Characterization and control of oocyte large-scale chromatin configuration in different cattle breeds

Factors defining developmental competence of bovine oocytes collected for in vitro embryo production†

Despite the currently relatively low effectiveness of producing bovine embryos in vitro, there is a growing interest in applying this laboratory method in the field of reproduction. Many aspects of the procedure need to be improved. One of the main problems is the inferior developmental competence of in vitro matured oocytes that are collected using the ovum pick-up method. The mechanisms of oocyte capacitation and maturation, as well as the in vivo conditions in which they grow and mature, should be carefully analyzed. A deliberate application of the identified mechanisms and beneficial factors affecting the in vitro procedures seems to be essential for achieving higher developmental competence of the oocytes that are subjected to fertilization. The results may be improved by developing and employing a laboratory maturation protocol that corresponds with appropriate preparation of donors before the ovum pick-up, an optimized hormonal treatment program, the appropriate size of ovarian follicles at the time of aspiration, and a fine-tuned coasting period.

Improving Cryopreservation Efficiency and Pregnancy Rate through Superovulation with Follicle-Stimulating Hormone in Korean Hanwoo Cows via Ovum Pick Up

The aim of this study was to devise an efficient technique for generating embryos from high-quality bovine females. Oocytes were collected from 20 control and 15 Hanwoo (Bos taurus coreanae) females treated with the FSH. A combination of decreasing FSH doses (36, 36, 24, and 24 mg, 12 h apart), progesterone, estrogen, and prostaglandins were administered to synchronize and mildly stimulate the animals. The FSH-treated group (1125 oocytes) and control group (1022 oocytes) exhibited a higher proportion of grade A and B oocytes (88.2%) than the other grades (p < 0.05), with most at the germinal vesicle 2 stage (64.0%). Moreover, the FSH-treated group achieved a notably higher blastocyst rate (44.7%) compared to the control group (31.1%) (p < 0.05). After undergoing vitrification and in vitro culture (IVC) warming, embryos in the FSH group exhibited higher re-expansion rates (grade 1: 86.9%; grades 2 and 3: 57.9%) compared to those in the control (p < 0.05). This highlights the positive impact of FSH treatment on in vitro embryo production (IVEP) and the OPU rate.

Advances in Oocyte Maturation In Vivo and In Vitro in Mammals

The quality and maturation of an oocyte not only play decisive roles in fertilization and embryo success, but also have long-term impacts on the later growth and development of the fetus. Female fertility declines with age, reflecting a decline in oocyte quantity. However, the meiosis of oocytes involves a complex and orderly regulatory process whose mechanisms have not yet been fully elucidated. This review therefore mainly focuses on the regulation mechanism of oocyte maturation, including folliculogenesis, oogenesis, and the interactions between granulosa cells and oocytes, plus in vitro technology and nuclear/cytoplasm maturation in oocytes. Additionally, we have reviewed advances made in the single-cell mRNA sequencing technology related to oocyte maturation in order to improve our understanding of the mechanism of oocyte maturation and to provide a theoretical basis for subsequent research into oocyte maturation.

Enhanced progesterone support during stimulated cycles of transvaginal follicular aspiration improves bovine in vitro embryo production

The in vitro production (IVP) of cattle embryos requires that germinal-vesicle stage oocytes undergo a period of maturation in vitro prior to fertilization and culture to the blastocyst stage. Success of IVP in taurine cattle is enhanced following ovarian stimulation prior to oocyte retrieval (OPU), particularly if preceded by a short period of FSH withdrawal ('coasting'). However, evidence regarding the importance of progesterone (P4) support during OPU-IVP is equivocal. The current study, therefore, determined the effects of increased peripheral P4 concentrations during FSH-stimulated ('coasted') cycles of OPU. Progesterone support was provided by either an active corpus luteum (CL) and/or one of two intravaginal P4 releasing devices (i.e., CIDR® [1.38 g P4] or PRID® Delta [1.55 g P4]). Expt. 1 established an initial estrus prior to OPU, allowing CL formation (single luteal phase) spanning the first two of five cycles of OPU; the remaining three cycles were supported by either a CIDR® or PRID® Delta. Expt. 2 commenced with two cycles of dominant follicle removal (including prostaglandin F_2α) undertaken seven days apart prior to six cycles of OPU. The absence of a CL meant that these cycles were supported only by a CIDR® or PRID® Delta. As each experiment involved several sequential cycles of OPU, the cumulative effects of device use on vaginal discharges were also assessed. Each experiment involved 10 sexually mature Holstein heifers. In the absence of a CL, peak plasma P4 concentrations were greater (P = 0.002) for the PRID® Delta (4.3 ± 0.22) than for the CIDR® (2.9 ± 0.22). In Expt. 1 there was an interaction (P < 0.05) between CL presence at OPU and P4 device on Day 8 blastocyst yields, indicating an effect of P4 device only when the CL was absent. The percentage hatching/hatched blastocysts of matured oocytes for the CIDR® and PRID® Delta was 44.3 ± 5.04 and 41.0 ± 5.40 in the presence, and 17.1 ± 3.48 and 42.2 ± 3.76 in the absence, of a CL (P = 0.018). Combined analyses of data from Expt. 1 and 2, when no CL was present, confirmed that Day 8 blastocyst yields were greater (P = 0.022) for the PRID® Delta than the CIDR®. Vaginal discharge scores were higher (P < 0.001) for the PRID® Delta than the CIDR® in Expt. 1 but not in Expt 2; however scores were low, did not increase with repeated use, and thus were deemed of no clinical or welfare concern. In conclusion, enhanced P4 support during FSH-stimulated cycles of OPU-IVP can improve in vitro embryo development.

Progress toward species-tailored prematuration approaches in carnivores

In the past four decades, the bovine model has been highly informative and inspiring to assisted reproductive technologies (ART) in other species. Most of the recent advances in ART have come from studies in cattle, particularly those unveiling the importance of several processes that must be recapitulated in vitro to ensure the proper development of the oocyte. The maintenance of structural and functional communications between the cumulus cells and the oocyte and a well-orchestrated chromatin remodeling with the gradual silencing of transcriptional activity represent essential processes for the progressive acquisition of oocyte developmental competence. These markers are now considered the milestones of physiological approaches to increase the efficiency of reproductive technologies. Different in vitro approaches have been proposed. In particular, the so-called "pre-IVM" or "prematuration" is a culture step performed before in vitro maturation (IVM) to support the completion of the oocyte differentiation process. Although these attempts only partially improved the embryo quality and yield, they currently represent a proof of principle that oocytes retrieved from an ovary or an ovarian batch shouldn't be treated as a whole and that tailored approaches can be developed for culturing competent oocytes in several species, including humans. An advancement in ART's efficiency would be desirable in carnivores, where the success is still limited. Since the progress in reproductive medicine has often come from comparative studies, this review highlights aspects that have been critical in other species and how they may be extended to carnivores.

Physiological parameters related to oocyte nuclear differentiation for the improvement of IVM/IVF outcomes in women and cattle

In vitro maturation (IVM) has been applied in numerous different contexts and strategies in humans and animals, but in both cases it represents a challenge still far from being overcome. Despite the large dataset produced over the last two decades on the mechanisms that govern antral follicular development and oocyte metabolism and differentiation, IVM outcomes are still unsatisfactory. This review specifically focuses on data concerning the potential consequences of using supraphysiological levels of FSH during IVM, as well as on the regulation of oocyte chromatin dynamics and its utility as a potential marker of oocyte developmental competence. Taken together, the data revisited herein indicate that a significant improvement in IVM efficacy may be provided by the integration of pre-OPU patient-specific protocols preparing the oocyte population for IVM and more physiological culture systems mimicking more precisely the follicular environment that would be experienced by the recovered oocytes until completion of metaphase II.

Effect of season on germinal vesicle stage, quality, and subsequent in vitro developmental competence in bovine cumulus-oocyte complexes

Although the reproductive performance of grazing cattle is lower in summer compared to winter, the effect of season on oocyte developmental competence has not been thoroughly examined. We measured the effect of season on oocyte chromatin compaction, cumulus cell quality, and embryonic development after in vitro fertilization. Cumulus oocytes-complexes (COCs) were collected from abattoir cows' ovaries during the winter and summer months. First, we evaluated the degree of chromatin compaction in germinal vesicle (GV) oocytes (GV1 through GV3), which is associated with different degrees of developmental competence. Then, we determined the apoptotic index in cumulus cells from immature and in vitro matured COCs. Finally, in vitro matured oocytes were fertilized to determine blastocyst rate and embryo quality. During the summer months, we observed a significantly lower proportion of oocytes reaching the GV3 stage and higher levels of DNA fragmentation in cumulus cell. As a result, blastocyst yield and quality were reduced during the summer months. In conclusion, summer negatively affected oocyte GV stage progression, cumulus cell quality, and embryo development. Increased cumulus cell DNA fragmentation during summer, may partially explain the reduced oocyte maturation capacity, considering the relevance of cumulus-oocyte communication during this stage.

Relationship between chromatin configuration and in vitro maturation ability in guinea pig oocytes

Background

Germinal vesicle (GV) chromatin configurations of oocytes are proposed to be related to oocyte competence and may reflect the quality of oocyte. Currently, a limited number of published studies investigated the GV chromatin configurations of guinea pig oocytes.

Objective

In this study on the in vitro maturation (IVM) of guinea pig oocytes, we examined the changes in their GV chromatin configurations during meiotic progression.

Methods

Based on the degree of chromatin compaction, the GV chromatin configurations of guinea pig oocytes could be divided into three categories depending on whether the nucleolus‐like body (NLB) was surrounded or partly surrounded by compacted chromatin, namely the uncondensed (NSN), the intermediate type (SN‐1) and the compacted type (SN‐2).

Results

The percentage of cells displaying the SN‐2 configuration increased with the growth of guinea pig oocytes, suggesting that this configuration presents the potential for maturation in oocytes. Oocytes derived from larger follicle exhibited increased meiotic potential. Serum starvation affected the GV chromatin configurations of guinea pig oocytes.

Conclusions

Collectively, these results suggest that the SN‐2 type might be a more mature form of configuration in guinea pig oocyte, whose proportion was associated with the follicle size and susceptible to the environment (e.g. serum concentration).

Recreating the Follicular Environment: A Customized Approach for In Vitro Culture of Bovine Oocytes Based on the Origin and Differentiation State

The mammalian ovary is a large source of oocytes organized into follicles at various stages of folliculogenesis. However, only a limited number of them can be used for in vitro embryo production (IVEP), while most have yet to complete growth and development to attain full meiotic and embryonic developmental competence. While the in vitro growth of primordial follicles in the ovarian cortex has the potential to produce mature oocytes, it is still at an experimental stage. The population of early antral follicles (EAFs), instead, may represent a reserve of oocytes close to completing the growth phase, which might be more easily exploited in vitro and could increase the number of female gametes dedicated to IVEP.Here we present in vitro culture strategies that have been developed utilizing physiological parameters to support the specific needs of oocytes at distinct stages of differentiation, in order to expand the source of female gametes for IVEP by maximizing the attainment of fertilizable oocytes. Furthermore, these culture systems provide powerful tools to dissect the molecular processes that direct the final differentiation of the mammalian oocyte.

Synchronization of germinal vesicle maturity improves efficacy of in vitro embryo production in Holstein cows

Germinal vesicle oocytes obtained by ovum pick-up (OPU) on a random day are heterogeneous in terms of chromatin maturity, and those with an intermediate degree of chromatin compaction present higher developmental competence. We previously developed a synchronization protocol combining follicle aspiration and FSH treatment capable of increasing the percentage of oocytes with intermediate chromatin compaction (classified as GV2 oocytes; within progressive stages of chromatin compaction ranging from GV0 to GV3) at the time of OPU. In this study, we tested the capacity of a similar protocol to synchronize oocyte chromatin maturity before OPU, as well as to improve the efficacy of in vitro embryo production (IVP) in Holstein cows. In the first experiment, eight non-lactating Holstein cows were subjected to the D5/4FSH, during which all follicles larger than 2 mm were aspirated and a progesterone intravaginal device was inserted on a random day (day 0). Subsequently, four IM injections of FSH (Folltropin; 40/40/20/20 mg) were administered 12h apart on days 2 and 3, and removal of the progesterone device and OPU were performed on day 5. Of the oocytes recovered by OPU, 83.2% were at the GV2 stage. In a second experiment, eighteen non-lactating Holstein cows (Synchro group) were subjected to the D5/4FSH protocol followed by IVM/IVF, and embryo production was compared with that of other seventeen cows submitted to OPU on a random day followed by IVM/IVF (Control group). Blastocyst rate in relation to total oocytes recovered was higher in the Synchro group (37.9%) compared to the Control group (21%; P < 0.05). The percentage of good quality blastocysts morphologically selected for freezing and later transfer in relation to the total number of oocytes recovered tended to be higher in the Synchro group (27.68%) compared to the Control group (14.34%; P = 0.1). These data suggest that synchronization protocols increasing the percentage of GV2 oocytes in the population subjected to IVM/IVF can improve the efficacy of embryo in vitro production in cattle.