Molecular cytogenetics of human single pronucleated zygotes

Healthy live birth following embryo transfer of a blastocyst of tetrapronuclear (4PN) origin: a case report

During IVF treatments, normal fertilization is generally evidenced by the appearance of two pronuclei, one arising from the oocyte and the other from the male gamete. Embryos derived from zygotes with a pronuclei number other than two are assumed to possess a ploidy abnormality and their transfer is usually avoided owing to increased risk of implantation failure, miscarriage, and molar pregnancies. Nonetheless, the inclusion of genotyping data in preimplantation genetic testing has revealed that a normal diploid configuration is possible in embryos deriving from zygotes with an abnormal pronuclei number such as tripronuclear and one pronucleus. Here, we present a one-of-a-kind transfer of a tetrapronuclear-derived embryo that was discovered to be diploid and negative for other whole chromosome or segmental aneuploidies during preimplantation genetic testing using a targeted next-generation sequencing approach. The transfer resulted in the live birth of a healthy infant who is now 4 years old and has no apparent health or developmental impairments.

All that glitters is not gold: a stereological study of human donor oocytes

Here we report a quantitative analysis of human metaphase II (MII) oocytes from a 22-year-old oocyte donor, retrieved after ovarian-controlled hyperstimulation. Five surplus donor oocytes were processed for transmission electron microscopy (TEM), and a stereological analysis was used to quantify the distribution of organelles, using the point-counting technique with an adequate stereological grid. Comparisons between means of the relative volumes (Vv) occupied by organelles in the three oocyte regions, cortex (C), subcortex (SC) and inner cytoplasm (IC), followed the Kruskal-Wallis test and Mann-Whitney U-test with Bonferroni correction. Life cell imaging and TEM analysis confirmed donor oocyte nuclear maturity. Results showed that the most abundant organelles were smooth endoplasmic reticulum (SER) elements (26.8%) and mitochondria (5.49%). Significant differences between oocyte regions were found for lysosomes (P = 0.003), cortical vesicles (P = 0.002) and large SER vesicles (P = 0.009). These results were quantitatively compared with previous results using prophase I (GV) and metaphase I (MI) immature oocytes. In donor MII oocytes there was a normal presence of cortical vesicles, SER tubules, SER small, medium and large vesicles, lysosomes and mitochondria. However, donor MII oocytes displayed signs of cytoplasmic immaturity, namely the presence of dictyosomes, present in GV oocytes and rare in MI oocytes, of SER very large vesicles, characteristic of GV oocytes, and the rarity of SER tubular aggregates. Results therefore indicate that the criterion of nuclear maturity used for donor oocyte selection does not always correspond to cytoplasmic maturity, which can partially explain implantation failures with the use of donor oocytes.

Prediction of live birth in vitrified-warmed 1PN-derived blastocyst transfer: Overall quality grade, ICM, TE, and expansion degree

Background: Numerous studies have reported that transfer of blastocysts derived from monopronuclear (1PN) zygotes achieved live births. However, the potential value of morphology grading for the prediction of 1PN blastocyst viability is unclear, and the blastocyst selection criterion for successful pregnancy has not been set up yet. The aim of this study is to assess the ability of the blastocyst morphology grading system based on three parameters, namely, inner cell mass (ICM), trophectoderm (TE), and expansion degree and to predict outcomes of a cycle with single 1PN blastocyst transfer.

Methods: A total of 266 vitrified-warmed 1PN-derived blastocyst transfer cycles for IVF treatment at Shanghai Ninth People’s Hospital between 2007 and 2020 were included. The study was performed on single blastocyst transfers. Electronic records of patients were retrospectively analyzed. In the current study, the blastocysts were classified into three groups: “good,” 3-6AA, 3-6AB, 3-6BA; “medium,” 3-6BB, 3-6AC, 3-6CA; and “poor,” 3-6BC, 3-6CB, 3-6CC. The basal characteristics, embryo grading, and clinical outcomes were compared between the three groups. The association of morphology parameters with pregnancies and live births was analyzed. Logistic regression was adopted to set up a prediction model of live births.

Results: Transfer of the good-quality blastocysts achieved significant higher pregnancies (biochemical pregnancy: 59%; clinical pregnancy: 56.4%, and live birth 48.7%) than those in the group of the medium (biochemical pregnancy: 59%; clinical pregnancy: 49.6%; live birth: 40.4%) or poor-quality (biochemical pregnancy: 38.4%; clinical pregnancy: 34.9%; live birth: 26.7%) blastocysts (p < 0.05). There was a significant association between ICM and live birth. A prediction model of live births involving ICM, TE, and expansion degree was set up.

Conclusion: In 1PN transfer cycles, a higher overall blastocyst quality is shown to correlate most strongly with optimal pregnancy and live birth outcomes. The selection of high-quality blastocysts for transfer should consider the ICM score first. The prediction model of live births based on ICM, TE, and expansion degree may help predict successful pregnancy in 1PN single-blastocyst transfer cycles.

Monopronucleated (1PN) and tripronuclear (3PN) zygotes formation during assisted reproduction in POSEIDON group 4 patients: Emphasizing on polar bodies

AIM

Abnormal fertilization (1PN/3PN) and its accompanying polar body (PB) conditions have been less discussed in poor ovarian responders. By observing the PBs, we analyzed the mechanisms of abnormal fertilization and aimed to explore the role of intracytoplasmic sperm injection/in vitro fertilization (ICSI/IVF) in POSEIDON group 4 patients.

METHODS

An observational study. All fresh IVF/ICSI cycles from January 2018 to December 2019 were evaluated. The inclusion criteria were POSEIDON group 4. Fertilization and PB conditions were assessed 16-18 h post-insemination. Primary observation endpoints including normal fertilization, abnormal fertilization, and total fertilization failure rate.

RESULTS

A total of 351 cycles involving 180 patients met the inclusion criteria. Of these, 15 cycles reported no retrieved oocytes. Finally, 336 cycles (IVF, n = 267; ICSI, n = 69) were included. A total of 1005 oocytes and 939 embryos were assessed. The mean female age was 40.8 years, and the mean AMH level was 0.6 ng/mL. The normal fertilization rate was 69.7%. The zygote distribution was 18.7% 0PN, 3.9% 1PN, 66.9% 2PN, 9.5% 3PN, and 1.0% ≥4PN. For 1PN zygotes, 59% were denoted as 1PN2PB. The mean 3PN rate was 8.9%.

CONCLUSIONS

In POSEIDON group 4, most of the monopronucleated zygotes were 1PN2PB. Digyny (3PN1PB), due to failure to extrude the second PB, was the major cause of triploidy in which ICSI could not circumvent. The distribution of abnormally fertilized zygotes was similar in IVF and ICSI. To investigate the mechanisms of abnormal fertilization and assess whether ICSI is necessary, analysis of PB will provide important clues.

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.

HIRA contributes to zygote formation in mice and is implicated in human 1PN zygote phenotype

Elucidating the mechanisms underpinning fertilisation is essential to optimising IVF procedures. One of the critical steps involves paternal chromatin reprogramming, in which compacted sperm chromatin packed by protamines is removed by oocyte factors and new histones, including histone H3.3, are incorporated. HIRA is the main H3.3 chaperone governing this protamine-to-histone exchange. Failure of this step results in abnormally fertilised zygotes containing only one pronucleus (1PN), in contrast to normal two-pronuclei (2PN) zygotes. 1PN zygotes are frequently observed in IVF treatments, but the genotype-phenotype correlation remains elusive. We investigated the maternal functions of two other molecules of the HIRA complex, Cabin1 and Ubn1, in mouse. Loss-of-function Cabin1 and Ubn1 mouse models were developed: their zygotes displayed an abnormal 1PN zygote phenotype. We then studied human 1PN zygotes and found that the HIRA complex was absent in 1PN zygotes that lacked the male pronucleus. This shows that the role of the HIRA complex in male pronucleus formation potentially has coherence from mice to humans. Furthermore, rescue experiments in mouse showed that the abnormal 1PN phenotype derived from Hira mutants could be resolved by overexpression of HIRA. We have demonstrated that HIRA complex regulates male pronucleus formation in mice and is implicated in humans, that both CABIN1 and UBN1 components of the HIRA complex are equally essential for male pronucleus formation, and that rescue is feasible.

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.

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.

Value of transferring embryos derived from monopronucleated (1PN) zygotes at the time of fertilization assessment

This paper is a retrospective analysis of the sole transfer of monopronucleated zygotes (1PN) embryos both in in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) to determine the value of transferring embryos formed from 1PN. In fresh cycles, 1PN cleavage-stage embryos (1PN cleavage fresh) were transferred. In frozen–thawed cycles, 1PN blastocyst-stage embryos (1PN blast frozen) were transferred. We used comparison groups: for fresh cycles, 2PN cleavage-stage embryos (2PN cleavage fresh) were transferred; and for frozen–thawed cycles, 2PN blastocyst-stage embryos (2PN blast frozen) were transferred. Comparison groups were matched for cycle and patient characteristics to the 1PN group. Finally, for fresh cycles, live birth rates (LBR) in the 1PN cleavage group were significantly lower than those in 2PN cleavage group, both for IVF [LBR = 7.64% vs. pregnancy rate (PR) = 22.12%, P = 0.003, respectively] and ICSI (LBR = 0% vs. LBR = 20.00%, P < 0.001, respectively). For frozen–thawed IVF cycles, the PR in the 1PN blastocyst group were comparable with those of the 2PN blastocyst group (1PN: LBR = 33.14% vs. 2PN: LBR = 37.24%, P = 0.289, respectively), while in ICSI, the PR in the 1PN blastocyst group were lower than those in the 2PN blastocyst group (LBR = 15.25% vs. LBR = 40.68%, P = 0.002, respectively). So, for IVF, blastocyst culture was capable of selecting normal 1PN embryos for transfer and achieves satisfying outcomes. However, for ICSI, blastocyst culture was not effective enough to eliminate abnormal embryos and 1PN embryo transfer needed to be treated with caution.

The influence of clinical and laboratory factors on the formation of monopronucleated zygotes after intracytoplasmic sperm injection (ICSI)

The aim of the present study was to determine whether clinical or laboratory factors can influence the development of single pronucleated zygotes (1PN) and two polar bodies (PB) after ICSI. In total, 341 ICSI cycles performed at FertiClinic-Villa Margherita from January 2012 to December 2014 were enrolled in the study. Group A included 240 cycles with no 1PN−2PB while group B included 101 cycles with one or more 1PN−2PB. Age, stimulation protocol, infertility factor, amount of gonadotropin administered, duration of therapy, peak estradiol levels, number of follicles at maturation triggering, oocytes retrieved and mature oocytes, time between retrieval and injection and sperm characteristics were compared between groups. In opposition to previous results showing no relationship between 1PN occurrence and clinical or laboratory variables, we observed that 1PN−2PB zygote formation seems to be associated with a lower female age, higher level of E2 and higher number of follicles on day of oocyte maturation triggering, higher number of astenozoospermic male patients, more oocytes retrieved at pick-up, more mature oocytes (MII) and longer time to injection.

Abnormally fertilized oocytes can result in healthy live births: improved genetic technologies for preimplantation genetic testing can be used to rescue viable embryos in in vitro fertilization cycles

OBJECTIVE

To test whether abnormally fertilized oocyte (AFO)-derived blastocysts are diploid and can be rescued for clinical use.

DESIGN

Longitudinal-cohort study from January 2015 to September 2016 involving IVF cycles with preimplantation genetic testing for aneuploidy (PGT-A). Ploidy assessment was incorporated whenever a blastocyst from a monopronuclear (1PN) or tripronuclear zygote (2PN + 1 smaller PN; 2.1 PN) was obtained.

SETTING

Private IVF clinics and genetics laboratories.

PATIENT(S)

A total of 556 women undergoing 719 PGT-A cycles.

INTERVENTION(S)

Conventional chromosome analysis was performed on trophectoderm biopsies by quantitative polymerase chain reaction. For AFO-derived blastocysts, ploidy assessment was performed on the same biopsy with the use of allele ratios for hetorozygous SNPs analyzed by means of next-generation sequencing (1:1 = diploid; 2:1 = triploid; loss of heterozygosity = haploid). Balanced-diploid 1PN- and 2.1PN-derived blastocysts were transferred in the absence of normally fertilized transferable embryos.

MAIN OUTCOME MEASURE(S)

Ploidy constitution and clinical value of AFO-derived blastocysts in IVF PGT-A cycles.

RESULT(S)

Of the 5,026 metaphase II oocytes injected, 5.2% and 0.7% showed 1PN and 2.1PN, respectively. AFOs showed compromised embryo development (P<.01). Twenty-seven AFO-derived blastocysts were analyzed for ploidy constitution. The 1PN-derived blastocysts were mostly diploid (n = 9/13; 69.2%), a few were haploid (n = 3/13; 23.1%), and one was triploid (n = 1/13; 7.7%). The 2.1PN-derived blastocysts were also mostly diploid (n = 12/14; 85.7%), and the remainder were triploid. Twenty-six PGT-A cycles resulted in one or more AFO-derived blastocysts (n = 26/719; 3.6%). Overall, eight additional balanced-diploid transferable embryos were obtained from AFOs. In three cycles, the only balanced-diploid blastocyst produced was from an AFO (n = 3/719; 0.4%). Three AFO-derived live births were achieved: one from a 1PN zygote and two from 2.1PN zygotes.

CONCLUSION(S)

Enhanced PGT-A technologies incorporating reliable ploidy assessment provide an effective tool to rescue AFO-derived blastocysts for clinical use.

Chromosomal analysis of blastocyst derived from monopronucleated ICSI zygotes: approach by double trophectoderm biopsy

Objective

This study aims to increase the knowledge about monopronucleated ICSI-derived blastocysts, analyzing trophectoderm biopsies by aCGH and FISH to evaluate their chromosome constitution.

Methods

Fifteen monopronucleated ICSI-derived blastocysts were studied. Double trophectoderm biopsy was performed and analyzed by FISH and aCGH. The blastocysts were classified according to chromosome constitution. Disagreements between the two techniques were assessed.

Results

Results obtained after FISH and aCGH analyses showed the following: 20% (3/15) and 60% (9/15) diploid females, respectively; 26.7% (4/15) and 26.7% (4/15) diploid males, respectively; and 53.3% (8/15) and 13.3% (2/15) mosaics, respectively. No mosaic male embryos were found using FISH or aCGH. There were disagreements in 40% (6/15) of the cases due to the higher detection of mosaicism by FISH compared to aCGH.

Conclusions

The combination of FISH and aCGH has been shown to be a suitable approach to increase the knowledge about monopronucleated ICSI-derived embryos. FISH analysis of blastocysts derived from monopronucleated ICSI zygotes enabled us to conclude that aCGH underestimates haploidy. Some diploid embryos diagnosed by aCGH are in fact mosaic. In cases where these embryos would be used for reproductive purposes, extra analysis of parental genome origin is recommended.

Could monopronucleated ICSI zygotes be considered for transfer? Analysis through time-lapse monitoring and PGS

PURPOSE

The purpose of this study was to investigate the chromosomal constitution and the developmental potential of intracytoplasmic sperm injection (ICSI) deriving embryos displaying a single pronucleus at the zygote stage.

METHODS

Eighty-eight embryos from single pronucleus (1PN) two polar bodies (2PB) ICSI zygotes from 64 preimplantational genetic screening (PGS) cycles (October 2012-December 2014), were retrospectively analyzed. Zygotes were cultured in a time-lapse incubator. Embryo biopsy was performed on day 3 and genetic analysis approached by array comparative genomic hybridization.

RESULTS

Chromosomal analysis revealed that 17% (15/88) of embryos derived from 1PN 2PB zygotes were diagnosed as euploid. After blastomere biopsy at day 3, the blastocyst rate at day 5 was 3.4% (3/88). Only 2.3% (2/88) euploid blastocysts were obtained. In two couples and after counseling and patient agreement, the transfer of a euploid blastocyst from a 1PN 2PB ICSI zygote was performed resulting in the birth of a healthy child.

CONCLUSIONS

These results open the possibility to consider embryos coming from 1PN 2PB ICSI zygotes for transfer when no other embryos from 2PN 2PB ICSI zygotes are available and if a PGS diagnosis of euploidy is obtained. Confirmation of biparental inheritance is strongly recommended.

Epigenetic reprogramming of the zygote in mice and men: on your marks, get set, go!

Gametogenesis (spermatogenesis and oogenesis) is accompanied by the acquisition of gender-specific epigenetic marks, such as DNA methylation, histone modifications and regulation by small RNAs, to form highly differentiated, but transcriptionally silent cell-types in preparation for fertilisation. Upon fertilisation, extensive global epigenetic reprogramming takes place to remove the previously acquired epigenetic marks and produce totipotent zygotic states. It is the aim of this review to delineate the cellular and molecular events involved in maternal, paternal and zygotic epigenetic reprogramming from the time of gametogenesis, through fertilisation, to the initiation of zygotic genome activation for preimplantation embryonic development. Recent studies have begun to uncover the indispensable functions of epigenetic players during gametogenesis, fertilisation and preimplantation embryo development, and a more comprehensive understanding of these early events will be informative for increasing pregnancy success rates, adding particular value to assisted fertility programmes.