Ultrastructural Dynamics of Human Reproduction, from Ovulation to Fertilization and Early Embryo Development1

Effects of intracellular Ca2+ on developmental potential and ultrastructure of cryopreserved-warmed oocyte in mouse

Maintaining appropriate intracellular calcium of oocytes is necessary to prevent ultrastructure and organelle damage caused by freezing and cryoprotectants. The present study aimed to investigate whether cryoprotectant-induced changes in the calcium concentrations of oocytes can be regulated to reduce damage to developmental potential and ultrastructure. A total of 33 mice and 1381 oocytes were used to explore the effects of intracellular calcium on the development and ultrastructures of oocytes subjected to 2-aminoethoxydiphenyl borate (2-APB) inhibition or thapsigargin (TG) stimulation. Results suggested that high levels intracellular calcium interfered with TG compromised oocyte survival (84.4 % vs. 93.4 %, p < 0.01) and blastocyst formation in fresh and cryopreservation oocytes (78.1 % vs. 86.4 %, and 60.5 % vs. 72.5 %, p < 0.05) compared with that of 2-APB pretreated oocytes in which Ca2+ was stabilized even though no differences in fertilization and cleavage was detected (p > 0.05). Examination by transmission electron microscopy indicated that the microvilli decreased and shortened, cortical granules considerably decreased in the cortex area, mitochondrial vesicles and vacuoles increased, and the proportion of vacuole mitochondria increased after oocytes were exposed to cryoprotectants. The cryopreservation-warming process deteriorated the negative effects on organelles of survival oocytes. By contrast, a low level of intracellular calcium mediated with 2-APB was supposed to contribute to the protection of organelles. These findings suggested oocyte injuries induced by cryoprotectants and low temperatures can be alleviated. More studies are necessary to confirm the relationship among Ca2+ concentration of the cytoplasm, ultrastructural injuries, and disrupted developmental potential in oocytes subjected to cryopreservation and warming.

Heavy Metals in Follicular Fluid Affect the Ultrastructure of the Human Mature Cumulus-Oocyte Complex

It is known that exposure to heavy metal such as lead (Pb) and cadmium (Cd) has several adverse effects, particularly on the human reproductive system. Pb and Cd have been associated with infertility in both men and women. In pregnant women, they have been associated with spontaneous abortion, preterm birth, and impairment of the development of the fetus. Since these heavy metals come from both natural and anthropogenic activities and their harmful effects have been observed even at low levels of exposure, exposure to them remains a public health issue, especially for the reproductive system. Given this, the present study aimed to investigate the potential reproductive effects of Pb and Cd levels in the follicular fluid (FF) of infertile women and non-smokers exposed to heavy metals for professional reasons or as a result of living in rural areas near landfills and waste disposal areas in order to correlate the intrafollicular presence of these metals with possible alterations in the ultrastructure of human cumulus-oocyte complexes (COCs), which are probably responsible for infertility. Blood and FF metals were measured using atomic absorption spectrometry. COCs corresponding to each FF analyzed were subjected to ultrastructural analyses using transmission electron microscopy. We demonstrated for the first time that intrafollicular levels of Pb (0.66 µg/dL-0.85 µg/dL) and Cd (0.26 µg/L-0.41 µg/L) could be associated with morphological alterations of both the oocyte and cumulus cells' (CCs) ultrastructure. Since blood Cd levels (0.54 µg/L-1.87 µg/L) were above the current reference values established by the guidelines of the Agency for Toxic Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA) (0.4 µg/L), whereas blood Pb levels (1.28 µg/dL-3.98 µg/dL) were below the ATSDR reference values (≤5 µg/dL), we believe that these alterations could be due especially to Cd, even if we cannot exclude a possible additional effect of Pb. Our results highlighted that oocytes were affected in maturation and quality, whereas CCs showed scarcely active steroidogenic elements. Regressing CCs, with cytoplasmic alterations, were also numerous. According to Cd's endocrine-disrupting activity, the poor steroidogenic activity of CCs might correlate with delayed oocyte cytoplasmic maturation. So, we conclude that levels of heavy metals in the blood and the FF might negatively affect fertilization, embryo development, and pregnancy, compromising oocyte competence in fertilization both directly and indirectly, impairing CC steroidogenic activity, and inducing CC apoptosis.

Spermatogenic cell-specific SPACA4 is essential for efficient sperm-zona pellucida binding in vitro

Fertilization is a complex and highly regulated process that involves a series of molecular interactions between sperm and oocytes. However, the mechanisms of proteins involved in human fertilization, such as that of testis-specific SPACA4, remain poorly understood. Here we demonstrated that SPACA4 is a spermatogenic cell-specific protein. SPACA4 is expressed during spermatogenesis, upregulated in early-stage spermatids, and downregulated in elongating spermatids. SPACA4 is an intracellular protein that locates in the acrosome and is lost during the acrosome reaction. Incubation with antibodies against SPACA4 inhibited the binding of spermatozoa to zona pellucida. SPACA4 protein expression levels across different semen parameters were similar but varied significantly among patients. A prospective clinical study found no association between SPACA4 protein levels and fertilization or cleavage rates. Thus, the study suggests a novel function for SPACA4 in human fertilization in a non-dose-dependent manner. However, a larger clinical trial is required to evaluate the potential use of sperm SPACA4 protein levels to predict fertilization potential.

Ultrastructural Evaluation of Mouse Oocytes Exposed In Vitro to Different Concentrations of the Fungicide Mancozeb

Mancozeb is a widely used fungicide, considered to be an endocrine disruptor. In vivo and in vitro studies evidenced its reproductive toxicity on mouse oocytes by altering spindle morphology, impairing oocyte maturation, fertilization, and embryo implantation. Mancozeb also induces dose-dependent toxicity on the ultrastructure of mouse granulosa cells, including chromatin condensation, membrane blebbing, and vacuolization. We evaluated the effects on the ultrastructure of mouse oocytes isolated from cumulus-oocyte complexes (COCs), exposed in vitro to increasing concentrations of mancozeb. COCs were matured in vitro with or without (control) low fungicide concentrations (0.001-1 μg/mL). All mature oocytes were collected and prepared for light and transmission electron microscopy. Results showed a preserved ultrastructure at the lowest doses (0.001-0.01 μg/mL), with evident clusters of round-to-ovoid mitochondria, visible electron-dense round cortical granules, and thin microvilli. Mancozeb concentration of 1 μg/mL affected organelle density concerning controls, with a reduction of mitochondria, appearing moderately vacuolated, cortical granules, and microvilli, short and less abundant. In summary, ultrastructural data revealed changes mainly at the highest concentration of mancozeb on mouse oocytes. This could be responsible for the previously described impaired capability in oocyte maturation, fertilization, and embryo implantation, demonstrating its impact on the reproductive health and fertility.

Effects of Simulated Microgravity In Vitro on Human Metaphase II Oocytes: An Electron Microscopy-Based Study

The Gravity Force to which living beings are subjected on Earth rules the functionality of most biological processes in many tissues. It has been reported that a situation of Microgravity (such as that occurring in space) causes negative effects on living beings. Astronauts returning from space shuttle missions or from the International Space Station have been diagnosed with various health problems, such as bone demineralization, muscle atrophy, cardiovascular deconditioning, and vestibular and sensory imbalance, including impaired visual acuity, altered metabolic and nutritional status, and immune system dysregulation. Microgravity has profound effects also on reproductive functions. Female astronauts, in fact, suppress their cycles during space travels, and effects at the cellular level in the early embryo development and on female gamete maturation have also been observed. The opportunities to use space flights to study the effects of gravity variations are limited because of the high costs and lack of repeatability of the experiments. For these reasons, the use of microgravity simulators for studying, at the cellular level, the effects, such as those, obtained during/after a spatial trip, are developed to confirm that these models can be used in the study of body responses under conditions different from those found in a unitary Gravity environment (1 g). In view of this, this study aimed to investigate in vitro the effects of simulated microgravity on the ultrastructural features of human metaphase II oocytes using a Random Positioning Machine (RPM). We demonstrated for the first time, by Transmission Electron Microscopy analysis, that microgravity might compromise oocyte quality by affecting not only the localization of mitochondria and cortical granules due to a possible alteration of the cytoskeleton but also the function of mitochondria and endoplasmic reticulum since in RPM oocytes we observed a switch in the morphology of smooth endoplasmic reticulum (SER) and associated mitochondria from mitochondria-SER aggregates to mitochondria-vesicle complexes. We concluded that microgravity might negatively affect oocyte quality by interfering in vitro with the normal sequence of morphodynamic events essential for acquiring and maintaining a proper competence to fertilization in human oocytes.

All that glitters is not gold: a stereological study of human donor oocytes

Here we report a quantitative analysis of human metaphase II (MII) oocytes from a 22-year-old oocyte donor, retrieved after ovarian-controlled hyperstimulation. Five surplus donor oocytes were processed for transmission electron microscopy (TEM), and a stereological analysis was used to quantify the distribution of organelles, using the point-counting technique with an adequate stereological grid. Comparisons between means of the relative volumes (Vv) occupied by organelles in the three oocyte regions, cortex (C), subcortex (SC) and inner cytoplasm (IC), followed the Kruskal-Wallis test and Mann-Whitney U-test with Bonferroni correction. Life cell imaging and TEM analysis confirmed donor oocyte nuclear maturity. Results showed that the most abundant organelles were smooth endoplasmic reticulum (SER) elements (26.8%) and mitochondria (5.49%). Significant differences between oocyte regions were found for lysosomes (P = 0.003), cortical vesicles (P = 0.002) and large SER vesicles (P = 0.009). These results were quantitatively compared with previous results using prophase I (GV) and metaphase I (MI) immature oocytes. In donor MII oocytes there was a normal presence of cortical vesicles, SER tubules, SER small, medium and large vesicles, lysosomes and mitochondria. However, donor MII oocytes displayed signs of cytoplasmic immaturity, namely the presence of dictyosomes, present in GV oocytes and rare in MI oocytes, of SER very large vesicles, characteristic of GV oocytes, and the rarity of SER tubular aggregates. Results therefore indicate that the criterion of nuclear maturity used for donor oocyte selection does not always correspond to cytoplasmic maturity, which can partially explain implantation failures with the use of donor oocytes.

The effects of vitrification on oocyte quality

Vitrification, is an ultra-rapid, manual cooling process that produces glass-like (ice crystal free) solidification. Water is prevented from forming intercellular and intracellular ice crystals during cooling as a result of oocyte dehydration and the use of highly concentrated cryoprotectant. Though oocytes can be cryopreserved without ice crystal formation through vitrification, it is still not clear whether the process of vitrification causes any negative impact (temperature change/chilling effect, osmotic stress, cryoprotectant toxicity, and/or phase transitions) on oocyte quality that translate to diminished embryo developmental potential or subsequent clinical outcomes. In this review, we attempt to assess the technique's potential effects and the consequence of these effects on outcomes.

The human sperm basal body is a complex centrosome important for embryo preimplantation development

The mechanism of conversion of the human sperm basal body to a centrosome after fertilization, and its role in supporting human early embryogenesis, has not been directly addressed so far. Using proteomics and immunofluorescence studies, we show here that the human zygote inherits a basal body enriched with centrosomal proteins from the sperm, establishing the first functional centrosome of the new organism. Injection of human sperm tails containing the basal body into human oocytes followed by parthenogenetic activation, showed that the centrosome contributes to the robustness of the early cell divisions, increasing the probability of parthenotes reaching the compaction stage. In the absence of the sperm-derived centrosome, pericentriolar material (PCM) components stored in the oocyte can form de novo structures after genome activation, suggesting a tight PCM expression control in zygotes. Our results reveal that the sperm basal body is a complex organelle which converts to a centrosome after fertilization, ensuring the early steps of embryogenesis and successful compaction. However, more experiments are needed to elucidate the exact molecular mechanisms of centrosome inheritance in humans.

The Role of Mitochondria in Oocyte Maturation

With the nucleus as an exception, mitochondria are the only animal cell organelles containing their own genetic information, called mitochondrial DNA (mtDNA). During oocyte maturation, the mtDNA copy number dramatically increases and the distribution of mitochondria changes significantly. As oocyte maturation requires a large amount of ATP for continuous transcription and translation, the availability of the right number of functional mitochondria is crucial. There is a correlation between the quality of oocytes and both the amount of mtDNA and the amount of ATP. Suboptimal conditions of in vitro maturation (IVM) might lead to changes in the mitochondrial morphology as well as alternations in the expression of genes encoding proteins associated with mitochondrial function. Dysfunctional mitochondria have a lower ability to counteract reactive oxygen species (ROS) production which leads to oxidative stress. The mitochondrial function might be improved with the application of antioxidants and significant expectations are laid on the development of new IVM systems supplemented with mitochondria-targeted reagents. Different types of antioxidants have been tested already on animal models and human rescue IVM oocytes, showing promising results. This review focuses on the recent observations on oocytes’ intracellular mitochondrial distribution and on mitochondrial genomes during their maturation, both in vivo and in vitro. Recent mitochondrial supplementation studies, aiming to improve oocyte developmental potential, are summarized.

iTRAQ-based proteomic analysis of bovine pre-ovulatory plasma and follicular fluid

Bovine follicular fluid (FF) creates a unique microenvironment in follicles necessary for follicle growth, oocyte maturation, and estradiol (E2) production. The objective of this study was to analyze changes in proteins in FF and plasma (PL) from animals with high E2 (HE2) or low E2 (LE2) during the preovulatory period. Beef cows were synchronized, and follicular dynamics and ovulatory response were monitored using transrectal ultrasonography. Nine cows were selected and slaughtered, blood samples were collected at slaughter and FF was aspirated from dominant follicles (DF; >10 mm). Abundant proteins (albumin, IgG, IgA, and alpha-1-antitrypsin) were depleted from both PL and FF. Peptides were labeled with iTRAQ reagents and quantified using 2-dimentional liquid chromatography ESI-based mass spectrometry. Estradiol was associated with protein changes in PL and FF. Protein expression changes between FF HE2 and FF LE2 were greater than between PL HE2 and PL LE2. There were 15 up-regulated proteins and 10 down-regulated proteins in FF HE2 compared to FF LE2, and 7 proteins up-regulated and 9 proteins down-regulated in PL HE2 compared to PL LE2. Several of the differentially expressed proteins function in follicle development and were mainly categorized under cellular process and metabolic process. Pathway analysis identified the up- and down-regulated proteins were predominantly associated with the complement and coagulation cascades. The data demonstrate E2 regulates a wide range of reproductive associated proteins in bovine PL and FF and can provide the basis for further investigation of specific processes involved in such regulation.

Biomechanics of Early Life in the Female Reproductive Tract

Early human life that starts at the onset of fertilization and ends with implantation of the embryo in the uterine wall is the foundation for a successful pregnancy. The different stages during this period require biomechanical mechanisms, which are mostly unknown due to difficulties to conduct in vivo studies in humans.

Failure of complete hatching of ICSI-derived human blastocyst by cell herniation via small slit and insufficient expansion despite ongoing cell proliferation

PURPOSE

To assess the effect of intracytoplasmic sperm injection (ICSI) on embryo hatching and visualise the effects of zona thinning (ZT) on the embryo using time-lapse monitoring.

METHODS

In vitro fertilisation (IVF) (n = 178) and ICSI (n = 110)-derived cryopreserved blastocysts were donated by patients who previously had a baby. This study investigated the impacts of IVF, ICSI, laser-assisted hatching by ZT and formation of ICSI penetration trace on zona pellucida of IVF-derived blastocyst on blastcyst diameter, the estimated number of trophectoderm (TE) cells and completed hatching rate.

RESULTS

The completed hatching rate and diameters of the completely hatched blastocysts at hatching commencement and at the maximum expansion were significantly greater in the IVF than in ICSI groups. The completed hatching rate significantly increased with ZT in both groups. The maximum diameters of the completely hatched blastocysts were significantly smaller in the ZT than in non-ZT groups. The estimated TE cell numbers increased from hatching commencement to their maximum expansion points. The incompletely hatched ICSI-derived blastocysts intermittently herniated cells via small slits until degeneration. The completed hatching rate significantly decreased by the formation of ICSI penetration trace on zona pellucida of IVF-derived blastocyst.

CONCLUSION

ICSI-derived blastocysts intermittently release proliferating cells and extracted TE cells and/or inner cell masses via a small slit; thus, blastocyst expansion is not sufficiently increased, leading to a reduced complete hatching rate. Therefore, the ICSI penetration trace potentially has negative effects on blastocyst expansion process in vitro and is a risk factor for the failure of completed hatching.

microRNA-125b Regulates Apoptosis by Targeting Bone Morphogenetic Protein Receptor 1B in Yak Granulosa Cells

The intronic microRNA, miR-125b, plays a vital role in promyelocytic and hematopoietic stem cells, and in the development and apoptosis of cancer cells. In this study, we showed that miR-125b regulates granulosa cell (GC) apoptosis in the yak ovary. Bioinformatic analyses and luciferase reporter assays demonstrated that bone morphogenetic protein receptor type 1B (BMPR1B) is an miR-125b target. miR-125b overexpression induced apoptosis in yak GC, and affected the mRNA and protein expression of BMPR1B and the ratio of Bcl2/Bax. Silencing of miR-125b decreased the rate of yak GC apoptosis and increased the ratio of Bcl2/Bax. In addition, the effects of an miR-125b inhibitor were overturned by cotransfection with siRNA-BMPR1B2 (siRNA-299) in yak GC. Together, these results demonstrated that miR-125b regulates GC apoptosis in the yak ovary by targeting BMPR1B.

Improving oocyte quality by transfer of autologous mitochondria from fully grown oocytes

Older women are often the most challenging group of patients in fertility clinics due to a decline in both number and overall quality of oocytes. The quality of oocytes has been linked to mitochondrial dysfunction. In this mini-review, we discuss this hypothesis and suggest alternative treatment options using autologous mitochondria to potentially augment pregnancy potential in ART. Autologous transfer of mitochondria from the patient's own germline cells has attracted much attention as a possible new treatment to revitalize deficient oocytes. IVF births have been reported after transfer of oogonial precursor cell-derived mitochondria; however, the source and quality of the mitochondria are still unclear. In contrast, fully grown oocytes are loaded with mitochondria which have passed the genetic bottleneck and are likely to be of high quality. An increased supply of such oocytes could potentially be obtained by in vitro follicle activation of ovarian cortical biopsies or from surplus immature oocytes collected from women undergoing ART or fertility preservation of ovarian tissue. Taken together, autologous oocytes are not necessarily a limiting resource in ART as fully grown oocytes with high quality mitochondria can be obtained from natural or stimulated ovaries and potentially be used to improve both quality and quantity of oocytes available for fertility treatment.

Freeze/thaw stress induces organelle remodeling and membrane recycling in cryopreserved human mature oocytes

PURPOSE

Our aim was to evaluate the ultrastructure of human metaphase II oocytes subjected to slow freezing and fixed after thawing at different intervals during post-thaw rehydration.

METHODS

Samples were studied by light and transmission electron microscopy.

RESULTS

We found that vacuolization was present in all cryopreserved oocytes, reaching a maximum in the intermediate stage of rehydration. Mitochondria-smooth endoplasmic reticulum (M-SER) aggregates decreased following thawing, particularly in the first and intermediate stages of rehydration, whereas mitochondria-vesicle (MV) complexes augmented in the same stages. At the end of rehydration, vacuoles and MV complexes both diminished and M-SER aggregates increased again. Cortical granules (CGs) were scarce in all cryopreserved oocytes, gradually diminishing as rehydration progressed.

CONCLUSIONS

This study also shows that such a membrane remodeling is mainly represented by a dynamic process of transition between M-SER aggregates and MV complexes, both able of transforming into each other. Vacuoles and CG membranes may take part in the membrane recycling mechanism.

A novel method for transmission electron microscopy study of cytoplasmic fragments from preimplantation human embryos

Transmission electron microscopy (TEM) is the main tool for exploring the intracellular damage and organelle distribution. The cause of producing embryo cytoplsamic fragmentation is not completely understood. Since the fragments have detrimental effects on embryo development, the ultrastructural analysis of fragments may play an important role in fragmentation etiology and in embryo development as well. There are no studies regarding the ultrastructure of fragments in transferable embryos, because the preparation for TEM is not vital and embryos are discarded inevitably. This study aims to introduce a new method for ultrastructural evaluation of fragments without damaging the human cleaving embryos. Microsc. Res. Tech. 79:459-462, 2016. © 2016 Wiley Periodicals, Inc.

Ultrastructure of human oocytes after in vitro maturation

STUDY HYPOTHESIS

How does the ultrastructure of human oocytes matured in vitro compare with oocytes collected from women after full hormonal stimulation?

STUDY FINDING

The ultrastructure of human oocytes matured in vitro is largely, but not entirely, similar to those matured in vivo.

WHAT IS KNOWN ALREADY

Embryos derived from in vitro-matured oocytes often have limited developmental potential, possibly as an effect of inappropriate in vitro maturation (IVM) conditions. Transmission electron microscopy (TEM) is a valuable research tool to compare in vivo and in vitro matured oocytes. However, previous studies on the ultrastructure of human IVM oocytes were done with inadequate material or inappropriate IVM conditions, and have limited significance.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Immature cumulus cell-enclosed oocytes, retrieved from mid-sized antral follicles of women requiring IVM treatment, were matured in vitro for 30 h. No leftover germinal vesicle-stage oocytes collected from fully stimulated cycles were used. Control in vivo matured oocytes were obtained from age-matched women undergoing full ovarian stimulation. In vitro and in vivo matured oocytes were analysed by TEM and compared according to previously established morphometric criteria of oocyte quality.

MAIN RESULTS AND THE ROLE OF CHANCE

All oocytes had normal ooplasm showing uniform distribution of organelles. Mitochondrial morphology appeared similar between the maturation conditions. Cortical granules were found typically stratified in a single, mostly continuous row just beneath the ooplasm in all oocytes. Microvilli were well preserved after IVM. Vacuoles were only occasionally found in all oocytes and, if present, they were frequently associated with lysosomes. Mitochondria-smooth endoplasmic reticulum (M-SER) aggregates and mitochondria-vesicles (MV) complexes were commonly found in in vivo matured oocytes. However, large MV complexes partially replaced M-SER aggregates in IVM oocytes.

LIMITATIONS, REASONS FOR CAUTION

As a note of caution it should be noticed that, being laborious and technically demanding, TEM cannot be applied to a large number of samples in a single investigation. Therefore, our data require further independent confirmation.

WIDER IMPLICATIONS OF THE FINDINGS

Our data suggests the notion that TEM remains a valuable research tool that can also offer quantitative data if associated with morphometric criteria of evaluation. Therefore, it can be adopted to test pre-clinically the performance of novel in vitro systems that are demanded to make oocytes IVM more successful in the human.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This study was independently funded by Biogenesi Reproductive Medicine Centre, Monza, Italy. All authors declare that their participation in the study did not involve factual or potential conflicts of interests.

Ultrastructural markers of quality are impaired in human metaphase II aged oocytes: a comparison between reproductive and in vitro aging

PURPOSE

Childbearing delay contributes to the increase of subfertile couples that require assisted reproductive technology (ART). Subfertility relates with reproductive aging (RA). In vitro aging (IvA) (due to extended culture) may also impair oocyte competence. Aims of this study were to evaluate and compare the oocyte ultrastructure after RA and IvA.

METHODS

Cumulus-oocyte complexes (COCs) (n = 68), with metaphase II oocyte and expanded cumulus, from consenting patients (<35 years old and ≥35 years old, n = 36), were selected by phase contrast microscopy and fixed at pick up, or after 24 h culture. COCs (n = 44) were studied by light and qualitative/morphometric transmission electron microscopy. Two-way ANOVA, with age and culture as grouping factors, was applied for statistical analysis (p < 0.05). Metaphase II cumulus-free oocytes (n = 24) were selected for confocal microscopy observations.

RESULTS

Significant decrease of mitochondria-smooth endoplasmic reticulum aggregates, increase of mitochondria-vesicle complexes size and amount, decrease of cortical granules and microvilli, and alterations of the spindle structure characterized both RA and IvA oocytes. These changes were significantly more evident in the RA oocytes submitted to IvA. RA oocytes also showed changes of the zona pellucida and occurrence of vacuoles after culture. Cumuli appeared re-compacted after culture, irrespective of the age of the patients.

CONCLUSIONS

These data demonstrated that aging is related to decay of oocyte ultrastructural quality, and that oocytes from elder women are more sensitive to prolonged culture (IvA) than the oocytes from younger women. These morphological results should be considered when applying ART in aged patients, rescue ICSI, or artificial oocyte activation.

Use of both cumulus cells' transcriptomic markers and zona pellucida birefringence to select developmentally competent oocytes in human assisted reproductive technologies

BACKGROUND

Selection of the best oocyte for subsequent steps of fertilization and embryo transfer was shown to be the crucial step in human infertility treatment procedure. Oocyte selection using morphological criteria mainly Zona pellucida (ZP) has been the gold standard method in assisted reproductive technologies (ART) clinics, but this selection approach has limitations in terms of accuracy, objectivity and constancy. Recent studies using OMICs-based approaches have allowed the identification of key molecular markers that quantitatively and non-invasively predict the oocyte quality for higher pregnancy rates and efficient infertility treatment. These biomarkers are a valuable reinforcement of the morphological selection criteria widely used in in vitro fertilization (IVF) clinics. In this context, this study was designed to investigate the relationship between transcriptomic predictors of oocyte quality found by our group and the conventional morphological parameters of oocyte quality mainly the ZP birefringence.

RESULTS

Microarray data revealed that 48 and 27 differentially expressed candidate genes in cumulus cells (CCs) were respectively overexpressed and underexpressed in the ZGP (Zona Good Pregnant) versus ZBNP (Zona Bad Non Pregnant) groups. More than 70% of previously reported transcriptomic biomarkers of oocyte developmental competence were confirmed in this study. The analysis of possible association between ZP birefringence versus molecular markers approach showed an absence of correlation between them using the current set of markers.

CONCLUSIONS

This study suggested a new integrative approach that matches morphological and molecular approaches used to select developmentally competent oocytes able to lead to successful pregnancy and the delivery of healthy baby. For each ZP birefringence score, oocytes displayed a particular CCs' gene expression pattern. However, no correlations were found between the 7 gene biomarkers of oocyte developmental potential and the ZP birefringence score. Further studies using larger lists of candidate markers are required to identify suitable genes that are highly correlated with the morphological criteria, and therefore able to reinforce the accuracy of oocyte selection and the effectiveness of infertility treatment.

Fine morphological assessment of quality of human mature oocytes after slow freezing or vitrification with a closed device: a comparative analysis

BACKGROUND

Human mature oocytes are very susceptible to cryodamage. Several reports demonstrated that vitrification might preserve oocyte better than slow freezing. However, this is still controversial. Thus, larger clinical, biological and experimental trials to confirm this concept are necessary. The aim of the study was to evaluate and compare fine morphological features in human mature oocytes cryopreserved with either slow freezing or vitrification.

METHODS

We used 47 supernumerary human mature (metaphase II) oocytes donated by consenting patients, aged 27-32 years, enrolled in an IVF program. Thirtyfive oocytes were cryopreserved using slow freezing with 1.5 M propanediol +0.2 M sucrose concentration (20 oocytes) or a closed vitrification system (CryoTip Irvine Scientific CA) (15 oocytes). Twelve fresh oocytes were used as controls. All samples were prepared for light and transmission electron microscopy evaluation.

RESULTS

Control, slow frozen/thawed and vitrified/warmed oocytes (CO, SFO and VO, respectively) were rounded, 90-100 μm in diameter, with normal ooplasm showing uniform distribution of organelles. Mitochondria-smooth endoplasmic reticulum (M-SER) aggregates and small mitochondria-vesicle (MV) complexes were the most numerous structures found in all CO, SFO and VO cultured for 3-4 hours. M-SER aggregates decreased, and large MV complexes increased in those SFO and VO maintained in culture for a prolonged period of time (8-9 hours). A slight to moderate vacuolization was present in the cytoplasm of SFO. Only a slight vacuolization was present in VO, whereas vacuoles were almost completely absent in CO. Amount and density of cortical granules (CG) appeared abnormally reduced in SFO and VO, irrespective of the protocol applied.

CONCLUSIONS

Even though, both slow freezing and vitrification ensured a good overall preservation of the oocyte, we found that: 1) prolonged culture activates an intracellular membrane "recycling" that causes the abnormal transformation of the membranes of the small MV complexes and of SER into larger rounded vesicles; 2) vacuolization appears as a recurrent form of cell damage during slow freezing and, at a lesser extent, during vitrification using a closed device; 3) premature CG exocytosis was present in both SFO and VO and may cause zona pellucida hardening.

Ultrastructure of immature and mature human oocytes after cryotop vitrification

In vitro maturation of vitrified immature germinal vesicle (GV) oocytes is a promising fertility preservation option. We analyzed the ultrastructure of human GV oocytes after Cryotop vitrification (GVv) and compared it with fresh GV (GVc), fresh mature metaphase II (MIIc) and Cryotop-vitrified mature (MIIv) oocytes. By phase contrast microscopy and light microscopy, the oolemmal and cytoplasmic organization of fresh and vitrified oocytes did not show significant changes. GVv oocytes showed significant ultrastructural alterations of the microvilli in 40% of the samples; small vacuoles and occasional large/isolated vacuoles were abnormally present in the ooplasm periphery of 50% of samples. The ultrastructure of nuclei and mitochondria-vesicle (MV) complexes, as well as the distribution and characteristics of cortical granules (CGs), were comparable with those of GVc oocytes. MIIv oocytes showed an abnormal ultrastructure of microvilli in 30% of the samples and isolated large vacuoles in 70% of the samples. MV complexes were normal, but mitochondria-smooth endoplasmic reticulum aggregates appeared to be of reduced size. CGs were normally located under the oolemma but presented abnormalities in distribution and matrix electron density. In conclusion, Cryotop vitrification preserved main oocyte characteristics in the GV and MII stages, even if peculiar ultrastructural alterations appeared in both stages. This study also showed that the GV stage appears more suitable for vitrification than the MII stage, as indicated by the good ultrastructural preservation of important structures that are present only in immature oocytes, like the nucleus and migrating CGs.

Oocytes with a dark zona pellucida demonstrate lower fertilization, implantation and clinical pregnancy rates in IVF/ICSI cycles

The morphological assessment of oocytes is important for embryologists to identify and select MII oocytes in IVF/ICSI cycles. Dysmorphism of oocytes decreases viability and the developmental potential of oocytes as well as the clinical pregnancy rate. Several reports have suggested that oocytes with a dark zona pellucida (DZP) correlate with the outcome of IVF treatment. However, the effect of DZP on oocyte quality, fertilization, implantation, and pregnancy outcome were not investigated in detail. In this study, a retrospective analysis was performed in 268 infertile patients with fallopian tube obstruction and/or male factor infertility. In 204 of these patients, all oocytes were surrounded by a normal zona pellucida (NZP, control group), whereas 46 patients were found to have part of their retrieved oocytes enclosed by NZP and the other by DZP (Group A). In addition, all oocytes enclosed by DZP were retrieved from 18 patients (Group B). No differences were detected between the control and group A. Compared to the control group, the rates of fertilization, good quality embryos, implantation and clinical pregnancy were significantly decreased in group B. Furthermore, mitochondria in oocytes with a DZP in both of the two study groups (A and B) were severely damaged with several ultrastructural alterations, which were associated with an increased density of the zona pellucida and vacuolization. Briefly, oocytes with a DZP affected the clinical outcome in IVF/ICSI cycles and appeared to contain more ultrastructural alterations. Thus, DZP could be used as a potential selective marker for embryologists during daily laboratory work.

Behaviour of cytoplasmic organelles and cytoskeleton during oocyte maturation

Assisted reproduction technology (ART) has become an attractive option for infertility treatment and holds tremendous promise. However, at present, there is still room for improvement in its success rates. Oocyte maturation is a process by which the oocyte becomes competent for fertilization and subsequent embryo development. To better understand the mechanism underlying oocyte maturation and for the future improvement of assisted reproduction technology, this review focuses on the complex processes of cytoplasmic organelles and the dynamic alterations of the cytoskeleton that occur during oocyte maturation. Ovarian stimulation and in-vitro maturation are the major techniques used in assisted reproduction technology and their influence on the organelles of oocytes is also discussed. Since the first birth by assisted reproduction treatment was achieved in 1978, numerous techniques involved in assisted reproduction have been developed and have become attractive options for infertility treatment. However, the unsatisfactory success rate remains as a main challenge. Oocyte maturation is a process by which the oocyte becomes competent for fertilization and subsequent embryo development. Oocyte maturation includes both nuclear and cytoplasmic maturation. Nuclear maturation primarily involves chromosomal segregation, which has been well studied, whereas cytoplasmic maturation involves a series of complicated processes, and there are still many parts of this process that remain controversial. Ovarian stimulation and in-vitro maturation (IVM) are the major techniques of assisted reproduction. The effect of ovarian stimulation or IVM on the behaviour of cell organelles of the oocyte has been postulated as the reason for the reduced developmental potential of in-vitro-produced embryos. To further understanding of the mechanism of oocyte maturation and future improvement of assisted reproduction treatment, the complex events of cytoplasmic organelles and the cytoskeleton that occur during oocyte maturation and the influence of ovarian stimulation and IVM on these organelles are described in this review.

Whole-depth change in bovine zona pellucida biomechanics after fertilization: how relevant in hindering polyspermy?

Polyspermy is a common problem in bovine in vitro fertilization (IVF) and has a still unclear etiology. In this specie, after IVF, despite the lack of a biochemical post-fertilization hardening, the stiffness of the outer ZP layer is significantly increased. Therefore, polyspermy might be related to an incomplete or insufficient stiffening of the ZP. We obtained, by using atomic force spectroscopy in physiological conditions, a complete characterization of the biomechanical changes of the inner and outer ZP layers occurring during oocyte maturation/fertilization and correlated them to the ultrastructural changes observed by transmission electron microscopy using ruthenium red and saponin technique. In both the inner and outer ZP layers, stiffness decreased at maturation while, conversely, increased after fertilization. Contextually, at the nanoscale, during maturation both ZP layers displayed a fine filaments network whose length increased while thickness decreased. After fertilization, filaments partially recovered the immature features, appearing again shorter and thicker. Overall, the observed biomechanical modifications were substantiated by ultrastructural findings in the ZP filament mesh. In fertilized ZP, the calculated force necessary to displace ZP filaments resulted quite similar to that previously reported as generated by bovine sperm flagellum. Therefore, in bovine IVF biomechanical modifications of ZP appear ineffective in hindering sperm transit, highlighting the relevance of additional mechanisms operating in vivo.

Ultrastructure of human mature oocytes after vitrification

Since the introduction of human assisted reproduction, oocyte cryopreservation has been regarded as an attractive option to capitalize the reproductive potential of surplus oocytes and preserve female fertility. However, for two decades the endeavor to store oocytes has been limited by the not yet optimized methodologies, with the consequence of poor clinical outcome or of uncertain reproducibility. Vitrification has been developed as the promising technology of cryopreservation even if slow freezing remains a suitable choice. Nevertheless, the insufficiency of clinical and correlated multidisciplinary data is still stirring controversy on the impact of this technique on oocyte integrity. Morphological studies may actually provide a great insight in this debate. Phase contrast microscopy and other light microscopy techniques, including cytochemistry, provided substantial morpho-functional data on cryopreserved oocyte, but are unable to unraveling fine structural changes. The ultrastructural damage is one of the most adverse events associated with cryopreservation, as an effect of cryo-protectant toxicity, ice crystal formation and osmotic stress. Surprisingly, transmission electron microsco py has attracted only limited attention in the field of cryopreservation. In this review, the subcellular structure of human mature oocytes following vitrification is discussed at the light of most relevant ultrastructural studies.

Nanoscale characterization of the biomechanical hardening of bovine zona pellucida

The zona pellucida (ZP) is an extracellular membrane surrounding mammalian oocytes. The so-called zona hardening plays a key role in fertilization process, as it blocks polyspermy, which may also be caused by an increase in the mechanical stiffness of the ZP membrane. However, structural reorganization mechanisms leading to ZP's biomechanical hardening are not fully understood yet. Furthermore, a correct estimate of the elastic properties of the ZP is still lacking. Therefore, the aim of the present study was to investigate the biomechanical behaviour of ZP membranes extracted from mature and fertilized bovine oocytes to better understand the mechanisms involved in the structural reorganization of the ZP that may lead to the biomechanical hardening of the ZP. For that purpose, a hybrid procedure is developed by combining atomic force microscopy nanoindentation measurements, nonlinear finite element analysis and nonlinear optimization. The proposed approach allows us to determine the biomechanical properties of the ZP more realistically than the classical analysis based on Hertz's contact theory, as it accounts for the nonlinearity of finite indentation process, hyperelastic behaviour and material heterogeneity. Experimental results show the presence of significant biomechanical hardening induced by the fertilization process. By comparing various hyperelastic constitutive models, it is found that the Arruda-Boyce eight-chain model best describes the biomechanical response of the ZP. Fertilization leads to an increase in the degree of heterogeneity of membrane elastic properties. The Young modulus changes sharply within a superficial layer whose thickness is related to the characteristic distance between cross-links in the ZP filamentous network. These findings support the hypothesis that biomechanical hardening of bovine ZP is caused by an increase in the number of inter-filaments cross-links whose density should be higher in the ZP inner side.

Ultrastructure of human gametes, fertilization and embryos in assisted reproduction: a personal survey

This extensively illustrated review will cover the progression of recent research on the ultrastructure of human gametes, fertilization and embryos performed in collaboration with colleagues in In vitro fertilization (IVF) centers over the past three decades, in Australia, Singapore, India, England, Sri Lanka, Spain and Italy. It will also include some aspects of gametogenesis and embryogenesis, particularly in relation to the centrosome that activates embryonic development, and is inherited from the father at fertilization. Assessment of both normal and abnormal gametes and embryos and some clinical aspects of assisted reproduction will be discussed. Full reference will also be made to the contribution of other groups to the ultrastructure of reproduction, particularly in humans.

Biomarkers of human oocyte developmental competence expressed in cumulus cells before ICSI: a preliminary study

PURPOSE

To identify reliable genomic biomarkers expressed in cumulus cells that accurately and non-invasively predict the oocyte developmental competence and reinforce the already used morphological criteria.

METHODS

Eight consenting patients were selected for ovarian stimulation and ICSI procedures. Cumulus-oocyte complexes were transvaginally punctured and individually selected based on both good morphological criteria and high zona pellucida birefringence. Following ICSI, two 3-day embryos per patient were transferred. Pregnancy outcome was recorded and proven implantation was thereafter confirmed. Differential gene expression was assessed using two microarray platforms. Further real-time PCR validation, Ingenuity pathways analysis and intra-patient analysis were performed on 17 selected candidates.

RESULTS

Seven genes were differentially (p ≤ 0.05) associated to successful pregnancy and implantation. These biomarkers could be used to predict the oocyte developmental competence.

CONCLUSIONS

These genomic markers are a powerful reinforcement of morphological approaches of oocyte selection. Their large-scale validation could increase pregnancy outcome and single embryo transfer efficiency.

Changes in the distribution of mitochondria before and after in vitro maturation of human oocytes and the effect of in vitro maturation on mitochondria distribution

OBJECTIVE

To clarify the relationship between oocyte maturation and mitochondria distribution and assess the effects of in vitro maturation (IVM) on the distribution of mitochondria in human oocytes.

DESIGN

Prospective randomized trial.

SETTING

Hospital-based IVF center.

PATIENT(S)

One hundred fifty-eight patients undergoing intracytoplasmic sperm injection (ICSI) treatment for male factors or combined with oviduct infertility and fifteen patients undergoing controlled ovarian hyperstimulation followed by coitus or IUI.

INTERVENTION(S)

Of all the 284 immature oocytes, 140 were fixed directly. The others were prepared for IVM before they were fixed. All the 21 oocytes matured in vivo were fixed directly and stained for mitochondria. Both immature and mature oocytes were stained by Mito Tracker Green FM. The distribution of mitochondria was observed using a confocal laser scanning microscope.

MAIN OUTCOME MEASURE(S)

Mitochondrial distribution.

RESULT(S)

Three mitochondria distribution patterns were identified: peripheral, semiperipheral, and evenly diffused. A peripheral distribution of mitochondria was presented by 64.1% (50/78) of the germinal vesicle (GV) oocytes; 45.2% (28/62) of the meiosis I oocytes maintained the peripheral distribution; and 38.7% (24/62) presented a diffused status. After IVM, 75.5% (80/106) of the oocytes displayed an evenly diffused type of distribution. The mitochondria were more abundant in the inner cytoplasm than in the peripheral region in most of the oocytes matured in vivo.

CONCLUSION(S)

There are obvious changes in the distribution of mitochondria in human oocytes before and after maturation. Distribution of mitochondria in oocytes matured in vitro is slightly different from that of oocytes matured in vivo. The results may partially explain the reduced developmental potential of oocytes matured in vitro compared with those matured in vivo.

Preservation of fertility in young cancer patients: contribution of transmission electron microscopy

During the last decade, new technologies in reproductive medicine have emerged to preserve the fertility of women whose gonadal function is threatened by premature menopause or gonadotoxic treatments. To offer an individualized approach to these patients, different experimental procedures are under investigation, including oocyte cryopreservation and cryopreservation and transplantation of ovarian tissue in the form of cortical fragments, whole ovary or isolated follicles. This review shows that transmission electron microscopy (TEM), combined with other in-vivo and in-vitro analysis techniques, is a valuable tool in the establishment of new experimental protocols to preserve female fertility. Ultrastructural studies allow in-depth evaluation of the oocyte's unique morpho-functional characteristics, which explain its low cryotolerance, and provide essential information on follicular, stromal and endothelial cell integrity, as well as cellular interactions crucial for normal folliculogenesis. In order to be able to offer appropriate and efficient options in every clinical situation, oocyte in-vitro maturation and ovarian tissue transplantation need to be optimized. Further development of new approaches, such as follicular isolation and whole ovary transplantation, should be encouraged. Fine ultrastructural details highlighted by TEM studies will be useful for the further optimization of these emerging technologies.

Molecular mechanisms of ovulation: co-ordination through the cumulus complex

Successful ovulation requires that developmentally competent oocytes are released with appropriate timing from the ovarian follicle. Somatic cells of the follicle sense the ovulatory stimulus and guide resumption of meiosis and release of the oocyte, as well as structural remodelling and luteinization of the follicle. Complex intercellular communication co-ordinates critical stages of oocyte maturation and links this process with release from the follicle. To achieve these outcomes, ovulation is controlled through multiple inputs, including endocrine hormones, immune and metabolic signals, as well as intrafollicular paracrine factors from the theca, mural and cumulus granulosa cells and the oocyte itself. This review focuses on the recent advances in understanding of molecular mechanisms that commence after the gonadotrophin surge and culminate with release of the oocyte. These mechanisms include intracellular signalling, gene regulation and remodelling of tissue structure in each of the distinct ovarian compartments. Most critical ovulatory mediators exert effects through the cumulus cell complex that surrounds and connects with the oocyte. The convergence of ovulatory signals through the cumulus complex co-ordinates the key mechanistic processes that mediate and control oocyte maturation and ovulation.

Ultrastructure of human mature oocytes after slow cooling cryopreservation using different sucrose concentrations†

BACKGROUND

We studied the ultrastructural characteristics of human mature oocytes frozen/thawed (F/T) using different concentrations of sucrose. Fresh human mature oocytes were used as controls.

METHODS

The oocytes (n = 48) were fixed in 1.5% glutaraldehyde at sampling (n = 16) or after freeze/thawing performed using a slow cooling method with propane-1,2-diol 1.5 mol/l and sucrose at either 0.1 mol/l (n = 16) or 0.3 mol/l (n = 16) in the freezing solution. The oocytes were then processed for electron microscopy observations.

RESULTS

Fresh and F/T oocytes belonging to both study groups were regularly rounded in sections, with a homogeneous cytoplasm and an intact zona pellucida (ZP). Organelles (mainly mitochondria-smooth endoplasmic reticulum aggregates and mitochondria-vesicle complexes) were abundant and uniformly dispersed in the ooplasm. The amount and density of cortical granules appeared to be abnormally reduced in some F/T samples, independently of the sucrose concentration in the freezing solution: this feature was frequently associated with an increased density of the inner ZP, possibly related to the occurrence of zona 'hardening'. Furthermore, slight to moderate microvacuolization was revealed in the ooplasm of some F/T oocytes, particularly in those treated with sucrose 0.3 mol/l.

CONCLUSIONS

Freeze/thawing procedures are associated with ultrastructural alterations in specific oocyte microdomains, presumably linked to the reduced developmental potential of mature cryopreserved oocytes. Further work is needed to determine whether or not a high concentration of sucrose plays a role, at least in part, in producing the above alterations.

Three-dimensional structure of the zona pellucida at ovulation

The mammalian zona pellucida (ZP) is an extracellular matrix surrounding oocytes and early embryos, which is critical for normal fertilization and preimplantation development. It is made up of three/four glycoproteins arranged in a delicate filamentous matrix. Scanning electron microscopy (SEM) studies have shown that ZP has a porous, net-like structure and/or nearly smooth and compact aspect. In this study, the fine 3-D structure of the human and mouse ZP is reviewed with the aim to integrate ultrastructural and molecular data, considering that the mouse is still used as a good model for human fertilization. By conventional SEM observations, numerous evidences support that the spongy ZP appearance well correlates with mature oocytes. When observed through more sophisticated techniques at high resolution SEM, ZP showed a delicate meshwork of thin interconnected filaments, in a regular alternating pattern of wide and tight meshes. In mature oocytes, the wide meshes correspond to "pores" of the "spongy" ZP, whereas the tight meshes correspond to the compact parts of the ZP surrounding the pores. In conclusion, the traditional "spongy" or "compact" appearance of the ZP at conventional SEM appears to be only the consequence of a prevalence of different arrangements of microfilament networks, according to the maturation stage of the oocyte, and in agreement with the modern supramolecular model of the ZP at the basis of egg-sperm recognition. Despite great differences in molecular characterization of ZP glycoproteins between human and mouse ZP, there are no differences in the 3-D organization of glycoproteic microfilaments in these species.