Messenger RNA expression patterns in bovine embryos derived from in vitro procedures and their implications for development

Gene expression profile differences in embryos derived from prepubertal and adult Japanese Black cattle during in vitro development

The present study was carried out to compare the gene expression profiles of in vitro-generated embryos derived from adult and prepubertal Japanese Black cattle oocytes using GeneChip Bovine Genome Array (containing 24072 probe sets representing over 23000 transcripts). Microarray experiments were performed on populations of 8- to 16-cell stage embryos and blastocysts derived from adult (24-35 months old) versus prepubertal (9-10 months old) Japanese Black cattle oocytes matured and fertilised in vitro. In total, 591 (2.4%) and 490 (2.0%) genes were differentially expressed in prepubertal and adult bovine in 8- to 16-cell and blastocyst stage embryos, respectively. Out of these, 218 and 248 genes were upregulated, while 373 and 242 were downregulated in prepubertal and adult 8- to 16-cell and blastocysts stage embryos, respectively. Gene ontology classification regarding biological process, molecular functions and cellular component revealed diversity in transcript abundances between prepubertal and adult groups in both the distinct developmental stages. Quantitative reverse transcription-PCR validated the expression differences of some selected transcripts as identified by microarray analysis. To our knowledge, this is the first report indicating the significant number of genes differentially expression (>2-fold, P<0.01) in preimplantition embryos between adult and prepubertal Japanese Black cattle during in vitro development.

Epigenetic status and full-term development of bovine cloned embryos treated with trichostatin A

We examined the comprehensive epigenetic status, including histone H3 and H4 acetylation, DNA methylation and level of mRNA transcripts of bovine somatic cell nuclear transfer (SCNT) embryos treated with trichostatin A (TSA), along with their full-term developmental efficacy. Treatment with 50 nM TSA enhanced early developmental competence; increased acetylation of two histones, H3K9K14 and H4K8, at the blastocyst stage; and maintained the DNA methylation status of the satelliteI sequence in bovine SCNT embryos. The difference in IGFBP-3 transcript levels between in vivo and SCNT embryos disappeared in SCNT embryos after treatment with 50 nM TSA. Pregnancy, full-term developmental competence and body weight at birth of offspring did not differ between SCNT embryos treated with 50 nM TSA and untreated embryos. These results suggest that treatment with TSA improves preimplantation development and changes the epigenetic status but does not promote the full-term development competence in bovine SCNT embryos.

New challenges in the analysis of gene transcription in bovine blastocysts

In the last years, enormous progress has been made in the analysis of gene transcription at the blastocyst stage. The study of gene expression at this early stage of development is challenging because of the very small amount of starting material, which limits the use of traditional mRNA analysis approaches such as Northern blot. Another problem is the difficulty for data normalization, particularly the identification of the best housekeeping gene with the lowest fluctuation under different developmental conditions. Moreover, the transcriptional analysis of embryo biopsies or individual embryos needs to take into consideration that the blastocyst is a transitional stage of development, which is composed of three different types of cells (trophoblast, epiblast and primitive ectoderm) with different patterns of gene expression, and that there are large differences between male and female blastocysts. In this review, we analyse the different specific and sensitive tools available to compare mRNA expression levels of specific genes at the blastocyst stage, and how the protocol and the analytical method used can influence the results dramatically. Finally, we describe future research challenges to identify candidate genes related to developmental competence of bovine blastocysts, not only in terms of pregnancy rates but also in relation to adverse long-term consequences in the adult animal.

Exploring the application of high-throughput genomics technologies in the field of maternal-embryo communication

Deciphering the complex molecular dialogue between the maternal tract and embryo is crucial to increasing our understanding of pregnancy failure, infertility problems and in the modulation of embryo development, which has consequences through adulthood. High-throughput genomic technologies have been applied to look for a holistic view of the molecular interactions occurring during this dialogue. Among these technologies, microarrays have been widely used, being one of the most popular tools in maternal-embryo communication. Today, next generation sequencing technologies are dwarfing the capabilities of microarrays. The application of these new technologies has broadened to almost all areas of genomics research, because of their massive sequencing capacity. We review the current status of high-throughput genomic technologies and their application to maternal-embryo communication research. We also survey next generation technologies and their huge potential in many research areas. Given the diversity of unanswered questions in the field of maternal-embryo communication and the wide range of possibilities that these technologies offer, here we discuss future perspectives on the use of these technologies to enhance maternal-embryo research.

Expression of apoptotic genes in immature and in vitro matured equine oocytes and cumulus cells

The gene expression of Bax, Bcl-2, survivin and p53, following in vitro maturation of equine oocytes, was compared in morphologically distinct oocytes and cumulus cells. Cumulus-oocyte complexes (COC) were harvested and divided into two groups: G1 - morphologically healthy cells; and G2 - less viable cells or cells with some degree of atresia. Total RNA was isolated from both immature and in vitro matured COC and real-time reverse transcription polymerase chain reaction (qRT-PCR) was used to quantify gene expression. Our results showed there was significantly higher expression of survivin (P < 0.05) and lower expression of p53 (P < 0.01) in oocytes compared with cumulus cells in G1. No significant difference in gene expression was observed following in vitro maturation or in COC derived from G1 and G2. However, expression of the Bax gene was significantly higher in cumulus cells from G1 (P < 0.02).

Components of oviduct physiology in eutherian mammals

Recalling the evolutionary sequence of development first of gonad and subsequently of oviducts, ovarian endocrine regulation of all known components of oviduct physiology is reviewed. Ovaries not only influence oviducts via the systemic blood circulation, but also locally by counter-current transfer of relatively high concentrations of steroid hormones and prostaglandins between the ovarian vein and oviduct branch of the ovarian artery. The efficiency and impact of such counter-current transfer is greatest around the time of ovulation, the transfer process receiving further inputs from hormones present in peritoneal fluid. Classical oviduct physiology is summarised, and the potential molecular consequences of temperature gradients within the duct lumen examined. At ovulation, an oocyte-cumulus complex is displaced in minutes from the follicular surface to the site of fertilisation at the ampullary-isthmic junction of the oviduct. This rapid initial phase is contrasted with the subsequent slow progression of embryos to the uterus in days, still encompassed within a zona pellucida. Regarding transport of spermatozoa, the formation of a pre-ovulatory reservoir in the caudal portion of the oviduct isthmus is noted, with suppression of motility and sperm-head binding to epithelial organelles acting to maintain fertilising ability. Completion of capacitation is prompted shortly before ovulation, predominantly by Ca(2+) influx into bound spermatozoa. A controlled release of spermatozoa coupled with their hyperactivation results in initial sperm:egg ratios at the site of fertilisation close to unity, thereby avoiding the pathological condition of polyspermy. Both the oviduct milieu and embryonic development are influenced by paracrine activity of follicular granulosa cells released at ovulation and remaining in suspension in the vicinity of the oocyte or embryo. These cells may amplify early pregnancy signals from a zygote to the endosalpinx. Beneficial effects of the oviduct on domestic animal embryos are contrasted with anomalies arising as a consequence of in vitro culture. Primate embryos do not require exposure to an oviduct for normal development, perhaps due to overlapping compositions of endosalpingeal and endometrial secretions. Additionally, primate endometrial secretions may be modified by viable gametes or an embryo in the presence of a cumulus cell suspension.

Cell arrest and cell death in mammalian preimplantation development: lessons from the bovine model

BACKGROUND

The causes, modes, biological role and prospective significance of cell death in preimplantation development in humans and other mammals are still poorly understood. Early bovine embryos represent a very attractive experimental model for the investigation of this fundamental and important issue.

METHODS AND FINDINGS

To obtain reference data on the temporal and spatial occurrence of cell death in early bovine embryogenesis, three-dimensionally preserved embryos of different ages and stages of development up to hatched blastocysts were examined in toto by confocal laser scanning microscopy. In parallel, transcript abundance profiles for selected apoptosis-related genes were analyzed by real-time reverse transcriptase-polymerase chain reaction. Our study documents that in vitro as well as in vivo, the first four cleavage cycles are prone to a high failure rate including different types of permanent cell cycle arrest and subsequent non-apoptotic blastomere death. In vitro produced and in vivo derived blastocysts showed a significant incidence of cell death in the inner cell mass (ICM), but only in part with morphological features of apoptosis. Importantly, transcripts for CASP3, CASP9, CASP8 and FAS/FASLG were not detectable or found at very low abundances.

CONCLUSIONS

In vitro and in vivo, errors and failures of the first and the next three cleavage divisions frequently cause immediate embryo death or lead to aberrant subsequent development, and are the main source of developmental heterogeneity. A substantial occurrence of cell death in the ICM even in fast developing blastocysts strongly suggests a regular developmentally controlled elimination of cells, while the nature and mechanisms of ICM cell death are unclear. Morphological findings as well as transcript levels measured for important apoptosis-related genes are in conflict with the view that classical caspase-mediated apoptosis is the major cause of cell death in early bovine development.

Transcriptome changes at the initiation of elongation in the bovine conceptus

The majority of embryonic loss in cattle occurs before maternal recognition of pregnancy, at around Day 16 postconception. The origin of the embryo can have a significant impact on the dynamics of embryo mortality. The aim of this study was to examine the temporal changes in transcriptional profile as the embryo develops from a spherical blastocyst on Day 7 to an ovoid conceptus at the initiation of elongation on Day 13 and to highlight differences in these temporal gene expression dynamics between in vivo- and in vitro-derived blastocysts that may be associated with embryonic survival/mortality using the bovine Affymetrix microarray. All embryos were produced either in vitro by in vitro fertilization or in vivo by superovulation. A proportion of Day 7 blastocysts were snap frozen, and the remainder were transferred (n = 10 per recipient) to synchronized heifers, recovered on Day 13, and snap frozen individually. Three pools of Day 7 blastocysts (n = 25 per pool) and Day 13 conceptuses (n = 5 per pool) were used for microarray analysis. In Day 7 blastocysts, 50 genes were found to be differentially expressed (P < 0.05), of which 19 were up-regulated and 31 down-regulated in the in vivo compared to in vitro embryos. In Day 13 conceptuses, 288 genes were found to be differentially expressed (P < 0.05), of which 133 were up-regulated and 155 down-regulated in the in vivo compared to in vitro embryos. The comparison between Day 7 and Day 13 embryos revealed significant temporal changes in transcript profile with 1806 and 909 transcripts differentially expressed in the in vitro- and in vivo-derived embryos, respectively. Across the three array comparisons between Day 7 and Day 13 embryos, 444 genes were consistently exclusively present in the in vivo embryos, whereas 1341 were exclusively present in the in vitro embryos. Regardless of the origin of the embryo, 465 differentially expressed genes between Day 7 and 13 were common to both in vivo- and in vitro-derived embryos; these genes are likely critical for the transition between the blastocyst (Day 7) and ovoid conceptus (Day 13) stages of embryo development. In order to validate the microarray findings, differences in the expression of six genes (CYP51A1, FADS1, TDGF1, HABP2, APOA2, and SLC12A2) were confirmed by quantitative real-time PCR on in vivo- and in vitro-derived embryos on Day 7 and Day 13 using independent samples from those used for the microarray. Subsequent mapping of these differentially expressed genes into relevant functional groups and pathways identified important pathways involved in conceptus elongation in cattle. In conclusion, this analysis has identified genes and pathways crucial for the transition from a spherical blastocyst to an ovoid conceptus as well as those uniquely associated with a greater likelihood of embryonic survival (those unique to in vivo embryos) or loss (those unique to in vitro embryos).

Epigenetic profile of developmentally important genes in bovine oocytes

Assisted reproductive technologies are associated with an increased incidence of epigenetic aberrations, specifically in imprinted genes. Here, we used the bovine oocyte as a model to determine putative epigenetic mutations at three imprinted gene loci caused by the type of maturation, either in vitro maturation (IVM) in Tissue Culture Medium 199 (TCM) or modified synthetic oviduct fluid (mSOF) medium, or in vivo maturation. We applied a limiting dilution approach and direct bisulfite sequencing to analyze the methylation profiles of individual alleles (DNA molecules) for H19/IGF2, PEG3, and SNRPN, which are each associated with imprinting defects in humans and/or the mouse model, and are known to be differentially methylated in bovine embryos. Altogether, we obtained the methylation patterns of 203 alleles containing 4,512 CpG sites from immature oocytes, 213 alleles with 4,779 CpG sites from TCM-matured oocytes, 215 alleles/4,725 CpGs in mSOF-matured oocytes, and 78 alleles/1,672 CpGs from in vivo-matured oocytes. The total rate of individual CpGs and entire allele methylation errors did not differ significantly between the two IVM and the in vivo group, indicating that current IVM protocols have no or only marginal effects on these critical epigenetic marks. Furthermore, the mRNA expression profiles of the three imprinted genes and a panel of eight other genes indicative of oocyte competence were determined by quantitative real-time PCR. We found different mRNA expression profiles between in vivo-matured oocytes versus their in vitro-matured counterparts, suggesting an influence on regulatory mechanisms other than DNA methylation.

DNA methylation status of bovine blastocyst embryos obtained from various procedures

DNA methylation is an important factor for the regulation of gene expression in early embryos. It is well known that the satellite I sequence is more heavily methylated in bovine somatic cell nuclear transfer (NT-SC) embryos than in embryos derived from in vitro fertilization (IVF). However, the methylation status of bovine embryos obtained by other procedures is not well known. To clarify DNA methylation levels of bovine embryos obtained from various procedures, we examined satellite I sequences in bovine blastocyst (BC) embryos derived from NT-SC, NT using embryonic blastomeres (NT-EM), in vivo (Vivo), IVF and parthenogenetic treatment (PA). Furthermore, in order to evaluate the efficacy of DNA demethylation by the NT procedure, we determined the DNA methylation levels in bovine embryos in which NT was recapitulated (Re-NT). Although the DNA methylation levels in the NT-SC embryos were higher than those in the other embryos, the NT-EM embryos exhibited lower DNA methylation levels. The satellite I sequence in the NT-SC embryos was more demethylated than that in the donor cells. Although the DNA methylation level in the individual NT-SC embryos showed variation, the full-term developmental efficacy of these embryos were not different. These findings suggest that the methylation level of the satellite I sequence at the BC stage is not related to the abnormalities of bovine embryos produced by NT-SC. There was no difference in methylation levels between Re-NT and NT-SC embryos. Our results indicated that the DNA methylation status differed among embryos produced by various methods and that at least some of the demethylation of the donor cell genome occurred in the recipient cytoplast after NT-SC, but the demethylation ability of the NT procedure was noted in the first NT but not in the second NT.

Female fertility in French dairy breeds: current situation and strategies for improvement

In most countries where genetic selection for milk production has been conducted, a continuous degradation of reproductive performances has been observed in dairy cattle. This paper describes the phenotypic evolution of fertility traits in France for the main 3 dairy breeds and the different actions undertaken to counteract this unfavourable evolution of reproductive performance. In 2001, functional traits (including conception rate as a fertility trait) have been included into the French total merit index ISU, to take these traits into account in the selection objective. In addition, a marker-assisted selection (MAS) program has been developed since 2001. Through this approach, a set of QTL related to fertility traits were identified using the phenotypic information for fertility collected on a national basis. Fine mapping of these QTL let to their introduction in 2008 in genomic evaluation and the routine selection for fertility traits in breeding schemes. This approach will evolve in the near future through the inclusion of results of several genomic research programs that were conducted either in station or under field conditions aiming at identifying genes and pathways controlling cattle fertility (based on genomics, transcriptomics and proteomics) and at enhancing phenotyping for reproductive performance. The paper describes the general approach behind these evolutions, the expected benefits of genomic selection and first results. Information on the potential use of reproductive technologies that may improve the efficiency of multiple trait genomic selection are also presented.

DNA methylation patterns reflect epigenetic reprogramming in bovine embryos

To understand the epigenetic alterations associated with assisted reproduction technology (ART) and the reprogramming of gene expression that follows somatic cell nuclear transfer (SCNT), we screened a panel of 41 amplicons representing 25 developmentally important genes on 15 different chromosomes (a total of 1079 CpG sites). Methylation analysis was performed on DNA from pools of 80 blastocysts representing three classes of embryos. This revealed a subset of amplicons that distinguish between embryos developing in vivo, produced in vitro, or reconstructed by SCNT. Following SCNT, we observed massive epigenetic reprogramming evidenced by reduced levels of methylation in the resultant embryos. Analysis of data from the 28 most informative amplicons (hotspot loci), representing more than 523 individual CpG sites, we discovered subsets of amplicons with methylation patterns that were unique to each class of embryo and may indicate metastable epialleles. Analysis of eight genes with respect to mRNA expression did not reveal a direct correlation with DNA methylation levels. In conclusion, this approach revealed a subset of amplicons that can be used to evaluate blastocyst quality and reprogramming following SCNT, and can also be employed for the localization of the epigenetic control regions within individual genes and for more general studies of stem cell differentiation.

Effect of in vitro culture of human embryos on birthweight of newborns

BACKGROUND

In animal models, in vitro culture of preimplantation embryos has been shown to be a risk factor for abnormal fetal outcome, including high and low birthweight. In the human, mean birthweight of singletons after in vitro fertilization (IVF) is considerably lower than after natural conception, but it is not known whether culture conditions play a role in this.

METHODS

We compared pregnancy rates and perinatal outcomes from singleton pregnancies resulting from a total of 826 first IVF treatment cycles in which oocytes and embryos were randomly allocated to culture in either of two commercially available sequential media systems.

RESULTS

When the 110 live born singletons in the Vitrolife group were compared with the 78 singletons in the Cook group, birthweight +/- SEM (3453 +/- 53 versus 3208 +/- 61 g, P = 0.003), and birthweight adjusted for gestational age and gender (mean z-score +/- SEM: 0.13 +/- 0.09 versus -0.31 +/- 0.10, P = 0.001) were both significantly higher in the Vitrolife group. When analyzed by multiple linear regression together with several other variables that could possibly affect birthweight as covariates, the type of culture medium was significantly (P = 0.01) associated with birthweight.

CONCLUSIONS

In vitro culture of human embryos can affect birthweight of live born singletons.

Embryo culture: can we perform better than nature?

Culture of preimplantation-stage embryos has always been a key element of laboratory embryology and has contributed substantially to the success of many assisted reproduction procedures. During the past decade, its importance has increased as extended in-vitro embryo culture and single blastocyst transfer have become indispensable parts of the approach to decreasing the chance of multiple pregnancy while preserving the overall efficiency of the treatment. However, in spite of the scientific and commercial challenge stimulating research worldwide to optimize embryo culture conditions, a consensus is missing even in the basic principles, including composition and exchange of media, the required physical and biological environment and even the temperature of incubation. This review attempts to summarize the controversies, demonstrate the fragility of some widely accepted dogmas and generate an open-minded debate towards rapid and efficient optimization. New approaches expanding the traditional frames of mammalian embryo culture are also discussed. Although some researchers suppose that the efficiency of the presently applied in-vitro culture systems have already approached the biological limits, authors are confident that substantial improvement may be achieved that may expand considerably the possibilities of future assisted reproduction in humans.

An in vivo culture system for human embryos using an encapsulation technology: a pilot study

BACKGROUND

Animal studies have demonstrated better embryo development in vivo than in vitro. This pilot study tested the feasibility of using a novel in utero culture system (IUCS) to obtain normal human fertilization and embryo development.

METHODS

The IUCS device comprised a perforated silicone hollow tube. The study included 13 patients (<36 years) undergoing a first intracytoplasmic sperm injection (ICSI) treatment and 167 metaphase II oocytes in three groups. In Group 1, 1-2 h after ICSI, sibling oocytes were assigned to IUCS or conventional in vitro culture. The device was retrieved on Day 1, and all zygotes were cultured in vitro till Day 5. In Group 2, fertilized oocytes were assigned on Day 1, embryos retrieved on Day 3 and all embryos cultured till Day 5. In Group 3, after Day 0 assignment, embryos were retrieved on Day 3 for blastomere biopsy and fluorescence in situ hybridization (FISH) and cultured until Day 5. The highest quality blastocysts were transferred on Day 5.

RESULTS

Fertilization and embryo development were comparable in the in vitro and IUCS arms, with a tendency towards better embryo quality in the IUCS. FISH analysis in Group 3 revealed more normal embryos using the IUCS (P = 0.049). Three clinical pregnancies and live births were obtained: two from the IUCS arm and one from the in vitro arm.

CONCLUSIONS

Our pilot study shows that this new IUCS appears to be feasible and safe, supporting normal fertilization, embryo development and normal chromosomal segregation. Furthermore, live births are possible after the transient presence of a silicone device in the uterus. Clinicaltrials.gov: NCT00480103.

Consequences of in vitro culture conditions on embryo development and quality

Despite major efforts directed at improving the yield of blastocysts from immature oocytes in vitro, the quality of such blastocysts continually lags behind that of blastocysts produced in vivo. These differences are manifested at the level of morphology, metabolism, gene expression and cryotolerance, and may have a knock-on effect further along the developmental axis. Evidence suggesting that in vitro culture conditions, while capable of producing blastocysts in relatively high numbers, are far from optimal with deficiencies being manifested in terms of abnormally large offspring. It is clear nowadays that modification of the post-fertilization culture environment in vitro can improve blastocyst quality to some extent.

Embryonic gene expression profiling using microarray analysis

Microarray technology enables the interrogation of thousands of genes at one time and therefore a systems level of analysis. Recent advances in the amplification of RNA, genome sequencing and annotation, and the lower cost of developing microarrays or purchasing them commercially, have facilitated the analysis of single preimplantation embryos. The present review discusses the components of embryonic expression profiling and examines current research that has used microarrays to study the effects of in vitro production and nuclear transfer.

Developmental capabilities of embryos produced in vitro from prepubertal lamb oocytes

Breeding from prepubertal females, known as juvenile in vitro embryo transfer (JIVET), reduces the generation interval and increases the rate of genetic gain in animal breeding programs. While the birth of the first lambs from prepubertal ewes occurred nearly 30 years ago; and there is considerable interest in the commercialization of this technology, its efficiency remains too low. The advent of in vitro production (IVP) of embryo resulted in the more widespread use of JIVET. Morphologic and metabolic differences coupled with reduced in vitro and in vivo development of oocytes derived from prepubertal animals have been reported. Research has been undertaken to optimize donor selection and hormone stimulation methods in an attempt to reduce the variability and increase the proportion of donors responding to hormone stimulation and increase oocyte developmental competence. Yet, this variation persists and the development of oocytes and embryos from prepubertal animals remains reduced when compared with adults. Recent improvements to JIVET, resulting from a modified hormone stimulation regime, have eliminated the failure of donors to respond to hormone stimulation, and increased both the number and developmental competence of oocytes harvested from very young prepubertal lambs. This increased efficiency has facilitated the incorporation of other reproductive technologies such as sperm sexing with JIVET, resulting in the birth of lambs of a pre-determined sex from prepubertal lambs. Increased rates of genetic gain in sheep breeding programs can be achieved by combining sexed sperm with oocytes obtained from lambs as young as 3-4 weeks of age. Continued increases in the efficiency of JIVET resulting from further improvements to hormone stimulation regimes and an increased understanding of the differences between oocytes from adult and prepubertal animals will result in the commercialization of this technology.

Genome-wide expression profiling reveals distinct clusters of transcriptional regulation during bovine preimplantation development in vivo

Bovine embryos can be generated by in vitro fertilization or somatic nuclear transfer; however, these differ from their in vivo counterparts in many aspects and exhibit a higher proportion of developmental abnormalities. Here, we determined for the first time the transcriptomes of bovine metaphase II oocytes and all stages of preimplantation embryos developing in vivo up to the blastocyst using the Affymetrix GeneChip Bovine Genome Array which examines approximately 23,000 transcripts. The data show that bovine oocytes and embryos transcribed a significantly higher number of genes than somatic cells. Several hundred genes were transcribed well before the 8-cell stage, at which the major activation of the bovine genome expression occurs. Importantly, stage-specific expression patterns in 2-cell, 4-cell, and 8-cell stages, and in morulae and blastocysts, were detected, indicating dynamic changes in the embryonic transcriptome and in groups of transiently active genes. Pathway analysis revealed >120 biochemical pathways that are operative in early preimplantation bovine development. Significant differences were observed between the mRNA expression profiles of in vivo and in vitro matured oocytes, highlighting the need to include in vivo derived oocytes/embryos in studies evaluating assisted reproductive techniques. This study provides the first comprehensive analysis of gene expression and transcriptome dynamics of in vivo developing bovine embryos and will serve as a basis for improving assisted reproductive technology.

[Parental imprinting related to Assisted Reproductive Technologies]

Until the introduction of Assisted Reproductive Technologies (ART), many studies were conducted in order to evaluate their impact upon the children's health born in such a way. The epigenetic-risk notion was invoked and a link between ART and diseases associated with imprinting alterations was suggested with different examples, such as Beckwith-Wiedemann syndrome (BWS), Angelman syndrome (AS) and Silver-Russell syndrome (SRS). The epigenetic "life cycle" of imprinting (germline erasure, germline establishment, and somatic maintenance) concerns all the phases from gametogenesis, gamete maturation, fertilization, to early embryo development and appears particularly vulnerable to perturbations induced by superovulation, in vitro fertilization, embryo culture and embryo transfer. The studies, performed in model animal, provide a basis of the understanding of imprinting alterations induced by the ART and clinically useful information in order to improve the ART.

Preimplantation embryo programming: transcription, epigenetics, and culture environment

Preimplantation development directs the formation of an implantation- or attachment-competent embryo so that metabolic interactions with the uterus can occur, pregnancy can be initiated, and fetal development can be sustained. The preimplantation embryo exhibits a form of autonomous development fueled by products provided by the oocyte and also from activation of the embryo's genome. Despite this autonomy, the preimplantation embryo is highly influenced by factors in the external environment and in extreme situations, such as those presented by embryo culture or nuclear transfer, the ability of the embryo to adapt to the changing environmental conditions or chromatin to become reprogrammed can exceed its own adaptive capacity, resulting in aberrant embryonic development. Nuclear transfer or embryo culture-induced influences not only affect implantation and establishment of pregnancy but also can extend to fetal and postnatal development and affect susceptibility to disease in later life. It is therefore critical to define the basic program controlling preimplantation development, and also to utilize nuclear transfer and embryo culture models so that we may design healthier environments for preimplantation embryos to thrive in and also minimize the potential for negative consequences during pregnancy and post-gestational life. In addition, it is necessary to couple gene expression analysis with the investigation of gene function so that effects on gene expression can be fully understood. The purpose of this short review is to highlight our knowledge of the mechanisms controlling preimplantation development and report how those mechanisms may be influenced by nuclear transfer and embryo culture.

Biological interpretations of transcriptomic profiles in mammalian oocytes and embryos

The characterization of gene-expression profiles in oocytes and embryos is critical to understand the influence of genetic and environmental factors on preimplantation and fetal development. Numerous gene-expression microarray studies using different platforms and species are offering insights into the biological processes extensively represented among the genes exhibiting differential expression. Major advances on understanding the direct relationship between gene expression and developmental competence are being reported. Integration of information across studies using meta-analysis techniques can increase the precision and accuracy to identify expression profiles associated with embryo development. Gene network and pathway analyses are offering insights into gene interactions and expression profiles of embryos. All these advances are cementing the way toward a comparative and systems approach to understanding the complex processes underlying vertebrate development.

Transmission ratio distortion at the growth hormone gene (GH1) in bovine preimplantation embryos: An in vitro culture-induced phenomenon?

The growth hormone gene (GH1) and its polypeptide product (GH) have a crucial role in reproduction, embryogenesis and general development. A polymorphism present in the fifth exon of the bovine GH1 gene (GH1 p.Leu127Val) has been associated with GH release and milk production in cattle. The objective of the present study was to examine the genotype frequencies of the GH1 p.Leu127Val polymorphism in bovine blastocysts produced in vitro and in vivo to determine if allelic variation of the GH1 gene affects embryo development and survival. A heterozygous (p.Leu127/Val127) sire was used for in vitro fertilization of oocytes of unknown maternal genotype (n = 104) and known maternal genotype (n = 115). PCR amplification and genotyping of the GH1 gene from Day 8 blastocysts derived from these fertilized oocytes demonstrated that there was significant over-representation from the expected Mendelian ratio of GH1 p.Leu127/Leu127 homozygotes from oocytes of known maternal genotype (P = 0.006). Contrary to this, analysis of in vivo-produced bovine blastocysts of known parental GH1 genotype (n = 69) did not reveal an overrepresentation of GH1 p.Leu127/Leu127 homozygotes. These results suggest that developing in vitro-produced embryos are exposed to a selection process, probably due to a less favorable culture environment, that acts to increase the number of GH1 p.Leu127/Leu127 homozygotes, thereby giving rise to the observed transmission ratio distortion (TRD) of GH1 genotypes when compared to in vivo produced embryos.

Bovine oocytes and early embryos express Staufen and ELAVL RNA-binding proteins

RNA-binding proteins (RBP) influence RNA editing, localization, stability and translation and may contribute to oocyte developmental competence by regulating the stability and turnover of oogenetic mRNAs. The expression of Staufen 1 and 2 and ELAVL1, ELAVL2 RNA-binding proteins during cow early development was characterized. Cumulus–oocyte complexes were collected from slaughterhouse ovaries, matured, inseminated and subjected to embryo culturein vitro. Oocyte or preimplantation embryo pools were processed for RT-PCR and whole-mount immunofluorescence analysis of mRNA expression and protein distribution. STAU1 and STAU2 and ELAVL1 mRNAs and proteins were detected throughout cow preimplantation development from the germinal vesicle (GV) oocyte to the blastocyst stage. ELAVL2 mRNAs were detectable from the GV to the morula stage, whereas ELAVL2 protein was in all stages examined and localized to both cytoplasm and nuclei. The findings provide a foundation for investigating the role of RBPs during mammalian oocyte maturation and early embryogenesis.

Effects of follicle size and stages of maturation on mRNA expression in bovine in vitro matured oocytes

Transcription in bovine oocytes: The goal of this study was to unravel the dynamics of transcripts thought to be critically involved in oocyte maturation. The relative abundance (RA) of DYNLL1 (cytoplasmic dynein light chain LC8), DYNC1I1 (cytoplasmic dynein 1 intermediate chain), DCTN1 (dynactin 1; pGlued homolog, the activator of the cytoplasmic dynein complex 1), PMSB1 (proteasome beta subunit 1), PMSA4 (proteasome alfa subunit 4), PAP (poly-A polymerase) and Cx43 (connexin 43) were determined by semi-quantitative endpoint RT-PCR at different stages of IVM, that is, GV, GVBD, MI and MII in oocytes collected from follicles of two different size categories, that is, <2 mm and 2-8 mm. The RA of DYNLL1 and DYNC1I1 were significantly higher in immature oocytes from bigger follicles than in oocytes from small follicles. Messenger RNA expression levels were similar for DCTN1, PMSB1, PMSA4, PAP, and Cx43 in the two groups during the maturation process. RA of DYNLL1, DYNC1I1 and PMSB1 decreased significantly during IVM in oocytes from follicles 2 to 8 mm. The RA for DYNLL1 was significantly higher in GVBD and MI in the oocytes from follicles 2 to 8 mm in size compared to the other group. The higher mRNA expression of DYNLL1 and DYNC1I1 and the diverging dynamics of DYNLL1, DYNC1I1, and PMSB1 mRNA expression during IVM in oocytes from the different follicle categories could be related to the developmental capacity, that is, development to blastocysts after IVF. The differences found between groups of oocytes could serve as a marker to assess the developmental capacity of bovine oocytes.

Epigenetic reprogramming in embryonic and foetal development upon somatic cell nuclear transfer cloning

The birth of ‘Dolly’, the first mammal cloned from an adult donor cell, has sparked a flurry of research activities to improve cloning technology and to understand the underlying mechanism of epigenetic reprogramming of the transferred somatic cell nucleus. Especially in ruminants, somatic cell nuclear transfer (SCNT) is frequently associated with pathological changes in the foetal and placental phenotype and has significant consequences for development both before and after birth. The most critical factor is epigenetic reprogramming of the transferred somatic cell nucleus from its differentiated status into the totipotent state of the early embryo. This involves an erasure of the gene expression program of the respective donor cell and the establishment of the well-orchestrated sequence of expression of an estimated number of 10 000–12 000 genes regulating embryonic and foetal development. The following article reviews the present knowledge on the epigenetic reprogramming of the transferred somatic cell nucleus, with emphasis on DNA methylation, imprinting, X-chromosome inactivation and telomere length restoration in bovine development. Additionally, we briefly discuss other approaches towards epigenetic nuclear reprogramming, including the fusion of somatic and embryonic stem cells and the overexpression of genes crucial in the formation and maintenance of the pluripotent status. Improvements in our understanding of this dramatic epigenetic reprogramming event will be instrumental in realising the great potential of SCNT for basic biological research and for various agricultural and biomedical applications.

Global gene expression analysis of bovine blastocysts produced by multiple methods

Reproductive efficiency using somatic cell nuclear transfer (SCNT) technology remains suboptimal. Of the various efforts to improve the efficiency, chromatin transfer (CT) and clone-clone aggregation (NTagg) have been reported to produce live cloned animals. To better understand the molecular mechanisms of somatic cell reprogramming during SCNT and assess the various SCNT methods on the molecular level, we performed gene expression analysis on bovine blastocysts produced via standard nuclear transfer (NT), CT, NTagg, in vitro fertilization (IVF), and artificial insemination (AI), as well as on somatic donor cells, using bovine genome arrays. The expression profiles of SCNT (NT, CT, NTagg) embryos were compared with IVF and AI embryos as well as donor cells. NT and CT embryos have indistinguishable gene expression patterns. In comparison to IVF or AI embryos, the number of differentially expressed genes in NTagg embryos is significantly higher than in NT and CT embryos. Genes that were differentially expressed between all the SCNT embryos and IVF or AI embryos are identified. Compared to AI embryos, more than half of the genes found deregulated between SCNT and AI embryos appear to be the result of in vitro culture alone. The results indicate that although SCNT methods have altered differentiated somatic nuclei gene expression to more closely resemble that of embryonic nuclei, combination of insufficient reprogramming and in vitro culture condition compromise the developmental potential of SCNT embryos. This is the first set of comprehensive data for analyzing the molecular impact of various nuclear transfer methods on bovine pre-implantation embryos.

The quiet embryo hypothesis: molecular characteristics favoring viability

It has been proposed that the viability of early mammalian embryos is associated with a metabolism that is "quiet" rather than "active" (Leese HJ, 2002:BioEssays 24:845-849). The data on which this hypothesis was based were largely drawn from measurements on the depletion and appearance of amino acids from the culture medium. Data on the de novo synthesis of protein in in vivo- and in vitro-derived bovine embryos, as determined from the flux of radiolabeled methionine, have provided further support of the hypothesis and are interpreted to provide a new set of testable propositions that could illuminate the molecular basis of the quiet metabolism phenotype. The propositions are based on the premise that the extent of DNA damage, and the RNA and protein content of the immature oocyte, are key factors in determining whether the zygote progresses to the blastocyst stage. We propose that stochastic events and environmental stresses determine whether the condition of the genome, transcriptome, and proteome of the zygote will support development. Several molecular components are identified that may determine the viability of a zygote, and we speculate that the cellular response to unfavorable events or excessive DNA damage may be the premature activation of the embryonic genome and of apoptosis.

In vitro-produced bovine embryos: dealing with the warts

Bovine in vitro embryo production is an inefficient process; while maturation and fertilization proceed apparently normally, the proportion of embryos reaching the transferable (blastocyst) stage is rarely over 40% and those that do reach this stage are often compromised in quality and competence. There is considerable evidence of a significant influence of follicular origin on oocyte developmental potential and it appears that once the oocyte is removed from the follicle its developmental capacity is capped. Evidence suggests that while culture conditions during bovine in vitro embryo production can impact somewhat the developmental potential of the early embryo, the intrinsic quality of the oocyte is the key factor determining the proportion of oocytes developing to the blastocyst stage. This paper highlights some of the problems associated with in vitro production of embryos and discusses some of the ways of overcoming these problems.