Potential of zygotes to produce live births can be identified by the size of the male and female pronuclei just before their membranes break down

Two types of cleavage, from zygote to three cells, result in different clinical outcomes and should be treated differently

Research Question

What is the utilization rate of embryos that exert inadequate zygote cleavage into three daughter cells?

Design

This study used a retrospective dataset from a single IVF Unit. A total of 3,060 embryos from 1,811 fresh IVF cycles were analyzed. The cleavage pattern, morphokinetics, and outcome were recorded. Only 2pn embryos, fertilized by ejaculated sperm, and cultured in a time-lapse system for at least 5 days were included. We generated three study groups according to the embryo's cleavage pattern: (I) Control, normal cleavage (n = 551); (II) fast cleavage, zygote to three cells within 5 h (n = 1,587); and (III) instant direct tripolar cleavage (IDC) from zygote to three cells (n = 922).

Results

The rate of usable fast cleavage blastocysts was 108/1,587 (6.81%) and usable control blastocysts was 180/551 (32.67%). The time of PN fading and from fading to first cleavage differed significantly between the three groups. Although the pregnancy rate of control and fast cleavage blastocysts were comparable (40.35% and 42.55%, respectively), the amount of instant direct cleavage embryos that reached blastocyst stage was neglectable (only four embryos out of 922 analyzed IDC embryos) and unsuitable for statistical comparison of pregnancy rates.

Conclusion

Our results indicate the need to culture instant direct cleavage embryos for 5 days, up to the blastocyst stage, and avoid transfer of embryos that are fated to arrest even when their morphological grade on day 3 is acceptable, whereas fast cleavage embryos could be transferred on day 3 when there is no alternative.

Altered morphokinetics and differential reproductive outcomes associated with cell exclusion events in human embryos

RESEARCH QUESTION

Can embryos harbouring cell exclusion and their reproductive outcomes be classified based on morphokinetic profiles?

DESIGN

A total of 469 time-lapse videos of embryos transferred between 2013 and 2019 from a single clinic were analysed. Videos were assessed and grouped according to the presence or absence of one or more excluded cells before compaction. Cell division timings, intervals between subsequent cell divisions and dynamic intervals were analysed to determine the morphokinetic profiles of embryos with cell exclusion (CE+), compared with fully compacted embryos without cell exclusion or extrusion (CE-).

RESULTS

Transfer of CE+ embryos resulted in lower proportions of fetal heartbeat (FHB) and live birth compared with CE- embryos (both, P < 0.001). CE+ embryos were associated with delays in t2 (P = 0.030), t6 (P = 0.018), t7 (P < 0.001), t8 (P = 0.001), tSC (P < 0.001) and tM (P < 0.001). Earlier timings for t3 (P = 0.014) and t5 (P < 0.001) were positively associated with CE+; CE+ embryos indicated prolonged S2, S3, ECC3, cc2 and cc4. Logistic regression analysis revealed that t5, tM, S2 and ECC3 were the strongest predictive indicators of cell exclusion. Timings for S2 and ECC3 were useful in identifying increased odds of FHB when a cell exclusion event was present.

CONCLUSION

Embryos harbouring cell exclusion indicated altered morphokinetic profiles. Their overall lower reproductive success was associated with two morphokinetic parameters. Morphokinetic profiles could be used as adjunct indicators for reproductive success during cycles producing few, low-quality embryos. This may allow more objective identification of cell exclusion and refinement of embryo ranking procedures before transfer.

Variation of Female Pronucleus Reveals Oocyte or Embryo Chromosomal Copy Number Variations

The characteristics of the human pronuclei (PNs), which exist 16-22 h after fertilization, appear to serve as good indicators to evaluate the quality of human oocyte and embryo, and may reflect the status of female and male chromosome composition. Here, a quantitative PN measurement method that is generated by applying expert experience combined with deep learning from large annotated datasets is reported. After mathematic reconstruction of PNs, significant differences are obtained in chromosome-normal rate and chromosomal small errors such as copy number variants by comparing the size of the reconstructive female PN. After integrating the whole procedure of PN dynamics and adjusting for errors that occur during PN identification, the results are robust. Notably, all positive prediction results are obtained from the female propositus population. Thus, the size of female PNs may mirror the internal quality of the chromosomal integrity of the oocyte. Embryos that develop from zygotes with larger female PNs may have a reduced risk of copy number variations.

Embryo classification beyond pregnancy: early prediction of first trimester miscarriage using machine learning

PURPOSE

First trimester miscarriage is a major concern in IVF-ET treatments, accounting for one out of nine clinical pregnancies and for up to one out of three recognized pregnancies. To develop a machine learning classifier for predicting the risk of cleavage-stage embryos to undergo first trimester miscarriage based on time-lapse images of preimplantation development.

METHODS

Retrospective study of a 4-year multi-center cohort of 391 women undergoing intra-cytoplasmatic sperm injection (ICSI) and fresh single or double embryo transfers. The study included embryos with positive indication of clinical implantation based on gestational sac visualization either with first trimester miscarriage or live-birth outcome. Miscarriage was determined based on negative fetal heartbeat indication during the first trimester. Data were recorded and obtained in hospital setting and research was performed in university setting.

RESULTS

A minimal subset of six non-redundant morphodynamic features were screened that maintained high prediction capacity. Features that account for the distribution of the nucleolus precursor bodies within the small pronucleus and pronuclei dynamics were highly predictive of miscarriage outcome as evaluated using the SHapley Additive exPlanations (SHAP) methodology. Using this feature subset, XGBoost and random forest models were trained following a 100-fold Monte-Carlo cross validation scheme. Miscarriage was predicted with AUC 0.68 to 0.69.

CONCLUSION

We report the development of a decision-support tool for identifying the embryos with high risk of miscarriage. Prioritizing embryos for transfer based on their predicted risk of miscarriage in combination with their predicted implantation potential is expected to improve live-birth rates and shorten time-to-pregnancy.

Sperm Energy Restriction and Recovery (SER) Alters Epigenetic Marks during the First Cell Cycle of Development in Mice

The sperm energy restriction and recovery (SER) treatment developed in our laboratory was shown to improve fertilization and blastocyst development following in vitro fertilization (IVF) in mice. Here, we investigated the effects of SER on early embryogenesis. Developmental events observed during the first cell cycle indicated that progression through the pronuclear stages of SER-generated embryos is advanced in comparison with control-generated embryos. These findings prompted further analysis of potential effects of SER on pronuclear chromatin dynamics, focusing on the key H3K4me3 and H3K27ac histone modifications. Nearly all the SER-generated embryos displayed H3K4me3 in the male pronuclei at 12 h post-insemination (HPI), while a subset of the control-generated embryos did not. Additionally, SER-generated embryos displayed a more homogenous intensity of H3K27ac at 8 and 12 HPI compared to control embryos. These changes in histone modifications during the first cell cycle were accompanied by differences in gene expression at the two-cell stage; both of these changes in early embryos could potentially play a role in the improved developmental outcomes of these embryos later in development. Our results indicate that sperm incubation conditions have an impact on early embryo development and can be useful for the improvement of assisted reproductive technology outcomes.

Fertilization signatures as biomarkers of embryo quality

Fertilization underpins the vital transition from gametic meiosis to embryonic mitosis. For decades, in human IVF, microscopic observation at a single time point has limited our appreciation of the morphokinetic complexity of this process. More recently, the introduction of time lapse technology-also enhanced by combination with artificial intelligence-has revealed the finest morphokinetic details of the beginning of human development. Overall, a picture has finally emerged in which the precise timing, morphology and geometry of several fertilization events offer clues to predict the fate of the embryo-a key aspect of assisted reproduction. In this scenario, correct unfolding of intra- and interpronuclear rearrangements emerge as a crucial factor to create a platform able to preserve genetic and cellular integrity at the first mitotic cleavage.

Morphometric and morphokinetic differences in the sperm- and oocyte-originated pronuclei of male and female human zygotes: a time-lapse study

PURPOSE

To study the morphometric and morphokinetic profiles of pronuclei (PN) between male and female human zygotes.

METHOD(S)

This retrospective cohort study included 94 consecutive autologous single day 5 transfer cycles leading to a singleton live birth. All oocytes were placed in the EmbryoScope + incubator post-sperm injection with all annotations performed retrospectively by one embryologist (L-SO). Timing parameters included 2nd polar body extrusion (tPB2), sperm-originated PN (tSPNa) or oocyte-originated PN (tOPNa) appearance, and PN fading (tPNF). Morphometrics were evaluated at 8 (stage 1), 4 (stage 2), and 0 h before PNF (stage 3), measuring PN area (um²), PN juxtaposition, and nucleolar precursor bodies (NPB) arrangement.

RESULTS

Male zygotes had longer time intervals of tPB2_tSPNa than female zygotes (4.8 ± 0.2 vs 4.2 ± 0.1 h, OR = 1.442, 95% CI 1.009-2.061, p = 0.044). SPN increased in size from stage 1 through 2 to 3 (435.3 ± 7.2, 506.7 ± 8.0, and 556.3 ± 8.9 um², p = 0.000) and OPN did similarly (399.0 ± 6.1, 464.3 ± 6.7, and 513.8 ± 6.5 um², p = 0.000), with SPN being significantly larger than OPN at each stage (p < 0.05 respectively). More male than female zygotes reached central PN juxtaposition at stage 1 (76.7% vs 51.0%, p = 0.010), stage 2 (97.7% vs 86.3%, p = 0.048), and stage 3 (97.7% vs 86.3%, p = 0.048). More OPN showed aligned NPBs than in SPN at stage 1 only (44.7% vs 28.7%, p = 0.023).

CONCLUSION(S)

Embryos with different sexes display different morphokinetic and morphometric features at the zygotic stage. Embryo selection using such parameters may lead to unbalanced sex ratio in resulting offspring.

Focus on time-lapse analysis: blastocyst collapse and morphometric assessment as new features of embryo viability

The main goal of assisted reproductive technology (ART) is to achieve a healthy singleton live birth after the transfer of one embryo. A major objective of IVF scientists has always been to use adequate criteria for selecting the embryo for transfer according to its implantation potential. Indeed, embryo quality is usually assessed by evaluating visual morphology, which relies on the removal of the embryo from the incubator and might include inter- and intra-evaluator variation among embryologists. Recently, an advancement in embryo culture has taken place with the introduction of a new type of incubator with an integrated time-lapse monitoring system, which enables embryologists to analyse the dynamic events of embryo development from fertilization to blastocyst formation. This novel practice is rapidly growing and has been used in many IVF centres worldwide. Therefore, the main aim of this review is to present the benefits of time-lapse monitoring in a modern embryology laboratory; in particular, we discuss blastocyst collapse and morphometric blastocyst assessment, and analyse their association with embryo viability and implantation potential. In addition, we highlight preliminary studies involving artificial intelligence and machine learning models as non-invasive markers of clinical pregnancy.

Nuclear-to-cytoplasmic ratios of 1PN and 2PN zygotes after in vitro fertilization of mouse oocytes

Numerous studies have reported comparisons of the nuclear-to-cytoplasmic (NC) ratio during mitosis. However, little information is known about how the pronuclear size is regulated and determined at the end of meiosis II in mammalian zygotes. The present study aims to analyze the NC ratio of female and male pronuclei, and also to compare the size of single pronuclei using photographs that were obtained during experiments to create chimeric hermaphrodites from 2-cell oocytes. The volume of both the female and the male pronucleus was found to correlate with the volume of the oocyte cytoplasm. The NC ratio of the male pronucleus was greater than that of the female pronucleus. The NC ratio of the average volume of the female and male pronuclei was greater than the NC ratio of the mononucleate oocytes. The occurrence of 1PN oocytes was significantly higher when the volume of cytoplasm was lower than the cut-off value. These results indicated that the NC ratio is retained during pronuclear formation. A higher NC ratio in male compared with the female pronucleus indicated structural and/or molecular difference between the two pronuclei. 1PN formation may occur when sperm enters close to the MII spindle.

Migration speed of nucleolus precursor bodies in human male pronuclei: a novel parameter for predicting live birth

PURPOSE

To study the relationship between the migration speed of nucleolus precursor bodies (NPBs) in male and female pronuclei (mPN; fPN) and human embryo development during assisted reproduction.

METHODS

The migration speed of 263 NPBs from 47 zygotes was quantitated, and embryonic development was observed until the blastocyst stage. The central coordinates of mPN, fPN, and NPBs were noted at multiple timepoints. Then, the distance traveled by the NPBs between two sequential images was measured, and migration speed was calculated. Additionally, we investigated the relationship between NPB migration speed and ploidy status (N = 33) or live birth/ongoing pregnancy (LB/OP) (N = 60) after assisted reproduction.

RESULTS

The NPB migration speed in both mPN and fPN was significantly faster in the zygotes that developed into blastocysts (N = 25) than that in the zygotes that arrested (N = 22). The timing of blastulation was negatively correlated with NPB migration speed in the mPN. Faster NPB migration was significantly correlated with LB/OP. In multivariate logistic analysis, NPB migration speed in the mPN was the only morphokinetic parameter associated with LB/OP. In a receiver-operating characteristic curve analysis of LB/OP by the NPB migration speed in the mPN, the cut-off value was 4.56 μm/h. When this cut-off value was applied to blastocysts with preimplantation genetic testing for aneuploidy, 100% of the blastocysts faster than or equal to the cut-off value were euploid.

CONCLUSION

The NPBs migrated faster in zygotes having the potential to develop into a blastocyst, and eventually into a baby. This predictor could be an attractive marker for non-invasive embryo selection.

Use of time-lapse monitoring in medically assisted reproduction treatments: a mini-review

During human in vitro culture, a morphological microscope analysis is normally performed to select the best embryo to transfer, with the hope of obtaining a successful pregnancy. The morphological evaluation may combine number and size of blastomeres, fragmentation, multinucleation, blastocyst expansion, inner-cell mass and trophectoderm appearance. However, standard microscopy evaluation involves the removal of the embryos from the incubator, exposing them to changes in pH, temperature, and oxygen level. Additionally, morphological assessments might include high inter-observer variability. Recently, continuous embryo culture using time-lapse monitoring (TLM) has allowed embryologists to analyse the dynamic and morphokinetic events of embryo development and, based on that, the embryologist is able to scrutinize the complete sequence of embryonic evolution, from fertilization to the blastocyst formation. Therefore, TLM allows an uninterrupted culture condition, reducing the need to remove embryos from the incubator. The monitoring system is normally composed of a standard incubator with an integrated microscope coupled to a digital camera, which is able to collect images at regular times, and subsequently processed into video. These data can be annotated and analyzed using an integrated software, therefore this allows embryologists to facilitate the process of embryo selection for transfer. The main aim of this paper is to discuss the potential benefits and uses of the TLM in the embryology laboratory.

Blastulation of a zygote to a hatched blastocyst without any clear cell division: an observational finding in a time-lapse system after in vitro fertilization

PURPOSE

To describe an interesting not previously described morphokinetic finding.

METHODS

Retrospective case report of a couple undergoing controlled ovarian stimulation (COS) followed by in vitro fertilization and blastocyst transfer.

RESULTS

We identified a unique finding of blastulation of a fertilized human zygote after conventional in vitro fertilization. The fertilized zygote did not show any clear cytokinesis until approximately 107 h post insemination, when it started dividing into a blastocyst. By 113 h post insemination, inner cell mass and trophectoderm cells could be clearly distinguished and the blastocyst was completely hatched by 136 h post insemination.

CONCLUSION

Time-lapse systems offer more detailed observations of embryonic development. Here, we report an atypical development of an embryo that was not described previously. We hope to become an insightful discussion among peers and incentive the publication of such findings in the future.

Noninvasive embryo selection: kinetic analysis of female and male pronuclear development to predict embryo quality and potential to produce live birth

OBJECTIVE

To evaluate a noninvasive method of examining euploid embryos, focusing on kinetic analyses, from second polar body extrusion to pronuclear membrane breakdown (PNMBD).

DESIGN

Retrospective embryo cohort study.

SETTING

Private IVF clinic.

PATIENT(S)

213 frozen-thawed single blastocyst transfers.

INTERVENTION(S)

Fertilized oocytes were recorded by means of time-lapse photography, followed by kinetic analysis of female and male pronuclei (PNs).

MAIN OUTCOME MEASURE(S)

The differences in size between the 2PNs in embryos resulting in live births compared with those of embryos from failed pregnancies were analyzed according to sequential size from early PN stages to PNMBD.

RESULT(S)

It was found that the difference in areas between male and female PNs immediately before PNMBD is a better predictor of embryo quality if this difference is below a known cutoff value. The size of male PNs 8 hours before the onset of PNMBD should be larger than female PNs (B). The difference in size between male and female PNs 8 hours before PNMBD should be larger than the difference in their size immediately before PNMBD. When normal embryos were defined using the equation (A∪C)∩B, the birth rates for in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) were 68.1% and 50.0%, respectively. For the remaining embryos, defined as abnormal according to the above criteria, birth rates were 9.4% for IVF and 4.2% for ICSI.

CONCLUSION(S)

We have developed a method for noninvasive embryo evaluation by means of the kinetic analysis of female and male PN growths. This method should enable us to select embryos that have a higher potential for healthy births.

Potential of zygotes to produce live births can be identified by the size of the male and female pronuclei just before their membranes break down

Aim

To determine whether there are differences in size between the male and female pronuclei immediately before the pronuclear membrane breakdown (PNMBD) and to evaluate whether pronuclear size differences influence normal birth rates.

Methods

Time‐lapse photography was used to measure the size of each pronucleus, while the outcome of 71 frozen‐thawed single blastocyst transfers in patients receiving hormone therapy was analyzed retrospectively. The pronuclear measurements were made 4 hours before the PNMBD, corresponding to 16‐20 hours after insemination or intracytoplasmic sperm injection, and immediately before the PNMBD. The differences in the areas between the pronuclei in the zygotes that were associated with the live births were compared with those that were associated with the failed pregnancies.

Results

The average difference in the area between the pronuclei 4 hours before and immediately before the PNMBD in the patients with a live birth was significantly smaller than in the patients with a failed birth. In addition, the average area difference in the patients with a successful birth was significantly smaller when the measurements were made immediately before the PNMBD, compared with the measurements 4 hours before the PNMBD. Such differences were not detected among the patients who did not achieve a birth.

Conclusion

The birth of healthy babies resulted from zygotes that contained pronuclei of similar size when the measurements were made immediately before the PNMBD. Evaluating the size of each pronucleus immediately before the PNMBD provides an effective indicator of the embryo's potential at an early stage of development.