suggest a role for FGF10 in the control of fetal folliculogenesis in cattle.

Transcriptome profiling of granulosa cells of bovine ovarian follicles during growth from small to large antral sizes

Background

At later stages of folliculogenesis, the mammalian ovarian follicle contains layers of epithelial granulosa cells surrounding an antral cavity. During follicle development granulosa cells replicate, secrete hormones and support the growth of the oocyte. In cattle, the follicle needs to grow > 10 mm in diameter to allow an oocyte to ovulate, following which the granulosa cells cease dividing and differentiate into the specialised cells of the corpus luteum. To better understand the molecular basis of follicular growth and granulosa cell maturation, we undertook transcriptome profiling of granulosa cells from small (< 5 mm; n = 10) and large (> 10 mm, n = 4) healthy bovine follicles using Affymetrix microarrays (24,128 probe sets).

Results

Principal component analysis for the first two components and hierarchical clustering showed clustering into two groups, small and large, with the former being more heterogeneous. Size-frequency distributions of the coefficient of variation of the signal intensities of each probe set also revealed that small follicles were more heterogeneous than the large. IPA and GO enrichment analyses revealed that processes of axonal guidance, immune signalling and cell rearrangement were most affected in large follicles. The most important networks were associated with: (A) Notch, SLIT/ROBO and PI3K signalling, and (B) ITGB5 and extracellular matrix signalling through extracellular signal related kinases (ERKs). Upstream regulator genes which were predicted to be active in large follicles included STAT and XBP1. By comparison, developmental processes such as those stimulated by KIT, IHH and MEST were most active in small follicles. MGEA5 was identified as an upstream regulator in small follicles. It encodes an enzyme that modifies the activity of many target proteins, including those involved in energy sensing, by removal of N-acetylglucosamine from serine and threonine residues.

Conclusions

Our data suggest that as follicles enlarge more genes and/or pathways are activated than are inactivated, and gene expression becomes more uniform. These findings could be interpreted that either the cells in large follicles are more uniform in their gene expression, or that follicles are more uniform or a combination of both and that additional factors, such as LH, are additionally controlling the granulosa cells.

Effects of growth differentiation factor-9 and FSH on in vitro development, viability and mRNA expression in bovine preantral follicles

The present study investigated the role of growth differentiation factor (GDF)-9 and FSH, alone or in combination, on the growth, viability and mRNA expression of FSH receptor, proliferating cell nuclear antigen (PCNA) and proteoglycan-related factors (i.e. hyaluronan synthase (HAS) 1, HAS2, versican, perlecan) in bovine secondary follicles before and after in vitro culture. After 12 days culture, sequential FSH (100 ng mL–1 from Days 0 to 6 and 500 ng mL–1 from Days 7 to 12) increased follicular diameter and resulted in increased antrum formation (P < 0.05). Alone, 200 ng mL–1 GDF-9 significantly reduced HAS1 mRNA levels, but increased versican and perlecan mRNA levels in whole follicles, which included the oocyte, theca and granulosa cells. Together, FSH and GDF-9 increased HAS2 and versican (VCAN) mRNA levels, but decreased PCNA mRNA expression, compared with levels in follicles cultured in α-minimum essential medium supplemented with 3.0 mg mL–1 bovine serum albumin, 10 µg mL–1 insulin, 5.5 µg mL–1 transferrin, 5 ng mL–1 selenium, 2 mM glutamine, 2 mM hypoxanthine and 50 μg mL–1 ascorbic acid (α-MEM+). Comparisons of uncultured (0.2 mm) and α-MEM+ cultured follicles revealed that HAS1 mRNA expression was higher, whereas VCAN expression was lower, in cultured follicles (P < 0.05). Expression of HAS1, VCAN and perlecan (HSPG2) was higher in cultured than in vivo-grown (0.3 mm) follicles. In conclusion, FSH and/or GDF-9 promote follicular growth and antrum formation. Moreover, GDF-9 stimulates expression of versican and perlecan and interacts positively with FSH to increase HAS2 expression.

In vitro activation of dormant follicles for fertility preservation

Recent advances in radiotherapy and chemotherapy have led to higher cure rates for female children and adolescents with cancer. However, these treatments adversely affect germ cell survival, and ovarian failure is thus a probable side effect of these anticancer therapies. Moreover, an increasing number of women are choosing to postpone childbearing until later in life, but their primordial follicle reserves degenerate with advancing age. Thus there is a pressing need for the development of fertility preservation methods for these individuals. Ovarian tissue cryopreservation prior to loss of the primordial follicle population either due to cancer treatments or normal aging is a promising option for safeguarding fertility. A complete in vitro maturation (IVM) system could help generate mature eggs for later use without the patient having to undergo the cumbersome process involved in current assisted reproduction methods to generate mature eggs. Cryopreserved ovarian cortical tissues have attracted the attention of reproductive biologists and clinicians because of the large number of safely frozen primordial follicles in them, and it is theoretically possible to use these follicles for in vitro activation (IVA) and subsequent IVM. Ovarian tissue collection is independent of patient age and social or personal conditions. Despite being widely accepted potential techniques for fertility preservation, IVA and IVM of human primordial follicles to obtain fertilizable eggs remains far from reality. This chapter highlights the current achievements and obstacles in obtaining growing follicles through activation of dormant follicles.

Equine chorionic gonadotropin alters luteal cell morphologic features related to progesterone synthesis

Exogenous eCG for stimulation of a single dominant follicle or for superovulation are common strategies to improve reproductive efficiency by increasing pregnancy rates and embryo production, respectively. Morphofunctional changes in the CL of eCG-treated cattle include increases in CL volume and plasma progesterone concentrations. Therefore, we tested the hypothesis that eCG alters the content of luteal cells and mitochondria related to hormone production. Twelve crossbred beef cows were synchronized and then allocated into three groups (four cows per group) and received no further treatment (control) or were given eCG either before or after follicular deviation (superovulation and stimulation of the dominant follicle, respectively). Six days after ovulation, cows were slaughtered and CL collected for morphohistologic and ultrastructural analysis. Mitochondrial volume per CL was highest in superovulated followed by stimulated and then control cows (18,500 ± 2630, 12,300 ± 2640, and 7670 ± 3400 μm(3); P < 0.001), and the density of spherical mitochondria and the total number of large luteal cells were increased (P < 0.05) in stimulated cows compared with the other two groups (110.32 ± 14.22, 72.26 ± 8.77, and 70.46 ± 9.58 mitochondria per μm(3) and 678 ± 147, 245 ± 199, and 346 ± 38 × 10(6) cells, respectively. However, the largest diameters of the large luteal cells were increased in superovulated and control cows versus stimulated ones (32.32 ± 0.06, 31.59 ± 0.81, and 29.44 ± 0.77 μm; P < 0.0001). In contrast, the total number of small luteal cells was increased in superovulated cows (1456 ± 268, 492 ± 181, and 822 ± 461 × 10(6), P < 0.05). In conclusion, there were indications of cellular changes related to increased hormonal production (stimulatory treatment) and increased CL volume (superovulatory treatment).