Global gene expression profiling of preimplantation embryos

Genome-wide methylation profile of mitochondrial DNA across bovine preimplantation development

This study characterized variations in the methylation profile of mitochondrial DNA (mtDNA) during initial bovine embryo development and correlated the presence of methylation with mtDNA transcription. Bovine oocytes were obtained from abattoir ovaries and submitted to in vitro culture procedures. Oocytes and embryos were collected at various stages (immature oocyte, IM; mature oocyte, MII; zygote, ZY; 4-cells, 4C; 16-cells, 16C and blastocysts, BL). Total DNA (including mtDNA) was used for Whole Genome Enzymatic Methyl Sequencing and for quantification of mtDNA copy number. Extracted RNA was used for quantification of mitochondrial transcripts using Droplet Digital PCR. We selected ND6, CYTB, tRNA-Phe and tRNA-Gln based on their location in the mitochondrial genome, functionality and/or previous literature associating these regions with cytosine methylation. The number of mtDNA copies per oocyte/embryo was found to be similar, while methylation levels in mtDNA varied among stages. Higher total methylation levels were found mainly at 4C and 16C. In specific gene regions, higher methylation levels were also observed at 4C and 16C (ND6, CYTB and tRNA-Phe), as well as an inverse correlation with the quantity of transcripts for these regions. This is a first description of epigenetic changes occurring in mtDNA during early embryonic development. Our results indicate that methylation might regulate the mtDNA transcription at a local level, particularly around the time of embryonic genome activation.

Unraveling the Consequences of Oxygen Imbalance on Early Embryo Development: Exploring Mitigation Strategies

Although well-established and adopted by commercial laboratories, the in vitro embryo production system still requires refinements to achieve its highest efficiency. Early embryonic development is a dynamic event, demanding suitable conditions to provide a high number of embryos with quality and competence. The first step to obtaining an optimized in vitro environment is to know the embryonic metabolism and energy request throughout the different stages of development. Oxygen plays a crucial role in several key biological processes necessary to sustain and complete embryonic development. Nonetheless, there is still controversy regarding the optimal in vitro atmospheric concentrations during culture. Herein, we discuss the impact of oxygen tension on the viability of in vitro-produced embryos during early development. The importance of oxygen tension is addressed as its roles regarding essential embryonic traits, including embryo production rates, embryonic cell viability, gene expression profile, epigenetic regulation, and post-cryopreservation survival. Finally, we highlight the damage caused by in vitro unbalanced oxygen tensions and strategies to mitigate the harmful effects.

Preimplantation embryo gene expression: 56 years of discovery, and counting

The biology of preimplantation embryo gene expression began 56 years ago with studies of the effects of protein synthesis inhibition and discovery of changes in embryo metabolism and related enzyme activities. The field accelerated rapidly with the emergence of embryo culture systems and progressively evolving methodologies that have allowed early questions to be re-addressed in new ways and in greater detail, leading to deeper understanding and progressively more targeted studies to discover ever more fine details. The advent of technologies for assisted reproduction, preimplantation genetic testing, stem cell manipulations, artificial gametes, and genetic manipulation, particularly in experimental animal models and livestock species, has further elevated the desire to understand preimplantation development in greater detail. The questions that drove enquiry from the earliest years of the field remain drivers of enquiry today. Our understanding of the crucial roles of oocyte-expressed RNA and proteins in early embryos, temporal patterns of embryonic gene expression, and mechanisms controlling embryonic gene expression has increased exponentially over the past five and a half decades as new analytical methods emerged. This review combines early and recent discoveries on gene regulation and expression in mature oocytes and preimplantation stage embryos to provide a comprehensive understanding of preimplantation embryo biology and to anticipate exciting future advances that will build upon and extend what has been discovered so far.

Translation regulatory factor BZW1 regulates preimplantation embryo development and compaction by restricting global non-AUG Initiation

Protein synthesis is an essential step in gene expression during the development of mammalian preimplantation embryos. This is a complex and highly regulated process. The accuracy of the translation initiation codon is important in various gene expression programs. However, the mechanisms that regulate AUG and non-AUG codon initiation in early embryos remain poorly understood. BZW1 is a key factor in determining the mRNA translation start codon. Here, we show that BZW1 is essential for early embryonic development in mice. Bzw1-knockdown embryos fail to undergo compaction, and show decreased blastocyst formation rates. We also observe defects in the differentiation capacity and implantation potential after Bzw1 interference. Further investigation revealed that Bzw1 knockdown causes the levels of translation initiation with CUG as the start codon to increase. The decline in BZW1 levels result in a decrease in protein synthesis in preimplantation embryos, whereas the total mRNA levels are not altered. Therefore, we concluded that BZW1 contributes to protein synthesis during early embryonic development by restricting non-AUG translational initiation.

The role of insulin-like growth factor 2 mRNA binding proteins in female reproductive pathophysiology

Insulin-like growth factor 2 (IGF2) mRNA binding proteins (IMPs) family belongs to a highly conserved family of RNA-binding proteins (RBPs) and is responsible for regulating RNA processing including localization, translation and stability. Mammalian IMPs (IMP1-3) take part in development, metabolism and tumorigenesis, where they are believed to play a major role in cell growth, metabolism, migration and invasion. IMPs have been identified that are expressed in ovary, placenta and embryo. The up-to-date evidence suggest that IMPs are involved in folliculogenesis, oocyte maturation, embryogenesis, implantation, and placentation. The dysregulation of IMPs not only contributes to carcinogenesis but also disturbs the female reproduction, and may participate in the pathogenesis of reproductive diseases and obstetric syndromes, such as polycystic ovary syndrome (PCOS), pre-eclampsia (PE), gestational diabetes mellitus (GDM) and gynecological tumors. In this review, we summarize the role of IMPs in female reproductive pathophysiology, and hope to provide new insights into the identification of potential therapeutic targets.

Transcription of rRNA in early mouse embryos promotes chromatin reorganization and expression of major satellite repeats

During the first cell cycles of the early development, the chromatin of the embryo is highly reprogrammed alongside that embryonic genome starts its own transcription. The spatial organization of the genome is a major process that contributes to regulating gene transcription in time and space, however, it is poorly studied in the context of early embryos. To study the cause and effect link between transcription and spatial organization in embryos, we focused on the ribosomal genes, that are first silent and begin to transcribe during the 2-cell stage in the mouse. We demonstrated that ribosomal sequences and early unprocessed rRNAs are spatially organized in a very peculiar manner from the 2-cell to the 16-cell. Using drugs interfering with ribosomal DNA transcription, we show that this organization, totally different from somatic cells, depends on an active transcription of ribosomal genes and induces a unique chromatin environment that favors transcription of major satellite sequences after the 4-cell stage.

Reprogramming of DNA methylation is linked to successful human preimplantation development

Human preimplantation development is characterized by low developmental rates that are poorly understood. Early mammalian embryogenesis is characterized by a major phase of epigenetic reprogramming, which involves global DNA methylation changes and activity of TET enzymes; the importance of DNA methylation reprogramming for successful human preimplantation development has not been investigated. Here, we analyzed early human embryos for dynamic changes in 5-methylcytosine and its oxidized derivatives generated by TET enzymes. We observed that 5-methylcytosine and 5-hydroxymethylcytosine show similar, albeit less pronounced, asymmetry between the parental pronuclei of human zygotes relative to mouse zygotes. Notably, we detected low levels of 5-formylcytosine and 5-carboxylcytosine, with no apparent difference in maternal or paternal pronuclei of human zygotes. Analysis of later human preimplantation stages revealed a mosaic pattern of DNA 5C modifications similar to those of the mouse and other mammals. Strikingly, using noninvasive time-lapse imaging and well-defined cell cycle parameters, we analyzed normally and abnormally developing human four-cell embryos for global reprogramming of DNA methylation and detected lower 5-methylcytosine and 5-hydroxymethylcytosine levels in normal embryos compared to abnormal embryos. In conclusion, our results suggest that DNA methylation reprogramming is conserved in humans, with human-specific dynamics and extent. Furthermore, abnormalities in the four-cell-specific DNA methylome in early human embryogenesis are associated with abnormal development, highlighting an essential role of epigenetic reprogramming for successful human embryogenesis. Further research should identify the underlying genomic regions and cause of abnormal DNA methylation reprogramming in early human embryos.

Erasing gametes to write blastocysts: metabolism as the new player in epigenetic reprogramming

Understanding preimplantation embryonic development is crucial for the improvement of assisted reproductive technologies and animal production. To achieve this goal, it is important to consider that gametes and embryos are highly susceptible to environmental changes. Beyond the metabolic adaptation, the dynamic status imposed during follicular growth and early embryogenesis may create marks that will guide the molecular regulation during prenatal development, and consequently impact the offspring phenotype. In this context, metaboloepigenetics has gained attention, as it investigates the crosstalk between metabolism and molecular control, i.e., how substrates generated by metabolic pathways may also act as players of epigenetic modifications. In this review, we present the main metabolic and epigenetic events of pre-implantation development, and how these systems connect to open possibilities for targeted manipulation of reproductive technologies and animal production systems.

Ashwin Gene Expression Profiles in Oocytes, Preimplantation Embryos, and Fetal and Adult Bovine Tissues

Simple Summary

Ashwin is a gene involved in the morphogenesis of the central nervous system and the early embryonic development of Xenopus laevis. The analysis of its phylogeny in silico has shown that its functions are restricted to vertebrates, but we lack additional information regarding its biological importance in higher vertebrates, such as mammals. The present study reveals the wide and variable expression of this gene in different bovine organs and confirms its significant expression during early embryonic development, with a pattern similar to that reported for maternal genes. In addition, specific expression of this gene throughout follicular development and during bovine spermatogenesis is revealed, leading to the proposal of ashwin as a new gene with important biological implications in bovine development and reproduction.

Abstract

The ashwin gene, originally identified in Xenopus laevis, was found to be expressed first in the neural plate and later in the embryonic brain, eyes, and spinal cord. Functional studies of ashwin suggest that it participates in cell survival and anteroposterior patterning. Furthermore, ashwin is expressed zygotically in this species, which suggests that it participates in embryonic development. Nevertheless, the expression of this gene has not been studied in mammals. Thus, the aim of this study was to analyze the ashwin expression pattern in bovine fetal and adult tissues, as well as in three independent samples of immature and mature oocytes, and in two- to four-, and eight-cell embryos, morula, and blastocysts. Spatiotemporal expression was analyzed using real-time polymerase chain reaction (PCR); ashwin mRNA was detected in all tissues analyzed, immature and mature oocytes, and two- to eight-cell embryos. It was down-regulated in morula and blastocysts, suggesting that this expression profile is similar to that of maternal genes. Immunohistochemical localization of the ashwin protein in fetal and adult ovaries and testes reveals that this protein is consistently present during all stages of follicular development and during bovine spermatogenesis. These observations lead us to propose ashwin as an important gene involved in mammalian reproduction.

Dynamic Alternative Splicing During Mouse Preimplantation Embryo Development

The mechanism of alternative pre-mRNA splicing (AS) during preimplantation development is largely unknown. In order to capture the dynamic changes of AS occurring during embryogenesis, we carried out bioinformatics analysis based on scRNA-seq data over the time-course preimplantation development in mouse. We detected numerous previously-unreported differentially expressed genes at specific developmental stages and investigated the nature of AS at both minor and major zygotic genome activation (ZGA). The AS and differential AS atlas over preimplantation development were established. The differentially alternatively spliced genes (DASGs) are likely to be key splicing factors (SFs) during preimplantation development. We also demonstrated that there is a regulatory cascade of AS events in which some key SFs are regulated by differentially AS of their own gene transcripts. Moreover, 212 isoform switches (ISs) during preimplantation development were detected, which may be critical for decoding the mechanism of early embryogenesis. Importantly, we uncovered that zygotic AS activation (ZASA) is in conformity with ZGA and revealed that AS is coupled with transcription during preimplantation development. Our results may provide a deeper insight into the regulation of early embryogenesis.

Developmental Biology: Embryos Need to Control Their Nucleotides Just Right

The maternal-to-zygotic transition in the Drosophila embryo requires accurate control of the levels of free nucleotides, arguing for an essential role of nucleotide metabolism in the regulation of the cell cycle during early embryogenesis.

RNA-Binding Protein IGF2BP2/IMP2 is a Critical Maternal Activator in Early Zygotic Genome Activation

A number of genes involved in zygotic genome activation (ZGA) have been identified, but the RNA-binding maternal factors that are directly related to ZGA in mice remain unclear. The present study shows that maternal deletion of Igf  2bp2 (also commonly known as Imp2) in mouse embryos causes early embryonic developmental arrest in vitro at the 2-cell-stage. Transcriptomics and proteomics analyses of 2-cell-stage embryos in mice reveal that deletion of IMP2 downregulates the expression of Ccar1 and Rps14, both of which are required for early embryonic developmental competence. IGF2, a target of IMP2, when added in culture media, increases the proportion of wild-type embryos that develop successfully to the blastocyst stage: from 29% in untreated controls to 65% (50 × 10-9 m IGF2). Furthermore, in an experiment related to embryo transfer, foster mothers receiving IGF2-treated embryos deliver more pups per female than females who receive untreated control embryos. In clinically derived human oocytes, the addition of IGF2 to the culture media significantly enhances the proportion of embryos that develop successfully. Collectively, the findings demonstrate that IMP2 is essential for the regulation and activation of genes known to be involved in ZGA and reveal the potential embryonic development-related utility of IGF2 for animal biotechnology and for assisted reproduction in humans.

SMCHD1 terminates the first embryonic genome activation event in mouse two-cell embryos and contributes to a transcriptionally repressive state

Embryonic genome activation (EGA) in mammals begins with transient expression of a large group of genes (EGA1). Importantly, entry into and exit from the 2C/EGA state is essential for viability. Dux family member genes play an integral role in EGA1 by activating other EGA marker genes such as Zscan4 family members. We previously reported that structural maintenance of chromosomes flexible hinge domain-containing protein 1 (Smchd1) is expressed at the mRNA and protein levels in mouse oocytes and early embryos and that elimination of Smchd1 expression inhibits inner cell mass formation, blastocyst formation and hatching, and term development. We extend these observations here by showing that siRNA knockdown of Smchd1 in zygotes results in overexpression of Dux and Zscan4 in two-cell embryos, with continued overexpression of Dux at least through the eight-cell stage as well as prolonged expression of Zscan4. These results are consistent with a role for SMCHD1 in promoting exit from the EGA1 state and establishing SMCHD1 as a maternal effect gene and the first chromatin regulatory factor identified with this role. Additionally, bioinformatics analysis reveals that SMCHD1 also contributes to the creation of a transcriptionally repressive state to allow correct gene regulation.

Mediator complex component MED13 regulates zygotic genome activation and is required for postimplantation development in the mouse

Understanding factors that regulate zygotic genome activation (ZGA) is critical for determining how cells are reprogrammed to become totipotent or pluripotent. There is limited information regarding how this process occurs physiologically in early mammalian embryos. Here, we identify a mediator complex subunit, MED13, as translated during mouse oocyte maturation and transcribed early from the zygotic genome. Knockdown and conditional knockout approaches demonstrate that MED13 is essential for ZGA in the mouse, in part by regulating expression of the embryo-specific chromatin remodeling complex, esBAF. The role of MED13 in ZGA is mediated in part by interactions with E2F transcription factors. In addition to MED13, its paralog, MED13L, is required for successful preimplantation embryo development. MED13L partially compensates for loss of MED13 function in preimplantation knockout embryos, but postimplantation development is not rescued by MED13L. Our data demonstrate an essential role for MED13 in supporting chromatin reprogramming and directed transcription of essential genes during ZGA.

Prostaglandin F2α (PGF2α) production possibility and its receptors expression in the early- and late-cleaved preimplantation bovine embryos

BACKGROUND

Prostaglandin F_2α (PGF_2α) is an important component for the physiology of female reproductive processes. In the literature, the data pertaining to the synthesis and action of PGF_2α in early embryonic bovine development are limited. In our study, we used the bovine in vitro culture model based on the time of first cleavage to determine the mRNA expression and immunolocalization of PGF_2α synthase and its receptor in bovine embryos from the 2-cell stage to the hatched blastocyst stage. We also evaluated PGF_2α production at 2, 5 and 7 days of in vitro culture.

RESULTS

We found a significantly higher proportion of blastocysts obtained from the early-cleaved embryos than from the late-cleaved embryos (37.7% vs. 26.1% respectively, P < 0.05). The PGFS mRNA expression was significantly higher in the late-cleaved group than in the early-cleaved group at the 2-, 4- and 16-cell stages (P < 0.05). For PTGFR, we observed that within the late-cleaved group, the mRNA abundance was significantly higher in embryos at the 2- and 16-cell stages than in embryos at the 4- and 8-cell stages (P < 0.05). We observed that PTGFR mRNA expression was significantly higher in the 2- and 16-cell embryos in the late-cleaved group than that in the early-cleaved group embryos (P < 0.05). Among the blastocysts, the PGFS and PTGFR expression levels showed a trend towards higher mRNA expression in the late-cleaved group than in the early-cleaved group. Analysis of PGF_2α production showed that within the early-cleaved group, the content of PGF_2α in the in vitro culture medium was significantly higher on day 7 than it was on day 2 (P < 0.05).

CONCLUSIONS

The mRNA expression levels of PGF_2α synthase and its receptor depend on the developmental stage and the embryo quality. Analyses of PGFS and PTGFR expression in bovine blastocysts and of PGF_2α embryo production suggest that prostaglandin F_2α can act in an autocrine and paracrine manner in bovine in vitro-produced preimplantation embryos. Moreover, the tendency of PTGFR and PGFS mRNA expression to be upregulated in embryos with low developmental potential can indicate a compensation mechanism related to high PGFS and PTGFR mRNA expression levels in low-quality embryos.

EmExplorer: a database for exploring time activation of gene expression in mammalian embryos

Understanding early development offers a striking opportunity to investigate genetic disease, stem cell and assisted reproductive technology. Recent advances in high-throughput sequencing technology have led to the rising influx of omics data, which have rapidly boosted our understanding of mammalian developmental mechanisms. Here, we review the database EmExplorer (a database for exploring time activation of gene expression in mammalian embryos), which systematically organizes the genes from development-related pathways, and which we have already established and continue to update it. The current version of EmExplorer incorporates over 26 000 genes obtained from 306 functional pathways in five species. The function annotations of development-related genes were also integrated into EmExplorer. To facilitate data extraction, the database also contains the following information. (i) The dynamic expression values for each development stage are matched to the corresponding genes. (ii) A two-layer search tool which supports multi-option searching, such as by official symbol, pathway name and function annotation. The returned entries can directly link to the analysis results for the corresponding gene or pathway in the analysis module. (iii) The analysis module provides different gene comparisons at the multi-species level and functional pathway level, which shows the species specificity and stage specificity at the gene or pathway level. (iv) The analysis based on the hypergeometric distribution test reveals the enrichment of gene functions at a particular stage of one organism's pathway. (v) The browser is designed for users with ambiguous searching goals and greatly helps new users to get a general idea of the contents of the database. (vi) The experimentally validated pathways are manually curated and shown on the home page. EmExplorer will be helpful for elucidating early developmental mechanisms and exploring time activation genes. EmExplorer is freely available at http://bioinfor.imu.edu.cn/emexplorer.

Towards Improving Embryo Prioritization: Parallel Next Generation Sequencing of DNA and RNA from a Single Trophectoderm Biopsy

Improved embryo prioritization is crucial in optimizing the results in assisted reproduction, especially in light of increasing utilization of elective single embryo transfers. Embryo prioritization is currently based on morphological criteria and in some cases incorporates preimplantation genetic testing for aneuploidy (PGT-A). Recent technological advances have enabled parallel genomic and transcriptomic assessment of a single cell. Adding transcriptomic analysis to PGT-A holds promise for better understanding early embryonic development and implantation, and for enhancing available embryo prioritization tools. Our aim was to develop a platform for parallel genomic and transcriptomic sequencing of a single trophectoderm (TE) biopsy, that could later be correlated with clinical outcomes. Twenty-five embryos donated for research were utilized; eight for initial development and optimization of our method, and seventeen to demonstrate clinical safety and reproducibility of this method. Our method achieved 100% concordance for ploidy status with that achieved by the classic PGT-A. All sequencing data exceeded quality control metrics. Transcriptomic sequencing data was sufficient for performing differential expression (DE) analysis. All biopsies expressed specific TE markers, further validating the accuracy of our method. Using PCA, samples clustered in euploid and aneuploid aggregates, highlighting the importance of controlling for ploidy in every transcriptomic assessment.

Ovarian extracellular MicroRNAs as the potential non-invasive biomarkers: An update

Through the reproductive system, it has been realized that the microRNAs (miRNAs) have emerged as one of the principal post-transcriptional gene regulators of the diverse developmental processes. The ovary, as a dynamic organ, co-ordinates follicle recruitment, selection, and ovulation, in which miRNAs play the central role almost in its all functions. Deregulation of these developmental procedures in ovary could lead to the ovarian dysfunction, infertility, decrease in the assisted reproductive treatment (ART) outcome, and death in some patients with ovarian cancer. In recent years, detection of ovarian extracellular miRNAs in body fluids such as follicular fluid and serum/plasma has opened a new era in the biomarker discovery field. Here through the present review, different aspects of the potential and proposed involvement of the extracellular miRNAs in both physiologic and pathologic contexts of the ovary have been discussed. Moreover, the researchers have addressed the relevant findings, challenges, and issues which associated with the extracellular miRNAs in the ovarian microenvironments to provide the better insight into understanding the molecular mechanisms which were involved in the pathophysiologic conditions. Finally, a comprehensive survey of the gaps has been discussed to hopefully shed new light and perspective on the development of the novel diagnostic and therapeutic platforms in the clinic.

Exploring timing activation of functional pathway based on differential co-expression analysis in preimplantation embryogenesis

Recent genome-wide omics studies have confirmed the early embryogenesis strictly dependent on the rigorous spatiotemporal activation and multilevel regulation. However, the full effect of functional pathway was not considered. To obtain complete understanding of the gene activation during early development, we performed systematic comparisons based on differential co-expression analysis for bovine preimplantation embryo development (PED). The results confirmed that the functional pathways actively transcribes as early as the 2-cell and 4-cell waves, which Basal transcription factor, Endocytosis and Spliceosome pathway can represent first signs of embryonic activity. Endocytosis act as one of master activators for uncovering a series of successive waves of maternal pioneer signal regulator with the help of Spliceosome complex. Furthermore, the results showed that pattern recognition receptors began to perform its essential function at 4-cell stage, which might be needed to coordinate the later major activation. And finally, our work presented a probable dynamic landscape of key functional pathways for embryogenesis. A clearer understanding of early embryo development will be helpful for Assisted Reproductive Technology (ART) and Regenerative Medicine (RM).

Cleavage-stage embryo micromanipulation in the clinical setting

Embryo micromanipulation was developed after introduction of microinjection to overcome infertility. Embryo micromanipulation may be performed at any embryo stage from pronuclear to blastocyst. The technique started out as basic and turned out to be increasingly more complex. Embryo micromanipulation at the cleavage-stage includes a wide range of techniques, from opening the zona pellucida in order to improve the chance of implantation, to removing detrimental components from the embryo to enhance embryo development or blastomeres for preimplantation genetic diagnosis and embryo splitting. Evaluating the impact(s) of different micromanipulation techniques on epigenetics of the embryo and considering quality control during these techniques are important issues in this regard. This review aims to discuss the micromanipulation of cleavage-stage embryos in clinical assisted reproductive technology (ART).

ABBREVIATIONS

ART: assisted reproductive technology; ICSI: intracytoplasmic sperm injection; IVF: in vitro fertilization; PGD: preimplantation genetic diagnosis; PZD: partial zona dissection; ZP: zona pellucida; SUZI: subzonal insemination; PVS: perivitelline space; AH: assisted hatching; LAH: laserassisted hatching; ZT: zona thinning; UV: ultraviolet; IR: infrared; PCR: polymerase chain reaction; FISH: fluorescent in situ hybridization; NGS: next generation sequencing; QC: quality control; QA: quality assurance.

Potential role of microRNAs in mammalian female fertility

Since the first evidence for the involvement of microRNAs (miRNAs) in various reproductive processes through conditional knockout of DICER, several studies have been conducted to investigate the expression pattern and role of miRNAs in ovarian follicular development, oocyte maturation, embryo development, embryo-maternal communication, pregnancy establishment and various reproductive diseases. Although advances in sequencing technology have fuelled miRNA studies in mammalian species, the presence of extracellular miRNAs in various biological fluids, including follicular fluid, blood plasma, urine and milk among others, has opened a new door in miRNA research for their use as diagnostic markers. This review presents data related to the identification and expression analysis of cellular miRNA in mammalian female fertility associated with ovarian folliculogenesis, oocyte maturation, preimplantation embryo development and embryo implantation. In addition, the relevance of miRNAs to female reproductive disorders, including polycystic ovary syndrome (PCOS), endometritis and abnormal pregnancies, is discussed for various mammalian species. Most importantly, the mechanism of release and the role of extracellular miRNAs in cell-cell communication and their potential role as non-invasive markers in female fertility are discussed in detail. Understanding this layer of regulation in female reproduction processes will pave the way to understanding the genetic regulation of female fertility in mammalian species.

Vitrification of human immature oocytes before and after in vitro maturation: a review

The use of immature oocytes subjected to in vitro maturation (IVM) opens interesting perspectives for fertility preservation where ovarian reserves are damaged by pathologies or therapies, as in PCO/PCOS and cancer patients. Human oocyte cryopreservation may offer some advantages compared to embryo freezing, such as fertility preservation in women at risk of losing fertility due to oncological treatment or chronic disease, egg donation and postponing childbirth. It also eliminates religious and/or other ethical, legal, and moral concerns of embryo freezing. In addition, a successful oocyte cryopreservation program could eliminate the need for donor and recipient menstrual cycle synchronization. Recent advances in vitrification technology have markedly improved the oocyte survival rate after warming, with fertilization and implantation rates comparable with those of fresh oocytes. Healthy live births can be achieved from the combination of IVM and vitrification, even if vitrification of in vivo matured oocytes is still more effective. Recently, attention is given to highlight whether vitrification procedures are more successful when performed before or after IVM, on immature GV-stage oocytes, or on in vitro matured MII-stage oocytes. In this review, we emphasize that, even if there are no differences in survival rates between oocytes vitrified prior to or post-IVM, reduced maturation rates of immature oocytes vitrified prior to IVM can be, at least in part, explained by underlying ultrastructural and biomolecular alterations.

Metabolite availability as a window to view the early embryo microenvironment in vivo

A preimplantation embryo exists independent of blood supply, and relies on energy sources from its in vivo environment (e.g., oviduct and uterine fluid) to sustain its development. The embryos can survive in this aqueous environment because it contains amino acids, proteins, lactate, pyruvate, oxygen, glucose, antioxidants, ions, growth factors, hormones, and phospholipids-albeit the concentration of each component varies by species, stage of the estrous cycle, and anatomical location. The dynamic nature of this environment sustains early development from the one-cell zygote to blastocyst, and is reciprocally influenced by the embryo at each embryonic stage. Focusing on embryo metabolism allowed us to identify how the local environment was deliberately selected to meet the dynamic needs of the preimplantation embryo, and helped reveal approaches to improve the in vitro culture of human embryos for improved implantation rates and pregnancy outcome.

Localisation of RNAs and proteins in nucleolar precursor bodies of early mouse embryos

Early embryos of all mammalian species contain morphologically distinct but transcriptionally silent nucleoli called the nucleolar precursor bodies (NPBs), which, unlike normal nucleoli, have been poorly studied at the biochemical level. To bridge this gap, here we examined the occurrence of RNA and proteins in early mouse embryos with two fluorochromes - an RNA-binding dye pyronin Y (PY) and the protein-binding dye fluorescein-5'-isothiocyanate (FITC). The staining patterns of zygotic NPBs were then compared with those of nucleolus-like bodies (NLBs) in fully grown surrounded nucleolus (SN)-type oocytes, which are morphologically similar to NPBs. We show that both entities contain proteins, but unlike NLBs, NPBs are significantly impoverished for RNA. Detectable amounts of RNA appear on the NPB surface only after resumption of rDNA transcription and includes pre-rRNAs and 28S rRNA as evidenced by fluorescence in situ hybridisation with specific oligonucleotide probes. Immunocytochemical assays demonstrate that zygotic NPBs contain rRNA processing factors fibrillarin, nucleophosmin and nucleolin, while UBF (the RNA polymerase I transcription factor) and ribosomal proteins RPL26 and RPS10 are not detectable. Based on the results obtained and data in the contemporary literature, we suggest a scheme of NPB assembly and maturation to normal nucleoli that assumes utilisation of maternally derived nucleolar proteins but of nascent rRNAs.

Regulation of DNA Replication in Early Embryonic Cleavages

Early embryonic cleavages are characterized by short and highly synchronous cell cycles made of alternating S- and M-phases with virtually absent gap phases. In this contracted cell cycle, the duration of DNA synthesis can be extraordinarily short. Depending on the organism, the whole genome of an embryo is replicated at a speed that is between 20 to 60 times faster than that of a somatic cell. Because transcription in the early embryo is repressed, DNA synthesis relies on a large stockpile of maternally supplied proteins stored in the egg representing most, if not all, cellular genes. In addition, in early embryonic cell cycles, both replication and DNA damage checkpoints are inefficient. In this article, we will review current knowledge on how DNA synthesis is regulated in early embryos and discuss possible consequences of replicating chromosomes with little or no quality control.

How do laboratory embryo transfer techniques affect IVF outcomes? A review of current literature

Over the last few years, many studies have focused on embryo selection methods, whereas little attention has been given to the standardization of the procedure of embryo transfer. In this review, several parameters of the embryo transfer procedure are examined, such as the: (i) culture medium volume and loading technique; (ii) syringe and catheters used for embryo transfer; (iii) viscosity and composition of the embryo transfer medium; (iv) environment of embryo culture; (v) timing of embryo transfer; (vi) and standardization of the embryo transfer techniques. The aim of this manuscript is to review these factors and compare the existing embryo transfer techniques and highlight the need for better embryo transfer standardization.

Novel embryo selection techniques to increase embryo implantation in IVF attempts

PURPOSE

The final success of an IVF attempt depends on several steps and decisions taken during the ovarian stimulation, the oocyte retrieval, the embryo culture and the embryo transfer. The final selection of the embryos most likely to implant is the final step in this process and the responsibility of the lab. Apart from strict morphologic criteria that historically have been used in embryo selection, additional information on genetic, metabolomic and morphokinetic characteristics of the embryo is recently combined to morphology to select the embryo most likely to produce a pregnancy. In this manuscript, we review the most recent information on the current methods used for embryo selection presenting the predictive capability of each one.

METHODS

A literature search was performed on Pubmed, Medline and Cochrane Database of Systematic Reviews for published studies using appropriate key words and phrases with no limits placed on time.

RESULTS

It seems that the combination of morphologic criteria in conjunction to embryo kinetics as documented by time-lapse technology provides the most reliable information on embryo quality. Blastocyst biopsy with subsequent comprehensive chromosome analysis allows the selection of the euploid embryos with the higher implantation potential.

CONCLUSION

Embryo time-lapse imaging and blastocyst biopsy combined to comprehensive chromosome analysis are the most promising technologies to increase pregnancy rates and reduce the possibility of multiple pregnancies. However, further studies will demonstrate the capability of routinely using these technologies to significantly improve IVF outcomes.

Factors affecting the gene expression of in vitro cultured human preimplantation embryos

STUDY QUESTION

What is the relative effect of common environmental and biological factors on transcriptome changes during human preimplantation development?

SUMMARY ANSWER

Developmental stage and maternal age had a larger effect on the global gene expression profile of human preimplantation embryos than the culture medium or oxygen concentration used in in vitro culture.

WHAT IS KNOWN ALREADY

Studies on mouse and bovine embryos have shown that different conditions in the in vitro culture of embryos can lead to changes in transcriptome profiles. For humans, an effect of developmental stage on the transcriptome profile of embryos has been demonstrated, but studies on the effect of maternal age or culture conditions are lacking.

STUDY DESIGN, SIZE, DURATION

Donated, good quality, day 4 cryopreserved human preimplantation embryos (N = 89) were randomized to be cultured in one of two culture media (G5 medium or HTF medium) and one of two oxygen concentrations (5% or 20%), with stratification for maternal age. Next to these variables, developmental stage after culture was taken into account in the analysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Embryos that developed to morula or blastocyst stage during these 2 days whose amplified mRNA passed our quality control criteria for microarray hybridization were individually examined for genome-wide gene expression (N = 37).

MAIN RESULTS AND THE ROLE OF CHANCE

Based on the number of differentially expressed genes (DEGs), developmental stage (3519 DEGs) and maternal age (1258 DEGs) had a larger effect on the global gene expression profile of human preimplantation embryos than either tested culture medium (596 DEGs) or oxygen concentration (492 DEGs) used during in vitro culture. Interactions between the factors were found, indicating that culture conditions might have a different effect depending on the developmental stage or the maternal age of the embryos. Affected pathways included metabolism, cell cycle processes and oxidative phosphorylation.

LIMITATIONS, REASONS FOR CAUTION

Culture of embryos for only 2 days might have limited the effect on global gene expression by the investigated culture conditions. Earlier stages of development (Day 0 until Day 4) were not analyzed and these embryos might respond differently to the experimental conditions. The freezing and thawing procedures might have had an effect on gene expression. RT-PCR validation was not performed due to scarcity of the material.

WIDER IMPLICATIONS OF THE FINDINGS

Our results show that when studying gene expression in single human preimplantation embryos under various experimental conditions, one should take into account the confounding effect of biological variables, such as developmental stage and maternal age. This makes these experiments different from gene expression experiments where these variables can be tightly controlled, for example when using cell lines.

STUDY FUNDING/COMPETING INTERESTS

This study received no external funding and there were no competing interests.

Gene Coexpression and Evolutionary Conservation Analysis of the Human Preimplantation Embryos

Evolutionary developmental biology (EVO-DEVO) tries to decode evolutionary constraints on the stages of embryonic development. Two models--the "funnel-like" model and the "hourglass" model--have been proposed by investigators to illustrate the fluctuation of selective pressure on these stages. However, selective indices of stages corresponding to mammalian preimplantation embryonic development (PED) were undetected in previous studies. Based on single cell RNA sequencing of stages during human PED, we used coexpression method to identify gene modules activated in each of these stages. Through measuring the evolutionary indices of gene modules belonging to each stage, we observed change pattern of selective constraints on PED for the first time. The selective pressure decreases from the zygote stage to the 4-cell stage and increases at the 8-cell stage and then decreases again from 8-cell stage to the late blastocyst stages. Previous EVO-DEVO studies concerning the whole embryo development neglected the fluctuation of selective pressure in these earlier stages, and the fluctuation was potentially correlated with events of earlier stages, such as zygote genome activation (ZGA). Such oscillation in an earlier stage would further affect models of the evolutionary constraints on whole embryo development. Therefore, these earlier stages should be measured intensively in future EVO-DEVO studies.

Distinct differences in global gene expression profiles in non-implanted blastocysts and blastocysts resulting in live birth

Results from animal models points towards the existence of a gene expression profile that is distinguishably different in viable embryos compared with non-viable embryos. Knowledge of human embryo transcripts is however limited, in particular with regard to how gene expression is related to clinical outcome. The purpose of the present study was therefore to determine the global gene expression profiles of human blastocysts. Next Generation Sequencing was used to identify genes that were differentially expressed in non-implanted embryos and embryos resulting in live birth. Three trophectoderm biopsies were obtained from morphologically high quality blastocysts resulting in live birth and three biopsies were obtained from non-implanting blastocysts of a comparable morphology. Total RNA was extracted from all samples followed by complete transcriptome sequencing. Using a set of filtering criteria, we obtained a list of 181 genes that were differentially expressed between trophectoderm biopsies from embryos resulting in either live birth or no implantation (negative hCG), respectively. We found that 37 of the 181 genes displayed significantly differential expression (p<0.05), e.g. EFNB1, CYTL1 and TEX26 and TESK1, MSL1 and EVI5 in trophectoderm biopsies associated with live birth and non-implanting, respectively. Out of the 181 genes, almost 80% (145 genes) were up-regulated in biopsies from un-implanted embryos, whereas only 20% (36 genes) showed an up-regulation in the samples from embryos resulting in live birth. Our findings suggest the presence of molecular differences visually undetectable between implanted and non-implanted embryos, and represent a proof of principle study.

Effective Oocyte Vitrification and Survival Techniques for Bovine Somatic Cell Nuclear Transfer

Bovine somatic cell nuclear transfer (SCNT) using vitrified-thawed (VT) oocytes has been studied; however, the cloning efficiency of these oocytes is not comparable with that of nonvitrified (non-V) fresh oocytes. This study sought to optimize the survival and cryopreservation of VT oocytes for SCNT. Co-culture with feeder cells that had been preincubated for 15 h significantly improved the survival of VT oocytes and their in vitro developmental potential following SCNT in comparison to co-culture with feeder cells that had been preincubated for 2, 5, or 24 h (p<0.05). Spindle assessment via the Oosight Microscopy Imaging System and microtubule staining revealed that vitrified metaphase II oocytes (VT group) were not suitable for SCNT. However, enucleating and/or activating oocytes prior to freezing enhanced their developmental potential and suitability for SCNT. The cloning efficiency of the enucleated-activated-vitrified-thawed (EAVT) group (21.6%) was better than that of the other vitrification groups [enucleated-vitrified-thawed (EVT) group, 13.7%; VT group, 15.0%; p<0.05] and was comparable with that of the non-V group (25.9%). The reactive oxygen species level was significantly lower in the EAVT group than in the other vitrification groups (p<0.05). mRNA levels of maternal genes (ZAR1, BMP15, and NLRP5) and a stress gene (HSF1) were lower in the vitrification groups than in the non-V group (p<0.05), whereas the level of phospho-p44/42 mitogen-activated protein kinase did not differ among the groups. Among the vitrification groups, blastocysts in the EAVT group had the best developmental potential, as judged by their high mRNA expression of developmental potential-related genes (POU5f1, Interferon-tau, and SLC2A5) and their low expression of proapoptotic (CASP3) and stress (Hsp70) genes. This study demonstrates that SCNT using bovine frozen-thawed oocytes can be successfully achieved using optimized vitrification and co-culture techniques.

Lactate production by the mammalian blastocyst: manipulating the microenvironment for uterine implantation and invasion?

The mammalian blastocyst exhibits a high capacity for aerobic glycolysis, a metabolic characteristic of tumours. It has been considered that aerobic glycolysis is a means to ensure a high carbon flux to fulfil biosynthetic demands. Here, alternative explanations for this pattern of metabolism are considered. Lactate creates a microenvironment of low pH around the embryo to assist the disaggregation of uterine tissues to facilitate trophoblast invasion. Further it is proposed that lactate acts as a signalling molecule (especially at the reduced oxygen tension present at implantation) to elicit bioactive VEGF recruitment from uterine cells, to promote angiogenesis. Finally it is suggested that the region of high lactate/low pH created by the blastocyst modulates the activity of the local immune response, helping to create immune tolerance. Consequently, the mammalian blastocyst offers a model to study the role of microenvironments, and how metabolites and pH are used in signalling.

New insights into the maternal to zygotic transition

The initial phases of embryonic development occur in the absence of de novo transcription and are instead controlled by maternally inherited mRNAs and proteins. During this initial period, cell cycles are synchronous and lack gap phases. Following this period of transcriptional silence, zygotic transcription begins, the maternal influence on development starts to decrease, and dramatic changes to the cell cycle take place. Here, we discuss recent work that is shedding light on the maternal to zygotic transition and the interrelated but distinct mechanisms regulating the onset of zygotic transcription and changes to the cell cycle during early embryonic development.

Identification and functional analysis of long non-coding RNAs in mouse cleavage stage embryonic development based on single cell transcriptome data

Background

Long non-coding RNAs (lncRNAs) regulate embryonic development and cell fate decision in various ways, such as modulation of chromatin modification and post-transcription regulation of gene expression. However, the profiles and roles of lncRNAs in early mammalian development have not yet been demonstrated. Here, we reported a comprehensive analysis of mouse cleavage stage embryonic lncRNA profiles based on public single-cell RNA-seq data.

Results

We reconstructed 50,006 high-confidence transcripts in 22,827 loci, and identified 5563 novel lncRNAs from 3492 loci expressed in mouse cleavage stage embryos. These lncRNAs share similar characteristics with previously reported vertebrate lncRNAs, such as relatively short length, low exon number, low expression level and low sequence conservation. Expression profile analysis revealed that the profiles of lncRNA vary considerably at different stages of cleavage stage embryos, suggesting that many lncRNAs in cleavage stage embryos are stage-specifically expressed. Co-expression network analysis suggested many lncRNAs in cleavage stage embryos are associated with cell cycle regulation, transcription, translation and oxidative phosphorylation to regulate the process of cleavage stage embryonic development.

Conclusions

This study provides the first catalog of lncRNAs expressed in mouse cleavage stage embryos and gives a revealing insight into the molecular mechanism responsible for early embryonic development.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-845) contains supplementary material, which is available to authorized users.

Biopsy of human morula-stage embryos: outcome of 215 IVF/ICSI cycles with PGS

Preimplantation genetic diagnosis (PGD) is commonly performed on biopsies from 6-8-cell-stage embryos or blastocyst trophectoderm obtained on day 3 or 5, respectively. Day 4 human embryos at the morula stage were successfully biopsied. Biopsy was performed on 709 morulae from 215 ICSI cycles with preimplantation genetic screening (PGS), and 3-7 cells were obtained from each embryo. The most common vital aneuploidies (chromosomes X/Y, 21) were screened by fluorescence in situ hybridization (FISH). No aneuploidy was observed in 72.7% of embryos, 91% of those developed to blastocysts. Embryos were transferred on days 5-6. Clinical pregnancy was obtained in 32.8% of cases, and 60 babies were born. Patients who underwent ICSI/PGS treatment were compared with those who underwent standard ICSI treatment by examining the percentage of blastocysts, pregnancy rate, gestational length, birth height and weight. No significant differences in these parameters were observed between the groups. Day 4 biopsy procedure does not adversely affect embryo development in vitro or in vivo. The increased number of cells obtained by biopsy of morulae might facilitate diagnostic screening. There is enough time after biopsy to obtain PGD results for embryo transfer on day 5-6 in the current IVF cycle.

Live pups from evaporatively dried mouse sperm stored at ambient temperature for up to 2 years

The purpose of this study is to develop a mouse sperm preservation method based on evaporative drying. Mouse sperm were evaporatively dried and stored at 4°C and ambient temperature for 3 months to 2 years. Upon rehydration, a single sperm was injected into a mature oocyte to develop into a blastocyst after culture or a live birth after embryo transfer to a recipient female. For the samples stored at 4°C for 3, 6, 12, 18, and 24 months, the blastocyst formation rate was 61.5%, 49.1%, 31.5%, 32.2%, and 41.4%, respectively. The blastocyst rate for those stored at ambient temperature (∼22°C) for 3, 6, 12, and 18 months was 57.8%, 36.2%, 33.6%, and 34.4%, respectively. Fifteen, eight and three live pups were produced from sperm stored at room temperature for 12, 18, and 24 months, respectively. This is the first report of live offspring produced from dried mouse sperm stored at ambient temperature for up to 2 years. Based on these results, we suggest that evaporative drying is a potentially useful method for the routine preservation of mouse sperm.

Genomic profiling to improve embryogenesis in the pig

Over the past decade the technology to characterize transcription during embryogenesis has progressed from estimating a single transcript to a reliable description of the entire transcriptome. Northern blots were followed by sequencing ESTs, quantitative real time PCR, cDNA arrays, custom oligo arrays, and more recently, deep sequencing. The amount of information that can be generated is overwhelming. The challenge now is how to glean information from these vast data sets that can be used to understand development and to improve methods for creating and culturing embryos in vitro, and for reducing reproductive loss. The use of ESTs permitted the identification of SPP1 as an oviductal component that could reduce polyspermy. Microarrays identified LDL and NMDA as components to replace BSA in embryo culture media. Deep sequencing implicated arginine, glycine, and folate as components that should be adjusted in our current culture system, and identified a characteristic of embryo metabolism that is similar to cancer and stem cells. Not only will these characterizations aid in improving in vitro production of embryos, but will also be useful for identifying, or creating conditions for donor cells that will be more likely to result in normal development of cloned embryos. The easily found targets have been identified, and now more sophisticated methods are being employed to advance our understanding of embryogenesis. Here the technology to study the global transcriptome is reviewed followed by specific examples of how the technology has been used to understand and improve porcine embryogenesis both in vitro and in vivo.

Fate of D3 mouse embryonic stem cells exposed to X-rays or carbon ions

The risk of radiation exposure during embryonic development is still a major problem in radiotoxicology. In this study we investigated the response of the murine embryonic stem cell (mESC) line D3 to two radiation qualities: sparsely ionizing X-rays and densely ionizing carbon ions. We analyzed clonogenic cell survival, proliferation, induction of chromosome aberrations as well as the capability of cells to differentiate to beating cardiomyocytes up to 3 days after exposure. Our results show that, for all endpoints investigated, carbon ions are more effective than X-rays at the same radiation dose. Additionally, in long term studies (≥8 days post-irradiation) chromosomal damage and the pluripotency state were investigated. These studies reveal that pluripotency markers are present in the progeny of cells surviving the exposure to both radiation types. However, only in the progeny of X-ray exposed cells the aberration frequency was comparable to that of the control population, while the progeny of carbon ion irradiated cells harbored significantly more aberrations than the control, generally translocations. We conclude that cells surviving the radiation exposure maintain pluripotency but may carry stable chromosomal rearrangements after densely ionizing radiation.

In-vitro model systems for the study of human embryo-endometrium interactions

Implantation requires highly orchestrated interactions between the developing embryo and maternal endometrium. The association between abnormal implantation and reproductive failure is evident, both in normal pregnancy and in assisted reproduction patients. Failure of implantation is the pregnancy rate-limiting step in assisted reproduction, but, as yet, empirical interventions have largely failed to address this problem. Better understanding of the mechanisms underlying human embryo-endometrium signalling is a prerequisite for the further improvement of assisted reproduction outcomes and the development of effective interventions to prevent early pregnancy loss. Studying human embryo implantation is challenging since in-vivo experiments are impractical and unethical, and studies in animal models do not always translate well to humans. However, in recent years in-vitro models have been shown to provide a promising way forward. This review discusses the principal models used to study early human embryo development and initial stages of implantation in vitro. While each model has limitations, exploiting these models will improve understanding of the molecular mechanisms and embryo-endometrium cross-talk at the early implantation site. They provide valuable tools to study early embryo development and pathophysiology of reproductive disorders and have revealed novel disease mechanisms such as the role of epigenetic modifications in recurrent miscarriage.

Transcriptomic signature to oxidative stress exposure at the time of embryonic genome activation in bovine blastocysts

In order to understand how in vitro culture affects embryonic quality, we analyzed survival and global gene expression in bovine blastocysts after exposure to increased oxidative stress conditions. Two pro-oxidant agents, one that acts extracellularly by promoting reactive oxygen species (ROS) production (0.01 mM 2,2'-azobis (2-amidinopropane) dihydrochloride [AAPH]) or another that acts intracellularly by inhibiting glutathione synthesis (0.4 mM buthionine sulfoximine [BSO]) were added separately to in vitro culture media from Day 3 (8-16-cell stage) onward. Transcriptomic analysis was then performed on resulting Day-7 blastocysts. In the literature, these two pro-oxidant conditions were shown to induce delayed degeneration in a proportion of Day-8 blastocysts. In our experiment, no morphological difference was visible, but AAPH tended to decrease the blastocyst rate while BSO significantly reduced it, indicating a differential impact on the surviving population. At the transcriptomic level, blastocysts that survived either pro-oxidant exposure showed oxidative stress and an inflammatory response (ARRB2), although AAPH induced higher disturbances in cellular homeostasis (SERPINE1). Functional genomics of the BSO profile, however, identified differential expression of genes related to glycine metabolism and energy metabolism (TPI1). These differential features might be indicative of pre-degenerative blastocysts (IGFBP7) in the AAPH population whereas BSO exposure would select the most viable individuals (TKDP1). Together, these results illustrate how oxidative disruption of pre-attachment development is associated with systematic up-regulation of several metabolic markers. Moreover, it indicates that a better capacity to survive anti-oxidant depletion may allow for the survival of blastocysts with a quieter metabolism after compaction.

Systems genetics implicates cytoskeletal genes in oocyte control of cloned embryo quality

Cloning by somatic cell nuclear transfer is an important technology, but remains limited due to poor rates of success. Identifying genes supporting clone development would enhance our understanding of basic embryology, improve applications of the technology, support greater understanding of establishing pluripotent stem cells, and provide new insight into clinically important determinants of oocyte quality. For the first time, a systems genetics approach was taken to discover genes contributing to the ability of an oocyte to support early cloned embryo development. This identified a primary locus on mouse chromosome 17 and potential loci on chromosomes 1 and 4. A combination of oocyte transcriptome profiling data, expression correlation analysis, and functional and network analyses yielded a short list of likely candidate genes in two categories. The major category-including two genes with the strongest genetic associations with the traits (Epb4.1l3 and Dlgap1)-encodes proteins associated with the subcortical cytoskeleton and other cytoskeletal elements such as the spindle. The second category encodes chromatin and transcription regulators (Runx1t1, Smchd1, and Chd7). Smchd1 promotes X chromosome inactivation, whereas Chd7 regulates expression of pluripotency genes. Runx1t1 has not been associated with these processes, but acts as a transcriptional repressor. The finding that cytoskeleton-associated proteins may be key determinants of early clone development highlights potential roles for cytoplasmic components of the oocyte in supporting nuclear reprogramming. The transcriptional regulators identified may contribute to the overall process as downstream effectors.

Physiological and molecular determinants of embryo implantation

Embryo implantation involves the intimate interaction between an implantation-competent blastocyst and a receptive uterus, which occurs in a limited time period known as the window of implantation. Emerging evidence shows that defects originating during embryo implantation induce ripple effects with adverse consequences on later gestation events, highlighting the significance of this event for pregnancy success. Although a multitude of cellular events and molecular pathways involved in embryo-uterine crosstalk during implantation have been identified through gene expression studies and genetically engineered mouse models, a comprehensive understanding of the nature of embryo implantation is still missing. This review focuses on recent progress with particular attention to physiological and molecular determinants of blastocyst activation, uterine receptivity, blastocyst attachment and uterine decidualization. A better understanding of underlying mechanisms governing embryo implantation should generate new strategies to rectify implantation failure and improve pregnancy rates in women.

Investigation of gene expression profiles before and after embryonic genome activation and assessment of functional pathways at the human metaphase II oocyte and blastocyst stage

OBJECTIVE

To compare the oocyte versus the blastocyst transcriptome and provide data on molecular pathways before and after embryonic genome activation.

DESIGN

Prospective laboratory research study.

SETTING

An IVF clinic and a specialist preimplantation genetics laboratory.

PATIENT(S)

Couples undergoing or having completed IVF treatment donating surplus oocytes or cryopreserved blastocysts after patient consent.

INTERVENTION(S)

Sets of pooled metaphase II (MII) oocytes or blastocysts were processed for RNA extraction, RNA amplification, and analysis with the use of the Human Genome Survey Microarrays v2.0 (Applied Biosystems).

MAIN OUTCOME MEASURE(S)

Association of cell type and gene expression profile.

RESULT(S)

Totals of 1,909 and 3,122 genes were uniquely expressed in human MII oocytes and human blastocysts respectively, and 4,910 genes were differentially expressed between the two sample types. Expression levels of 560 housekeeping genes, genes involved in the microRNA processing pathway, as well as hormones and hormone receptors were also investigated.

CONCLUSION(S)

The lists of genes identified may be of use for understanding the processes involved in early embryo development and blastocyst implantation, and for identifying any dysregulation leading to infertility.

Comparison of gene expression in fresh and frozen–thawed human preimplantation embryos

Identification and characterisation of differentially regulated genes in preimplantation human embryonic development are required to improve embryo quality and pregnancy rates in IVF. In this study, we examined expression of a number of genes known to be critical for early development and compared expression profiles in individual preimplantation human embryos to establish any differences in gene expression in fresh compared to frozen–thawed embryos used routinely in IVF. We analysed expression of 19 genes by cDNA amplification followed by quantitative real-time PCR in a panel of 44 fresh and frozen–thawed human preimplantation embryos. Fresh embryos were obtained from surplus early cleavage stage embryos and frozen–thawed embryos from cryopreserved 2PN embryos. Our aim was to determine differences in gene expression between fresh and frozen–thawed human embryos, but we also identified differences in developmental expression patterns for particular genes. We show that overall gene expression among embryos of the same stage is highly variable and our results indicate that expression levels between groups did differ and differences in expression of individual genes was detected. Our results show that gene expression from frozen–thawed embryos is more consistent when compared with fresh, suggesting that cryopreserved embryos may represent a reliable source for studying the molecular events underpinning early human embryo development.