Calcium concentration in vitrification medium affects the developmental competence of in vitro matured ovine oocytes

Transcriptomics Reveal Molecular Differences in Equine Oocytes Vitrified before and after In Vitro Maturation

In the last decade, in vitro embryo production in horses has become an established clinical practice, but blastocyst rates from vitrified equine oocytes remain low. Cryopreservation impairs the oocyte developmental potential, which may be reflected in the messenger RNA (mRNA) profile. Therefore, this study aimed to compare the transcriptome profiles of metaphase II equine oocytes vitrified before and after in vitro maturation. To do so, three groups were analyzed with RNA sequencing: (1) fresh in vitro matured oocytes as a control (FR), (2) oocytes vitrified after in vitro maturation (VMAT), and (3) oocytes vitrified immature, warmed, and in vitro matured (VIM). In comparison with fresh oocytes, VIM resulted in 46 differentially expressed (DE) genes (14 upregulated and 32 downregulated), while VMAT showed 36 DE genes (18 in each category). A comparison of VIM vs. VMAT resulted in 44 DE genes (20 upregulated and 24 downregulated). Pathway analyses highlighted cytoskeleton, spindle formation, and calcium and cation ion transport and homeostasis as the main affected pathways in vitrified oocytes. The vitrification of in vitro matured oocytes presented subtle advantages in terms of the mRNA profile over the vitrification of immature oocytes. Therefore, this study provides a new perspective for understanding the impact of vitrification on equine oocytes and can be the basis for further improvements in the efficiency of equine oocyte vitrification.

Effect of bovine oocyte vitrification with EGTA and post-warming recovery with resveratrol on meiotic spindle, mitochondrial function, reactive oxygen species, and developmental competence

The present study aimed to determine the effects of the addition of EGTA to vitrification solutions and a post-warming recovery period supplemented with 1 μM resveratrol on meiotic spindle integrity, mitochondrial activity, ATP content, reactive oxygen species (ROS) levels, and developmental potential of partially denuded, vitrified-warmed bovine oocytes. Results of microtubule distribution and chromosomal arrangement indicated that resveratrol supplementation, irrespective to EGTA addition, reduced the incidence of abnormal meiotic spindles to similar levels of the control group. Mitochondrial membrane potential was similar in all groups, but ATP content was negatively affected by the vitrification-warming procedure and failed to recover after 4 h of post-warming culture. Resveratrol caused the reduction of ROS to lower levels of the control group, and showed the lowest ROS levels when combined with EGTA treatment. Oocytes in all vitrification groups presented lower developmental potential when compared to fresh oocytes. However, oocytes that underwent vitrification supplemented with EGTA and post-warming culture along with resveratrol showed higher developmental competence compared with vitrified-warmed oocytes not supplemented with resveratrol. The results of our study indicate that submitting vitrified-warmed, partially denuded bovine oocytes to a post-warming recovery period supplemented with 1 μM resveratrol improves vitrification outcomes. However, the benefits of EGTA on vitrification and warming of bovine oocytes need to be further investigated.

Potential Development of Vitrified Immature Human Oocytes: Influence of the Culture Medium and the Timing of Vitrification

How does the in vitro maturation (IVM) medium and the vitrification procedure affect the survival of germinal vesicle (GV) oocytes obtained from stimulated cycles and their development to the blastocyst stage? In total, 1085 GV human oocytes were obtained after women underwent a cycle of controlled ovarian stimulation, and these oocytes were subjected to IVM before or after their vitrification. IVM was carried out in two commercial culture media not specifically designed for maturation. MII oocytes were then activated and embryo development until day 6 was evaluated. According to the results, a higher percentage of oocytes reach the MII stage if they are vitrified before they undergo IVM. Nevertheless, the medium used and the sample size determine whether these differences become significant or not. Similar survival rates and development to blastocysts were observed in all the conditions studied.

Oocyte Cryopreservation in Domestic Animals and Humans: Principles, Techniques and Updated Outcomes

Oocyte cryopreservation plays important roles in basic research and the application of models for genetic preservation and in clinical situations. This technology provides long-term storage of gametes for genetic banking and subsequent use with other assisted reproductive technologies. Until recently, oocytes have remained the most difficult cell type to freeze, as the oocytes per se are large with limited surface area to cytoplasm ratio. They are also highly sensitive to damage during cryopreservation, and therefore the success rate of oocyte cryopreservation is generally poor when compared to noncryopreserved oocytes. Although advancement in oocyte cryopreservation has progressed rapidly for decades, the improvement of cryosurvival and clinical outcomes is still required. This review focuses on the principles, techniques, outcomes and prospects of oocyte cryopreservation in domestic animals and humans.

Highly successful production of viable mice derived from vitrified germinal vesicle oocytes

The vitrification of immature germinal vesicle (GV) oocytes is an important way to preserve genetic resources and female fertility. However, it is well known that cryopreserved GV oocytes have very poor developmental ability and that further improvement in this technique is needed. We previously reported the successful vitrification of matured mouse oocytes with enclosed cumulus cells using the calcium-free vitrification solution supplemented with ethylene glycol (EG) by the minimal volume cooling (MVC) method. In this study, we investigated whether our method is applicable to the vitrification of mouse oocytes at the GV stage (GV oocytes). Following maturation and fertilization in vitro, vitrified GV oocytes showed high survival (94.3 ± 2.0%) and maturation (94.3 ± 2.1%) rates. Although the fertilization and blastocyst rates of vitrified oocytes (fertilization: 46.6 ± 4.9% and blastocyst: 46.6 ± 3.0%) were significantly lower than those of fresh oocytes (fertilization: 73.0 ± 7.1% and blastocyst: 71.6 ± 8.0%) (P < 0.01), there were no differences in the ability to develop to term between fresh oocytes (50.0 ± 8.4%) and vitrified oocytes (37.5 ± 4.6%) (P > 0.05). In conclusion, we here show, for the first time, the efficient production of live mice derived from vitrified GV oocytes.

In vitro production of small ruminant embryos: latest improvements and further research

This review presents the latest advances in and main obstacles to the application of invitro embryo production (IVEP) systems in small ruminants. This biotechnology is an extremely important tool for genetic improvement for livestock and is essential for the establishment of other biotechnologies, such as cloning and transgenesis. At present, the IVEP market is almost non-existent for small ruminants, in contrast with the trends observed in cattle. This is probably related to the lower added value of small ruminants, lower commercial demand and fewer qualified professionals interested in this area. Moreover, there are fewer research groups working on small ruminant IVEP than those working with cattle and pigs. The heterogeneity of oocytes collected from growing follicles in live females or from ovaries collected from abattoirs remains a challenge for IVEP dissemination in goats and sheep. Of note, although the logistics of oocyte collection from live small ruminant females are more complex than in the bovine, in general the IVEP outcomes, in terms of blastocyst production, are similar. We anticipate that after appropriate training and repeatable results, the commercial demand for small ruminant invitro -produced embryos may increase.

Modulation of P2Y2 receptors in bovine cumulus oocyte complexes: effects on intracellular calcium, zona hardening and developmental competence

The improvement of cryopreserved oocyte survival is imperative for the preservation of female fertility. In this study, we investigate whether P2Y2 receptors (P2Y2R) can be directly implicated in calcium (Ca²⁺) homeostasis misbalances observed during the cryopreservation process of cumulus oocyte complexes (COC). Firstly, RNA was extracted from bovine immature and mature oocytes and cumulus cells and real-time PCR performed to identify P2Y2R transcripts (experiment 1). Changes in intracellular calcium concentration [Ca2+]i of mature COC and oocytes (experiment 2) were measured upon exposure to cryoprotectants (CPA), UTP (P2Y2R stimulator, 100 μM), and/or suramin (P2Y2R inhibitor, 100 and 300 μM). The functional role of P2Y2R was investigated by analyzing the effect on oocyte viability of its modulation prior and during oocyte exposure to CPA (experiment 3). Mature COC were randomly divided into groups, and exposed to CPA and different P2Y2 modulators. Oocytes' viability, cortical granules location, and competence for development were assessed. Results showed that P2Y2R mRNAs are expressed in both oocytes and cumulus cells. Stimulation with UTP and CPA led to [Ca2+]i increase, and this effect was totally or partially blocked by suramin (P2Y2R inhibitor). Oocyte exposure to CPA and UTP reduced embryo rates compared with control and suramin100μM (P ≤ 0.04). The observed enhanced premature zona hardening in oocytes exposed to CPA (P = 0.04) and UTP (P = 0.005) stimulus was inhibited by suramin 100 μM. In conclusion, inhibition of P2Y2R during cryoprotectant exposure reduces premature intracellular Ca²⁺ release and significantly improves the developmental competence of exposed bovine oocytes.

Cumulus cells during in vitro fertilization and oocyte vitrification in sheep: Remove, maintain or add?

The objective was to evaluate the developmental competence of immature and matured ovine oocytes after removing, maintaining or adding cumulus cells (CC) associated to vitrification by Cryotop method. Three experiments were performed involving 3,144 oocytes. In Experiment 1, CC were removed from immature, matured or fertilized oocytes subjected to in vitro embryo production. In Experiment 2, oocytes were vitrified either in MI or MII stage with or without CC, while a control group with CC remained unvitrified. In Experiment 3, oocytes partially denuded from CC were vitrified either in MI or MII stage, and a co-culture of fresh CC was added or not soon after warming to complete in vitro maturation (IVM) and in vitro fertilization (IVF), or IVF, respectively, while a control group remained unvitrified. In Experiment 1, the cleavage rate, development rate on Day 6 and blastocyst rate on Day 8 were improved when CC were maintained until the end of IVF (P < 0.05). In Experiment 2, vitrification of oocytes with enclosed CC showed a tendency to increase cleavage (P = 0.06) and improved blastocyst rate (P < 0.05). In Experiment 3, adding CC as co-culture after vitrification-warming tended to improve cleavage rate (P = 0.06) and increased hatching rate (P < 0.05). Regarding oocyte stage, vitrification of in vitro matured oocytes resulted in greater developmental competence than immature stages (P < 0.05). In conclusion, CC seems to have a relevant role for in vitro embryo development in either fresh or vitrified oocytes.

Morphological features and microtubular changes in vitrified ovine oocytes

Cryobanking of oocytes collected from prepubertal donors may supply a virtually unlimited number of female gametes for both basic research and commercial applications. Prepubertal oocytes show some structural and functional limitations compared to the adult ones that may impair their ability to recover damages from cryopreservation. In oocytes, the meiotic spindle is acutely sensitive to temperature deviation, but capable of regeneration following cryopreservation. In the present work, we studied the effects of vitrification and post-warming incubation on the microtubular cytoskeleton and the tubulin post-translational modifications (tyrosination and acetylation) in prepubertal and adult oocytes. Obtained results showed that prepubertal oocytes are more affected by vitrification-induced injuries than adult ones. In fact, prepubertal oocytes showed more severe alterations of the meiotic spindle conformation and a higher percentage of parthenogenetic activation compared to adult ones. Moreover, in the adult oocytes the equilibrium between tyrosinated and acetylated α-tubulin was restored after 4 h of post-warming incubation. Diversely, in prepubertal oocytes the imbalance between tyrosinated and acetylated α-tubulin was increased during post-warming incubation. Our study shows that prepubertal oocytes react differently to the insults provoked by vitrification compared to adult oocytes, showing an impaired ability to recover from vitrification-induced injuries. In the evaluation of oocyte ability to recover from vitrification-induced injuries, tubulin post-translational modifications represent an important indicator for assessing oocyte quality.

Vitrification negatively affects the Ca2+-releasing and activation potential of mouse oocytes, but vitrified oocytes are potentially useful for diagnostic purposes

RESEARCH QUESTION

To what extent does vitrification affect the Ca²⁺-releasing and activation potential of mouse oocytes, which are commonly used to determine the oocyte activation potential of human spermatozoa?

DESIGN

The effect of mouse oocyte vitrification on Ca²⁺ dynamics and developmental competence after oocyte activation was assessed and compared with fresh mouse oocytes. Moreover, the Ca²⁺ store content of the endoplasmic reticulum was determined at different time points during the vitrification-warming procedure. Finally, the Ca²⁺ pattern induced by cryoprotectant exposure was determined.

RESULTS

After human sperm injection into mouse oocytes, Ca²⁺ dynamics but not fertilization rates were significantly altered by vitrification warming (P < 0.05). Ca²⁺ dynamics in response to SrCl₂ or ionomycin were also altered by oocyte vitrification. In contrast, activation and blastocyst rates after SrCl₂ exposure were not affected (P > 0.05), whereas activation rates after ionomycin exposure were significantly lower in vitrified-warmed oocytes (P < 0.05); blastocyst rates were not affected (P > 0.05). Cryoprotectant exposure was associated with a strong drop in endoplasmic reticulum Ca²⁺ store content. Oocytes rapidly recovered during warming and recovery in Ca²⁺-containing media; a threshold area under the curve of Ca²⁺ dynamics to obtain activation rates above 90% was determined.

CONCLUSIONS

Vitrified-warmed mouse oocytes display reduced Ca²⁺-releasing potential upon oocyte activation, caused by cryoprotectant exposure. With adapted classification criteria, these oocytes could be used for diagnosing oocyte activation deficiencies in patients. Evaluating the Ca²⁺-signalling machinery in vitrified-warmed human oocytes is required.

An improved method for vitrification of in vitro matured ovine oocytes; beneficial effects of Ethylene Glycol Tetraacetic acid, an intracellular calcium chelator

Vitrification affects fertilization ability and developmental competence of mammalian oocytes. This effect may be more closely associated with an intracellular calcium rise induced by cryoprotectants. The present study aimed to assess whether addition of Ethylene Glycol Tetraacetic acid (EGTA) to vitrification solution could improve quality and developmental competence of in vitro matured ovine oocytes. Vitrified groups were designed according to the presence or absence of EGTA and/or calcium in base media, including: mPB1+ (modified PBS with Ca²⁺), mPB1^- (modified PBS without Ca²⁺), mPB1⁺/EGTA (mPB1⁺ containing EGTA), mPB1^-/EGTA (mPB1^- containing EGTA). In vitro development, numerical chromosome abnormalities, hardening of zona pellucida, mitochondrial distribution and function of viable oocytes were evaluated and compared between groups. Quality of blastocysts was assessed by differential and TUNEL staining. Also, mRNA expression levels of six candidate genes (KIF11, KIF2C, CENP-E, KIF20A, KIF4A and KIF2A), were quantitatively evaluated by RT-PCR. Our results showed that calcium-free vitrification and EGTA supplementation can significantly increase the percentage of normal haploid oocytes and maintain normal distribution and function of mitochondria in vitrified ovine oocytes, consequently improving developmental rate after in vitro fertilization. qRT-PCR analysis showed no significant difference in mRNA expression levels of kinesin genes between vitrified and fresh oocytes. Also, the presence of calcium in vitrification solution significantly increased zona hardening. In conclusion, we have shown for the first time that supplementation of vitrification solution with EGTA, as a calcium chelator, improved the ability of vitrified ovine oocytes to preserve mitochondrial distribution and function, as well as normal chromosome segregation.

Calcium ion regulation by BAPTA-AM and ruthenium red improved the fertilisation capacity and developmental ability of vitrified bovine oocytes

Vitrification reduces the fertilisation capacity and developmental ability of mammalian oocytes; this effect is closely associated with an abnormal increase of cytoplasmic free calcium ions ([Ca²⁺]i). However, little information about the mechanism by which vitrification increases [Ca²⁺]i levels or a procedure to regulate [Ca²⁺]i levels in these oocytes is available. Vitrified bovine oocytes were used to analyse the effect of vitrification on [Ca²⁺]i, endoplasmic reticulum Ca²⁺ (ER Ca²⁺), and mitochondrial Ca²⁺ (mCa²⁺) levels. Our results showed that vitrification, especially with dimethyl sulfoxide (DMSO), can induce ER Ca²⁺ release into the cytoplasm, consequently increasing the [Ca²⁺]i and mCa²⁺ levels. Supplementing the cells with 10 μM 1,2-bis (o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA-AM or BAPTA) significantly decreased the [Ca²⁺]i level and maintained the normal distribution of cortical granules in the vitrified bovine oocytes, increasing their fertilisation ability and cleavage rate after in vitro fertilisation (IVF). Treating vitrified bovine oocytes with 1 μM ruthenium red (RR) significantly inhibited the Ca²⁺ flux from the cytoplasm into mitochondria; maintained normal mCa²⁺ levels, mitochondrial membrane potential, and ATP content; and inhibited apoptosis. Treating vitrified oocytes with a combination of BAPTA and RR significantly improved embryo development and quality after IVF.

Effect of vitrification on the zona pellucida hardening and follistatin and cathepsin B genes expression and developmental competence of in vitro matured bovine oocytes

The purpose of our study was to assess the effect of vitrification with or without the presence of calcium in the vitrification solution on the: 1) diameter of oocytes and thickness of the zona pellucida, 2) zona pellucida hardening, 3) expression of mRNA follistatin (FST) and cathepsin B (CTSB) in oocytes and 4) developmental competence of embryos derived from in vitro matured and vitrified oocytes. The results of our study demonstrate, that vitrification did not alter thickness of the zona pellucida and diameter of the oocytes, however it triggered hardening of the zona pellucida. The presence of calcium in the vitrification solutions intensified hardening of zona in immature and mature oocytes (P < 0.04, P < 0.001, respectively) and provoked increased mRNA FST expression in oocytes matured in vitro compared to immature oocytes (P < 0.01) and those vitrified without calcium (P < 0.004). CTSB mRNA expression was increased in immature oocytes and oocytes vitrified with calcium compare to mature oocytes (P < 0.02). The developmental potential of vitrified oocytes was impaired compared to non-vitrified oocytes, being more evident in oocytes vitrified with calcium. In summary, vitrification did not change the oocyte diameter and thickness of the zona pellucida and expression of FST and CTSB mRNA. It diminished developmental potential of the vitrified oocytes. The presence of calcium in the vitrification solutions increased hardening of zona pellucida as well as affected the level of FST and CTSB mRNA in oocytes and developmental potential of these oocytes.

Osmotic-shock produced by vitrification solutions improves immature human oocytes in vitro maturation

BACKGROUND

During cytoplasmic oocyte maturation, Ca(2+) currents are vital for regulating a broad range of physiological processes. Recent studies have demonstrated that DMSO and EG cause large transient increases in intracellular Ca(2+) in mouse oocytes. The CP used in vitrifying protocols also increases the intracellular calcium transient. The aim of this study is to evaluate the effects of vitrifying time (before and after IVM) and exposure to the vitrification solutions and ionomycin on oocyte quality and embryonic development.

METHODS

221 GV-oocytes unsuitable for IVF-ICSI cycles were randomly distributed into one of the following three groups. G1 (control group): 41 GV-oocytes IVM until MII; G2: 43 oocytes vitrified at GV stage and IVM until MII stage; and G3: 53 GV-oocytes IVM until MII and then vitrified. In order to clarify the effect of vitrification solutions (VS) on human oocyte IVM through the intracellular Ca(2+) oscillation, the following two groups were also included. G4: 43 GV-oocytes exposed to VS and IVM until MII; and G5: 41 GV-oocytes exposed to ionomycin and IVM until MII. All GV-oocytes that reached MII-stage were parthenogenetically activated to assess oocyte viability. IVM was performed in IVF-medium (24-48 h). Chemical treatment (ionomycin) and osmotic treatment (vitrification solutions) were performed without liquid-N2 immersion. The following rates were evaluated: survival (SR), in-vitro maturation (IVMR), activation (AR), development to 2-cell (DRC), development to morula (DRCM) and development to blastocyst (DRB). Ratios between the different treatment groups were compared using contingency tables analysis (chi-square test).

RESULTS

A high survival rate was obtained in G2 (95.5 %) and G4 (96.6 %). In-vitro maturation rate was significantly higher for G4 (86 %) and G2 (83.7 %) compared to G1 (63.4 %), G3 (56.6 %) and G5 (48.8 %). DRCM was significantly higher for G1 and G2 compared to G3 (G1: 15.8 %, G2: 20.7 % and G3: 0 %). DRB was only obtained for the oocytes vitrified before IVM (G2: 3.4 %). AR was also significantly higher for G2 and G4 compared to G5 (G2: 80.5 %, G4: 86.5 % and G5: 55 %). DRCM and DRB were only obtained in G2 and G4. DRCM was significantly higher for oocytes vitrified at GV stage (G2) and for oocytes exposed to the VS in G4 compared to the oocytes exposed to the ionomycin in G5 (G2: 20.7 %; G4: 37.5 % and G5: 0 %).

CONCLUSIONS

Vitrifying GV-oocytes improves their IVM. Further investigation could look to increase the oocyte pool and improve fertility preservation options.

The effects of permeating cryoprotectants on intracellular free-calcium concentrations and developmental potential of in vitro-matured feline oocytes

Embryos produced from vitrified feline oocytes have resulted in pregnancies, but the efficiency of oocyte vitrification in cats is still low. Our objectives were to evaluate the effects of exposing feline oocytes to ethylene glycol (EG), propanediol (PrOH) and dimethyl sulfoxide (DMSO) on changes in intracellular free-calcium concentrations ([Ca2+]i), the time needed for enzymatic digestion of the zona pellucida (ZP), the incidence of parthenogenetic activation and degeneration and embryonic development following in vitro fertilisation (IVF). All of the chemicals tested altered [Ca2+]i, but changes in [Ca2+]i, resistance of the ZP to enzymatic digestion and the incidence of parthenogenetic activation (<5% for all treatments) were not affected (P > 0.05) by extracellular Ca2+. Exposure to EG (>44.1%) and DMSO (19.7%) increased (P < 0.05) oocyte degeneration compared with control oocytes and oocytes exposed to PrOH (≤2.5%). Following exposure to a combination of PrOH and DMSO (10% v/v each), blastocyst development (per cleaved embryo; 52.1%) was similar (P > 0.05) to control oocytes (64.4%). When oocytes were vitrified with PrOH and DMSO, 28.3% of surviving (intact plasma membrane) oocytes cleaved following IVF, but no blastocyst developed. When a non-permeating cryoprotectant (galactose, 0.25 M) was added to the vitrification medium, 47.7% of surviving oocytes cleaved and 14.3% developed to the blastocyst stage.

Dose-dependent effect of melatonin on postwarming development of vitrified ovine embryos

After cryopreservation, embryos become sensitive to the oxidative stress, resulting in lipid peroxidation, membrane injury, and structural destruction. The present study aimed to assess the effect of increasing concentration of melatonin during postwarming culture on embryo's ability to restore its functions after cryopreservation. In vitro-produced blastocysts were vitrified, warmed, and cultured in vitro in TCM 199 with 5 different supplementations: control (CTR): 10% fetal calf serum; bovine serum albumin (BSA): 0.04% (wt/vol) BSA; and MEL(-3), MEL(-6), MEL(-9): BSA plus melatonin 10(-3), 10(-6), and 10(-9) M. The medium with the highest melatonin concentration had the highest trolox equivalent antioxidant capacity, whose values were comparable with those determined in plasma sampled from adult ewes (8.7 ± 2.4 mM). The other media had lower trolox equivalent antioxidant capacity values (P < 0.01), below the range of the plasma. At the same time, embryos cultured with the highest melatonin concentration reported a lower in vitro viability, as evaluated by lower re-expansion and hatching rates, and lower total cell number compared with the other groups (P < 0.05). Their metabolic status was also affected, as evidenced by higher oxidative and apoptotic index and lower ATP concentration. The beneficial effects of melatonin on embryo development during postwarming culture were observed only at low concentration (10(-9) M). These results suggest that melatonin at high concentration may exert some degree of toxic activity on pre-implantation embryos. Thus, the dose at which the embryos are exposed is pivotal to obtain the desiderate effect.