Spindle abnormalities and chromosome misalignment in bovine oocytes after exposure to low doses of bisphenol A or bisphenol S

STUDY QUESTION

What are the effects of exposure to bisphenol A (BPA) or bisphenol S (BPS) during IVM on bovine oocyte maturation, spindle morphology and chromosome alignment?

SUMMARY ANSWER

Exposure to BPA or BPS during IVM resulted in increased spindle abnormalities and chromosome misalignment, even at very low concentrations.

WHAT IS KNOWN ALREADY

BPA is an endocrine disrupting chemical that alters oocyte maturation, spindle morphology and chromosome alignment in a range of species. The use of BPA substitutes, such as BPS, is increasing and these substitutes often display different potencies and mechanisms of action compared with BPA.

STUDY DESIGN, SIZE, DURATION

Bovine cumulus-oocyte complexes (COCs) underwent IVM with BPA or BPS for 24 h, together with vehicle-only controls. Overall, 10 different concentrations of BPA or BPS were used ranging from 1 fM to 50 μM in order to detect low dose or non-monotonic effects. An incomplete block design was utilized for the study, with at least three replicates per block. A total of 939 oocytes (250 of which were controls) were used for the BPA experiments, and 432 (110 controls) for the BPS experiments. Following the IVM period, the oocytes were denuded and fixed for immunocytochemistry.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immunocytochemistry was used to label the chromatin, actin, and microtubules in the fixed oocytes. The meiotic stage was assessed using immunofluorescence, and the metaphase-II (MII) oocytes were further assessed for spindle morphology and chromosome alignment (in all MII oocytes regardless of spindle morphology) using immunofluorescence and confocal microscopy. Significant differences between the treatment and control groups were determined using chi-square and Fisher's exact tests.

MAIN RESULTS AND THE ROLE OF CHANCE

There was no effect of BPA or BPS on the proportion of bovine oocytes that reached MII (P > 0.05). BPA and BPS increased spindle abnormalities in MII oocytes at almost all concentrations tested, including those as low as 1 fM (P = 0.013) or 10 fM (P < 0.0001), respectively, compared to control. Oocytes with flattened spindles with broad poles were observed at a higher frequency at some concentrations of BPA (P = 0.0002 and P = 0.002 for 10 nM and 50 μM, respectively) or BPS (P = 0.01 for 100 nM BPS), while this spindle phenotype was absent in the controls. BPA increased chromosome misalignment at concentrations of 10 fM, 10 nM and 50 μM (P < 0.0001 to P = 0.043 depending on the dose). BPS increased chromosome misalignment at concentrations of 10 fM, 100 pM, 10 nM, 100 nM and 50 μM (P < 0.0001 to P = 0.013 depending on the dose).

LIMITATIONS REASONS FOR CAUTION

Exposures to BPA or BPS were performed during the IVM of COCs to allow for determination of direct effects of these chemicals on oocyte maturation. Whole follicle culture or in vivo studies will confirm whether follicular cell interactions modify the effects of BPA or BPS on oocyte meiotic maturation. Investigation into the effects of BPA or BPS on other oocyte functions will determine whether these chemicals alter oocyte quality via mechanisms independent of the meiotic endpoints characterized here.

WIDER IMPLICATIONS OF THE FINDINGS

The findings of this study show that both BPA and BPS induce spindle abnormalities and chromosome misalignment in bovine in a non-monotonic manner, and at concentrations that are orders of magnitude below those measured in humans. Taken in context with previous studies on the effects of BPA in a range of species, our data support the literature that BPA may reduce oocyte quality and lead to subsequent infertility. Additionally, these results contribute to the burgeoning field of research on BPS and suggest that BPS may indeed be a 'regrettable substitution' for BPA.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by funding from the National Institutes of Health (NIH) (Grant 1R15ES024520-01). The authors declare no conflict of interest.

Bisphenol-A exposure and gene expression in human luteinized membrana granulosa cells in vitro

STUDY QUESTION

Does bisphenol-A (BPA) affect gene expression in human membrana granulosa cells (MGC)?

SUMMARY ANSWER

In vitro, short exposure to supra-physiological concentrations of BPA alters human MGC gene expression.

WHAT IS KNOWN ALREADY

Exposure to BPA may interfere with reproductive endocrine signaling. In vitro studies, mostly in animal models, have shown an inverse correlation between exposure to BPA and follicular growth, meiosis, and steroid hormone production in granulosa cells.

STUDY DESIGN, SIZE, DURATION

Primary cultures of MGC obtained from 24 patients undergoing IVF (for PGD, male factor infertility or unexplained infertility) were exposed to various concentrations of BPA (0, 0.02, 0.2, 2 or 20 µg/ml) for 48 h.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The study was conducted in a university-affiliated hospital. Microarray analysis was used to identify genes exhibiting expression changes following BPA exposure. Genes significantly altered were identified based on changes greater than 2-fold relative to the control group (not treated by BPA) and a Student's t-test P-value <0.05. Statistical significance was adjusted for multiple comparisons using the Benjamini-Hochberg method. Alterations in the expression of genes that are involved in the enriched functional annotations altered by BPA at the concentration of 20 µg/ml were confirmed by real-time PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

A distinct pattern of gene expression was observed in primary cultures of MGC exposed to the highest BPA concentration compared with untreated cells. We identified 652 genes that exhibited at least 2-fold differences in expression after BPA exposure (all P < 0.05 versus untreated). These genes were significantly enriched for annotations related to cell cycle progression, segregation of chromosomes, steroid metabolism, apoptosis, lipid synthesis, oocyte maturation and chromosomal alignment. No significant changes in gene expression were found at the lower doses of BPA most relevant to human exposure.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Human exposure to BPA in vivo occurs over long periods of time. In this in vitro model, cells were exposed to the chemical for 48 h only. Thus, the effects of BPA on the human follicle might be underestimated.

WIDER IMPLICATIONS OF THE FINDINGS

As BPA exposure is ubiquitous, understanding the effects of the chemical on the ovary, specifically in women of reproductive age, has public health significance. The clinical evidence to date points to an association between BPA exposure and impaired IVF outcome, although not all studies have shown negative effects. Our study adds valuable mechanistic information showing that exposure to BPA alters granulosa cell gene expression at high and supra-physiological doses.

STUDY FUNDING/COMPETING INTERESTS

This study was supported by grant number 1936/12 from the ISF. The authors have nothing to disclose.

The antral follicle: a microenvironment for oocyte differentiation

Mammalian reproduction hinges upon the timely ovulation of a fully differentiated oocyte. This event is the culmination of a complex and dynamic developmental relationship between the oocyte and the antral follicle housing it; the antral follicle constitutes a specialized microenvironment or niche, uniquely suited to the needs of the oocyte as it approaches ovulation. During this time, the oocyte must complete its final growth, capacitation, and nuclear and cytoplasmic maturation. Its microenvironment--the antral follicle--is in turn responsible for the integrity of these processes and the production of a high quality oocyte. Components of the antral follicle, including three distinct somatic cell types (theca, granulosa and cumulus), the basal lamina, and follicular fluid, each have active and regulatory roles in oocyte differentiation. Several milestones in antral folliculogenesis also have an influence on oocyte development. This review will discuss the antral follicle microenvironment with specific attention to its importance in oocyte differentiation. As assisted reproductive technologies (ART) often require stages of oocyte differentiation to occur in vitro rather than in vivo, current knowledge of the antral follicle microenvironment will also be discussed with respect to its clinical applications.

Over 40 years of mentoring, educating, and researching in the world of oocytes

David Albertini has dedicated his life to illuminating our understanding of the most wondrous of cells--the oocyte. Beyond his powerful scientific contributions, he has mindfully and tirelessly mentored and educated scientists and clinicians in our field. In this essay which reports a dialogue, David Albertini shares some of the key experiences that have governed his career path. He has been a spokesperson to the public to ensure the accurate conveying of current findings in our field, and he has always strived to help others in communicating effectively. He also reflects (notably in light of where funding priorities may lie) on the imperative to use animal model systems that will be most suitable for addressing the pressing questions of reproductive biology today. Dr. Albertini pioneered the use of live cell imaging approaches over 30 years ago, and he has eagerly passed on his expertise to many others while these techniques were in their infancies. His career has been fueled by his passion for visualizing cellular events in live cells and tissues, as never undertaken or seen before. He took chances while always embracing opportunities as they arose. Dr. Albertini has also delineated the intersection between basic research on the oocyte and emerging trends in reproductive medicine--such as oocyte cryopreservation. Not only does he continue to advance the field of human oocyte biology, but he is also, and yet again, extending his role as educator and mentor by taking a lead in reproductive medicine as a journal editor and as a mentor to young and rising clinicians in the field.

Follicular fluid hydrogen peroxide and lipid hydroperoxide in bovine antral follicles of various size, atresia, and dominance status

PURPOSE

To avoid inducing a state of oxidative stress (OS), assisted reproductive technologies (ART) must maintain a balance of reactive oxygen species (ROS) and antioxidants during the in vitro culture of oocytes. However, oocyte requirements and tolerance thresholds for ROS during in vivo development are still unclear. Previous studies have examined ROS levels in follicular fluid (FF) using pooled samples or according to follicle size. This study sought to examine two OS markers, lipid hydroperoxides (LPO) and hydrogen peroxide (H2O2), in FF of individually sampled follicles from bovine ovary pairs according to follicle size, atresia, and dominance status.

METHODS

TUNEL and cleaved Caspase-3 labeling were used to identify apoptotic granulosa cells and determine follicle atresia status. LPO were measured directly for the first time in FF.

RESULTS

Non-atretic follicles and dominant follicles contained more FF H2O2 than atretic follicles and corresponding subordinate follicles, respectively. FF LPO did not vary in relation to atretic status, and no difference existed between dominant and subordinate follicles. However, FF LPO was significantly lower in first subordinate follicles than in the second subordinate follicles from each ovary pair. Neither H2O2 nor LPO levels correlated with follicle size.

CONCLUSIONS

These data provide clear evidence that the events of antral folliculogenesis are relevant to ROS dynamics in vivo. Furthermore, such studies will help to optimize in vitro conditions for oocyte culture protocols, particularly when combined with a comparison of oocyte quality with respect to source follicle characteristics.

The association between severe obesity and characteristics of failed fertilized oocytes

STUDY QUESTION

Is the cytoskeletal and chromosomal organization of failed fertilized oocytes from severely obese patients (BMI ≥ 35 kg/m²) altered compared with that in patients with normal BMI (BMI 18.5-24.9 kg/m²)?

SUMMARY ANSWER

Compared with normal BMI patients, severe obesity was associated with a greater prevalence of spindle anomalies and non-aligned chromosomes in failed fertilized oocytes.

WHAT IS KNOWN AND WHAT THIS PAPER ADDS

Obesity is associated with poor reproductive outcomes, but little is known regarding the underlying mechanisms. To address potential mechanisms, our study compared the cytoskeletal and chromosome organization in failed fertilized oocytes from severely obese and normal BMI patients.

DESIGN

The study population was drawn from IVF patients treated in a hospital-based infertility clinic between February 2010 and July 2011. The prevalence of meiotic spindle and chromosome alignment anomalies in failed fertilized oocytes from patients with severe obesity (i.e. Class II and III; BMI 35.0-50.1 kg/m²) was compared with those from patients with normal BMI (BMI 18.5-24.9 kg/m²). Oocytes were fixed and then labeled for tubulin, actin and chromatin. Spindle number and integrity, as well as chromosome alignment, were assessed using immunofluorescence microscopy and, in some cases, confocal microscopy. Generalized estimating equations were applied, which account for the correlation among oocytes from the same patient to estimate odds ratio (OR), 95% confidence intervals (CIs) and two-sided Wald P-values. Models were adjusted for continuous age at cycle start, cycle type (IVF or ICSI) and polycystic ovarian syndrome (PCOS) a priori.

PARTICIPANTS AND SETTING

University-affiliated infertility clinic. A total of 276 oocytes that failed to fertilize from 137 patients were evaluated: 105 oocytes from severely obese women (n = 47) and 171 oocytes from normal BMI patients (n = 90).

MAIN RESULTS AND THE ROLE OF CHANCE

(i) Significantly more oocytes from the severely obese group exhibited two spindles compared with those from the normal BMI group (58.9 versus 35.1%; OR = 2.68, CI = 1.39-5.15, P-value = 0.003). (ii) Among oocytes with a single spindle, those from severely obese patients showed a significantly higher prevalence of disarranged spindles with non-aligned chromosomes compared with those from normal BMI patients (28.6 versus 8.6%; OR = 4.58, CI = 1.05-19.86, P-value = 0.04).

BIAS, CONFOUNDING AND OTHER REASONS FOR CAUTION

Inclusion of only failed fertilized oocytes, small sample size, unknown factors such as non-PCOS comorbidity.

GENERALIZABILITY TO OTHER POPULATIONS

For this study, by design, it is unclear whether the findings are generalizable to successfully fertilized oocytes, and whether this oocyte-level influence of obesity is generalizable to infertile women who do not undergo stimulation or, more broadly, to spontaneous conceptions in fertile women.

STUDY FUNDING/COMPETING INTEREST(S)

none.

TRIAL REGISTRATION NUMBER

n/a.

Is it best to cryopreserve human cumulus-free immature oocytes before or after in vitro maturation?

Freezing unfertilized oocytes is an option for females without a partner, either to preserve their fertility prior to sterilizing cancer treatment or for social reasons. Our study considered whether it is best to freeze immature human oocytes at the germinal vesicle (GV) stage, prior to in vitro maturation (IVM) or at metaphase-II (M-II), after IVM. Sibling GV-stage oocytes from stimulated ICSI cycles were allocated to freezing either prior to (n=109) or after (n=107) IVM. Cumulus-free oocytes were cryopreserved using a choline-substituted slow-freezing protocol and matured in a defined medium, with analysis of chromatin, microtubules, and microfilaments by three-dimensional imaging. Cryopreserved oocytes were compared with oocytes matured in vitro but never frozen (n=114). Survival was similar between oocytes frozen before or after IVM (69.7% vs. 70.5%). Polar body extrusion after IVM was lower in oocytes frozen at the GV stage versus those matured and then frozen (51.3% vs. 75.7%) or not frozen (75.4%). Stratification by patient age (<36 and ⩾36year) showed no difference in oocyte survival or maturation. Oocytes frozen as GVs showed elevated proportions of spontaneous activation (with or without polar body), an effect augmented by patient age. Spindle and chromosome configurations were disrupted to similar extents in both groups of frozen oocytes, with no further detrimental effect of patient age. The length, width, and volume of bipolar M-II spindles were comparable in all three groups. When frozen as GVs, oocytes exhibited decreased maturation and increased spontaneous activation, suggesting that it is best to freeze oocytes at M-II.

Release of superoxide dismutase-1 by day 3 embryos of varying quality and implantation potential

PURPOSE

To determine if the antioxidant superoxide dismutase-1 (SOD1 or Cu,Zn-SOD) is released by cultured human cleavage-stage embryos and to assess any link between SOD1 and implantation potential.

METHODS

Women (n = 91; ≤40 years old) undergoing IVF treatment with transfer of one or two 8-cell embryos that resulted in 0 or 100% implantation were included. Following individual embryo culture, spent medium samples (n = 122) were collected and levels of SOD1 protein were measured by an enzyme-linked immunosorbent assay. SOD1 detection and concentration in embryo spent medium were analyzed in relation to embryo fragmentation and symmetry scores, and implantation (viable fetus at >12 weeks).

RESULTS

Cleavage-stage embryos release SOD1 protein into the spent culture medium. Neither detection nor concentration of SOD1 was related to implantation. There was a positive relationship between increased embryo fragmentation scores and SOD1 release, with no apparent association with symmetry. In non-pregnant cycles, the release of SOD1 decreased with increasing maternal age.

CONCLUSIONS

While SOD1 does not predict implantation potential of select good-quality embryos, our data support the need to evaluate the biological significance of released SOD1 by embryos of varying quality and from patients of varying age.

Maturation outcomes are improved following Cryoleaf vitrification of immature human oocytes when compared to choline-based slow-freezing

PURPOSE

The cryopreservation of immature oocytes permits oocyte banking for patients at risk of losing their fertility. However, the optimum protocol for such fertility preservation remains uncertain.

METHODS

The present study investigated the survival, maturation, cytoskeletal and chromosome organization of sibling immature oocytes leftover from controlled ovarian stimulation cycles, that were either slow-frozen (with choline-substitution) or vitrified. A comparison group included oocytes that were never cryopreserved.

RESULTS

Among the three groups, comparable rates were observed for both survival (67-70%) and polar body extrusion (59-79%). Significantly more oocytes underwent spontaneous activation after IVM following slow-freezing compared with either vitrification or no cryopreservation. Likewise, the incidence of spindle abnormalities was greatest in the slow-frozen group, with no differences in spindle morphometrics or chromosome organization.

CONCLUSIONS

While the overall incidence of mature oocytes with normal bipolar spindles from warmed immature oocytes was low, the yield using Cryoleaf vitrification was slightly superior to choline-based slow-freezing.

Fluctuations in total antioxidant capacity, catalase activity and hydrogen peroxide levels of follicular fluid during bovine folliculogenesis

Follicular fluid is an important environment for oocyte development, yet current knowledge regarding its in vivo oxidant and antioxidant levels remains limited. Examining follicular fluid oxidants and antioxidants will improve understanding of their changes in vivo and contribute to optimisation of in vitro maturation conditions. The aim of the present study was to consider selected markers, namely catalase (CAT) enzyme activity, total antioxidant capacity (TAC) and hydrogen peroxide (H(2)O(2)) in follicular fluid samples (n = 503) originating from bovine antral follicles. The dynamic changes in two relevant antioxidant measures and one reactive oxygen species (ROS) were measured through stages of bovine follicular development and the oestrous cycle. CAT activity and H(2)O(2) levels decreased significantly as follicle size increased, whereas TAC increased significantly as follicle size increased. Lower TAC and higher H(2)O(2) in small follicles suggest increased ROS in the initial stages of folliculogenesis. Because CAT levels are highest in the follicular fluid of small follicles in the setting of an overall low TAC, CAT may represent a dominant antioxidant defence in the initial stages of folliculogenesis. Future studies must focus on other reactive oxygen species and their various scavenger types during antral folliculogenesis.

Cellular and genetic analysis of oocytes and embryos in a human case of spontaneous oocyte activation

Unusual and consistent defects in infertility patients merit attention as these may indicate an underlying genetic abnormality, in turn necessitating tailored management strategies. We describe a case of repeated early pregnancy loss from in vivo conceptions, followed by cancelled embryo transfers after one IVF and one ICSI/PGD cycle. Following the unexpected presence of cleaved embryos at the fertilization check in the first IVF attempt, oocytes and embryos were subsequently analyzed in an ICSI/PGD case. Part of the oocyte cohort was fixed at retrieval for a cellular evaluation of microtubules, microfilaments and chromatin. The remaining oocytes were injected with sperm, and resultant embryos were biopsied for genetic analysis by fluorescence in situ hybridization (FISH), single-nucleotide polymorphism (SNP) microarray for 23 chromosome pairs, as well as with PCR for sex chromosomes. The presence of interphase microtubule networks and pronuclear structures indicated that oocytes were spontaneously activated by the time of retrieval. FISH revealed aneuploidy in all seven blastomeres analyzed, with all but two lacking Y chromosomes. Microarray SNP analysis showed an exclusively maternal origin of all blastomeres analyzed, which was further confirmed by PCR. From our multi-faceted analyses, we conclude that spontaneous activation, or parthenogenesis, was probably the pathology underlying our patient's recurrent inability to maintain a normal pregnancy. Such analyses may prove beneficial not only in diagnosing case-specific aberrations for other patients with similar or related failures, but also for furthering our general understanding of oocyte activation.

Profiling of superoxide dismutase isoenzymes in compartments of the developing bovine antral follicles

The antral follicle constitutes a complex and regulated ovarian microenvironment that influences oocyte quality. Oxidative stress is a cellular state that may play a role during folliculogenesis and oogenesis, although direct supporting evidence is currently lacking. We thus evaluated the expression of the three isoforms (SOD1, SOD2, and SOD3) of the enzymatic antioxidant superoxide dismutase in all the cellular (granulosa cells, cumulus cells, and oocytes) and extracellular (follicular fluid) compartments of the follicle. Comparisons were made in bovine ovaries across progressive stages of antral follicular development. Follicular fluid possessed increased amounts of SOD1, SOD2, and SOD3 in small antral follicles when compared with large antral follicles; concomitantly, total SOD activity was highest in follicular fluids from smaller diameter follicles. SOD1, SOD2, and SOD3 proteins were expressed in granulosa cells without any fluctuations in follicle sizes. All three SOD isoforms were present, but were distributed differently in oocytes from small, medium, or large antral follicles. Cumulus cells expressed high levels of SOD3, some SOD2, but no detectable SOD1. Our studies provide a temporal and spatial expression profile of the three SOD isoforms in the different compartments of the developing bovine antral follicles. These results lay the ground for future investigations into the potential regulation and roles of antioxidants during folliculogenesis and oogenesis.

Could oxidative stress influence the in-vitro maturation of oocytes?

In the efforts aimed at improving the quality of in-vitro-matured human oocytes, the dynamic balance and roles of pro-/antioxidants merit further consideration. In-vitro maturation (IVM) is emerging as a popular technology at the forefront of fertility treatment and preservation. However, standard in-vitro culture conditions exert oxidative stress or an imbalance between oxidants and antioxidants. Reactive oxygen species (ROS) are oxygen-derived molecules formed as intermediary products of cellular metabolism. By acting as powerful oxidants, ROS can oxidatively modify any molecule, resulting in structural and functional alterations. ROS are neutralized by an elaborate defence system consisting of enzymatic and nonenzymatic antioxidants. This review captures the inherent and external factors that may modulate the oxidative stress status of oocytes. It discusses the suspected impacts of oxidative stress on the gamut of events associated with IVM, including prematuration arrest, meiotic progression, chromosomal segregation, cytoskeletal architecture and gene expression. In-vivo and in-vitro strategies that may overcome the potential influences of oxidative stress on oocyte IVM are presented. Future studies profiling the oxidative stress status of the oocyte may permit not only the formulation of a superior IVM medium that maintains an adequate pro-/antioxidant balance, but also the identification of predictors of oocyte quality.

Optimum number of embryos to transfer in women more than 40 years of age undergoing treatment with assisted reproductive technologies

OBJECTIVE

To determine whether increasing the number of embryos transferred beyond five increases pregnancy rates in women aged > 40 years.

DESIGN

Retrospective analysis of cycles performed between January 1998 and July 2003.

SETTING

University-affiliated teaching hospital.

PATIENT(S)

Women aged > 40 years undergoing a fresh cycle with a day-3 ET (n = 863).

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Pregnancy, chemical pregnancy, miscarriage rates, number of viable fetuses at 12 weeks' gestation, live birth rates, and number of babies delivered.

RESULT(S)

Compared with patients with fewer than five embryos transferred, those having five or more embryos transferred had significantly increased pregnancy rates and live birth rates, more viable fetuses at 12 weeks, and significantly decreased miscarriage rates. None of these outcome variables differed between the five-embryo and more-than-five-embryo groups. There were no differences in outcome when only five embryos were transferred, regardless of whether five or more than five embryos were available. The number of embryos transferred did not significantly influence multiple birth rates.

CONCLUSION(S)

The present study demonstrates that in women aged > 40 years, five embryos is the optimum number to transfer, and transferring more than five does not confer any additional benefit to clinical outcome.

Genetic strain variations in the metaphase-II phenotype of mouse oocytes matured in vivo or in vitro

The interplay between genetic and epigenetic factors plays a central role in mammalian embryo production strategies that superimposeex vivoorin vivomanipulations upon strain background characteristics. In this study, we examined the relationship between genetic background and the phenotypic properties of mouse metaphase-II (M-II) oocytes that were matured underin vivo(IVO) orin vitroconditions, either in a basal (IVM) or a supplemented (IVM + ) medium. Differences existed amongst inbred (C57BL/6), outbred (CF-1, Black Swiss, NU/NU) and hybrid lines (B6D2F1) induced to superovulate with regard to cytoplasmic microtubule organizing center (MTOC) number but not spindle size or shape, except for larger and asymmetrical spindles in Black Swiss oocytes. When oocytes were matured in culture, meiotic spindle and cytoplasmic phenotypic properties of M-II oocytes were affected relative toin vivoconditions and between strains. Specifically, measures of meiotic spindle size, shape, polar pericentrin distribution and cytoplasmic MTOC number all revealed characteristic variations. Interestingly, the overall reduction in cytoplasmic MTOC number noted upon IVM was concomitant with an overall increase in spindle and polar body size. Maturation under IVM + conditions resulted in a further decrease in cytoplasmic MTOC number, but spindle and polar body characteristics were intermediate between IVO and IVM. How these oocyte phenotypic properties of maternal origin may be linked to predictive assessments of fecundity remains to be established.

Assessment and Optimization of Oocyte Quality during Assisted Reproductive Technology Treatment

The health of fetuses, neonates, and adults depends on normal development of the oocytes and embryos from which they arise. In addition, the ability of an embryo to implant, sustain a pregnancy, and give rise to a viable offspring is unquestionably rooted in oogenesis. Therefore, defining markers that can reliably predict the best quality oocytes will prove invaluable in the care and management of infertility patients. Furthermore, identification of the best quality oocyte will also permit the transfer of a single embryo, thereby increasing overall pregnancy rates and reducing multiple rates. Although a few potentially important predictors of oocyte quality have been identified, their application for routine use in the assisted reproductive technology (ART) laboratory remains to be established. With the critical need for markers to be noninvasive, reliable, and assayable with a rapid turnaround, key areas that will take advantage of recent technological advances are identified in this article. Oocyte quality also needs to be improved in the management of human infertility by ART programs. Given that mechanisms determining oocyte quality are multifactorial and complex, several possible areas that continue to necessitate optimization are discussed.

In vitro maturation of human oocytes and cumulus cells using a co-culture three-dimensional collagen gel system

BACKGROUND

Deficiencies remain in the ability of in vitro-matured human oocytes to acquire full developmental competence and give rise to a healthy pregnancy. A clear deficiency of current systems utilizing human oocytes has been the absence of cumulus cells. In the present study, a three-dimensional (3D) co-culture system exploiting an extracellular matrix was developed and compared to conventional methods for its ability to support maturation of human oocytes.

METHODS AND RESULTS

Cumulus cells were embedded into a 3D collagen gel matrix with individual oocytes added to each gel. Oocytes from the same patient cultured in the gel matrix matured to metaphase II at rates similar to those of cumulus-free oocytes cultured in individual microdrops. Following maturation of oocytes and fixation of intact gels, chromatin and cytoskeletal elements were assessed in oocytes and cumulus cells. The activities of the key cell cycle kinases, maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK), were compared in oocytes matured under the two culture conditions. Compared with denuded oocytes, co-cultured oocytes exhibited increased MAPK activity, but no difference in MPF levels.

CONCLUSIONS

This work characterizes a novel and efficacious culture system that takes advantage of the unique properties of the extracellular matrix, a 3D microenvironment, and the presence of cumulus cells for maturing human oocytes in vitro.

Hormonal control of somatic cell oocyte interactions during ovarian follicle development

In the mammalian ovarian follicle, paracrine signaling between the oocyte and somatic granulosa cells is bidirectional but the structural basis and physiological regulations of communication between gametic and somatic compartments remain unknown. The present experiments were designed to test the hypothesis that follicle stimulating hormone (FSH) regulates the ability of granulosa cells to make connections with the oocyte. We show that in prepubertal unprimed mice and mice carrying a targeted deletion of the FSHbeta subunit gene, granulosa cells exhibit orientation towards the oocyte manifest by the elaboration of transzonal projections (TZPs) and "apical" centrosome positioning at sites of granulosa-zona contact. In vivo FSH treatment results in a retraction of TZPs. Coincident with TZP retraction induced by FSH are changes in oocyte transcriptional activity and meiotic competence, which suggests one means by which the oocyte-granulosa cell dialogue may be modulated during development of ovarian follicles.

Distinct microtubule and chromatin characteristics of human oocytes after failed in-vivo and in-vitro meiotic maturation

BACKGROUND

While a complete failure of meiotic maturation following hCG administration is rare during IVF cycles, cases arise in which patients repeatedly display a high incidence of failure to complete maturation to metaphase II (MII) in vivo. For the immature oocytes of such patients, our objectives were (i) to ask whether progression to MII could be supported in vitro, and (ii) to define their microtubule/chromatin properties following in-vitro maturation (IVM). Together, these studies were aimed at augmenting our understanding of factors underlying meiotic arrest in the human.

METHODS

Cases are presented here for two patients (A and B) producing oocytes that recurrently showed the inability to mature to metaphase II in vivo. Following IVM attempts, chromatin and microtubule characteristics were identified in those oocytes that remained arrested during meiosis I.

RESULTS

In patient A, meiotically arrested oocytes exhibited clear defects in spindle and chromatin arrangements. In contrast, the majority of oocytes from patient B displayed normal MI and MII spindles with aligned chromosomes, although some oocytes exhibited indications for possible defects in cell cycle control.

CONCLUSIONS

Together, these analyses illustrate two cases with oocytes exhibiting a common gross defect, that is meiotic maturation arrest, but revealing different aetiologies or manifestations as evidenced by the presence or absence of abnormal spindle/chromatin organization. This work reinforces the existence of intrinsic defects in oocytes of some patients, the molecular and cellular bases of which merit further investigation.

Origins and manifestations of oocyte maturation competencies

Mammalian oocytes acquire a series of competencies during follicular development that play critical roles at fertilization and subsequent stages of preimplantation embryonic development. These competencies involve remodelling of chromatin and the cytoskeleton in the oocyte at critical stages of folliculogenesis when gametes and somatic cells communicate by paracrine and junctional mechanisms. While the detailed steps involved in bi-directional signalling between oocytes and granulosa cells remain unknown, studies from mice bearing targeted deletions in essential 'communication' genes reveal selective disturbances in oocyte maturation competencies that compromise the oocyte's developmental potential. Recent data are reviewed that illustrate the general principle that competencies acquired at sequential stages of oogenesis are manifest during oocyte growth, maturation, or following fertilization. The recognition that oocyte-specific genes are called into play at key developmental transitions in mammalian embryogenesis emphasizes the importance of monitoring genetic and epigenetic determinants when using current assisted reproductive technologies manipulations.

Day 3 and day 5 morphological predictors of embryo viability

Controlling multiple pregnancies in patients undergoing artificial reproductive procedures requires consideration of single embryo transfers. Therefore, refinements for embryo evaluation are needed that select for the most developmentally competent embryo. The present study was designed to identify day 3 and day 5 morphological predictors of viability following transfers in which the morphology and fate of each embryo was precisely determined. Assessments on day 3 included cell number, and the extent of fragmentation and asymmetry, and on day 5, the developmental stage. Embryos resulting in a viable fetus at 11 weeks gestation were considered developmentally competent. The relationships among individual and collective embryo morphological characteristics were evaluated. Analysis of the interactions among morphological characteristics of embryos transferred on day 3 enabled identification of a multivariable selection order. Assessment of day 5 embryos revealed that expanding and expanded blastocysts exhibited comparable developmental potential that was superior to that of either morulae or early blastocysts. However, expanding or expanded blastocysts derived from 7-cell or 8-cell embryos were developmentally superior to those derived from other cleavage stages, regardless of fragmentation or asymmetry. Collectively, these findings further understanding of morphological predictors of viability, thereby improving the ability to select the most viable embryo for transfer.

Assessment of oocyte quality following repeated gonadotropin stimulation in the mouse

The present study assessed the effects of repeated ovarian stimulation on oocyte quality. Female mice were stimulated with eCG and hCG at 1-wk intervals for 4 wk. Germinal vesicle (GV)-stage oocytes were evaluated in relation to size, somatic cell association, and chromatin organization after each week of stimulation. In addition, ATP content and expression of meiotic competence were monitored in GV and in vivo (IVO) or in vitro (IVM)-matured oocytes. The developmental competence of ovulated oocytes was determined after in vitro fertilization and embryo culture, and reproductive outcome was evaluated after mating following repeated cycles of stimulation. In GV oocytes, the degree of somatic cell association, size, and timing of transcriptional repression were altered when comparing repeated with single cycle(s) of stimulation. Meiotic competence expression was unaffected for IVO oocytes while IVM oocytes exhibited a progressive decrease in meiotic competence with repeated stimulation. The ATP content of immature and IVO oocytes decreased with repeated stimulation. Although after one cycle of stimulation ATP content was lower in IVM than IVO oocytes, IVM oocytes exhibited stable levels of ATP across cycles of stimulation. Last, the in vitro developmental competence of IVO oocytes retrieved after repeated stimulation was not significantly different, and in vivo, similar implantation and resorption rates were observed following mating of animals subjected to repeated stimulation. Therefore, despite measurable consequences of repeated stimulation on specific parameters of follicular oocyte quality, compensatory mechanisms may exist in vivo to optimize the developmental competence of ovulated oocytes in the mouse.

Assessment of nuclear and cytoplasmic maturation in in-vitro matured human oocytes

BACKGROUND

With improved prospects for the use of human oocyte in-vitro maturation in assisted reproductive technologies, the need to define more clearly the coordination of nuclear and cytoplasmic maturation has arisen.

METHODS

Immunofluorescence and confocal microscopy were used to evaluate cell cycle-dependent modifications in chromatin and microtubules in human germinal vesicle oocytes (n = 455) undergoing in-vitro maturation.

RESULTS

Four distinct classes of germinal vesicle stage oocytes were identified based on the expression of G2/interphase characteristics, but, of these, only one class of oocytes was competent to complete meiotic progression to metaphase-II in vitro. The majority of germinal vesicle stage oocytes resumed meiosis within 6 h (88.9%) of culture and exhibited an accelerated pace of progression to metaphase-II (66.7%) over 24 h, but in general were unable to maintain meiotic arrest and defaulted into interphase within 24 h of polar body emission. Characterization of microtubule dynamics and chromatin phosphorylation demonstrates specific cell cycle deficiencies in in-vitro matured human oocytes.

CONCLUSION

This work forms a basis for future studies aimed at optimizing nuclear and cytoplasmic maturation during in-vitro maturation.