Rescuing monopronucleated-derived human blastocysts: a model to study chromosomal topography and fingerprinting

An expert opinion on rescuing atypically pronucleated human zygotes by molecular genetic fertilization checks in IVF

IVF laboratories routinely adopt morphological pronuclear assessment at the zygote stage to identify abnormally fertilized embryos deemed unsuitable for clinical use. In essence, this is a pseudo-genetic test for ploidy motivated by the notion that biparental diploidy is required for normal human life and abnormal ploidy will lead to either failed implantation, miscarriage, or significant pregnancy complications, including molar pregnancy and chorionic carcinoma. Here, we review the literature associated with ploidy assessment of human embryos derived from zygotes displaying a pronuclear configuration other than the canonical two, and the related pregnancy outcome following transfer. We highlight that pronuclear assessment, although associated with aberrant ploidy outcomes, has a low specificity in the prediction of abnormal ploidy status in the developing embryo, while embryos deemed abnormally fertilized can yield healthy pregnancies. Therefore, this universal strategy of pronuclear assessment invariably leads to incorrect classification of over 50% of blastocysts derived from atypically pronucleated zygotes, and the systematic disposal of potentially viable embryos in IVF. To overcome this limitation of current practice, we discuss the new preimplantation genetic testing technologies that enable accurate identification of the ploidy status of preimplantation embryos and suggest a progress from morphology-based checks to molecular fertilization check as the new gold standard. This alternative molecular fertilization checking represents a possible non-incremental and controversy-free improvement to live birth rates in IVF as it adds to the pool of viable embryos available for transfer. This is especially important for the purposes of 'family building' or for poor-prognosis IVF patients where embryo numbers are often limited.

Micronuclei in 2-cell embryos show higher blastocyst formation rates on human embryonic development

OBJECTIVE

Multinucleated blastomeres at the two-cell stage (2MNB) represent a frequently observed nuclear abnormality in early human embryos. This abnormality has been reported to significantly impact on the embryo's developmental potential to reach the blastocyst stage. However, our understanding of the embryo's developmental potential and the morphokinetics of 2MNB remains limited. This study investigates the influence of 2MNB and its subtypes on the blastocyst formation.

STUDY DESIGN

A non-interventional retrospective study was performed in the Reproductive Medical Center of the First Affiliated Hospital of Hainan Medical University, using a time-lapse incubator. The study involved the evaluation of 4416 embryos, including 628 multinucleated embryos, from 1521 intracytoplasmic sperm injection (ICSI) cycles conducted between October 2019 and October 2021. The morphokinetic characteristics of multinucleated embryos were analyzed.

RESULTS

The results show multinucleation was the most common abnormal mitotic event during embryo development (14.22 %) in 4416 embryos. A control group of 3210 developmentally normal embryos was used in the study. The multinucleated blastomeres caused a lower blastocyst rate (52.48 % VS 64.02 %) compared to the control group. Whereas, 2MNBcause a higher blastocyst rate thanthemat the 4-cell stage (4MNB) (58.89 % VS 43.64 %). 2MNB can be further be further divided into 2MNB1/2cell and 2MNB2/2cell based on one multinucleated blastomere or two multinucleated blastomere appeared. Time to pronuclei fading (tPNf) is significantly longer in 2MNB2/2cell compared to 2MNB1/2cell. Furthermore, the 2MNB1/2cell embryos were divided into four subgroups (Bi-: two nuclei with almost the same size, Micro-: two nuclei with varying sizes, Poly-: more than two nuclei with almost the same size, and Cluster-: more than two nuclei with varying sizes) based on the number of nuclei and relative size. The results show that the Bi- and Micro- groups had a significantly increased blastocyst rate. The Cluster-, and Poly- groups showed significantly delayed embryonic development compared to normal controls. Bi-group has significant delays at t3, t5, and t8 and the Micro-group had a significant delay only at t8.

CONCLUSION

2MNB cause higher blastocyst rate than them at 4MNB. 2MNB1/2cell shows shorter tPNf compared to 2MNB2/2cell. Moreover, the Micro-, Bi- groups had a significantly increased blastocyst rate and different kinetic parameters compared to Cluster-, Poly-groups, suggesting that it is necessary to distinguish the nucleus status within 2MNB to increase the blastocyst rate. When selecting embryos for transformation from the 2MNB1/2cell, Micro- is the best choice.

The utility of human two plus one small pronucleated zygotes (2.1PN) based on clinical outcomes and the focused ploidy analysis

PURPOSE

Are human embryos arising from two plus one small pronucleated zygotes, called 2.1 pronuclei (PN), clinically useful?

METHODS

In a retrospective embryo cohort study and prospective experimental study, a total of 287 cycles in which at least one 2.1PN was identified in the fertilization check were included. Embryonic development and clinical outcome were compared for the 1395 2PN zygotes and 304 2.1PN zygotes that were siblings. All embryos were individually cultured in time-lapse systems. Twenty-five 2.1PN-derived blastocysts, donated for research, were used in focused single-nucleotide variant ploidy analysis to identify the distribution pattern of heterozygosity.

RESULTS

The average diameter of PN was 24.9 ± 2.4 µm for large PN and 10.2 ± 2.4 µm for small PN; 79.9% of small PN was derived from female pronuclei. Blastocyst formation rate and good-quality blastocyst rate were significantly lower with 2.1PN embryos than with 2PN embryos (40.0% vs. 57.7%, 21.4% vs. 33.5%, respectively). A total of 13 embryos derived from 2.1PN were transferred, and three healthy babies were born. In ploidy constitutions of trophectoderm (TE), 2.1PN-derived blastocyst TE was shown to be mostly diploid (95.8%, 23/24), and only one blastocyst showed triploid.

CONCLUSIONS

It was suggested that 2.1PN embryos have lower embryonic developmental potential than 2PN embryos, but most of the 2.1PN were diploid, indicating that they are likely to be clinically usable. It is recommended to perform embryo transfer following a combination of PGT-A and ploidy analysis.

Developmental potential of non- and mono-pronuclear zygotes and associated clinical outcomes in IVF cycles

Purpose

This study aims to evaluate the developmental potential of 0PN, 1PN, and 2PN zygotes in IVF cycles and compare their clinical outcomes.

Methods

We conducted a retrospective cohort study involving IVF patients. Blastocyst formation rates were assessed with 0PN, 1PN, and 2PN zygotes. Subsequently, we collected clinical outcome data following the transfer of these zygotes.

Results

The overall blastulation rate was similar between 0PN (29.6%) and 2PN (32.1%) zygotes, but 1PN zygotes exhibited a significantly lower blastulation rate (17.0%) compared to both 0PN and 2PN zygotes. Similarly, the overall rate of good-quality blastulation was comparable between 0PN (15.3%) and 2PN (17.5%) zygotes, while 1PN zygotes showed a significantly lower rate (7.0%) compared to both 0PN and 2PN. Clinical pregnancy, ectopic pregnancy, implantation, and live birth rates were similar among single blastocyst frozen embryo transfers (FET) of 0PN, 1PN, and 2PN. Additionally, no significant differences were observed between single- and double-blastocyst FET of 0PN and 2PN.

Conclusions

Our findings suggest that 0PN and 2PN zygotes have comparable developmental potential, while 1PN embryos exhibit lower developmental potential. Blastocyst FET outcomes appear similar among 0PN, 1PN, and 2PN zygotes.