Developmental potential of aneuploid human embryos cultured beyond implantation

A systematic review and meta-analysis of the diagnostic accuracy after preimplantation genetic testing for aneuploidy

OBJECTIVE

Aneuploidy accounts for many pregnancy failures and congenital anomalies. Preimplantation genetic testing for aneuploidy (PGT-A) is a screening test applied to embryos created from in vitro fertilization to diminish the chance of an aneuploid conception. The rate of misdiagnosis for both false aneuploidy (false positive) and false euploidy (false negative) test results is unknown. The objective of this study was to determine the rate of misclassification of both aneuploidy and euploidy after PGT-A.

DATA SOURCES

We conducted a systematic review and meta-analysis. We searched Medline, Embase, Cochrane Central, CINAHL and WHO Clinical Trials Registry from inception until April 10, 2024. The protocol was registered in International Prospective Register of Systematic Reviews (PROSPERO CRD 42020219074).

METHODS OF STUDY SELECTION

We included studies that conducted either a pre-clinical validation of the genetic platform for PGT-A using a cell line, studies that compared the embryo biopsy results to those from the whole dissected embryo or its inner cell mass (WE/ICM), and studies that compared the biopsy results to prenatal or postnatal genetic testing.

TABULATION, INTEGRATION, AND RESULTS

Two independent reviewers extracted true and false positives and negatives comparing biopsy results to the reference standard (known karyotype, WE/ICM, pregnancy outcome). For preclinical studies, the main outcome was the positive and negative predictive values. Misdiagnosis rate was the outcome for pregnancy outcome studies. The electronic search yielded 6674 citations, of which 109 were included. For WE/ICM studies (n=40), PPV was 89.2% (95% CI 83.1-94.0) and NPV was 94.2% (95% CI 91.1-96.7, I2=42%) for aneuploid and euploid embryos, respectively. The PPV for mosaic embryos of either a confirmatory mosaic or aneuploid result was 52.8% (95% CI 37.9-67.5). For pregnancy outcome studies (n=43), the misdiagnosis rate after euploid embryo transfer was 0.2% (95% CI 0.0-0.7%, I2=65%). However, the rate for mosaic transfer, with a confirmatory euploid pregnancy outcome, was 21.7% (95% CI: 9.6-36.9, I2=95%).

CONCLUSION

The accuracy of an aneuploid result from PGT-A is excellent and can be relied upon as a screening tool for embryos to avoid aneuploid pregnancies. Similarly, the misdiagnosis rate after euploid embryo transfer is less than 1%. However, there is a significant limitation in the accuracy of mosaic embryos.

Allelic transcriptomic profiling identifies the role of PRD-like homeobox genes in human embryonic-cleavage-stage arrest

Cleavage-stage arrest in human embryos substantially limits the success rate of infertility treatment, with maternal-to-zygotic transition (MZT) abnormalities being a potential contributor. However, the underlying mechanisms and regulators remain unclear. Here, by performing allelic transcriptome analysis on human preimplantation embryos, we accurately quantified MZT progression by allelic ratio and identified a fraction of 8-cell embryos, at the appropriate developmental time point and exhibiting normal morphology, were in transcriptionally arrested status. Furthermore, we identified PAIRED (PRD)-like homeobox transcription factors divergent paired-related homeobox (DPRX) and arginine-fifty homeobox (ARGFX) as factors involved in MZT, whose deficiency severely impairs MZT and lineage specification and leads to aberrant retention of histone acetylation. By reversing the acetylation retention caused by DPRX and ARGFX defects, embryonic arrest can be partially rescued. Our study identifies factors involved in human MZT and elucidates the etiology underlying human cleavage-stage arrest.

Frontier progress and translational challenges of pluripotent differentiation of stem cells

Stem cell research has significantly transformed regenerative medicine, with pluripotent stem cells (PSCs) serving as the cornerstone for disease modeling, drug screening, and therapeutic applications. Embryonic stem cells (ESCs) exhibit unparalleled self-renewal and tri-lineage differentiation, while induced pluripotent stem cells (iPSCs) bypass ethical constraints through somatic cell reprogramming. Clinical trials highlight the potential of mesenchymal stem cells (MSCs) in osteoarthritis and graft-versus-host disease, which leverage their immunomodulatory and paracrine effects. Despite advancements, challenges persist: iPSCs face epigenetic instability and tumorigenic risks, and adult stem cells struggle with inefficient differentiation. This paper systematically reviews stem cell source classification, differentiation regulatory mechanisms, cutting-edge technologies such as CRISPR/Cas9, and explores field-specific controversies (e.g., epigenetic stability of iPSCs) and future directions (e.g., integration of organoids and biomaterials). By analyzing current progress and challenges, it provides a multidimensional perspective for stem cell research.

Fetal karyotyping in adolescent pregnancies: a population-based cohort study on outcomes of invasive prenatal testing

Background

Adolescent pregnancies present unique challenges in prenatal diagnostics, yet data on the prevalence and types of chromosomal abnormalities in this population remain limited.

Objective

This study aimed to assess the prevalence and spectrum of chromosomal abnormalities and evaluate the effectiveness of invasive prenatal diagnostic procedures.

Methods

A retrospective cohort study analyzed data from invasive prenatal diagnostic procedures (amniocentesis and transabdominal chorionic villus sampling) and fetal karyotyping in adolescent pregnancies, comparing them with data obtained from pregnancies in older women.

Results

Abnormal karyotype prevalence varied by age. Trisomies were least frequent in adolescents (5.9%) vs. women 20-34 (9.3%) and ≥35 years (12.1%). Turner syndrome was more common in adolescents (4.6%) than in women 20-34 (2.8%) or ≥35 years (0.1%). Adolescents had a higher risk of unspecified fetal sex (RR = 2.25, 95% CI: 1.16-4.35) and culture failure (RR = 4.32, 95% CI: 2.07-9.00). Ultrasound abnormalities were the main reason for invasive testing (86.3%, p < 0.001). More chorionic villus sampling procedures were needed per abnormal karyotype in adolescents (3.25) vs. women 20-34 (2.42) or ≥35 years (2.19), while fewer amniocenteses were required (6.68 vs. 7.37 and 8.44).

Conclusion

Adolescents show unique chromosomal abnormalities, underscoring the need for tailored prenatal counseling and diagnostics.

Biopsy vs comprehensive embryo/blastocyst analysis: a closer look at embryonic chromosome evaluation

STUDY QUESTION

Compared with embryonic cytogenetic constitution of biopsied samples in human pre-implantation embryos, are there any differences in whole embryos?

SUMMARY ANSWER

Whole embryos exhibit a significantly higher euploidy rate and reduction in mosaic aneuploidy rate compared to biopsied samples.

WHAT IS KNOWN ALREADY

Much of the existing evidence of cytogenetic constitution of human pre-implantation embryos is based on biopsied cells obtained from blastomeres or trophectoderm biopsies. The mosaic rate of biopsies taken from blastocyst trophectoderm ranges widely, from 2% to 25%.

STUDY DESIGN SIZE DURATION

We investigated the embryonic cytogenetic constitution of 221 whole human embryos/blastocysts from 2019 to 2022, including 41 high-quality blastocysts, 57 low-quality blastocysts, and 123 arrested embryos/blastocysts.

PARTICIPANTS/MATERIALS SETTING METHODS

The cytogenetic constitution of whole embryos/blastocysts was assessed using next-generation sequencing. Mosaicism was diagnosed using a cut-off threshold of 30-70%, with embryos displaying 30-70% aneuploid cells classified as mosaic.

MAIN RESULTS AND THE ROLE OF CHANCE

Among high-quality blastocysts, the euploidy rate was 82.9%, with a remarkably low mosaic aneuploidy of only 2.5%. The euploidy rates of viable low-quality blastocysts and arrested embryos/blastocysts were 38.6% and 13.0%, respectively. The mosaic aneuploidy rate decreased progressively with embryonic development, from 93.2% at the cleavage stage to 40% at the blastocyst stage. Chaotic aneuploidy was the primary factor (66.1%, 39/59) contributing to embryonic arrest at the cleavage stage. Additionally, 26.1% of embryos/blastocysts exhibited segmental aneuploidy, with segmental duplications (30.6%, 22/72) and deletions (54.2%, 39/72) being the most common types of segmental aneuploidy.

LIMITATIONS REASONS FOR CAUTION

The sample size in this study is relatively small, especially in the subgroup analysis. Furthermore, whole-embryo analysis is not a foolproof diagnostic method, since it may underestimate the presence of mosaicism.

WIDER IMPLICATIONS OF THE FINDINGS

The cytogenetic constitution of whole embryos provides a more accurate reflection of their physiological state compared to biopsied samples. The low mosaic aneuploidy rate in high-quality blastocysts supports the practice of transferring mosaic embryos in patients without euploid embryos. If blastocysts reach stage III by Day 6 post-fertilization, nearly half are euploid, suggesting that extending embryo culture to Day 7 may be beneficial in cases where high-quality embryos are lacking.

STUDY FUNDING/COMPETING INTERESTS

This study was supported by the Natural Science Foundation of Guangdong Province (No. 2023A1515010250) and Pilot Program-China Reproductive Health Public Welfare Fund Project (No. SZ202413). The authors report no conflicts of interest.

TRIAL REGISTRATION NUMBER

N/A.

A pilot study of transcriptomic preimplantation genetic testing (PGT-T): towards a new step in embryo selection?

STUDY QUESTION

Is it possible to predict an euploid chromosomal constitution and identify a transcriptomic profile compatible with extended embryonic development from RNA sequencing (RNA-Seq) data?

SUMMARY ANSWER

It has been possible to obtain a karyotype comparable to preimplantation genetic testing for aneuploidy (PGT-A), in addition to a transcriptomic signature of embryos which might be suggestive of improved implantation capacity.

WHAT IS KNOWN ALREADY

Conventional assessment of embryo competence, based on morphology and morphokinetic, lacks knowledge of molecular aspects and faces controversy in predicting ploidy status. Understanding the embryonic transcriptome is crucial, as gene expression influences development and implantation. PGT has improved pregnancy rates, but problems persist when high-quality euploid embryos do not reach term. In fact, only around 50-60% implant, of which 10% result in miscarriage. Comprehensive approaches, including RNA-Seq, offer the potential to discover molecular markers of reproductive competence, and could theoretically be combined with extended-embryo culture platforms up to Day 14 that can be utilized as a proxy to study embryo development at post-implantation stages.

STUDY DESIGN, SIZE, DURATION

This prospective pilot cohort study was conducted from March 2023 to August 2023. A total of 30 vitrified human blastocysts with previous PGT-A diagnosis on Day 5 (D5) or Day 6 (D6) of development were analysed: n = 15 euploid and n = 15 aneuploid. Finally, 21 embryo samples were included in the study; the rest (n = 9) were excluded due to poor quality pre-sequencing data (n = 7) or highly discordant data (n = 2).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Following warming and re-expansion, embryos underwent a second trophectoderm (TE) biopsy. The embryos were then cultured until day 11 to assess their development. Biopsy analysis by RNA-Seq, studied the differential expressed genes (DEG) to compare embryos which did not or did attach to the plate: unattached embryos (n = 12) versus attached embryos (n = 9). Thus, we also obtained a specific transcriptomic signature of embryos with a "theoretical" capacity for sustained implantation, based on plate attachment on day 11.

MAIN RESULTS AND THE ROLE OF CHANCE

The digital karyotype obtained by RNA-Seq showed good concordance with the earlier PGT-A data, with a sensitivity of 0.81, a specificity of 0.83, a Cohen's Kappa of 0.66, and an area under the ROC of 0.9. At the gene level, 76 statistically significant DEGs were found in the comparison unattached versus attached embryos (Padj < 0.05; FC > 1). To address the functional implications of these differences, significantly deregulated pathways according to GO and KEGG categories were identified. The mural trophectoderm (TE) of the unattached blastocysts showed 63 significantly deregulated terms, displaying upregulation in autophagy, apoptosis, protein kinase and ubiquitin-like protein ligase activity, and downregulation of ribosome, spliceosome, kinetochore, segregation, and chromosome condensation processes. The overall transcriptomic signature specific to embryos still attached to the plate on day 11 (with a theoretically higher implantation capacity) consists of 501 genes, including: EMP2, AURKB, FOLR1, NOTCH3, LRP2, FZD5, MDH1, APOD, GPX8, COLEC12, HSPA1A, CMTM7, BEX3, which are related to implantation and embryonic development (raw P-value < 0.05; shrunk LFC > 1.1). These findings indicate that it might be possible to identify euploid embryos with a greater capacity for implantation and development, after excluding those embryos that present chromosomal alterations.

LIMITATIONS, REASONS FOR CAUTION

This study included a small sample size, remarkable variability between samples, and low success rate of RNA amplification. Also, structural chromosomal abnormalities were not included, and it was not possible to diagnose mosaic embryos. TE biopsy does not assure the chromosomal status of the whole embryo. The maximum day for in vitro development was Day 11, and attachment to the plate on this day does not provide a clear indication of implantation capacity and viability, which was not tested in this study.

WIDER IMPLICATIONS OF THE FINDINGS

The short-term goals following on from this pilot study is to expand the sample size with embryos of more complex abnormalities, and to perform a prospective in vitro preclinical validation. In a more distant future and with optimal results, this technique could have clinical application, thus increasing clinical outcomes by assessing both chromosomal content and transcriptomic profiling.

STUDY FUNDING/COMPETING INTEREST(S)

The Institut Valencià de Competitivitat Empresarial (IVACE) (IMIDCA/2022/39) and Generalitat Valenciana (CIACIF/2021/11) supported the present study. A.C. is an employee of JUNO Genetics. He has received honoraria for an IBSA lecture and a Merck lecture. He is also a minor shareholder of IVIRMA Global. The other authors have no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Role of spindle assembly checkpoint proteins in gametogenesis and embryogenesis

The spindle assembly checkpoint (SAC) is a surveillance mechanism that prevents uneven segregation of sister chromatids between daughter cells during anaphase. This essential regulatory checkpoint prevents aneuploidy which can lead to various congenital defects observed in newborns. Many studies have been carried out to elucidate the role of proteins involved in the SAC as well as the function of the checkpoint during gametogenesis and embryogenesis. In this review, we discuss the role of SAC proteins in regulating both meiotic and mitotic cell division along with several factors that influence the SAC strength in various species. Finally, we outline the role of SAC proteins and the consequences of their absence or insufficiency on proper gametogenesis and embryogenesis in vivo.

Generation of Monosomy 21q Human iPS Cells by CRISPR/Cas9-Mediated Interstitial Megabase Deletion

Missing an entire chromosome or chromosome arm in normal diploid cells has a deleterious impact on cell viability, which may contribute to the development of specific birth defects. Nevertheless, the effects of chromosome loss in human cells have remained unexplored due to the lack of suitable model systems. Here, we developed an efficient, selection-free approach to generate partial monosomy in human induced pluripotent stem cells (iPSCs). The introduction of Cas9 proteins and a pair of gRNAs induces over megabase-sized interstitial chromosomal deletions. Using human chromosome 21 (HSA21) as a model, partial monosomy 21q (PM21q) iPSC lines with deletions ranging from 4.5 to 27.9 Mb were isolated. A 33.6 Mb deletion, encompassing all protein-coding genes on 21q, was also achieved, establishing the first 21q monosomy human iPSC line. Transcriptome and proteome analyses revealed that the abundances of mRNA and protein encoded by the majority of genes in the monosomic regions are half of the diploid expression level, indicating an absence of dosage compensation. The ability to generate customized partial monosomy cell lines on an isogenic, karyotypically normal background should facilitate the gain of novel insights into the impact of chromosome loss on cellular fitness.

Chromosome architecture and low cohesion bias acrocentric chromosomes towards aneuploidy during mammalian meiosis

Aneuploidy in eggs is a leading cause of miscarriages or viable developmental syndromes. Aneuploidy rates differ between individual chromosomes. For instance, chromosome 21 frequently missegregates, resulting in Down Syndrome. What causes chromosome-specific aneuploidy in meiosis is unclear. Chromosome 21 belongs to the class of acrocentric chromosomes, whose centromeres are located close to the chromosome end, resulting in one long and one short chromosome arm. We demonstrate that acrocentric chromosomes are generally more often aneuploid than metacentric chromosomes in porcine eggs. Kinetochores of acrocentric chromosomes are often partially covered by the short chromosome arm during meiosis I in human and porcine oocytes and orient less efficiently toward the spindle poles. These partially covered kinetochores are more likely to be incorrectly attached to the spindle. Additionally, sister chromatids of acrocentric chromosomes are held together by lower levels of cohesin, making them more vulnerable to age-dependent cohesin loss. Chromosome architecture and low cohesion therefore bias acrocentric chromosomes toward aneuploidy during mammalian meiosis.

Preimplantation development analysis of aneuploid embryos with different chromosomal abnormalities

Background

The change of morphokinetic pattern in aneuploid embryos will facilitate the non-invasive selection of euploid embryos. In this study, we investigated the impact of different chromosomal abnormalities on the morphokinetic patterns of embryonic development.

Methods

Our cohort includes 939 time-lapse preimplantation genetic testing cycles performed between January 2019 and July 2022 at a single academic fertility center, with a total of 2876 biopsied blastocysts. Intracytoplasmic sperm injection, blastocyst culture, trophectoderm biopsy, time-lapse monitoring, and next-generation sequencing were performed.

Results

After adjusting for patient- and cycle-related factors, six morphokinetic parameters (t5, P = 0.006; t8, P = 0.048; tSB, P < 0.001; tB,P < 0.001; t5-t2, P = 0.004; tB-tSB, P < 0.001) were significant in multilevel mixed-effects logistic regression model analysis for morphokinetic parameters to predict euploid or aneuploid embryos. None of the patient- or cycle-related factors systematically affected any morphokinetic parameter. Morphokinetic parameters of late cleavage and blastocyst stages in embryos with chromosome fragment deletion (t4 to t8, tB, t5-t2, tB-tSB, ECC2, ECC3, s2, P < 0.05) or duplication (t4, t5, tSB, tB, t5-t2, P < 0.05) were prolonged, and the morphokinetic parameters of the blastocyst stage in monosomic embryos (tSB, tB, tB-tSB, P < 0.01) were prolonged. Partial or complete chromosome 20 or 22 deletion can cause significant delays in multiple parameters of cleavage and blastocyst stages (from t4 to tB, P < 0.05).

Conclusions

Our study found that different chromosomal abnormalities have different effects on the morphokinetic parameters. Significant delays in morphokinetic parameters at different stages were found in fragment-mutated embryos and monosomic embryos. This can provide insights into the pre-implantation development pattern of aneuploid embryos and help non-invasive embryo selection.

Complex aneuploidy triggers autophagy and p53-mediated apoptosis and impairs the second lineage segregation in human preimplantation embryos

About 70% of human cleavage stage embryos show chromosomal mosaicism, falling to 20% in blastocysts. Chromosomally mosaic human blastocysts can implant and lead to healthy new-borns with normal karyotypes. Studies in mouse embryos and human gastruloids showed that aneuploid cells are eliminated from the epiblast by p53-mediated apoptosis while being tolerated in the trophectoderm. These observations suggest a selective loss of aneuploid cells from human embryos, but the underlying mechanisms are not yet fully understood. Here, we investigated the cellular consequences of aneuploidy in a total of 125 human blastocysts. RNA-sequencing of trophectoderm cells showed activated p53 pathway and apoptosis proportionate to the level of chromosomal imbalance. Immunostaining corroborated that aneuploidy triggers proteotoxic stress, autophagy, p53-signaling, and apoptosis independent from DNA damage. Total cell numbers were lower in aneuploid embryos, due to a decline both in trophectoderm and in epiblast/primitive endoderm cell numbers. While lower cell numbers in trophectoderm may be attributed to apoptosis, aneuploidy impaired the second lineage segregation, particularly primitive endoderm formation. This might be reinforced by retention of NANOG. Our findings might explain why fully aneuploid embryos fail to further develop and we hypothesize that the same mechanisms lead to the removal of aneuploid cells from mosaic embryos.

Early human development and stem cell-based human embryo models

The use of stem cells to model the early human embryo promises to transform our understanding of developmental biology and human reproduction. In this review, we present our current knowledge of the first 2 weeks of human embryo development. We first focus on the distinct cell lineages of the embryo and the derivation of stem cell lines. We then discuss the intercellular crosstalk that guides early embryo development and how this crosstalk is recapitulated in vitro to generate stem cell-based embryo models. We highlight advances in this fast-developing field, discuss current limitations, and provide a vision for the future.

Human embryos harbor complex mosaicism with broad presence of aneuploid cells during early development

Pre-implantation genetic testing for aneuploidy (PGT-A) is used in approximately half of in vitro fertilization cycles. Given the limited understanding of the genetics of human embryos, the current use of PGT-A is based on biologically uncertain assumptions and unvalidated guidelines, leading to the possibility of disposing of embryos with pregnancy potential. We isolated and sequenced all single cells (1133) from in vitro cultured 20 human blastocysts. We found that all blastocysts exhibited mosaicism with mitotic-induced aneuploid cells and showed an ~25% aneuploidy rate per embryo. Moreover, 70% (14/20) of blastocysts contained 'chromosome-complementary' cells, suggesting genetic mosaicism is underestimated in routine PGT-A. Additionally, the analysis of 20,945 single cells from day 8-14 embryos (in vitro cultured) and embryonic/fetal organs showed that 97% of the analyzed embryos/organs were mosaic. Over 96% of their aneuploid cells harbored ≤ 2 chromosome errors. Our findings have revealed a high prevalence of mosaicism in human embryos.

Aligning genotyping and copy number data in single trophectoderm biopsies for aneuploidy prediction: uncovering incomplete concordance

STUDY QUESTION

To what extent can genotype analysis aid in the classification of (mosaic) aneuploid embryos diagnosed through copy number analysis of a trophectoderm (TE) biopsy?

SUMMARY ANSWER

In a small portion of embryos, genotype analysis revealed signatures of meiotic or uniform aneuploidy in those diagnosed with intermediate copy number changes, and signatures of presumed mitotic or putative mosaic aneuploidy in those diagnosed with full copy number changes.

WHAT IS KNOWN ALREADY

Comprehensive chromosome screening (CCS) for preimplantation genetic testing has provided valuable insights into the prevalence of (mosaic) chromosomal aneuploidy at the blastocyst stage. However, diagnosis of (mosaic) aneuploidy often relies solely on (intermediate) copy number analysis of a single TE biopsy. Integrating genotype information allows for independent assessment of the origin and degree of aneuploidy. Yet, studies aligning both datasets to predict (putative mosaic) aneuploidy in embryos remain scarce.

STUDY DESIGN SIZE DURATION

A single TE biopsy was collected from 1560 embryos derived from 221 couples tested for a monogenic disorder (n = 218) or microdeletion-/microduplication syndrome (n = 3). TE samples were subjected to both copy number and genotyping analysis.

PARTICIPANTS/MATERIALS SETTING METHODS

Copy number and SNP genotyping analysis were conducted using GENType. Unbalanced chromosomal anomalies ≥10 Mb (or ≥20 Mb for copy number calls <50%) were classified by degree, based on low-range intermediate (LR, 30-50%), high-range intermediate (HR, 50-70%) or full (>70%) copy number changes. These categories were further subjected to genotyping analysis to ascertain the origin (and/or degree) of aneuploidy. For chromosomal gains, the meiotic division of origin (meiotic I/II versus non-meiotic or presumed mitotic) was established by studying the haplotypes. The level of monosomy (uniform versus putative mosaic) in the biopsy could be ascertained from the B-allele frequencies. For segmental aneuploidies, genotyping was restricted to deletions.

MAIN RESULTS AND THE ROLE OF CHANCE

Of 1479 analysed embryos, 24% (n = 356) exhibited a whole-chromosome aneuploidy, with 19% (n = 280) showing full copy number changes suggestive of uniform aneuploidy. Among 258 embryos further investigated by genotyping, 95% of trisomies with full copy number changes were identified to be of meiotic origin. For monosomies, a complete loss of heterozygosity (LOH) in the biopsy was observed in 97% of cases, yielding a 96% concordance rate at the embryo level (n = 248/258). Interestingly, 4% of embryos (n = 10/258) showed SNP signatures of non-meiotic gain or putative mosaic loss instead. Meanwhile, 5% of embryos (n = 76/1479) solely displayed HR (2.5%; n = 37) or LR (2.6%; n = 39) intermediate copy number changes, with an additional 2% showing both intermediate and full copy number changes. Among embryos with HR intermediate copy number changes where genotyping was feasible (n = 25/37), 92% (n = 23/25) showed SNP signatures consistent with putative mosaic aneuploidy. However, 8% (n = 2/25) exhibited evidence of meiotic trisomy (9%) or complete LOH in the biopsy (7%). In the LR intermediate group, 1 of 33 (3%) genotyped embryos displayed complete LOH. Furthermore, segmental aneuploidy was detected in 7% of embryos (n = 108/1479) (or 9% (n = 139) with added whole-chromosome aneuploidy). These errors were often (52%) characterized by intermediate copy number values, which closely aligned with genotyping data when examined (94-100%).

LARGE SCALE DATA

N/A.

LIMITATIONS REASONS FOR CAUTION

The findings were based on single TE biopsies and the true extent of mosaicism was not validated through embryo dissection. Moreover, evidence of absence of a meiotic origin for a trisomy should not be construed as definitive proof of a mitotic error. Additionally, a genotyping diagnosis was not always attainable due to the absence of a recombination event necessary to discern between meiotic II and non-meiotic trisomy, or the unavailability of DNA from both parents.

WIDER IMPLICATIONS OF THE FINDINGS

Interpreting (intermediate) copy number changes of a single TE biopsy alone as evidence for (mosaic) aneuploidy in the embryo remains suboptimal. Integrating genotype information alongside the copy number status could provide a more comprehensive assessment of the embryo's genetic makeup, within and beyond the single TE biopsy. By identifying meiotic aberrations, especially in presumed mosaic embryos, we underscore the potential value of genotyping analysis as a deselection tool, ultimately striving to reduce adverse clinical outcomes.

STUDY FUNDING/COMPETING INTERESTS

L.D.W. was supported by the Research Foundation Flanders (FWO; 1S74621N). M.B., K.T., F.V.M., S.J., A.V.T., V.S., D.S., A.D., and S.S. are supported by Ghent University Hospital. B.M. was funded by Ghent University. The authors have no conflicts of interest.

Integrin signaling in pluripotent cells acts as a gatekeeper of mouse germline entry

Primordial germ cells (PGCs) are the precursors of gametes and the sole mechanism by which animals transmit genetic information across generations. In the mouse embryo, the transcriptional and epigenetic regulation of PGC specification has been extensively characterized. However, the initial event that triggers the soma-germline segregation remains poorly understood. Here, we uncover a critical role for the basement membrane in regulating germline entry. We show that PGCs arise in a region of the mouse embryo that lacks contact with the basement membrane, and the addition of exogenous extracellular matrix (ECM) inhibits both PGC and PGC-like cell (PGCLC) specification in mouse embryos and stem cell models, respectively. Mechanistically, we demonstrate that the engagement of β1 integrin with laminin blocks PGCLC specification by preventing the Wnt signaling-dependent down-regulation of the PGC transcriptional repressor, Otx2. In this way, the physical segregation of cells away from the basement membrane acts as a morphogenetic fate switch that controls the soma-germline bifurcation.

Advanced maternal age (AMA) and pregnancy: a feasible but problematic event

This narrative review aimed to summarize all adverse outcomes of pregnancy in advanced maternal age (AMA) to assess the age of the mother as a potentially crucial risk factor. AMA refers to women older than 35 years. While expectations and the role of women in society have undergone significant changes today, the biology of aging remains unchanged. Various pathologic changes occur in the human body with age, including chronic noncommunicable diseases, as well as notable changes in reproductive organs, that significantly affect fertility. Despite substantial advancements in technology and medicine, pregnancy in AMA remains a formidable challenge. Although there are some advantages to postponing childbirth, they primarily relate to maternal maturity and economic stability. However, regrettably, there are also many adverse aspects of pregnancy at advanced ages. These include complications affecting both the mother and the fetus. Pregnants in AMA were more prone to suffer from gestational diabetes mellitus, preeclampsia, and eclampsia during pregnancy compared to younger women. In addition, miscarriages and ectopic pregnancies were more prevalent. Delivery was more frequently completed via cesarean section, and postpartum complications and maternal mortality were also higher. Unfortunately, there were also complications concerning the fetus, such as chromosomal abnormalities, premature birth, low birth weight, admission to the neonatal intensive care unit, and stillbirth.

Leveraging chorionic villus biopsies for the derivation of patient-specific trophoblast stem cells

Human trophoblast stem (TS) cells are an informative in vitro model for the generation and testing of biologically meaningful hypotheses. The goal of this project was to derive patient-specific TS cell lines from clinically available chorionic villus sampling biopsies. Cell outgrowths were captured from human chorionic villus tissue specimens cultured in modified human TS cell medium. Cell colonies emerged early during the culture and cell lines were established and passaged for several generations. Karyotypes of the newly established chorionic villus-derived trophoblast stem (TS CV ) cell lines were determined and compared to initial genetic diagnoses from freshly isolated chorionic villi. Phenotypes of TSCV cells in the stem state and following differentiation were compared to cytotrophoblast-derived TS (TS CT ) cells. TSCV and TSCT cells uniformly exhibited similarities in the stem state and following differentiation into syncytiotrophoblast and extravillous trophoblast cells. Chorionic villus tissue specimens provide a valuable source for TS cell derivation. They expand the genetic diversity of available TS cells and are associated with defined clinical outcomes. TSCV cell lines provide a new set of experimental tools for investigating trophoblast cell lineage development.

HIF1A contributes to the survival of aneuploid and mosaic pre-implantation embryos

Human fertility is suboptimal, partly due to error-prone divisions in early cleavage-stages that result in aneuploidy. Most human pre-implantation are mosaics of euploid and aneuploid cells, however, mosaic embryos with a low proportion of aneuploid cells have a similar likelihood of developing to term as fully euploid embryos. How embryos manage aneuploidy during development is poorly understood. This knowledge is crucial for improving fertility treatments and reducing developmental defects. To explore these mechanisms, we established a new mouse model of chromosome mosaicism to study the fate of aneuploid cells during pre-implantation development. We previously used the Mps1 inhibitor reversine to generate aneuploidy in embryos. Here, we found that treatment with the more specific Mps1 inhibitor AZ3146 induced chromosome segregation defects in pre-implantation embryos, similar to reversine. However, AZ3146-treated embryos showed a higher developmental potential than reversine-treated embryos. Unlike reversine-treated embryos, AZ3146-treated embryos exhibited transient upregulation of Hypoxia Inducible-Factor-1A (HIF1A) and lacked p53 upregulation. Pre-implantation embryos develop in a hypoxic environment in vivo, and hypoxia exposure in vitro reduced DNA damage in response to Mps1 inhibition and increased the proportion of euploid cells in the mosaic epiblast. Inhibiting HIF1A in mosaic embryos also decreased the proportion of aneuploid cells in mosaic embryos. Our work illuminates potential strategies to improve the developmental potential of mosaic embryos.

A single-cell transcriptome atlas of human euploid and aneuploid blastocysts

Aneuploidy is frequently detected in early human embryos as a major cause of early pregnancy failure. However, how aneuploidy affects cellular function remains elusive. Here, we profiled the transcriptomes of 14,908 single cells from 203 human euploid and aneuploid blastocysts involving autosomal and sex chromosomes. Nearly all of the blastocysts contained four lineages. In aneuploid chromosomes, 19.5% ± 1.2% of the expressed genes showed a dosage effect, and 90 dosage-sensitive domains were identified. Aneuploidy leads to prevalent genome-wide transcriptome alterations. Common effects, including apoptosis, were identified, especially in monosomies, partially explaining the lower cell numbers in autosomal monosomies. We further identified lineage-specific effects causing unstable epiblast development in aneuploidies, which was accompanied by the downregulation of TGF-β and FGF signaling, which resulted in insufficient trophectoderm maturation. Our work provides crucial insights into the molecular basis of human aneuploid blastocysts and may shed light on the cellular interaction during blastocyst development.

Trophectoderm cells of human mosaic embryos display increased apoptotic levels and impaired differentiation capacity: a molecular clue regarding their reproductive fate?

STUDY QUESTION

Are there cell lineage-related differences in the apoptotic rates and differentiation capacity of human blastocysts diagnosed as euploid, mosaic, and aneuploid after preimplantation genetic testing for aneuploidy (PGT-A) based on concurrent copy number and genotyping analysis?

SUMMARY ANSWER

Trophectoderm (TE) cells of mosaic and aneuploid blastocysts exhibit significantly higher levels of apoptosis and significantly reduced differentiation capacity compared to those of euploid blastocysts.

WHAT IS KNOWN ALREADY

Embryos diagnosed as mosaic after PGT-A can develop into healthy infants, yet understanding the reasons behind their reproductive potential requires further research. One hypothesis suggests that mosaicism can be normalized through selective apoptosis and reduced proliferation of aneuploid cells, but direct evidence of these mechanisms in human embryos is lacking. Additionally, data interpretation from studies involving mosaic embryos has been hampered by retrospective analysis methods and the high incidence of false-positive mosaic diagnoses stemming from the use of poorly specific PGT-A platforms.

STUDY DESIGN, SIZE, DURATION

Prospective cohort study performing colocalization of cell-lineage and apoptotic markers by immunofluorescence (IF). We included a total of 64 human blastocysts donated to research on Day 5 or 6 post-fertilization (dpf) by 43 couples who underwent in vitro fertilization treatment with PGT-A at IVI-RMA Valencia between September 2019 and October 2022. A total of 27 mosaic blastocysts were analyzed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The study consisted of two phases: Phase I (caspase-3, n = 53 blastocysts): n = 13 euploid, n = 22 mosaic, n = 18 aneuploid. Phase II (terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL), n = 11 blastocysts): n = 2 euploid, n = 5 mosaic, n = 4 aneuploid. Following donation for research, vitrified blastocysts were warmed, cultured until re-expansion, fixed, processed for IF, and imaged using confocal microscopy. For each blastocyst, the following cell counts were conducted: total cells (DAPI+), TE cells (GATA3+), inner cell mass (ICM) cells (GATA3-/NANOG+), and apoptotic cells (caspase-3+ or TUNEL+). The incidence of apoptosis was calculated for each blastocyst by dividing the number of caspase-3+ cells (Phase I) or TUNEL+ cells (Phase II) by the number of TE or ICM cells. Statistical analysis was performed according to data type and distribution (P < 0.05 was considered statistically significant).

MAIN RESULTS AND THE ROLE OF CHANCE

Phase I: Mosaic blastocysts displayed a similar number of total cells (49.6 ± 15 cells at 5 dpf; 58.8 ± 16.9 cells at 6 dpf), TE cells (38.8 ± 13.7 cells at 5 dpf; 49.2 ± 16.2 cells at 6 dpf), and ICM cells (10.9 ± 4.2 cells at 5 dpf; 9.7 ± 7.1 cells at 6 dpf) compared to euploid and aneuploid blastocysts (P > 0.05). The proportion of TE cells retaining NANOG expression increased gradually from euploid blastocysts (9.7% = 63/651 cells at 5 dpf; 0% = 0/157 cells at 6 dpf) to mosaic blastocysts (13.1% = 104/794 cells at 5 dpf; 3.4% = 12/353 cells at 6 dpf) and aneuploid blastocysts (27.9% = 149/534 cells at 5 dpf; 4.6% = 19/417 cells at 6 dpf) (P < 0.05). At the TE level, caspase-3+ cells were frequently observed (39% = 901/2310 cells). The proportion of caspase-3+ TE cells was significantly higher in mosaic blastocysts (44.1% ± 19.6 at 5 dpf; 43% ± 16.8 at 6 dpf) and aneuploid blastocysts (45.9% ± 16.1 at 5 dpf; 49% ± 15.1 at 6 dpf) compared to euploid blastocysts (26.6% ± 16.6 at 5 dpf; 17.5% ± 14.8 at 6 dpf) (P < 0.05). In contrast, at the ICM level, caspase-3+ cells were rarely observed (1.9% = 11/596 cells), and only detected in mosaic blastocysts (2.6% = 6/232 cells) and aneuploid blastocysts (2.5% = 5/197 cells) (P > 0.05). Phase II: Consistently, TUNEL+ cells were only observed in TE cells (32.4% = 124/383 cells). An increasing trend was identified toward a higher proportion of TUNEL+ cells in the TE of mosaic blastocysts (37.2% ± 21.9) and aneuploid blastocysts (39% ± 41.7), compared to euploid blastocysts (23% ± 32.5), although these differences did not reach statistical significance (P > 0.05).

LIMITATIONS, REASONS FOR CAUTION

The observed effects on apoptosis and differentiation may not be exclusive to aneuploid cells. Additionally, variations in aneuploidies and unexplored factors related to blastocyst development and karyotype concordance may introduce potential biases and uncertainties in the results.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings demonstrate a cell lineage-specific effect of aneuploidy on the apoptotic levels and differentiation capacity of human blastocysts. This contributes to unravelling the biological characteristics of mosaic blastocysts and supports the concept of clonal depletion of aneuploid cells in explaining their reproductive potential.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by grants from Centro para el Desarrollo Tecnológico Industrial (CDTI) (20190022) and Generalitat Valenciana (APOTIP/2019/009). None of the authors has any conflict of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Chromosome Division in Early Embryos-Is Everything under Control? And Is the Cell Size Important?

Chromosome segregation in female germ cells and early embryonic blastomeres is known to be highly prone to errors. The resulting aneuploidy is therefore the most frequent cause of termination of early development and embryo loss in mammals. And in specific cases, when the aneuploidy is actually compatible with embryonic and fetal development, it leads to severe developmental disorders. The main surveillance mechanism, which is essential for the fidelity of chromosome segregation, is the Spindle Assembly Checkpoint (SAC). And although all eukaryotic cells carry genes required for SAC, it is not clear whether this pathway is active in all cell types, including blastomeres of early embryos. In this review, we will summarize and discuss the recent progress in our understanding of the mechanisms controlling chromosome segregation and how they might work in embryos and mammalian embryos in particular. Our conclusion from the current literature is that the early mammalian embryos show limited capabilities to react to chromosome segregation defects, which might, at least partially, explain the widespread problem of aneuploidy during the early development in mammals.

Single-cell DNA sequencing reveals a high incidence of chromosomal abnormalities in human blastocysts

Aneuploidy, a deviation from the normal chromosome copy number, is common in human embryos and is considered a primary cause of implantation failure and early pregnancy loss. Meiotic errors lead to uniformly abnormal karyotypes, while mitotic errors lead to chromosomal mosaicism: the presence of cells with at least 2 different karyotypes within an embryo. Knowledge about mosaicism in blastocysts mainly derives from bulk DNA sequencing (DNA-Seq) of multicellular trophectoderm (TE) and/or inner cell mass (ICM) samples. However, this can only detect an average net gain or loss of DNA above a detection threshold of 20%-30%. To accurately assess mosaicism, we separated the TE and ICM of 55 good-quality surplus blastocysts and successfully applied single-cell whole-genome sequencing (scKaryo-Seq) on 1,057 cells. Mosaicism involving numerical and structural chromosome abnormalities was detected in 82% of the embryos, in which most abnormalities affected less than 20% of the cells. Structural abnormalities, potentially caused by replication stress and DNA damage, were observed in 69% of the embryos. In conclusion, our findings indicated that mosaicism was prevalent in good-quality blastocysts, whereas these blastocysts would likely be identified as normal with current bulk DNA-Seq techniques used for preimplantation genetic testing for aneuploidy.

Genetic mechanisms of fertilization failure and early embryonic arrest: a comprehensive review

BACKGROUND

Infertility and pregnancy loss are longstanding problems. Successful fertilization and high-quality embryos are prerequisites for an ongoing pregnancy. Studies have proven that every stage in the human reproductive process is regulated by multiple genes and any problem, at any step, may lead to fertilization failure (FF) or early embryonic arrest (EEA). Doctors can diagnose the pathogenic factors involved in FF and EEA by using genetic methods. With the progress in the development of new genetic technologies, such as single-cell RNA analysis and whole-exome sequencing, a new approach has opened up for us to directly study human germ cells and reproductive development. These findings will help us to identify the unique mechanism(s) that leads to FF and EEA in order to find potential treatments.

OBJECTIVE AND RATIONALE

The goal of this review is to compile current genetic knowledge related to FF and EEA, clarifying the mechanisms involved and providing clues for clinical diagnosis and treatment.

SEARCH METHODS

PubMed was used to search for relevant research articles and reviews, primarily focusing on English-language publications from January 1978 to June 2023. The search terms included fertilization failure, early embryonic arrest, genetic, epigenetic, whole-exome sequencing, DNA methylation, chromosome, non-coding RNA, and other related keywords. Additional studies were identified by searching reference lists. This review primarily focuses on research conducted in humans. However, it also incorporates relevant data from animal models when applicable. The results were presented descriptively, and individual study quality was not assessed.

OUTCOMES

A total of 233 relevant articles were included in the final review, from 3925 records identified initially. The review provides an overview of genetic factors and mechanisms involved in the human reproductive process. The genetic mutations and other genetic mechanisms of FF and EEA were systematically reviewed, for example, globozoospermia, oocyte activation failure, maternal effect gene mutations, zygotic genome activation abnormalities, chromosome abnormalities, and epigenetic abnormalities. Additionally, the review summarizes progress in treatments for different gene defects, offering new insights for clinical diagnosis and treatment.

WIDER IMPLICATIONS

The information provided in this review will facilitate the development of more accurate molecular screening tools for diagnosing infertility using genetic markers and networks in human reproductive development. The findings will also help guide clinical practice by identifying appropriate interventions based on specific gene mutations. For example, when an individual has obvious gene mutations related to FF, ICSI is recommended instead of IVF. However, in the case of genetic defects such as phospholipase C zeta1 (PLCZ1), actin-like7A (ACTL7A), actin-like 9 (ACTL9), and IQ motif-containing N (IQCN), ICSI may also fail to fertilize. We can consider artificial oocyte activation technology with ICSI to improve fertilization rate and reduce monetary and time costs. In the future, fertility is expected to be improved or restored by interfering with or supplementing the relevant genes.

Preimplantation genetic testing: A remarkable history of pioneering, technical challenges, innovations, and ethical considerations

Preimplantation genetic testing (PGT) has emerged as a powerful companion to assisted reproduction technologies. The origins and history of PGT are reviewed here, along with descriptions of advances in molecular assays and sampling methods, their capabilities, and their applications in preventing genetic diseases and enhancing pregnancy outcomes. Additionally, the potential for increasing accuracy and genome coverage is considered, as well as some of the emerging ethical and legislative considerations related to the expanding capabilities of PGT.

Embryo drop-out rates in preimplantation genetic testing for aneuploidy (PGT-A): a retrospective data analysis from the DoLoRes study

PURPOSE

To evaluate the decline in transferable embryos in preimplantation genetic testing for aneuploidy (PGT-A) cycles due to (a) non-biopsable blastocyst quality, (b) failure of genetic analysis, (c) diagnosis of uniform numerical or structural chromosomal aberrations, and/or (d) chromosomal aberrations in mosaic constitution.

METHODS

This retrospective multicenter study comprised outcomes of 1562 blastocysts originating from 363 controlled ovarian stimulation cycles, respectively, 226 IVF couples in the period between January 2016 and December 2018. Inclusion criteria were PGT-A cycles with trophectoderm biopsy (TB) and next generation sequencing (NGS).

RESULTS

Out of 1562 blastocysts, 25.8% were lost due to non-biopsable and/or non-freezable embryo quality. In 10.3% of all biopsied blastocysts, genetic analysis failed. After exclusion of embryos with uniform or chromosomal aberrations in mosaic, only 18.1% of those originally yielded remained as diagnosed euploid embryos suitable for transfer. This translates into 50.4% of patients and 57.6% of stimulated cycles with no euploid embryo left for transfer. The risk that no transfer can take place rose significantly with a lower number of oocytes and with increasing maternal age. The chance for at least one euploid blastocyst/cycle in advanced maternal age (AMA)-patients was 33.3% compared to 52.1% in recurrent miscarriage (RM), 59.8% in recurrent implantation failure (RIF), and 60.0% in severe male factor (SMF).

CONCLUSIONS

The present study demonstrates that PGT-A is accompanied by high embryo drop-out rates. IVF-practitioners should be aware that their patients run a high risk of ending up without any embryo suitable for transfer after (several) stimulation cycles, especially in AMA patients. Patients should be informed in detail about the frequency of inconclusive or mosaic results, with the associated risk of not having an euploid embryo available for transfer after PGT-A, as well as the high cost involved in this type of testing.

Isoform-resolved transcriptome of the human preimplantation embryo

Human preimplantation development involves extensive remodeling of RNA expression and splicing. However, its transcriptome has been compiled using short-read sequencing data, which fails to capture most full-length mRNAs. Here, we generate an isoform-resolved transcriptome of early human development by performing long- and short-read RNA sequencing on 73 embryos spanning the zygote to blastocyst stages. We identify 110,212 unannotated isoforms transcribed from known genes, including highly conserved protein-coding loci and key developmental regulators. We further identify 17,964 isoforms from 5,239 unannotated genes, which are largely non-coding, primate-specific, and highly associated with transposable elements. These isoforms are widely supported by the integration of published multi-omics datasets, including single-cell 8CLC and blastoid studies. Alternative splicing and gene co-expression network analyses further reveal that embryonic genome activation is associated with splicing disruption and transient upregulation of gene modules. Together, these findings show that the human embryo transcriptome is far more complex than currently known, and will act as a valuable resource to empower future studies exploring development.

epiAneufinder identifies copy number alterations from single-cell ATAC-seq data

Single-cell open chromatin profiling via scATAC-seq has become a mainstream measurement of open chromatin in single-cells. Here we present epiAneufinder, an algorithm that exploits the read count information from scATAC-seq data to extract genome-wide copy number alterations (CNAs) for individual cells, allowing the study of CNA heterogeneity present in a sample at the single-cell level. Using different cancer scATAC-seq datasets, we show that epiAneufinder can identify intratumor clonal heterogeneity in populations of single cells based on their CNA profiles. We demonstrate that these profiles are concordant with the ones inferred from single-cell whole genome sequencing data for the same samples. EpiAneufinder allows the inference of single-cell CNA information from scATAC-seq data, without the need of additional experiments, unlocking a layer of genomic variation which is otherwise unexplored.

Compromised Mitotic Fidelity in Human Pluripotent Stem Cells

Human pluripotent stem cells (PSCs), which include both embryonic and induced pluripotent stem cells, are widely used in fundamental and applied biomedical research. They have been instrumental for better understanding development and cell differentiation processes, disease origin and progression and can aid in the discovery of new drugs. PSCs also hold great potential in regenerative medicine to treat or diminish the effects of certain debilitating diseases, such as degenerative disorders. However, some concerns have recently been raised over their safety for use in regenerative medicine. One of the major concerns is the fact that PSCs are prone to errors in passing the correct number of chromosomes to daughter cells, resulting in aneuploid cells. Aneuploidy, characterised by an imbalance in chromosome number, elicits the upregulation of different stress pathways that are deleterious to cell homeostasis, impair proper embryo development and potentiate cancer development. In this review, we will summarize known molecular mechanisms recently revealed to impair mitotic fidelity in human PSCs and the consequences of the decreased mitotic fidelity of these cells. We will finish with speculative views on how the physiological characteristics of PSCs can affect the mitotic machinery and how their suboptimal mitotic fidelity may be circumvented.

Restricting epigenetic activity promotes the reprogramming of transformed cells to pluripotency in a line-specific manner

Somatic cell reprogramming and oncogenic transformation share surprisingly similar features, yet transformed cells are resistant to reprogramming. Epigenetic barriers must block transformed cells from reprogramming, but the nature of those barriers is unclear. In this study, we generated a systematic panel of transformed mouse embryonic fibroblasts (MEFs) using oncogenic transgenes and discovered transformed cell lines compatible with reprogramming when transfected with Oct4/Sox2/Klf4/Myc. By comparing the reprogramming-capable and incapable transformed lines we identified multiple stages of failure in the reprogramming process. Some transformed lines failed at an early stage, whilst other lines seemed to progress through a conventional reprogramming process. Finally, we show that MEK inhibition overcomes one critical reprogramming barrier by indirectly suppressing a hyperacetylated active epigenetic state. This study reveals that diverse epigenetic barriers underly resistance to reprogramming of transformed cells.

Human embryo implantation

Embryo implantation in humans is interstitial, meaning the entire conceptus embeds in the endometrium before the placental trophoblast invades beyond the uterine mucosa into the underlying inner myometrium. Once implanted, embryo survival pivots on the transformation of the endometrium into an anti-inflammatory placental bed, termed decidua, under homeostatic control of uterine natural killer cells. Here, we examine the evolutionary context of embryo implantation and elaborate on uterine remodelling before and after conception in humans. We also discuss the interactions between the embryo and the decidualising endometrium that regulate interstitial implantation and determine embryo fitness. Together, this Review highlights the precarious but adaptable nature of the implantation process.

Novel Time-Lapse Parameters Correlate with Embryo Ploidy and Suggest an Improvement in Non-Invasive Embryo Selection

Selecting the best embryo for transfer is key to success in assisted reproduction. The use of algorithms or artificial intelligence can already predict blastulation or implantation with good results. However, ploidy predictions still rely on invasive techniques. Embryologists are still essential, and improving their evaluation tools can enhance clinical outcomes. This study analyzed 374 blastocysts from preimplantation genetic testing cycles. Embryos were cultured in time-lapse incubators and tested for aneuploidies; images were then studied for morphokinetic parameters. We present a new parameter, "st₂, start of t₂", detected at the beginning of the first cell cleavage, as strongly implicated in ploidy status. We describe specific cytoplasmic movement patterns associated with ploidy status. Aneuploid embryos also present slower developmental rates (t₃, t₅, t_SB, t_B, cc3, and t₅-t₂). Our analysis demonstrates a positive correlation among them for euploid embryos, while aneuploids present non-sequential behaviors. A logistic regression study confirmed the implications of the described parameters, showing a ROC value of 0.69 for ploidy prediction (95% confidence interval (CI), 0.62 to 0.76). Our results show that optimizing the relevant indicators to select the most suitable blastocyst, such as by including st₂, could reduce the time until the pregnancy of a euploid baby while avoiding invasive and expensive methods.

The human embryonic genome is karyotypically complex, with chromosomally abnormal cells preferentially located away from the developing fetus

STUDY QUESTION

Are chromosome abnormalities detected at Day 3 post-fertilization predominantly retained in structures of the blastocyst other than the inner cell mass (ICM), where chromosomally normal cells are preferentially retained?

SUMMARY ANSWER

In human embryos, aneuploid cells are sequestered away from the ICM, partly to the trophectoderm (TE) but more significantly to the blastocoel fluid within the blastocoel cavity (Bc) and to peripheral cells (PCs) surrounding the blastocyst during Day 3 to Day 5 progression.

WHAT IS KNOWN ALREADY

A commonly held dogma in all diploid eukaryotes is that two gametes, each with 'n' chromosomes (23 in humans), fuse to form a '2n' zygote (46 in humans); a state that remains in perpetuity for all somatic cell divisions. Human embryos, however, display high levels of chromosomal aneuploidy in early stages that reportedly declines from Day 3 (cleavage stage) to Day 5 (blastocyst) post-fertilization. While this observation may be partly because of aneuploid embryonic arrest before blastulation, it could also be due to embryo 'normalization' to a euploid state during blastulation. If and how this normalization occurs requires further investigation.

STUDY DESIGN, SIZE, DURATION

A total of 964 cleavage-stage (Day 3) embryos underwent single-cell biopsy and diagnosis for chromosome constitution. All were maintained in culture, assessing blastulation rate, both for those assessed euploid and aneuploid. Pregnancy rate was assessed for those determined euploid, blastulated and subsequently transferred. For those determined aneuploid and blastulated (174 embryos), ICM (all 174 embryos), TE (all 174), Bc (47 embryos) and PC (38 embryos) were analyzed for chromosome constitution. Specifically, concordance with the original Day 3 diagnosis and determination if any 'normalized' to euploid karyotypes within all four structures was assessed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

All patients (144 couples) were undergoing routine preimplantation genetic testing for aneuploidy in three IVF clinical settings. Cleavage-stage biopsy preceded chromosome analysis by next-generation sequencing. All patients provided informed consent. Additional molecular testing was carried out on blastocyst embryos and was analyzed for up to four embryonic structures (ICM, TE, Bc and PC).

MAIN RESULTS AND THE ROLE OF CHANCE

Of 463/964 embryos (48%) diagnosed as euploid at Day 3, 70% blastulated (leading to a 59% pregnancy rate) and 30% degenerated. Conversely, of the 501 (52%) diagnosed as aneuploid, 65% degenerated and 35% (174) blastulated, a highly significant difference (P < 0.0001). Of the 174 that blastulated, the ratio of '(semi)concordant-aneuploid' versus 'normalized-euploid' versus 'other-aneuploid' embryos was, respectively, 39%/57%/3% in the ICM; 49%/48%/3% in the TE; 78%/21%/0% in the PC; and 83%/10%/5% in the Bc. The TE karyotype therefore has a positive predictive value of 86.7% in determining that of the ICM, albeit with marginally higher aneuploid rates of abnormalities (P = .071). Levels of abnormality in Bc/PC were significantly higher (P < 0.0001) versus the ploidy of the ICM and TE and nearly all chromosome abnormalities were (at least partially) concordant with Day 3 diagnoses.

LIMITATIONS, REASONS FOR CAUTION

The results only pertain to human IVF embryos so extrapolation to the in vivo situation and to other species is not certain. We acknowledge (rather than lineage-specific survival, as we suggest here) the possibility of other mechanisms, such as lineage-specific movement of cells, during blastulation. Ethical considerations, however, make investigating this mechanism difficult on human embryos.

WIDER IMPLICATIONS OF THE FINDINGS

Mosaic human cleavage-stage embryos can differentiate into a euploid ICM where euploid cell populations predominate. Sequestering of aneuploid cells/nuclei to structures no longer involved in fetal development has important implications for preimplantation and prenatal genetic testing. These results also challenge previous fundamental understandings of mitotic fidelity in early human development and indicate a complex and fluid nature of the human embryonic genome.

STUDY FUNDING/COMPETING INTEREST(S)

This research was funded by Organon Pharmaceuticals and Merck Serono by grants to W.G.K. W.G.K. is also an employee of AdvaGenix, who could, potentially, indirectly benefit financially from publication of this manuscript. R.C.M. is supported by the National Institute of General Medical Sciences of the National Institutes of Health under award number R35GM133747. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. D.K.G. provides paid consultancy services for Care Fertility.

TRIAL REGISTRATION NUMBER

: N/A.

Human embryos donated for human embryonic stem cell derivation

Human embryonic stem cells (hESCs), produced from human embryos, are demonstrating: utility and promise in disease modeling; enhanced and unique understanding of early events in basic genetic or molecular or cellular or epigenetic development; novel human approaches to pharmaceutical screening; pathways toward the discoveries of disease treatments and cures; and foundational importance for regenerative medicine. The regulatory landscape is rigorous, and rightly so. Here, we discuss the current US federal and state regulatory environment. A unique approach of presenting anonymized embryo donor statements is provided to personalize the decision-making process of human embryo donation for hESC derivation. From the uses of preimplantation genetic-tested and affected human embryos to derived disease-specific hESCs, one can glean the much needed information on early human genetics and developmental biology, which are presented here. Finally, we discuss the future uses of hESCs, and other pluripotent stem cells, in general and reproductive medicine.

Mosaic results after preimplantation genetic testing for aneuploidy may be accompanied by changes in global gene expression

Aneuploidy in preimplantation embryos is a major cause of human reproductive failure. Unlike uniformly aneuploid embryos, embryos diagnosed as diploid-aneuploid mosaics after preimplantation genetic testing for aneuploidy (PGT-A) can develop into healthy infants. However, the reason why these embryos achieve full reproductive competence needs further research. Current RNA sequencing techniques allow for the investigation of the human preimplantation transcriptome, providing new insights into the molecular mechanisms of embryo development. In this prospective study, using euploid embryo gene expression as a control, we compared the transcriptome profiles of inner cell mass and trophectoderm samples from blastocysts with different levels of chromosomal mosaicism. A total of 25 samples were analyzed from 14 blastocysts with previous PGT-A diagnosis, including five low-level mosaic embryos and four high-level mosaic embryos. Global gene expression profiles visualized in cluster heatmaps were correlated with the original PGT-A diagnosis. In addition, gene expression distance based on the number of differentially expressed genes increased with the mosaic level, compared to euploid controls. Pathways involving apoptosis, mitosis, protein degradation, metabolism, and mitochondrial energy production were among the most deregulated within mosaic embryos. Retrospective analysis of the duration of blastomere cell cycles in mosaic embryos revealed several mitotic delays compared to euploid controls, providing additional evidence of the mosaic status. Overall, these findings suggest that embryos with mosaic results are not simply a misdiagnosis by-product, but may also have a genuine molecular identity that is compatible with their reproductive potential.

Recent insights into mammalian natural and synthetic ex utero embryogenesis

Research on early postimplantation mammalian development has been limited by the small size and intrauterine confinement of the developing embryos. Owing to the inability to observe and manipulate living embryos at these stages in utero, the establishment of robust ex utero embryo-culture systems that capture prolonged periods of mouse development has been an important research goal. In the last few years, these methods have been significantly improved by the optimization and enhancement of in vitro culture systems sustaining embryo development during peri-implantation stages for several species, and more recently, proper growth of natural mouse embryos from pregastrulation to late organogenesis stages and of embryonic stem cell (ES)-derived synthetic embryo models until early organogenesis stages. Here, we discuss the most recent ex utero embryo-culture systems established to date for rodents, nonhuman primates, and humans. We emphasize their technical aspects and developmental timeframe and provide insights into the new opportunities that these methods will contribute to the study of natural and synthetic mammalian embryogenesis and the stem-cell field.

Rare autosomal trisomies detected by non-invasive prenatal testing: an overview of current knowledge

Non-invasive prenatal testing has been introduced for the detection of Trisomy 13, 18, and 21. Using genome-wide screening also other "rare" autosomal trisomies (RATs) can be detected with a frequency about half the frequency of the common trisomies in the large population-based studies. Large prospective studies and clear clinical guidelines are lacking to provide adequate counseling and management to those who are confronted with a RAT as a healthcare professional or patient. In this review we reviewed the current knowledge of the most common RATs. We compiled clinical relevant parameters such as incidence, meiotic or mitotic origin, the risk of fetal (mosaic) aneuploidy, clinical manifestations of fetal mosaicism for a RAT, the effect of confined placental mosaicism on placental function and the risk of uniparental disomy (UPD). Finally, we identified gaps in the knowledge on RATs and highlight areas of future research. This overview may serve as a first guide for prenatal management for each of these RATs.

Comment on the recent PGDIS Position Statement on the Transfer of Mosaic Embryos 2021

The worldwide demand of preimplantation genetic testing for aneuploidy (PGT-A) is still growing. However, chromosomal mosaic results greatly challenge the clinical practice. The recently published PGDIS Position Statement on the Transfer of Mosaic Embryos is the third PGDIS position statement on how to deal with embryos diagnosed as chromosomal mosaics (CM) and, one of many attempts of different societies and working groups to provide a guideline for clinicians, laboratories, clinics, and genetic counselors. But still, as in previous statements, many issues remained unresolved. Moreover, from our point of view, the question how to deal with embryos diagnosed as CM, consisting of two or more karyological cell lines cannot be separated from all the other aspects of PGT-A including its accuracy. The paucity of clearcut indications for PGT-A and evidence of benefit as well as an overall cost-benefit assessment is given below.

Maternal selection of human embryos in early gestation: Insights from recurrent miscarriage

Compared to most mammals, human pregnancy is unusual in that it involves chromosomally diverse embryos, cyclical breakdown and regeneration of the uterine mucosa, and intimate integration of fetal and maternal cells at the uteroplacental interface. Not surprisingly, pregnancy often falters in early gestation. Whether these losses result in clinical miscarriages depends on the origins and impacts of chromosomal errors on fetal development and the ability of the decidualizing endometrium to engage in embryo biosensing and selection. Aneuploidy originating in oocytes during meiosis drives the age-related risk of miscarriage. By contrast, the frequency of endometrial cycles with an impaired decidual response may account for the stepwise increase in miscarriage rates with each pregnancy loss independently of maternal age. Additional physiological mechanisms operate in early gestation to ensure that most failing pregnancies are lost before vascular maternal-fetal connections are established by the end of the first trimester. Here, we summarise how investigations into the mechanisms that cause miscarriage led to new insights into the processes that govern maternal selection of human embryos in early gestation.

ESHRE Working Group on Chromosomal Mosaicism, De Rycke M, Capalbo A, Coonen E, Coticchio G, Fiorentino F, Goossens V, Mcheik S, Rubio C, Sermon K, Sfontouris I, Spits C, Vermeesch JR, Vermeulen N, Wells D, Zambelli F, Kakourou G. ESHRE survey results and good practice recommendations on managing chromosomal mosaicism

STUDY QUESTION

How should ART/preimplantation genetic testing (PGT) centres manage the detection of chromosomal mosaicism following PGT?

SUMMARY ANSWER

Thirty good practice recommendations were formulated that can be used by ART/PGT centres as a basis for their own policy with regards to the management of ‘mosaic’ embryos.

WHAT IS KNOWN ALREADY

The use of comprehensive chromosome screening technologies has provided a variety of data on the incidence of chromosomal mosaicism at the preimplantation stage of development and evidence is accumulating that clarifies the clinical outcomes after transfer of embryos with putative mosaic results, with regards to implantation, miscarriage and live birth rates, and neonatal outcomes.

STUDY DESIGN, SIZE, DURATION

This document was developed according to a predefined methodology for ESHRE good practice recommendations. Recommendations are supported by data from the literature, a large survey evaluating current practice and published guidance documents. The literature search was performed using PubMed and focused on studies published between 2010 and 2022. The survey was performed through a web-based questionnaire distributed to members of the ESHRE special interest groups (SIG) Reproductive Genetics and Embryology, and the ESHRE PGT Consortium members. It included questions on ART and PGT, reporting, embryo transfer policy and follow-up of transfers. The final dataset represents 239 centres.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The working group (WG) included 16 members with expertise on the ART/PGT process and chromosomal mosaicism. The recommendations for clinical practice were formulated based on the expert opinion of the WG, while taking into consideration the published data and results of the survey.

MAIN RESULTS AND THE ROLE OF CHANCE

Eighty percent of centres that biopsy three or more cells report mosaicism, even though only 66.9% of all centres have validated their technology and only 61.8% of these have validated specifically for the calling of chromosomal mosaicism. The criteria for designating mosaicism, reporting and transfer policies vary significantly across the centres replying to the survey. The WG formulated recommendations on how to manage the detection of chromosomal mosaicism in clinical practice, considering validation, risk assessment, designating and reporting mosaicism, embryo transfer policies, prenatal testing and follow-up. Guidance is also provided on the essential elements that should constitute the consent forms and the genetic report, and that should be covered in genetic counselling. As there are several unknowns in chromosomal mosaicism, it is recommended that PGT centres monitor emerging data on the topic and adapt or refine their policy whenever new insights are available from evidence.

LIMITATIONS, REASONS FOR CAUTION

Rather than providing instant standardized advice, the recommendations should help ART/PGT centres in developing their own policy towards the management of putative mosaic embryos in clinical practice.

WIDER IMPLICATIONS OF THE FINDINGS

This document will help facilitate a more knowledge-based approach for dealing with chromosomal mosaicism in different centres. In addition to recommendations for clinical practice, recommendations for future research were formulated. Following up on these will direct research towards existing research gaps with direct translation to clinical practice. Emerging data will help in improving guidance, and a more evidence-based approach of managing chromosomal mosaicism.

STUDY FUNDING/COMPETING INTEREST(S)

The WG received technical support from ESHRE. M.D.R. participated in the EQA special advisory group, outside the submitted work, and is the chair of the PGT WG of the Belgian society for human genetics. D.W. declared receiving salary from Juno Genetics, UK. A.C. is an employee of Igenomix, Italy and C.R. is an employee of Igenomix, Spain. C.S. received a research grant from FWO, Belgium, not related to the submitted work. I.S. declared being a Co-founder of IVFvision Ltd, UK. J.R.V. declared patents related to ‘Methods for haplotyping single-cells’ and ‘Haplotyping and copy number typing using polymorphic variant allelic frequencies’, and being a board member of Preimplantation Genetic Diagnosis International Society (PGDIS) and International Society for Prenatal Diagnosis (ISPD). K.S. reported being Chair-elect of ESHRE. The other authors had nothing to disclose.

DISCLAIMER

This Good Practice Recommendations (GPR) document represents the views of ESHRE, which are the result of consensus between the relevant ESHRE stakeholders and are based on the scientific evidence available at the time of preparation. 

ESHRE GPRs should be used for information and educational purposes. They should not be interpreted as setting a standard of care or be deemed inclusive of all proper methods of care, or be exclusive of other methods of care reasonably directed to obtaining the same results. They do not replace the need for application of clinical judgement to each individual presentation, or variations based on locality and facility type. 

Furthermore, ESHRE GPRs do not constitute or imply the endorsement, or favouring, of any of the included technologies by ESHRE. 

The AHNAK induces increased IL-6 production in CD4+ T cells and serves as a potential diagnostic biomarker for recurrent pregnancy loss

Disorganized maternal-fetal immune tolerance contributes to the occurrence of unexplained recurrent pregnancy loss (RPL). AHNAK is a scaffolding protein participating in the regulation of Ca2+ entry into T cells and the pathophysiology of diverse diseases. We performed differential gene expression analysis in decidual immune cells (DICs) isolated from three patients with RPL and from three healthy controls via RNA-sequencing (RNA-seq), which revealed 407 differentially expressed genes (DEGs). Among these DEGs, we underscored the clinical significance of elevated AHNAK mRNA and protein levels in DICs, peripheral blood mononuclear cells (PBMCs), and decidua of the patients with RPL, suggesting its potential use as a biomarker for the diagnosis of RPL. Especially, the ratios of decidual and blood AHNAK+CD4+ T cells in the CD4+ T cell population were significantly increased in patients with RPL, and the loss of AHNAK was further shown to inhibit interleukin (IL)-6 secretion in the CD4+ Jurkat cell line. Similar patterns were also observed in the clinical decidual and blood specimens. We uncovered that the AHNAK+CD4+ T cells could secrete more IL-6 than that the corresponding AHNAK-CD4+ T cells. Moreover, the frequencies of decidual and blood IL-6+CD4+ T cells in the CD4+ T-cell population were also increased in patients with RPL and showed significant positive correlations with the frequencies of AHNAK+CD4+ T cells. Our findings suggest that the elevated AHNAK expressed by CD4+ T cells may be involved in the immune dysregulation of RPL by increasing IL-6 production, illustrating its potential as a novel intervention target for RPL.

Stem Cell-Based Trophoblast Models to Unravel the Genetic Causes of Human Miscarriages

Miscarriage affects approximately 15% of clinically recognized pregnancies, and 1-3% of couples experience pregnancy loss recurrently. Approximately 50-60% of miscarriages result from chromosomal abnormalities, whereas up to 60% of euploid recurrent abortions harbor variants in candidate genes. The growing number of detected genetic variants requires an investigation into their role in adverse pregnancy outcomes. Since placental defects are the main cause of first-trimester miscarriages, the purpose of this review is to provide a survey of state-of-the-art human in vitro trophoblast models that can be used for the functional assessment of specific abnormalities/variants implicated in pregnancy loss. Since 2018, when primary human trophoblast stem cells were first derived, there has been rapid growth in models of trophoblast lineage. It has been found that a proper balance between self-renewal and differentiation in trophoblast progenitors is crucial for the maintenance of pregnancy. Different responses to aneuploidy have been shown in human embryonic and extra-embryonic lineages. Stem cell-based models provide a powerful tool to explore the effect of a specific aneuploidy/variant on the fetus through placental development, which is important, from a clinical point of view, for deciding on the suitability of embryos for transfer after preimplantation genetic testing for aneuploidy.

Metabolic and epigenetic dysfunctions underlie the arrest of in vitro fertilized human embryos in a senescent-like state

Around 60% of in vitro fertilized (IVF) human embryos irreversibly arrest before compaction between the 3- to 8-cell stage, posing a significant clinical problem. The mechanisms behind this arrest are unclear. Here, we show that the arrested embryos enter a senescent-like state, marked by cell cycle arrest, the down-regulation of ribosomes and histones and down-regulation of MYC and p53 activity. The arrested embryos can be divided into 3 types. Type I embryos fail to complete the maternal-zygotic transition, and Type II/III embryos have low levels of glycolysis and either high (Type II) or low (Type III) levels of oxidative phosphorylation. Treatment with the SIRT agonist resveratrol or nicotinamide riboside (NR) can partially rescue the arrested phenotype, which is accompanied by changes in metabolic activity. Overall, our data suggests metabolic and epigenetic dysfunctions underlie the arrest of human embryos.

Deconstructing human peri-implantation embryogenesis based on embryos and embryoids

The peri-implantation period from blastula to gastrula is one of the crucial stages of human embryo and stem cell development. During development, human embryos undergo many crucial events, such as embryonic lineage differentiation and development, structural self-assembly, pluripotency state transition, cell communication between lineages, and crosstalk between the embryo and uterus. Abnormalities in these developmental events will result in implantation failure or pregnancy loss. However, because of ethical and technical limits, the developmental dynamics of human peri-implantation embryos and the underlying mechanisms of abnormal development remain in a "black box". In this review, we summarize recent progress made towards our understanding of human peri-implantation embryogenesis based on extended in vitro cultured embryos and stem cell-based embryoids. These findings lay an important foundation for understanding early life, promoting research into human stem cells and their application, and preventing and treating infertility. We also propose key scientific issues regarding peri-implantation embryogenesis and provide an outlook on future study directions. Finally, we sum up China's contribution to the field and future opportunities.

Consequences of Chromosome Loss: Why Do Cells Need Each Chromosome Twice?

Aneuploidy is a cellular state with an unbalanced chromosome number that deviates from the usual euploid status. During evolution, elaborate cellular mechanisms have evolved to maintain the correct chromosome content over generations. The rare errors often lead to cell death, cell cycle arrest, or impaired proliferation. At the same time, aneuploidy can provide a growth advantage under selective conditions in a stressful, frequently changing environment. This is likely why aneuploidy is commonly found in cancer cells, where it correlates with malignancy, drug resistance, and poor prognosis. To understand this "aneuploidy paradox", model systems have been established and analyzed to investigate the consequences of aneuploidy. Most of the evidence to date has been based on models with chromosomes gains, but chromosome losses and recurrent monosomies can also be found in cancer. We summarize the current models of chromosome loss and our understanding of its consequences, particularly in comparison to chromosome gains.

On the origins and fate of chromosomal abnormalities in human preimplantation embryos: an unsolved riddle

About 8 out of 10 human embryos obtained in vitro harbour chromosomal abnormalities of either meiotic or mitotic origin. Abnormalities of mitotic origin lead to chromosomal mosaicism, a phenomenon which has sparked much debate lately as it confounds results obtained through preimplantation genetic testing for aneuploidy (PGT-A). PGT-A in itself is still highly debated, not only on the modalities of its execution, but also on whether it should be offered to patients at all.

We will focus on post-zygotic chromosomal abnormalities leading to mosaicism. First, we will summarize what is known of the rates of chromosomal abnormalities at different developmental stages. Next, based on the current understanding of the origin and cellular consequences of chromosomal abnormalities, which is largely based on studies on cancer cells and model organisms, we will offer a number of hypotheses on which mechanisms may be at work in early human development. Finally, and very briefly, we will touch upon the impact our current knowledge has on the practice of PGT-A. What is the level of abnormal cells that an embryo can tolerate before it loses its potential for full development? And is blastocyst biopsy as harmless as it seems?

Bioethics in human embryology: the double-edged sword of embryo research

There has been a significant increase in the use of assisted reproductive therapies (ARTs) over the past several decades, allowing many couples with infertility to conceive. Despite the achievements in this field, a mounting body of evidence concerning the epigenetic risks associated with ART interventions such as ovarian hormonal stimulation, intracytoplasmic sperm injection (ICSI), and in vitro culture (IVC) of oocytes and embryos has also emerged. Induced development of multiple follicles, the IVC media itself, and extended culture may alter the epigenome of both gametes and embryos, resulting in yet to be fully understood developmental, postnatal, and adult life health consequences. Investigators have attempted to decipher the molecular mechanisms mediating ART-induced epigenetic changes using either human samples or animal models with some success. As research in this field continues to expand, the ethical responsibilities of embryologists and researchers have become critically important. Here, we briefly discuss the ethical aspects of ART research, concentrating on the constraints arising from the perceived 'unnaturalness' of many of these procedures. Secondly, we focus on the bioethics and morality of human embryo research in general and how ethically acceptable model systems may be used to mimic early human embryogenesis. Lastly, we review the 14-day culture limit of human embryos and the notion that this rule could be considered of taken into account using new technologies and cues from animal models. The 'black box' of early post-implantation embryogenesis might be revealed using embryo models. As long as this distinct moral line has been drawn and closely followed, we should not fear scientific growth in embryo research. Although in vitro fertilization (IVF) is ethically acceptable, research with human embryos to improve its success raises serious ethical concerns that are in need of constant revisiting.Glossary index: Moral status: the ascription of obligations and rights to embryos on the basis of sentience; Sentience: the capacity of the developing embryo to experience feelings and sensations, such as the awareness of pain; Ectogenesis: the growth of the embryo in an artificial environment outside the mother's body.

Cleavage-stage human embryo arrest, is it embryo genetic composition or others?

Embryo transfer is a crucial step in IVF cycle, with increasing trend during the last decade of transferring a single embryo, preferably at the blastocyst stage. Despite increasing evidence supporting Day 5 blastocyst-stage transfer, the optimal day of embryo transfer remains controversial. The crucial questions are therefore, whether the mechanisms responsible to embryos arrest are embryo aneuploidy or others, and whether those embryos arrested in-vitro between the cleavage to the blastocyst stage would survive in-vivo if transferred on the cleavage-stage. We therefore aim to explore whether aneuploidy can directly contribute to embryo development to the blastocyst stage. Thirty Day-5 embryos, that their Day-3 blastomere biopsy revealed a single-gene defect, were donated by 10 couples undergoing preimplantation genetic testing treatment at our center. Affected high quality Day-3 embryos were cultured to Day-5, and were classified to those that developed to the blastocyst-stage and those that were arrested. Each embryo underwent whole genome amplification. Eighteen (60%) embryos were arrested, did not develop to the blastocyst stage and 12 (40%) have developed to the blastocyst stage. Nineteen embryos (63.3%) were found to be euploid. Of them, 12 (66.6%) were arrested embryos and 7 (58.3%) were those that developed to the blastocyst-stage. These figures were not statistically different (p = 0.644). Our observation demonstrated that the mechanism responsible to embryos arrest in vitro is not embryo aneuploidy, but rather other, such as culture conditions. If further studies will confirm that Day-5 blastocyst transfer might cause losses of embryos that would have been survived in vivo, cleavage-stage embryo transfer would be the preferred timing. This might reduce the cycle cancellations due to failure of embryo to develop to the blastocyst stage and will provide the best cumulative live birth-rate per started cycle.

In vitro culture of ovine embryos up to early gastrulating stages

Developmental failures occurring shortly after blastocyst hatching from the zona pellucida constitute a major cause of pregnancy losses in both humans and farm ungulates. The developmental events occurring following hatching in ungulates include the proliferation and maturation of extra-embryonic membranes – trophoblast and hypoblast – and the formation of a flat embryonic disc, similar to that found in humans, which initiates gastrulation prior to implantation. Unfortunately, our understanding of these key processes for embryo survival is limited because current culture systems cannot sustain ungulate embryo development beyond hatching. Here, we report a culture system that recapitulates most developmental landmarks of gastrulating ovine embryos: trophoblast maturation, hypoblast migration, embryonic disc formation, disappearance of the Rauber's layer, epiblast polarization and mesoderm differentiation. Our system represents a highly valuable platform for exploring the cell differentiation, proliferation and migration processes governing gastrulation in a flat embryonic disc and for understanding pregnancy failures during the second week of gestation.

This article has an associated ‘The people behind the papers’ interview.