Transcriptomic differences between human 8-cell-like cells reprogrammed with different methods

Embryonic genome activation (EGA) is a critical step in embryonic development. However, while EGA has been studied in mice using mouse 2-cell-like cells, human EGA remains incompletely elucidated due to the lack of an in vitro cell model recapitulating the early blastomere stage in humans. Recently, five groups independently reported human 8-cell-like cells (8CLCs, also called induced blastomere-like cells) developed from pluripotent stem cells and used single-cell RNA sequencing (scRNA-seq) to specify their cellular identities. Here we summarize the methods developed to produce the 8CLCs and compare their transcriptomic profiles by integrating them with the scRNA-seq datasets of human embryos. These observations will allow comparison and validation of the models, stimulate further in-depth research to characterize the genes involved in human EGA and pre-implantation development, and facilitate studies on human embryogenesis.

Transient DUX4 expression in human embryonic stem cells induces blastomere-like expression program that is marked by SLC34A2

Embryonic genome activation (EGA) is critical for embryonic development. However, our understanding of the regulatory mechanisms of human EGA is still incomplete. Human embryonic stem cells (hESCs) are an established model for studying developmental processes, but they resemble epiblast and are sub-optimal for modeling EGA. DUX4 regulates human EGA by inducing cleavage-stage-specific genes, while it also induces cell death. We report here that a short-pulsed expression of DUX4 in primed hESCs activates an EGA-like gene expression program in up to 17% of the cells, retaining cell viability. These DUX4-induced cells resembled eight-cell stage blastomeres and were named induced blastomere-like (iBM) cells. The iBM cells showed marked reduction of POU5F1 protein, as previously observed in mouse two-cell-like cells. Finally, the iBM cells were successfully enriched using an antibody against NaPi2b (SLC34A2), which is expressed in human blastomeres. The iBM cells provide an improved model system to study human EGA transcriptome.

Bioinspired 3D Culture in Nanoliter Hyaluronic Acid-Rich Core-Shell Hydrogel Microcapsules Isolates Highly Pluripotent Human iPSCs

Human induced pluripotent stem cells (iPSCs) are ideal for developing personalized medicine. However, the spontaneous differentiation of human iPSCs under conventional 2D and 3D cultures results in significant heterogeneity and compromised quality. Therefore, a method for effectively isolating and expanding high-quality human iPSCs is critically needed. Here, a biomimetic microencapsulation approach for isolating and culturing high-quality human iPSCs is reported. This is inspired by the natural proliferation and development of blastomeres into early blastocyst where the early embryonic stem cells-containing core is enclosed in a semipermeable hydrogel shell known as the zona pellucida (Zona). Blastomere cluster-like human iPSC clusters are encapsulated in a miniaturized (≈10 nanoliter) hyaluronic acid (HA)-rich core of microcapsules with a semipermeable Zona-like hydrogel shell and subsequently cultured to form pluripotent human iPSC spheroids with significantly improved quality. This is indicated by their high expression of pluripotency markers and highly efficient 3D cardiac differentiation. In particular, HA is found to be crucial for isolating the high-quality human iPSCs with the biomimetic core-shell microencapsulation culture. Interestingly, the isolated human iPSCs can maintain high pluripotency even after being cultured again in 2D. These discoveries and the bioinspired culture method may be valuable to facilitate the human iPSC-based personalized medicine.

On Mammalian Totipotency: What Is the Molecular Underpinning for the Totipotency of Zygote?

The mammalian zygote is described as a totipotent cell in the literature, but this characterization is elusive ignoring the molecular underpinnings. Totipotency can connote genetic totipotency, epigenetic totipotency, or the reprogramming capacity of a cell to epigenetic totipotency. Here, the implications of these concepts are discussed in the context of the properties of the zygote. Although genetically totipotent as any diploid somatic cell is, a zygote seems not totipotent transcriptionally, epigenetically, or functionally. Yet, a zygote may retain most of the key factors from its parental oocyte to reprogram an implanted differentiated genome or the zygote genome toward totipotency. This totipotent reprogramming process may extend to blastomeres in the two-cell-stage embryo. Thus, a revised alternative model of mammalian cellular totipotency is proposed, in which an epigenetically totipotent cell exists after the major embryonic genome activation and before the separation of the first two embryonic lineages.

Differences in blastomere totipotency in 2-cell mouse embryos are a maternal trait mediated by asymmetric mRNA distribution

It is widely held that the first two blastomeres of mammalian embryos are equally totipotent and that this totipotency belongs to the group of regulative properties. However, this interpretation neglects an important aspect: evidence only came from successful monozygotic twins which can speak only for those pairs of half-embryos that are able to regulate in the first place. Are the frequently occurring incomplete pairs simply an artefact, or do they represent a real difference, be it in the imperfect blastomere's ability to regulate growth or in the distribution of any compound X that constrains regulation? Using the model system of mouse embryos bisected at the 2-cell stage after fertilization, we present evidence that the interblastomere differences evade regulation by external factors and are already latent in oocytes. Specifically, an interblastomere imbalance of epiblast production persists under the most diverse culture conditions and applies to the same extent in parthenogenetic counterparts. As a result, cases in which twin blastocysts continued to develop in only one member account for 65 and 57% of zygotic and parthenogenetic pairs, respectively. The interblastomere imbalance is related to the subcellular distribution of gene products, as documented for the epiblast-related gene Cops3, using mRNA FISH in super-resolution mode confocal microscopy. Blastomere patterns of Cops3 mRNA distribution are α-amanitin-resistant. Thus, the imbalance originates not from de novo transcription, but from influences which are effective before fertilisation. These data expose previously unrecognized limits of regulative capacities of 2-cell stage blastomeres and point to aspects of cytoplasmic organization of the mouse oocyte that segregate unequally to blastomeres during cleavage.

The impacts of the number of prefreeze and postthaw blastomeres on embryo implantation potential: A systematic analysis

To systematically analyze the potential of embryo implantation through comparison between the number of surviving blastomeres, the growth, and implantation rate.Retrospective analysis on implantation rate and the growth of prefreeze-postthaw embryos with different blastomeres in 1487 frozen embryo transfer cycles.In groups of postthaw embryos without damage, implantation rate and the average number of blastomere growth increased significantly with increasing number of blastomeres. The implantation rate and the number of blastomeres of embryos with 8-8c (the number of blastomeres in prefreeze embryo-the number of blastomeres in postthaw embryo) continued to grow at a significantly higher rate than that of 5-5c and 6-6c (P < .05). In groups of embryos with the same number of blastomeres before freezing and with partial damage after resuscitation, the implantation rates were lower and the average numbers of blastomere growth reduced as the number of damaged blastomeres increased. For embryos with good quality before freezing, 1 to 3 damaged blastomeres in postthawed embryos did not affect the development and implantation rate. Both implantation rate and growth rate of embryos with 8-6c were significantly higher than those of embryos with 6-6c (P < .05).The number of surviving blastomeres and growth in frozen-thawed embryos could be important index to predict embryo development potential and clinical outcome of implantation. For embryos with good quality, a small amount of damaged blastomeres would not weaken embryo development potential and implantation rate after being thawed.

Enhanced development of mouse single blastomeres into blastocysts via the simultaneous inhibition of TGF-β and ERK pathways in microdroplet culture

Optimization of an in vitro culture that supports blastocyst (BL) development from single blastomeres (SBs) is essential to generate additional embryos for farm animals and humans and unravel the mechanisms that underlie totipotency. In this study, we have examined BL development from SBs that were derived from 2-cell and 4-cell mouse embryos in different media. Moreover, BLs were assessed for inner cell mass (ICM) by staining with Oct4. We found that BL development was improved in a lower volume of medium (1 µL) compared with a higher volume (5 µL). Furthermore, the supplementation of medium with the inhibitors of ERK1/2 and TGFβ (R2i) signaling pathways in 1 µL droplets of T6 medium improved BL development. The co-culture of SBs with intact embryos in the presence of R2i showed more BL development and ICM to trophectoderm cell number ratio in comparison with SB culture and SB group culture. We also observed reduced total cell number, ICM, and trophectoderm cell numbers in all of the SB culture conditions versus intact embryo development. These findings might facilitate the successful generation of additional embryos for biomedical applications and elucidate the mechanisms that underlie totipotency.

Timing embryo biopsy for PGD - before or after cryopreservation?

OBJECTIVE

Pre-implantation genetic diagnosis (PGD) is required in order to screen and diagnose embryos of patients at risk of having a genetically affected offspring. A biopsy to diagnose the genetic profile of the embryo may be performed either before or after cryopreservation. The aim of this study was to determine which biopsy timing yields higher embryo survival rates.

STUDY DESIGN

Retrospective cohort study of all PGD patients in a public IVF unit between 2010 and 2013. Inclusion criteria were patients with good-quality embryos available for cryopreservation by the slow freezing method. Embryos were divided into two groups: biopsy before and biopsy after cryopreservation. The primary outcome was embryo survival rates post thawing.

RESULTS

Sixty-five patients met inclusion criteria. 145 embryos were biopsied before cryopreservation and 228 embryos were cryopreserved and biopsied after thawing. Embryo survival was significantly greater in the latter group (77% vs. 68%, p < 0.0001).

CONCLUSION

Cryopreservation preceding biopsy results in better embryo survival compared to biopsy before cryopreservation.

Preimplantation genetic testing: polar bodies, blastomeres, trophectoderm cells, or blastocoelic fluid?

OBJECTIVE

To investigate the blastocoelic fluid (BF) for the presence of DNA that could be amplified and analyzed; the extent to which its chromosomal status corresponds to that found in trophectoderm (TE) cells, polar bodies (PBs), or blastomeres; and the identification of segmental abnormalities.

DESIGN

Longitudinal cohort study.

SETTING

In vitro fertilization unit.

PATIENT(S)

Fifty-one couples undergoing preimplantation genetic screening or preimplantation genetic diagnosis for translocations by array-comparative genomic hybridization on PBs (n = 21) or blastomeres (n = 30).

INTERVENTION(S)

BFs and TE cells were retrieved from 116 blastocysts, whose chromosome status had already been established by PB or blastomere assessment. Separate chromosome analysis was performed in 70 BFs.

MAIN OUTCOME MEASURE(S)

Presence of DNA in BFs, evaluation of the chromosome condition, and comparison with the diagnosis made in TE cells and at earlier stage biopsies.

RESULT(S)

DNA detection was 82%, with a net improvement after refinement of the procedure. In 97.1% of BFs, the ploidy condition corresponded to that found in TE cells, with one false positive and one false negative. The rate of concordance per single chromosome was 98.4%. Ploidy and chromosome concordance with PBs were 94% and 97.9%, respectively; with blastomeres, the concordances were 95% and 97.7%, respectively. Segmental abnormalities, which were detected in PBs or blastomeres of 16 blastocysts, were also identified in the corresponding BFs.

CONCLUSION(S)

BF represents to a good extent the blastocyst ploidy condition and chromosome status when compared with TE cells. If the proportion of clinically useful BFs is improved, blastocentesis could become the preferred source of DNA for chromosomal testing.

Blastocentesis: a source of DNA for preimplantation genetic testing. Results from a pilot study

OBJECTIVE

To investigate the presence of DNA in blastocyst fluids (BFs) and to estimate whether the chromosomal status predicted by its analysis corresponds with the ploidy condition in trophectoderm (TE) cells, the whole embryo, and that predicted by polar bodies (PBs) or blastomeres.

DESIGN

Prospective study.

SETTING

In vitro fertilization unit.

PATIENT(S)

Seventeen couples undergoing preimplantation genetic screening with the use of array comparative genomic hybridization on PBs (n = 12) or blastomeres (n = 5).

INTERVENTION(S)

BFs and TE cells were retrieved from 51 blastocysts for separate chromosomal analysis.

MAIN OUTCOME MEASURE(S)

Presence of DNA in BFs and assessment of the corresponding chromosome condition; correlation with the results in TE cells and those predicted by the analysis done at earlier stages.

RESULT(S)

DNA was detected in 39 BFs (76.5%). In 38 of 39 cases (97.4%) the ploidy condition of BFs was confirmed in TE cells, and the rate of concordance per single chromosome was 96.6% (904/936). In relation to the whole embryo, the ploidy condition corresponded in all cases with a per-chromosome concordance of 98.1%. The testing of PBs and blastomeres had 93.3% and 100% prediction of BF ploidy condition with a concordance per chromosome of 93.5% and 94%, respectively.

CONCLUSION(S)

Blastocentesis could represent an alternative source of material for chromosomal testing, because the BF is highly predictive of the embryo ploidy condition and chromosome content. Our data confirm the relevance of the oocyte and of the early-cleavage embryo in determining the ploidy condition of the resulting blastocyst.

Blastomere explants to test for cell fate commitment during embryonic development

Fate maps, constructed from lineage tracing all of the cells of an embryo, reveal which tissues descend from each cell of the embryo. Although fate maps are very useful for identifying the precursors of an organ and for elucidating the developmental path by which the descendant cells populate that organ in the normal embryo, they do not illustrate the full developmental potential of a precursor cell or identify the mechanisms by which its fate is determined. To test for cell fate commitment, one compares a cell's normal repertoire of descendants in the intact embryo (the fate map) with those expressed after an experimental manipulation. Is the cell's fate fixed (committed) regardless of the surrounding cellular environment, or is it influenced by external factors provided by its neighbors? Using the comprehensive fate maps of the Xenopus embryo, we describe how to identify, isolate and culture single cleavage stage precursors, called blastomeres. This approach allows one to assess whether these early cells are committed to the fate they acquire in their normal environment in the intact embryo, require interactions with their neighboring cells, or can be influenced to express alternate fates if exposed to other types of signals.

Zonula occludens-1 (ZO-1) is involved in morula to blastocyst transformation in the mouse

It is unknown whether or not tight junction formation plays any role in morula to blastocyst transformation that is associated with development of polarized trophoblast cells and fluid accumulation. Tight junctions are a hallmark of polarized epithelial cells and zonula occludens-1 (ZO-1) is a known key regulator of tight junction formation. Here we show that ZO-1 protein is first expressed during compaction of 8-cell embryos. This stage-specific appearance of ZO-1 suggests its participation in morula to blastocyst transition. Consistent with this idea, we demonstrate that ZO-1 siRNA delivery inside the blastomeres of zona-weakened embryos using electroporation not only knocks down ZO-1 gene and protein expressions, but also inhibits morula to blastocyst transformation in a concentration-dependent manner. In addition, ZO-1 inactivation reduced the expression of Cdx2 and Oct-4, but not ZO-2 and F-actin. These results provide the first evidence that ZO-1 is involved in blastocyst formation from the morula by regulating accumulation of fluid and differentiation of nonpolar blastomeres to polar trophoblast cells.

There is a cutoff limit in diameter between a blastomere and a small anucleate fragment

PURPOSE

To document the DNA content of blastomeres/fragments from early human preembryos and to determine if there is a "cutoff" diameter at which a cell should be considered an anucleate fragment rather than a blastomere.

METHODS

Surplus embryos from in vitro fertilization were used. Individual cells were measured, fixated, and stained for DNA.

RESULTS

In day 2 preembryos, only 2% of cells with a diameter <45 microm contained DNA, compared with 67% of those > or =45 microm. In day 3 preembryos, 3% of cells <40 microm contained DNA, compared with 66% of those > or =40 microm.

CONCLUSIONS

It is suggested that cells <45 microm in day 2 preembryos, and <40 microm in day 3 preembryos should be classified as fragments, and cells larger than this, as blastomeres. This may influence the embryo scoring system for in vitro fertilization. We therefore recommend that cells within this critical range should be measured when scoring preembryos for embryo transfer.

Zona pellucida thickness variation and occurrence of visible mononucleated blastomers in preembryos are associated with a high pregnancy rate in IVF treatment

PURPOSE

The ability of six morphological criteria (embryo development rate, fragmentation, regularity of blastomere shape, equality of blastomere size, zona pellucida thickness variation [ZPTV], and visible mononucleated blastomeres [VMBs]) to predict pregnancy in IVF treatment cycles was evaluated.

METHODS

In order to select a homogeneous study group, 85 consecutive nulliparous couples with single tubal infertility undergoing their first IVF treatment and receiving three preembryos at embryo replacement 2 days after ovum pickup were included.

RESULTS

A total of 255 preembryos was replaced two days after ovum pickup and resulted in 34 clinical pregnancies (40%). By logistic regression analysis, ZPTV and VMBs showed highly significant and strong predictive values, whereas none of the other parameters was a significant predictor of pregnancy. In the treatments in which all replaced preembryos had a ZPTV of less than 15%, the pregnancy rate was extremely low (1/22). If the maximum ZPTV of any of the replaced preembryos was in the interval between 15 and 20%, the pregnancy rate was 24.1% (7129). In the treatments in which at least one preembryo had a ZPTV of more than 20%, the pregnancy rate was 76.5% (26/34). When VMBs were added to the results of the ZPTV analysis, the pregnancy rate was as high as 92.3% (24/26).

CONCLUSIONS

ZPTV and VMBs seem to be strong predictors of pregnancy in IVF treatment and thus important indicators of good embryo quality.