Can 'abnormally' fertilized zygotes give rise to viable embryos?

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.

Developmental competence and neonatal outcomes of nonpronuclear zygotes following single vitrified-warmed blastocyst transfers using propensity score matching analysis

PURPOSE

To investigate developmental competence and neonatal outcomes of nonpronuclear (0PN) zygotes following single vitrified-warmed blastocyst transfers (VBT).

METHODS

The clinical, laboratorial and neonatal data of 996 patients with ≤ 38 years who underwent blastocyst culture and single VBT were retrospectively analyzed. The pregnancy and neonatal outcomes of VBT were compared between 0PN and 2PN blastocysts using propensity score matching (PSM). Moreover, Day 3 (D3) embryo development and blastocyst formation were compared between 0PN and 2PN zygotes.

RESULTS

There were no significant differences in clinical pregnancy rate (CPR), live birth rate (LBR) and neonatal outcomes of VBT between the 0PN and 2PN blastocysts irrespectively of whether PSM was used. However, early abortion rate (EAR) was higher in blastocysts from 0PN D3 embryos > 10 cells (p < 0.05) before PSM. Moreover, the early developmental competence of 0PN zygotes was different from that of 2PN zygotes presenting higher percentages of D3 embryos ≤ 6 cells (p < 0.01) and > 10 cells (p < 0.01), lower available blastocyst formation rate (ABFR) (p < 0.01) and good-quality blastocyst formation rate (GBFR) (p < 0.01) in D3 embryos with 4-6 cells. ABFR and GBFR increased with cell number when compared among embryos with 4-6 cells, 7-10 cells and > 10 cells, irrespectively of 0PN or 2PN embryos.

CONCLUSION

The early developmental competence of 0PN zygotes was different from that of 2PN zygotes, but did not influence pregnancy and neonatal outcomes following VBT. ABFR and GBFR increased with cell number, irrespectively of 0PN or 2PN embryos.

Assessing the clinical viability of micro 3 pronuclei zygotes

PURPOSE

What is the rate of euploidy and clinical viability of embryos resulting from micro 3 pronuclei zygotes?

METHODS

Retrospective cohort analysis in a single, academic in vitro fertilization (IVF) center from March 2018 to June 2021. Cohorts were separated by fertilization as either a 2 pronuclear zygote (2PN) or micro 3 pronuclear zygote (micro 3PN). PGT-A was performed to identify embryonic ploidy rates in embryos created from micro 3PN zygotes. The clinical outcomes of all transferred euploid micro 3PN zygotes were evaluated from frozen embryo transfer (FET) cycles.

RESULTS

During the designated study period, 75,903 mature oocytes were retrieved and underwent ICSI. Of these, 60,161 were fertilized as 2PN zygotes (79.3%) and 183 fertilized as micro 3PN zygotes (0.24%). Of the micro 3PN-derived embryos that underwent biopsy, 27.5% (n=11/42) were deemed euploid by PGT-A, compared to 51.4% (n=12,301/23,923) of 2PN-derived embryos, p=0.06. Four micro 3PN-derived embryos were transferred in subsequent single euploid FET cycles, which includes one live birth and one ongoing pregnancy.

CONCLUSION

Micro 3PN zygotes that develop to the blastocyst stage and meet the criteria for embryo biopsy have the potential to be euploid by preimplantation genetic testing for aneuploidy (PGT-A) and if selected for transfer can achieve a live birth. Although there are a significantly lower number of micro 3PN embryos that make it to blastocyst biopsy, the potential to continue to culture abnormally fertilized oocytes may give these patients a chance at pregnancy that they previously did not have.

Does the cell number of 0PN embryos on day 3 affect pregnancy and neonatal outcomes following single blastocyst transfer?

Background

0PN zygotes have a low cleavage rate, and the clinical outcomes of cleavage-stage embryo transfers are unsatisfactory. Blastocyst culturing is used to screen 0PN embryos, but whether the cell number of 0PN embryos on day 3 affects the clinical outcomes following single blastocyst transfer is unknown and would be helpful in evaluating the clinical value of these embryos.

Methods

This retrospective study compared 46,804 0PN zygotes, 242 0PN frozen-thawed single blastocyst transfers, and 92 corresponding 0PN singletons with 232,441 2PN zygotes, 3563 2PN frozen-thawed single blastocyst transfers, and 1250 2PN singletons from January 2015 to October 2019 at a tertiary-care academic medical centre. The 0PN and 2PN embryos were divided into two groups: the group with < 6 cells on day 3 and that with ≥ 6 cells. Embryo development, subsequent pregnancy and neonatal outcomes were compared between the two groups.

Results

The cleavage and available blastocyst rates of the 0PN zygotes were much lower than those of the 2PN zygotes (25.9% vs. 97.4%, P < 0.001; 13.9% vs. 23.4%, P < 0.001). In the < 6 cells group, the available blastocyst rate of the cleaved 0PN embryos was significantly lower than that of the 2PN embryos (2.5% vs. 12.7%, P < 0.001). However, in the ≥ 6 cells group, the available blastocyst rate of the 0PN cleaved embryos significantly improved, although it was slightly lower than that of the 2PN embryos (33.9% vs. 35.7%, P = 0.014). Importantly, compared to those of the 2PN single blastocyst transfers, the clinical pregnancy rate, live birth rate, Z-score and malformation rate of the 0PN single blastocyst transfers were not significantly different in either the < 6 cells group (30.4% vs. 39.8%, P = 0.362; 30.4% vs. 31.3%, P = 0.932; 0.89 ± 0.90 vs. 0.42 ± 1.02, P = 0.161; 0% vs. 2.6%, P = 1.000) or the ≥ 6 cells group (50.7% vs. 46.6%, P = 0.246; 39.7% vs. 38.3%, P = 0.677; 0.50 ± 1.23 vs. 0.47 ± 1.11, P = 0.861; 2.4% vs. 1.8%, P = 1.000).

Conclusions

The cell number on day 3 of 0PN embryos affected the subsequent formation of blastocysts but did not influence the subsequent pregnancy and neonatal outcomes of 0PN single blastocyst transfers, which may be beneficial to clinicians counselling patients on the clinical value of 0PN embryos.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12884-022-04492-7.

Development and frozen-thawed transfer of non-pronuclear zygotes-derived embryos in IVF cycles

OBJECTIVE

To explore the development and pregnancy potential of non-pronuclear (0PN) zygote-derived embryos in conventional in vitro fertilization (IVF) cycles.

STUDY DESIGN

Embryonic development in 1039 oocyte retrieval cycles and clinical outcomes of 659 frozen-thawed blastocyst transfer cycles were retrospectively studied.

RESULTS

Developmental potential of embryos with different blastomere numbers on day 3 were inconsistent in 0PN and 2PN groups. For 0PN-derived embryos, blastocyst rate of fast developing embryos (75.4%) was similar to that of intermediately developing embryos (72.9%), but good quality blastocyst rate of the former (49.2%) was significantly higher than that of the later (39.6%). In 2PN group, intermediately developing embryos had the highest blastocyst rate (77.9%) and good quality blastocyst rate (51.5%) (statistically significant). Comparison of frozen-thawed transfer was carried out between 0PN- and 2PN-derived blastocysts. For both single (SBT) and double blastocyst transfer (DBT) groups, no statistical differences existed between 0PN- and 2PN-derived blastocysts in clinical pregnancy rates (45.2% and 49.1% in SBT group, 64.7% and 66.4% in DBT group), implantation rates (45.2% and 49.1% in SBT group, 41.2% and 47.7% in DBT group) and live birth rates (35.5% and 36.8% in SBT group, 52.9% and 51.2% in DBT group).

CONCLUSION

The developmental characteristic of 0PN-derived embryos was different from that of 2PN-derived embryos in IVF cycles. 0PN-derived blastocysts could obtain acceptable clinical pregnancy and live birth, but more studies are needed to confirm the safety..

Comparison of the rates for reaching the blastocyst stage between normal and abnormal pronucleus embryos monitored by a time-lapse system in IVF patients

Objective

To compare the rates of blastocyst stage development between embryos fertilized after one (MPN) or more than two pronucleus (PN) (3PN, 4PN-multiPN) with those after 2PN in the same patients.

Material and Methods

The embryos of patients who had both abnormal PN (MPN, 3PN or 4PN) and normal fertilized (2PN) embryos after intracytoplasmic sperm injection (ICSI) fertilization, were followed with a time-lapse system following the ICSI procedure. The rates of reaching the blastocyst stage were compared between normal and abnormally fertilized embryos.

Results

One thousand eight hundred and twenty oocytes were collected from 140 patients and 1280 (70.3%) of them were fertilized. MPN, 2PN and 3PN, 4PN (multiPN) ratios of the embryos in the pronuclear stage were 11.4%, 83.13% and 5.47%, respectively. The rates of reaching the blastocyst stage among these embryos were 17.1%, 60.8% and 42.8% for MPN, 2PN and multiPN, respectively. The proportion reaching blastocyst development was significantly higher following 2PN compared to those after MPN and multiPN (p<0.05). Embryos developing after multiPN had significantly higher rates of reaching the blastocyst stage compared to those after MPN (p<0.01).

Conclusion

The majority of abnormally pronucleated embryos arrest without reaching the blastocyst stage. MultiPN embryos have a higher rate of blastocyst development than MPN embryos.

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

OBJECTIVE

To quantify the percentage of monopronuclear-derived blastocysts (MNBs) that are potentially useful for reproductive purposes using classic and state-of-the-art chromosome analysis approaches, and to study chromosomal distribution in the inner cell mass (ICM) and trophectoderm (TE) for intertissue/intratissue concordance comparison.

DESIGN

Prospective experimental study.

SETTING

Single-center in vitro fertilization clinic and reproductive genetics laboratory.

PATIENT(S)

A total of 1,128 monopronuclear zygotes were obtained between June 2016 and December 2018.

INTERVENTION(S)

MNBs were whole-fixed or biopsied to obtain a portion of ICM and 2 TE portions (TE1 and TE2) and were subsequently analyzed by fluorescence in situ hybridization, new whole-genome sequencing, and fingerprinting by single-nucleotide polymorphism array-based techniques (a-SNP).

MAIN OUTCOME MEASURE(S)

We assessed MNB rate, ploidy rate, and chromosomal constitution by new whole-genome sequencing, and parental composition by comparative a-SNP, performed in a "trio"-format (embryo/parents). The 24-chromosome distribution was compared between the TE and the ICM and within the TE.

RESULT(S)

A total of 18.4% of monopronuclear zygotes progressed to blastocysts; 77.6% of MNBs were diploid; 20% of MNBs were male and euploid, which might be reproductively useful. Seventy-five percent of MNBs were biparental and half of them were euploid, indicating that 40% might be reproductively useful. Intratissue concordance (TE1/TE2) was established for 93.3% and 73.3% for chromosome matching. Intertissue concordance (TE/ICM) was established for 78.8%, but 57.6% for chromosome matching. When segmental aneuploidy was not considered, intratissue concordance and chromosome matching increased to 100% and 80%, respectively, and intertissue concordance and chromosome matching increased to 84.8% and 75.8%, respectively.

CONCLUSION(S)

The a-SNP-trio strategy provides information about ploidy, euploidy, and parental origin in a single biopsy. This approach enabled us to identify 40% of MNBs with reproductive potential, which can have a significant effect in the clinical setting. Additionally, segmental aneuploidy is relevant for mismatched preimplantation genetic testing of aneuploidies, both within and between MNB tissues. Repeat biopsy might clarify whether segmental aneuploidy is a prone genetic character.

Abnormal fertilization in ICSI and its association with abnormal semen parameters: a retrospective observational study on 1855 cases

Intracytoplasmic sperm injection (ICSI) efficiently addresses male factor infertility. However, the occurrence of abnormal fertilization, mainly characterized by abnormal pronuclei (PN) patterns, merits investigation. To investigate abnormal fertilization patterns following ICSI and identify their respective associations with abnormal parameters in semen analysis (SA), a retrospective observational study including 1855 cycles was performed. Male infertility diagnosis relied on the 2010 WHO criteria. The population was divided into groups based on their SA results. The presence of 2PNs and extrusion of the second polar body (PB) indicated normal fertilization. A Kruskal–Wallis test along with a Wilcoxon post hoc evaluation and Bonferroni correction was employed for comparison among the groups. For the pregnancy rate, logistic regression was employed. No correlation was established between the SA abnormalities and the 1PN or 3PN formation rates. The highest and lowest 0PN rates were reported for the oligoasthenoteratozoospermic and normal groups, respectively. The lowest cleavage formation rates were identified in the oligoasthenozoospermic and oligoasthenoteratozoospermic groups. The aforementioned groups along with the oligoteratozoospermic group similarly presented the lowest blastocyst formation rates. For the clinical pregnancy rate, no statistically significant difference was observed. In conclusion, the incidence of two or more abnormal SA parameters – with the common denominator being oligozoospermia – may jeopardize normal fertilization, cleavage, and blastocyst rates. Once the developmental milestone of achieving blastocyst stage status was achieved, only oligoasthenozoospermia and oligoasthenoteratozoospermia were associated with lower rates. Interestingly, following adjustment for the number of blastocysts, no statistically significant differences were observed.

Copy number analysis of meiotic and postzygotic mitotic aneuploidies in trophectoderm cells biopsied at the blastocyst stage and arrested embryos

Preimplantation genetic testing for aneuploidy (PGT-A) by copy number analysis is now widely used to select euploid embryos for transfer. Whole or partial chromosome aneuploidy can arise in meiosis, predominantly female meiosis, or in the postzygotic, mitotic divisions during cleavage and blastocyst formation, resulting in chromosome mosaicism. Meiotic aneuploidies are almost always lethal, however, the clinical significance of mitotic aneuploidies detected by PGT-A is not fully understood and healthy live births have been reported following transfer of mosaic embryos. Here, we used single nucleotide polymorphism genotyping of both polar bodies and embryo samples to identify meiotic aneuploidies and compared copy number changes for meiotic and presumed mitotic aneuploidies in trophectoderm cells biopsied at the blastocyst stage and arrested embryos. PGT-A detected corresponding full copy number changes (≥70%) for 36/37 (97%) maternal meiotic aneuploidies. The number of presumed mitotic copy number changes detected exceeded those of meiotic origin. Although mainly in the mosaic range, some of these mitotic aneuploidies had copy number changes ≥70% and would have been identified as full aneuploidies. Interestingly, many arrested embryos had multiple mitotic aneuploidies across a broad range of copy number changes, which may have arisen through tripolar spindle and other mitotic abnormalities.

Strictly selected Mono- and non-pronuclear blastocysts could result in appreciable clinical outcomes in IVF cycles

The aim of the study was to examine the clinical value of blastocysts derived from mono-pronuclear (1PN) or non-pronuclear (0PN) zygotes with two polar bodies (2PB), which were selected by our criteria. We retrospectively analysed 610 frozen-thawed blastocyst transfer (FET) cycles and the corresponding oocyte retrieval cycles from 2014 to 2017. Developmental potential and clinical outcomes of embryos derived from zygotes with various numbers of pronuclei were analysed. Based on more detailed pre-selection settings, blastulation rates of 1PN/2PB and 0PN/2PB-derived embryos were 70.18% and 69.17%, respectively. Blastocyst FET results were not significantly different between 2PN/2PB, 1PN/2PB and 0PN/2PB groups in terms of clinical pregnancy rates (59.79%, 47.06% and 56.25%), implantation rates (47.24%, 40.00% and 47.62%), live birth rates (49.39%, 29.41% and 43.75%) or malformation rates (0%, 0% and 0%). In conclusion, after strict morphological selection and blastocyst culture, 1PN/2PB and 0PN/2PB-derived embryos in IVF cycles can have considerable clinical value. Blastocysts derived from 1PN/2PB or 0PN/2PB zygotes are worthwhile FET option for patients who have no available 2PN-derived embryos.

Obstetric and neonatal outcomes after the transfer of vitrified-warmed blastocysts developing from nonpronuclear and monopronuclear zygotes: a retrospective cohort study

OBJECTIVE

To evaluate the obstetric and neonatal outcomes after the transfer of vitrified-warmed single blastocysts developing from nonpronuclear (0PN) and monopronuclear (1PN) zygotes.

DESIGN

Cohort study.

SETTING

Affiliated hospital.

PATIENT(S)

This study was a retrospective analysis of 435 0PN and 281 1PN vitrified-warmed single blastocyst transfers, and 151 0PN and 75 1PN singletons, compared with 13,167 two-pronuclear (2PN) vitrified-warmed single blastocyst transfers and 4,559 2PN singletons, respectively.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Pregnancy rate (PR), abortion rate (AR), live birth rate (LBR), and singleton birthweight were the primary outcome measures.

RESULT(S)

PR, AR, and LBR were similar when compared between the 0PN and 2PN groups after vitrified-warmed blastocyst transfer. However, the 0PN group had a higher birthweights, higher z scores, and a greater proportion of very large for gestational age newborns. When comparing the 1PN and 2PN groups, we found that the PR was similar whereas the AR was higher and the LBR was lower. No differences were detected in the other neonatal outcomes.

CONCLUSION(S)

The results of the present study show that the transfer of 2PN blastocysts should be prioritized because of a higher AR and a lower LBR after 1PN blastocyst transfers and a higher birthweight after 0PN blastocyst transfers when compared with 2PN blastocyst transfers. Our data indicate the need for concern about the safety of 1PN and 0PN embryo transfers.

Blastocysts Derived From 0PN Oocytes: Genetic And Clinical Results

Objective

To analyze the genetic and clinical outcomes of blastocysts derived from 0PN oocytes after IVF/ICSI.

Methods

This retrospective observational study included patients aged 40 years or younger submitted to IVF/ICSI with their own oocytes and with blastocysts derived from 0PN oocytes between January 2015 and April 2018. The clinical outcomes of 0PN blastocyst transfers were analyzed. Genetic tests were performed on biopsied 0PN blastocysts with Next Generation Sequencing.

Results

A total of 27 0PN blastocysts were transferred, yielding an implantation rate of 48.0% and an ongoing pregnancy rate of 50.0%. The transfers resulted in 13 live births (59.0% live birth rate). Genetic test results revealed that four of the 17 0PN blastocysts biopsied were 46XX; three were 46XY; and 10 were aneuploid embryos, awarding a diploid rate to 76.4% (13/17).

Conclusion

Almost half of the 0PN blastocysts implanted (48.0%) and 13 healthy babies were born. More than three quarters (76.4%) of the 0PN blastocysts were diploid, thus ruling out the possibility of parthenogenetic activation. Our study indicated that the transfer of 0PN blastocysts is a safe, worthy option when the number of normal 2PN embryos is insufficient.

Embryos Arising from Apronuclear (0PN) and Unipronuclear (1PN) Have Similar Euploidy Rates with Those from 2PN and Should be Considered for Transfer

Background: Fertilisation assessment is routinely made at 16–18 hours post-ICSI and 18–20 hours post-insemination. However, the absence of pronuclei (PN) during standard fertilisation assessment does not necessarily indicate fertilisation failure. The aim of this study is to assess the chromosomal status of blastocysts derived from 0PN and 1PN zygotes as well as to assess the clinical outcome after transfer of such embryos.

Methods: In this study, we use microarray comparative genomic hybridisation (MaCGH) or next generation sequencing (NGS) to analyse the chromosomal status of 271 blastocysts (204 from 2PN, 41 from 0PN, 26 from 1PN) obtained from 42 patients who underwent conventional IVF (cIVF) and ICSI cycles with preimplantation genetic testing for aneuploidy (PGT-A).

Results: Euploidy was confirmed in 126 (126/204; 61.8%), 31 (31/41; 75.6%) and 18 (18/26; 69.2%) 2PN-, 0PN- and 1PN-derived blastocysts respectively while the remaining 96 blastocysts displayed various chromosomal abnormalities. A Y-chromosome was observed in 0PN-derived blastocysts (19/41; 46.3%) and 1PN-derived blastocysts (13/26; 50%), indicating that sperm had penetrated the oocyte and not due to parthenogenetic activation. Four euploid 0PN-derived blastocysts were transferred to 4 patients and 3 healthy live births were achieved. Four euploid 1PN-derived blastocysts were transferred to 4 patients and 1 on-going pregnancy was achieved.

Conclusion(s): 0PN- and 1PN-derived zygotes can be chromosomally normal and result in healthy live births. Such zygotes should not be discarded but instead be subjected to extended culture with PGT-A to ascertain the chromosomal and ploidy status and be considered for transfer.

Genome-wide haplotyping embryos developing from 0PN and 1PN zygotes increases transferrable embryos in PGT-M

STUDY QUESTION

Can genome-wide haplotyping increase success following preimplantation genetic testing for a monogenic disorder (PGT-M) by including zygotes with absence of pronuclei (0PN) or the presence of only one pronucleus (1PN)?

SUMMARY ANSWER

Genome-wide haplotyping 0PNs and 1PNs increases the number of PGT-M cycles reaching embryo transfer (ET) by 81% and the live-birth rate by 75%.

WHAT IS KNOWN ALREADY

Although a significant subset of 0PN and 1PN zygotes can develop into balanced, diploid and developmentally competent embryos, they are usually discarded because parental diploidy detection is not part of the routine work-up of PGT-M.

STUDY DESIGN, SIZE, DURATION

This prospective cohort study evaluated the pronuclear number in 2229 zygotes from 2337 injected metaphase II (MII) oocytes in 268 cycles. PGT-M for 0PN and 1PN embryos developing into Day 5/6 blastocysts with adequate quality for vitrification was performed in 42 of the 268 cycles (15.7%). In these 42 cycles, we genome-wide haplotyped 216 good quality embryos corresponding to 49 0PNs, 15 1PNs and 152 2PNs. The reported outcomes include parental contribution to embryonic ploidy, embryonic aneuploidy, genetic diagnosis for the monogenic disorder, cycles reaching ETs, pregnancy and live birth rates (LBR) for unaffected offspring.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Blastomere DNA was whole-genome amplified and hybridized on the Illumina Human CytoSNP12V2.1.1 BeadChip arrays. Subsequently, genome-wide haplotyping and copy-number profiling was applied to investigate the embryonic genome architecture. Bi-parental, unaffected embryos were transferred regardless of their initial zygotic PN score.

MAIN RESULTS AND THE ROLE OF CHANCE

A staggering 75.51% of 0PN and 42.86% of 1PN blastocysts are diploid bi-parental allowing accurate genetic diagnosis for the monogenic disorder. In total, 31% (13/42) of the PGT-M cycles reached ET or could repeat ET with an unaffected 0PN or 1PN embryo. The LBR per initiated cycle increased from 9.52 to 16.67%.

LIMITATIONS, REASONS FOR CAUTION

The clinical efficacy of the routine inclusion of 0PN and 1PN zygotes in PGT-M cycles should be confirmed in larger cohorts from multicenter studies.

WIDER IMPLICATIONS OF THE FINDINGS

Genome-wide haplotyping allows the inclusion of 0PN and 1PN embryos and subsequently increases the cycles reaching ET following PGT-M and potentially PGT for aneuploidy (PGT-A) and chromosomal structural rearrangements (PGT-SR). Establishing measures of clinical efficacy could lead to an update of the ESHRE guidelines which advise against the use of these zygotes.

STUDY FUNDING/COMPETING INTEREST(S)

SymBioSys (PFV/10/016 and C1/018 to J.R.V. and T.V.), the Horizon 2020 WIDENLIFE: 692065 to J.R.V., T.V., E.D., A.D. and M.Z.E. M.Z.E., T.V. and J.R.V. co-invented haplarithmisis (‘Haplotyping and copy-number typing using polymorphic variant allelic frequencies’), which has been licensed to Agilent Technologies. H.M. is fully supported by the (FWO) (ZKD1543-ASP/16). The authors have no competing interests to declare.

Mitochondrial activity and cytoskeleton organization in three pronuclei oocytes after intracytoplasmic sperm injection

Digyny, the presence of a third pronucleus due to the failure of second polar body extrusion, is problematic after intracytoplasmic sperm injection (ICSI) practices. Mitochondria have critical roles such as production of adenosine triphosphate (ATP) and regulation of Ca2+ homeostasis during oocyte maturation, fertilization and the following development, while the regulation of meiotic spindle formation, chromosome segregation, pronuclear apposition and cytokinesis is closely associated with the cytoskeleton. In this study, mitochondrial membrane potential, distribution of F-actin and γ-tubulin, and the ultrastructure of three pronuclear (3PN) oocytes were investigated. 3PN oocytes after ICSI procedure were taken from patients who were enrolled in assisted reproduction programmes. For mitochondrial membrane potential analysis, fresh oocytes stained with the mitochondrial membrane potential probe JC-1, were evaluated under fluorescence microscopy. The mitochondrial membrane potential of three pronuclear oocytes showed similar results to normal zygotes. γ-Tubulin was stained strongly at the subplasmalemmal domain and microfilaments were localized at the cortical, but not the perinuclear, area. Cytoplasmic halos were moderately or not detected by electron microscopy; lipofuscin granules, degenerated mitochondria, and multilamellated bodies were seen in the ooplasm. Immunohistochemistry and electron microscopic findings suggested that mitochondrial membrane potential has no direct effect on second polar body extrusion. This abnormality can be associated with an altered cytoskeleton due to poor oocyte quality.

Introducing precise genetic modifications into human 3PN embryos by CRISPR/Cas-mediated genome editing

PURPOSE

As a powerful technology for genome engineering, the CRISPR/Cas system has been successfully applied to modify the genomes of various species. The purpose of this study was to evaluate the technology and establish principles for the introduction of precise genetic modifications in early human embryos.

METHODS

3PN zygotes were injected with Cas9 messenger RNA (mRNA) (100 ng/μl) and guide RNA (gRNA) (50 ng/μl). For oligo-injections, donor oligo-1 (99 bp) or oligo-2 (99 bp) (100 ng/μl) or dsDonor (1 kb) was mixed with Cas9 mRNA (100 ng/μl) and gRNA (50 ng/μl) and injected into the embryos.

RESULTS

By co-injecting Cas9 mRNA, gRNAs, and donor DNA, we successfully introduced the naturally occurring CCR5Δ32 allele into early human 3PN embryos. In the embryos containing the engineered CCR5Δ32 allele, however, the other alleles at the same locus could not be fully controlled because they either remained wild type or contained indel mutations.

CONCLUSIONS

This work has implications for the development of therapeutic treatments of genetic disorders, and it demonstrates that significant technical issues remain to be addressed. We advocate preventing any application of genome editing on the human germline until after a rigorous and thorough evaluation and discussion are undertaken by the global research and ethics communities.

Live births resulting from 0PN-derived embryos in conventional IVF cycles

PURPOSE

The aim of this study was to (1) investigate the incidence of embryos derived from "unfertilized oocytes" i.e., oocytes not displaying pronuclei (0PN) at the time of the fertilization check and (2) determine the clinical pregnancy rates when transferring 0PN-derived embryos.

METHODS

In this retrospective study, 4424 IVF-ET cycles were reviewed.

RESULTS

In total, 11.3% (4966/43,949) 0PN-derived embryos were observed. It was found that female age, number of oocytes, and the top-quality embryo rate were significantly correlated with 0PN-derived embryo occurrence. The source of embryos transferred did not impact significantly on clinical pregnancy and live-birth rates. Of the 183 cycles included in this study where 275 0PN-derived embryos were transferred in total, only 0PN-derived embryos were available in 70 of those cycles. It was noteworthy that 13 healthy infants resulted from 0PN-derived embryos with an implantation rate of 17.0%.

CONCLUSION

These results indicate that the traditional method of excluding embryos because of those oocytes originally lacking any sign of a pronucleus at the fertilization check should be re-considered as transferring 0PN-derived embryos with subsequent expected developmental performance may be considered as an option for those patients where no other embryos are available.

Artificial oocyte activation with calcium ionophore does not cause a widespread increase in chromosome segregation errors in the second meiotic division of the oocyte

OBJECTIVE

To study the effect of artificial oocyte activation (AOA) on chromosome segregation errors in the meiotic divisions.

DESIGN

Prospective cohort study with historical control.

SETTING

Private/academic IVF centers.

PATIENT(S)

Fifty-six metaphase II oocytes were donated from 12 patients who had undergone IVF between June 2008 and May 2009.

INTERVENTION(S)

Oocytes were activated by 40 minutes' exposure to 100 μM calcium-ionophore. The activated oocyte was tubed and analyzed by array comparative genomic hybridization and/or single-nucleotide polymorphism genotyping and maternal haplotyping (meiomapping). A control sample of embryos derived from normally fertilized oocytes was included for comparison.

MAIN OUTCOME MEASURE(S)

Incidence of chromosome segregation errors in artificially activated and normally fertilized oocytes in relation to pronuclear evaluation.

RESULT(S)

Of 49 oocytes that survived the warming procedure, thirty-nine (79.6%) activated. Most activated normally, resulting in extrusion of the second polar body and formation of a single or no pronucleus (2PB1PN: 30 of 39, 76.9%; or 2PB0PN: 5 of 39, 12.8%). Twenty-seven of these were analyzed, and 16 (59.3%) were euploid, showing no effect of AOA on meiotic segregation. Single-nucleotide polymorphism analysis of normally activated oocytes confirmed normal segregation of maternal chromosomes. No difference in the proportion of meiosis II type errors was observed between artificially activated oocytes (28.6%; 95% confidence interval 3.7%-71.0%) compared with embryos obtained from normally fertilized oocytes (44.4%; 95% confidence interval 13.7%-78.8%). The abnormally activated oocytes, with ≥2PN (4 of 39, 10.3%) were diploid, indicating a failure to coordinate telophase of meiosis II with polar body extrusion.

CONCLUSION(S)

From this preliminary dataset, there is no evidence that AOA causes a widespread increase in chromosome segregation errors in meiosis II. However, we recommend that it be applied selectively to patients with specific indications.

Paternal heterochromatin formation in human embryos is H3K9/HP1 directed and primed by sperm-derived histone modifications

The different configurations of maternal and paternal chromatin, acquired during oogenesis and spermatogenesis, have to be rearranged after fertilization to form a functional embryonic genome. In the paternal genome, nucleosomal chromatin domains are re-established after the protamine-to-histone exchange. We investigated the formation of constitutive heterochromatin (cHC) in human preimplantation embryos. Our results show that histones carrying canonical cHC modifications are retained in cHC regions of sperm chromatin. These modified histones are transmitted to the oocyte and contribute to the formation of paternal embryonic cHC. Subsequently, the modifications are recognized by the H3K9/HP1 pathway maternal chromatin modifiers and propagated over the embryonic cleavage divisions. These results are in contrast to what has been described for mouse embryos, in which paternal cHC lacks canonical modifications and is initially established by Polycomb group proteins. Our results show intergenerational epigenetic inheritance of the cHC structure in human embryos.

The chromosomal constitution of embryos arising from monopronuclear oocytes in programmes of assisted reproduction

The assessment of oocytes showing only one pronucleus during assisted reproduction is associated with uncertainty. A compilation of data on the genetic constitution of different developmental stages shows that affected oocytes are able to develop into haploid, diploid, and mosaic embryos with more or less complex chromosomal compositions. In the majority of cases (~80%), haploidy appears to be caused by gynogenesis, whereas parthenogenesis or androgenesis is less common. Most of the diploid embryos result from a fertilization event involving asynchronous formation of the two pronuclei or pronuclear fusion at a very early stage. Uniparental diploidy may sometimes occur if one pronucleus fails to develop and the other pronucleus already contains a diploid genome or alternatively a haploid genome undergoes endoreduplication. In general, the chance of obtaining a biparental diploid embryo appears higher after conventional in vitro fertilization than after intracytoplasmic sperm injection. If a transfer of embryos obtained from monopronuclear oocytes is envisaged, it should be tried to culture them up to the blastocyst since most haploid embryos are not able to reach this stage. Comprehensive counselling of patients on potential risks is advisable before transfer and a preimplantation genetic diagnosis could be offered if available.

Atypical embryo phenotypes identified by time-lapse microscopy: high prevalence and association with embryo development

OBJECTIVE

To characterize atypical dynamic embryo phenotypes identified by time-lapse microscopy, evaluate their prevalence, and determine their association with embryo development.

DESIGN

Retrospective multicenter cohort study.

SETTING

Five IVF clinics in the United States.

PATIENT(S)

Sixty-seven women undergoing IVF treatment with 651 embryos.

INTERVENTION(S)

Embryo videos were retrospectively analyzed for atypical phenotypes.

MAIN OUTCOME MEASURE(S)

Identification of four groups of atypical embryo phenotypes: abnormal syngamy (AS), abnormal first cytokinesis (A1(cyt)), abnormal cleavage (AC), and chaotic cleavage (CC). Prevalence and association with embryo morphology and development potential were evaluated.

RESULT(S)

A high prevalence of atypical phenotypes was observed among embryos: AS 25.1% (163/649), A1(cyt) 31.0% (195/639), AC 18% (115/639) and CC 15% (96/639). A high percentage of embryos with atypical phenotype(s) had good quality on day 3 (overall grade good or fair): AS 78.6% (70/89); A1(cyt) 79.7% (94/119), AC 86.4% (70/81), and CC 35.2% (19/54), but the blastocyst formation rates for these embryos were significantly lower compared with their respective control groups: AS 21.5% vs. 44.9%, A1(cyt) 21.7% vs. 44.6%, AC 11.7% vs. 43.1%, and CC 14.0% vs. 42.3%.

CONCLUSION(S)

Embryos exhibiting atypical phenotypes are highly prevalent in human embryos and show significantly lower developmental potential than control embryos.

CLINICAL TRIAL REGISTRATION NUMBER

NCT01369446.

Molecular cytogenetics of human single pronucleated zygotes

The aim of the present study was to use fluorescence in situ hybridization to analyze the chromosome status of zygotes with a single pronucleus from in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) treatment cycles. In addition, we performed immunocytochemical detection of nuclear lamins and histone H3 trimethylated at lysine-9, Me(3)H3K9. Zygotes were processed 24 hours after insemination or injection to assure the absence of asynchrony. In opposition to previous results, we observed 2 pronuclei in 16 of 18 IVF zygotes and 40 of 64 ICSI zygotes, suggesting premature pronuclear breakdown. In IVF and ICSI zygotes, the rate of normal diploidy was only 6 of 16 and 27 of 56, respectively, suggesting that monopronucleated zygotes should not be used in assisted reproductive treatments. The possible mechanisms are discussed and compared to previous studies of monopronucleated zygotes.

Aneuploidy analysis of non-pronuclear embryos from IVF with use of array CGH: a case report

By using array comparative genomic hybridization (array CGH), to analyze the aneuploidy of the single blastomeres from non-pronuclear embryos on cleavage-stage in IVF cycle. Four non-pronuclear embryos were got from an IVF cycle, and the each single cell was biopsied from the four cleavage-stage embryos on the third day after the insemination which was investigated by using array CGH. After the biopsy, all the embryos continued to cleave, and lately entered the morula stage on the fifth day, just one embryo 3 was developed to early blastocyst stage on the sixth day. The four blastomere 24 chromosomes showed one X monomer and three normal XY diploids; the autosome chromosomes of blastomeres were abnormally gained or lost at different chromosome from four embryos, such as Embryo 1 : 49,X (-1, -5, -11, -19, -20, -21, -Y, +3, +6, +7, +8, +10, +13, +14, +16, +17, +18); Embryo 2 : 44,XY (-12, -15); Embryo 3: 47,XY (-3, -8, -9, -21, +7, +17, +18, +19, +20); Embryo 4 : 54,XY (+4, +7, +10, +12, +13, +16, +17, +22). With the use of the array CGH, the aneuploidy analysis could review the abnormal chromosomes of single blastomere from the non-pronuclear embryos, which can harbor the risk of abnormal sex chromosome and autosome chromosomes.

Could time-lapse embryo imaging reduce the need for biopsy and PGS?

PURPOSE

To review relevant studies examining the relationship between embryo morpho-kinetics and aneuploidy.

METHODS

Search of Pubmed and Medline using relevant keywords pertaining to morphology, morphokinetics and embryonic aneuploidy, as well as examination of various reference lists and conference proceedings.

RESULTS

An abundance of publications, both preliminary and peer-reviewed, have emerged regarding the usefulness of time-lapse imaging in tracking embryo development and improving embryo selection. Recently, these publications have explored ability to not only predict blastocyst formation and implantation, but also the ability to detect embryonic chromosomal aneuploidy. Of the two peer-reviewed retrospective studies on morpho-kinetics and embryonic aneuploidy, one demonstrates that early cleavage timings can indicate chromosomal complement, while the other demonstrates that key events following the maternal-zygotic transition can be markers of aneuploidy. A recent paper also demonstrates improved outcomes following IVF using a selection algorithm to identify embryos at "low risk" of chromosomal abnormalities. However, the predictive nature of these events and timings is far from ideal. Additionally, results may be dependent upon the day of biopsy and method utilized for chromosomal assessment.

CONCLUSION

With continued effort, the combination of multiple morphologic endpoint assessments and developmental timings and refinement of modeling systems may improve the predictive ability to determine embryonic aneuploidy. This may help select a subset of embryos that are less likely to carry chromosomal abnormalities and improve assisted reproductive outcomes. However, embryo biopsy, followed by preimplantation genetic screening/comprehensive chromosomal screening still remains the most reliable method to assess chromosomal complement of preimplantation embryos.

Early compaction at day 3 may be a useful additional criterion for embryo transfer

PURPOSE

The reduction of the number of embryos transferred while maintaining a satisfactory rate of pregnancy (PR) with in vitro fertilization calls for a refined technique of embryonic selection. This prospective study investigates the significance of early embryonic compaction at day 3 as a marker of the chances of implantation.

METHODS

We examined 317 transfers and their outcome involving 509 embryos including 91 compacted embryos.

RESULTS

Early compaction seems linked with the ovarian response to stimulation and embryonic quality. The PR is significantly increased when the embryonic cohort contains at least one compacted embryo (44% versus 29.5%, p = 0.01), and when at least one compacted embryo is transferred (44% versus 31%, p < 0.05). The analysis of our single embryo transfers shows that the implantation rates are significantly better for compacted embryos (50% versus 30%, p < 0.05) (OR 2.98; CI 1.02-5.28).

CONCLUSION

Thus, early compaction, sometimes observed at day 3, may serve as a useful additional criterion for selecting the embryos transferred.

Triploid and diploid embryonic stem cell lines derived from tripronuclear human zygotes

PURPOSE

Human embryonic stem cells (hESCs) are self-renewing, pluripotent cells that are valuable research tools and hold promise for use in regenerative medicine. The need for new hESC lines motivated our attempts to find a new resource for the derivation of hESC lines. The aim of this work was to establish more hESC lines from abnormal fertilized zygotes and to meet the emerging requirements for their use in cell replacement therapies, disease modeling, and basic research.

METHODS

A total of 130 tripronuclear human zygotes was collected 18-20 h post-insemination and cultured in a modified culture medium. The inner cell mass of 12 blastocysts were isolated by a mechanical method in order to establish embryonic stem cell lines.

RESULTS

We established four hESC lines derived from 130 trinuclear zygotes, one of which was triploid and the others were diploid. The efficiency of deriving hESC lines is 3.08 %. The ratio of deriving triploid and diploid hESC lines is 1:3. All of these hESC lines exhibited similar markers of undifferentiated hESCs and had the typical morphology of hESCs, a capacity for long-term proliferation, and pluripotent differentiation potential both in vivo and in vitro.

CONCLUSIONS

These abnormal zygotes, which otherwise would have been discarded, can serve as an alternative source for normal euploid hESC lines.

Cellular and genetic analysis of oocytes and embryos in a human case of spontaneous oocyte activation

Unusual and consistent defects in infertility patients merit attention as these may indicate an underlying genetic abnormality, in turn necessitating tailored management strategies. We describe a case of repeated early pregnancy loss from in vivo conceptions, followed by cancelled embryo transfers after one IVF and one ICSI/PGD cycle. Following the unexpected presence of cleaved embryos at the fertilization check in the first IVF attempt, oocytes and embryos were subsequently analyzed in an ICSI/PGD case. Part of the oocyte cohort was fixed at retrieval for a cellular evaluation of microtubules, microfilaments and chromatin. The remaining oocytes were injected with sperm, and resultant embryos were biopsied for genetic analysis by fluorescence in situ hybridization (FISH), single-nucleotide polymorphism (SNP) microarray for 23 chromosome pairs, as well as with PCR for sex chromosomes. The presence of interphase microtubule networks and pronuclear structures indicated that oocytes were spontaneously activated by the time of retrieval. FISH revealed aneuploidy in all seven blastomeres analyzed, with all but two lacking Y chromosomes. Microarray SNP analysis showed an exclusively maternal origin of all blastomeres analyzed, which was further confirmed by PCR. From our multi-faceted analyses, we conclude that spontaneous activation, or parthenogenesis, was probably the pathology underlying our patient's recurrent inability to maintain a normal pregnancy. Such analyses may prove beneficial not only in diagnosing case-specific aberrations for other patients with similar or related failures, but also for furthering our general understanding of oocyte activation.

Pronuclear transfer in human embryos to prevent transmission of mitochondrial DNA disease

Mutations in mitochondrial DNA (mtDNA) are a common cause of genetic disease. Pathogenic mutations in mtDNA are detected in approximately 1 in 250 live births and at least 1 in 10,000 adults in the UK are affected by mtDNA disease. Treatment options for patients with mtDNA disease are extremely limited and are predominantly supportive in nature. Mitochondrial DNA is transmitted maternally and it has been proposed that nuclear transfer techniques may be an approach for the prevention of transmission of human mtDNA disease. Here we show that transfer of pronuclei between abnormally fertilized human zygotes results in minimal carry-over of donor zygote mtDNA and is compatible with onward development to the blastocyst stage in vitro. By optimizing the procedure we found the average level of carry-over after transfer of two pronuclei is less than 2.0%, with many of the embryos containing no detectable donor mtDNA. We believe that pronuclear transfer between zygotes, as well as the recently described metaphase II spindle transfer, has the potential to prevent the transmission of mtDNA disease in humans.

Mechanisms giving rise to triploid zygotes during assisted reproduction

OBJECTIVE

To review information on the origin of triploid zygotes as gathered from assisted reproduction techniques.

DESIGN

Identification of relevant literature by a MEDLINE search and own experience on the basis of cytogenetic studies of abnormally fertilized oocytes.

SETTING

None.

PATIENT(S)

None.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

None.

RESULT(S)

Penetration of two haploid spermatozoa or of a single diploid spermatozoon into the oocyte causes diandric triploidy. The first case can be discerned by formation of a total of three pronuclei, whereas the second process will remain undetected, because it involves a female and a single but diploid male pronucleus. Digynic triploidy after intracytoplasmic sperm injection is characterized by nonextrusion of the second polar body and formation of three pronuclei. Digyny can also result from the fertilization of diploid giant oocytes. Depending on how maturation of these gametes proceeds, three or only two pronuclei will be observed. Thus, the size of the pronuclear stage must be considered for a successful identification of the abnormality. Endoreduplication within the female pronucleus is not detectable and may represent another, albeit rare, origin of digynic triploidy.

CONCLUSION(S)

Routine inspection of the number of pronuclei is not an absolutely reliable tool for excluding the development of triploid embryos. Observations during assisted reproduction may yield valuable information on the origin of triploidy.