The capability of reprogramming the male chromatin after fertilization is dependent on the quality of oocyte maturation

Long-Term Effects of ART on the Health of the Offspring

Assisted reproductive technologies (ART) significantly increase the chance of successful pregnancy and live birth in infertile couples. The different procedures for ART, including in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), intrauterine insemination (IUI), and gamete intrafallopian tube transfer (GIFT), are widely used to overcome infertility-related problems. In spite of its inarguable usefulness, concerns about the health consequences of ART-conceived babies have been raised. There are reports about the association of ART with birth defects and health complications, e.g., malignancies, high blood pressure, generalized vascular functional disorders, asthma and metabolic disorders in later life. It has been suggested that hormonal treatment of the mother, and the artificial environment during the manipulation of gametes and embryos may cause genomic and epigenetic alterations and subsequent complications in the health status of ART-conceived babies. In the current study, we aimed to review the possible long-term consequences of different ART procedures on the subsequent health status of ART-conceived offspring, considering the confounding factors that might account for/contribute to the long-term consequences.

A refined culture system of oocytes from early antral follicles promotes oocyte maturation and embryo development in cattle

In brief

The proposed culture system improves the current state of in vitro culture of growing oocytes in the bovine species and allows access to the untapped gamete reserve, thus improving reproductive efficiency.

Abstract

The present study aimed to improve the in vitro culture of bovine oocytes collected from early antral follicles (EAFs) to support the progressive acquisition of meiotic and developmental competence. The rationale that drove the development of such a culture system was to maintain as much as possible the physiological conditions that support the oocyte growth and differentiation in vivo. To this extent, oocytes were cultured for 5 days, which parallels the transition from early to medium antral follicles (MAFs) in the bovine, and supports promoting a 3D-like structure were provided. Additionally, the main hormones (follicle-stimulating hormone, estradiol, progesterone, and testosterone) were added in concentrations similar to the ones previously observed in bovine EAFs. The meiotic arrest was imposed using cilostamide. The cultured cumulus-oocyte complexes (COCs) reached a mean diameter of 113.4 ± 0.75 µm and showed a progressive condensation of the chromatin enclosed in the germinal vesicle (GV), together with a gradual decrease in the global transcriptional activity, measured by 5-ethynyl uridine incorporation. The described morpho-functional changes were accompanied by an increased ability to mature and develop to the blastocyst stage in vitro, although not matching the rates obtained by MAF-retrieved oocytes. The described system improves the current state of in vitro culture of growing oocytes in the bovine species, and it can be used to increase the number of gametes usable for in vitro embryo production in animals of high genetic merit or with specific desirable traits.

Meta-Analysis of Heifer Traits Identified Reproductive Pathways in Bos indicus Cattle

Fertility traits measured early in life define the reproductive potential of heifers. Knowledge of genetics and biology can help devise genomic selection methods to improve heifer fertility. In this study, we used ~2400 Brahman cattle to perform GWAS and multi-trait meta-analysis to determine genomic regions associated with heifer fertility. Heifer traits measured were pregnancy at first mating opportunity (PREG1, a binary trait), first conception score (FCS, score 1 to 3) and rebreeding score (REB, score 1 to 3.5). The heritability estimates were 0.17 (0.03) for PREG1, 0.11 (0.05) for FCS and 0.28 (0.05) for REB. The three traits were highly genetically correlated (0.75-0.83) as expected. Meta-analysis was performed using SNP effects estimated for each of the three traits, adjusted for standard error. We identified 1359 significant SNPs (p-value < 9.9 × 10-6 at FDR < 0.0001) in the multi-trait meta-analysis. Genomic regions of 0.5 Mb around each significant SNP from the meta-analysis were annotated to create a list of 2560 positional candidate genes. The most significant SNP was in the vicinity of a genomic region on chromosome 8, encompassing the genes SLC44A1, FSD1L, FKTN, TAL2 and TMEM38B. The genomic region in humans that contains homologs of these genes is associated with age at puberty in girls. Top significant SNPs pointed to additional fertility-related genes, again within a 0.5 Mb region, including ESR2, ITPR1, GNG2, RGS9BP, ANKRD27, TDRD12, GRM1, MTHFD1, PTGDR and NTNG1. Functional pathway enrichment analysis resulted in many positional candidate genes relating to known fertility pathways, including GnRH signaling, estrogen signaling, progesterone mediated oocyte maturation, cAMP signaling, calcium signaling, glutamatergic signaling, focal adhesion, PI3K-AKT signaling and ovarian steroidogenesis pathway. The comparison of results from this study with previous transcriptomics and proteomics studies on puberty of the same cattle breed (Brahman) but in a different population identified 392 genes in common from which some genes-BRAF, GABRA2, GABR1B, GAD1, FSHR, CNGA3, PDE10A, SNAP25, ESR2, GRIA2, ORAI1, EGFR, CHRNA5, VDAC2, ACVR2B, ORAI3, CYP11A1, GRIN2A, ATP2B3, CAMK2A, PLA2G, CAMK2D and MAPK3-are also part of the above-mentioned pathways. The biological functions of the positional candidate genes and their annotation to known pathways allowed integrating the results into a bigger picture of molecular mechanisms related to puberty in the hypothalamus-pituitary-ovarian axis. A reasonable number of genes, common between previous puberty studies and this study on early reproductive traits, corroborates the proposed molecular mechanisms. This study identified the polymorphism associated with early reproductive traits, and candidate genes that provided a visualization of the proposed mechanisms, coordinating the hypothalamic, pituitary, and ovarian functions for reproductive performance in Brahman cattle.

Sperm Methylome Profiling Can Discern Fertility Levels in the Porcine Biomedical Model

A combined Genotyping By Sequencing (GBS) and methylated DNA immunoprecipitation (MeDIP) protocol was used to identify—in parallel—genetic variation (Genomic-Wide Association Studies (GWAS) and epigenetic differences of Differentially Methylated Regions (DMR) in the genome of spermatozoa from the porcine animal model. Breeding boars with good semen quality (n = 11) and specific and well-documented differences in fertility (farrowing rate, FR) and prolificacy (litter size, LS) (n = 7) in artificial insemination programs, using combined FR and LS, were categorized as High Fertile (HF, n = 4) or Low Fertile (LF, n = 3), and boars with Unknown Fertility (UF, n = 4) were tested for eventual epigenetical similarity with those fertility-proven. We identified 165,944 Single Nucleotide Polymorphisms (SNPs) that explained 14–15% of variance among selection lines. Between HF and LF individuals (n = 7, 4 HF and 3 LF), we identified 169 SNPs with p ≤ 0.00015, which explained 58% of the variance. For the epigenetic analyses, we considered fertility and period of ejaculate collection (late-summer and mid-autumn). Approximately three times more DMRs were observed in HF than in LF boars across these periods. Interestingly, UF boars were clearly clustered with one of the other HF or LF groups. The highest differences in DMRs between HF and LF experimental groups across the pig genome were located in the chr 3, 9, 13, and 16, with most DMRs being hypermethylated in LF boars. In both HF and LF boars, DMRs were mostly hypermethylated in late-summer compared to mid-autumn. Three overlaps were detected between SNPs (p ≤ 0.0005, n = 1318) and CpG sites within DMRs. In conclusion, fertility levels in breeding males including FR and LS can be discerned using methylome analyses. The findings in this biomedical animal model ought to be applied besides sire selection for andrological diagnosis of idiopathic sub/infertility.

Abundances of autophagy-related protein LC3B in granulosa cells, cumulus cells, and oocytes during atresia of pig antral follicles

In mammals, apoptosis has been accepted as the type of programmed cell death (PCD) that occurs in ovarian follicles undergoing atresia. Results of recent studies, however, indicate autophagy may be an alternative mechanism involved in follicle depletion through independent or tandem actions with apoptosis. Western blotting and immunofluorescence procedures were used in the present study to investigate the abundances of LC3B protein in freshly collected granulosa cells (GCs), cumulus cells (CCs), and oocytes to evaluate whether autophagy is an important process of antral follicle atresia in sexually mature sows. Furthermore, apoptosis was analyzed using annexin V and TUNEL assays in the same cellular cohorts to evaluate the correlation between the two processes. Immunostaining results indicate autophagy was induced in the majority of GCs, CCs, and oocytes from early and advanced stage atretic follicles. The quantitative results of western blot analysis indicate there is a progressive increase (P <  0.05) in abundance of autophagy-related protein (LC3B-II) in these cells compared with cells in non-atretic follicles. Furthermore, there is confirmation that apoptosis occurs in the GCs of atretic follicles, thus indicating that in pigs apoptosis and autophagy are processes in GCs that regulate PCD and as a consequence antral follicle depletion. There was a greater abundance of LC3B-II in CCs and oocytes of atretic follicles, while apoptosis was not detected. It, therefore, is suggested that in these cells the two processes function independently, with autophagy having a cytoprotective rather than PCD mechanism of action.

Genome mutation after introduction of the gene editing by electroporation of Cas9 protein (GEEP) system in matured oocytes and putative zygotes

The application of CRISPR/Cas9 strategy promises to rapidly increase the production of genetically engineered animals since it yields stably integrated transgenes. In the present study, we investigated the efficiency of target mutations after electroporation with the CRISPR/Cas9 system using sgRNAs to target the MSTN or FGF10 genes in porcine-matured oocytes and putative zygotes. Effects of pulse number (3-7 pulse repetitions) during electroporation on the embryonic development and mutation efficiency were also investigated. Our results showed that the cleavage rate of matured oocytes with electroporation treatment significantly decreased as compared with electroporated putative zygotes (p < 0.05). Moreover, the rates of blastocyst formation from oocytes/zygotes electroporated with more than 5 pulses decreased. Mutation efficiency was then assessed after sequencing the target sites in individual blastocysts derived from oocytes/zygotes electroporated by 3 and 5 pulses. No bi-allelic mutations in all examined blastocysts were observed in this study. There were no differences in the mutation rates (50-60%) between blastocysts derived from matured oocytes electroporated by 3 and 5 pulses, irrespective of targeting gene. In the targeting MSTN gene, however, the mutation rate (12.5%) of blastocysts derived from putative zygotes electroporated by 3 pulses tended to be lower than that (60%) from 5-pulsed electroporated putative zygotes. These data indicate that the type of eggs may influence not only their development after electroporation treatment but also the mutation rate in the resulting blastocysts.

Reduced oxygen concentration during in vitro oocyte maturation alters global DNA methylation in the maternal pronucleus of subsequent zygotes in cattle

Preimplantation epigenetic reprogramming is sensitive to the environment of the gametes and the embryo. In vitro maturation (IVM) of bovine oocytes is a critical step of embryo in vitro production procedures and several factors influence its efficiency, including atmospheric oxygen tension. The possibility that the IVM environment can alter this process is tested by determining whether the global DNA methylation pattern (measured via immunofluorescent labeling of 5-methylcytosine [5meC]) in the parental pronuclei of bovine zygotes produced from cumulus-oocyte complexes matured under low (5%) and atmospheric (~20%) oxygen tension. Normalized 5meC signals differed significantly between maternal and paternal pronuclei of oocytes matured in vitro at 5% oxygen (p ≤ 0.05). There was a significant difference of 5meC between maternal pronuclei of oocytes matured at 5% oxygen and 20% oxygen ( p ≤ 0.05). The relative methylation level (normalized fluorescence intensity of paternal pronucleus divided by the normalized fluorescence intensity of maternal pronucleus) subsequent to maturation in vitro at 5% and 20% oxygen was also significantly altered ( p ≤ 0.05). Our results show that the pattern of global DNA methylation in the maternal pronucleus of bovine zygotes is affected by maturing the oocytes under low oxygen tension which may have an impact on early embryonic development. These data may contribute to the understanding of possible effects of IVM conditions on pronucleus reprogramming.

Defective sperm head decondensation undermines the success of ICSI in the bovine

The efficiency of intracytoplasmic sperm injection (ICSI) in the bovine is low compared to other species. It is unknown whether defective oocyte activation and/or sperm head decondensation limit the success of this technique in this species. To elucidate where the main obstacle lies, we used homologous and heterologous ICSI and parthenogenetic activation procedures. We also evaluated whether in vitro maturation negatively impacted the early stages of activation after ICSI. Here we showed that injected bovine sperm are resistant to nuclear decondensation by bovine oocytes and this is only partly overcome by exogenous activation. Remarkably, when we used heterologous ICSI, in vivo-matured mouse eggs were capable of mounting calcium oscillations and displaying normal PN formation following injection of bovine sperm, although in vitro-matured mouse oocytes were unable to do so. Together, our data demonstrate that bovine sperm are especially resistant to nuclear decondensation by in vitro-matured oocytes and this deficiency cannot be simply overcome by exogenous activation protocols, even by inducing physiological calcium oscillations. Therefore, the inability of a suboptimal ooplasmic environment to induce sperm head decondensation limits the success of ICSI in the bovine. Studies aimed to improve the cytoplasmic milieu of in vitro-matured oocytes and to replicate the molecular changes associated with in vivo capacitation and acrosome reaction will deepen our understanding of the mechanism of fertilization and improve the success of ICSI in this species.

Current perspectives on in vitro maturation and its effects on oocyte genetic and epigenetic profiles

In vitro maturation (IVM), the maturation in culture of immature oocytes, has been used in clinic for more than 20 years. Although IVM has the specific advantages of low cost and minor side effects over controlled ovarian stimulation, the prevalence of IVM is less than 1% of routine in vitro fertilization and embryo transfer techniques in many reproductive centers. In this review, we searched the MEDLINE database for all full texts and/or abstract articles published in English with content related to oocyte IVM mainly between 2000 and 2016. Many different aspects of the IVM method may influence oocyte potential, including priming, gonadotrophin, growth factors, and culture times. The culture conditions of IVM result in alterations in the oocyte or cumulus cell transcriptome that are not observed under in vivo culture conditions. Additionally, epigenetic modifications, such as DNA methylation or acetylation, are also different between in vitro and in vivo cultured oocytes. In sum, current IVM technique is still not popular and requires more systematic and intensive research to improve its effects and applications. This review will help point our problems, supply evidence or clues for future improving IVM technique, thus assist patients for fertility treatment or preservation as an additional option.

Dynamic reprogramming of 5-hydroxymethylcytosine during early porcine embryogenesis

DNA active demethylation is an important epigenetic phenomenon observed in porcine zygotes, yet its molecular origins are unknown. Our results show that 5-methylcytosine (5mC) converts into 5-hydroxymethylcytosine (5hmC) during the first cell cycle in porcine in vivo fertilization (IVV), IVF, and SCNT embryos, but not in parthenogenetically activated embryos. Expression of Ten-Eleven Translocation 1 (TET1) correlates with this conversion. Expression of 5mC gradually decreases until the morula stage; it is only expressed in the inner cell mass, but not trophectoderm regions of IVV and IVF blastocysts. Expression of 5mC in SCNT embryos is ectopically distinct from that observed in IVV and IVF embryos. In addition, 5hmC expression was similar to that of 5mC in IVV cleavage-stage embryos. Expression of 5hmC remained constant in IVF and SCNT embryos, and was evenly distributed among the inner cell mass and trophectoderm regions derived from IVV, IVF, and SCNT blastocysts. Ten-Eleven Translocation 3 was highly expressed in two-cell embryos, whereas TET1 and TET2 were highly expressed in blastocysts. These data suggest that TET1-catalyzed 5hmC may be involved in active DNA demethylation in porcine early embryos. In addition, 5mC, but not 5hmC, participates in the initial cell lineage specification in porcine IVV and IVF blastocysts. Last, SCNT embryos show aberrant 5mC and 5hmC expression during early porcine embryonic development.

In vivo and in vitro environmental effects on mammalian oocyte quality

The oocyte is at the center of the equation that results in female fertility. Many factors influence oocyte quality, including external factors such as maternal nutrition, stress, and environmental exposures, as well as ovarian factors such as steroids, intercellular communication, antral follicle count, and follicular fluid composition. These influences are interconnected; changes in the external environment of the female translate into ovarian changes that affect the oocyte. The lengthy period during which the oocyte remains arrested in the ovary provides ample time and opportunity for environmental factors to take their toll. An appropriate environment for growth and maturation of the oocyte, in vivo and in vitro, is critical to ensure optimal oocyte quality, which determines the success of fertilization and preimplantation embryo development, and has long-term implications for implantation, fetal growth, and offspring health.

Active DNA demethylation in mammalian preimplantation embryos: new insights and new perspectives

DNA methylation and demethylation are crucial for modulating gene expression and regulating cell differentiation. Functions and mechanisms of DNA methylation/demethylation in mammalian embryos are still far from being understood clearly. In this review we firstly describe new insights into DNA demethylation mechanisms, and secondly introduce the differences in active DNA methylation patterns in zygotes and early embryos in various mammalian species. Thirdly, we attempt to clarify the functions of DNA demethylation in early embryos. Most importantly we summarize the importance of active DNA demethylation and its possible relevance to human IVF clinics. Finally research perspectives regarding DNA demethylation are also discussed.

In vitro grown sheep preantral follicles yield oocytes with normal nuclear-epigenetic maturation

Background

Assisted reproductive technologies allow to utilize a limited number of fully grown oocytes despite the presence in the ovary of a large pool of meiotically incompetent gametes potentially able to produce live births. In vitro folliculogenesis could be useful to recruit these oocytes by promoting their growth and differentiation.

Methodology/Principal Findings

In vitro folliculogenesis was performed starting from sheep preantral (PA) follicles to evaluate oocyte nuclear/epigenetic maturation. Chromatin configuration, quantification of global DNA methylation, and epigenetic remodelling enzymes were evaluated with immunocytochemistry, telomere elongation was assessed with the Q-FISH technique, while the DNA methylation status at the DMRs of maternally IGF2R and BEGAIN, and paternally H19 methylated imprinted genes was determined by bisulfite sequencing and COBRA. Specifically, 70% of PA underwent early antrum (EA) differentiation and supported in culture oocyte global DNA methylation, telomere elongation, TERT and Dnmt3a redistribution thus mimicking the physiological events that involve the oocyte during the transition from secondary to tertiary follicle. Dnmt1 anticipated cytoplasmic translocation in in vitro grown oocytes did not impair global and single gene DNA methylation. Indeed, the in vitro grown oocytes acquired a methylation profile of IGF2R and BEGAIN compatible with the follicle/oocyte stage reached, and maintained an unmethylated status of H19. In addition, the percentage of oocytes displaying a condensed chromatin configuration resulted lower in in vitro grown oocytes, however, their ability to undergo meiosis and early embryo development after IVF and parthenogenetic activation was similar to that recorded in EA follicle in vivo grown oocytes.

Conclusions/Significance

In conclusion, the in vitro folliculogenesis was able to support the intracellular/nuclear mechanisms leading the oocytes to acquire a meiotic and developmental competence. Thus, the in vitro culture may increase the availability of fertilizable oocytes in sheep, and become an in vitro translational model to investigate the mechanisms governing nuclear/epigenetic oocyte maturation.

Insufficient histone-3 lysine-9 deacetylation in human oocytes matured in vitro is associated with aberrant meiosis

OBJECTIVE

To characterize histone acetylation during meiosis in human oocytes matured in vitro or in vivo.

DESIGN

Experimental study.

SETTING

University reproductive medical center.

PATIENT(S)

Patients undergoing routine intracytoplasmic sperm injection (ICSI) cycles.

INTERVENTION(S)

Immature and mature oocytes were collected from patients undergoing intracytoplasmic sperm injection.

MAIN OUTCOME MEASURE(S)

Immunohistochemical assessment of the levels of acetylated lysine-9 of histone-3 (H3K9) and lysine-12 of histone-4 (H4K12) combined with spindle and chromosome configurations in in vitro- and in vivo-matured human oocytes. Transcript levels of histone deacetylases (HDACs) 1 and 2 were measured by single-cell quantitative polymerase chain reaction.

RESULT(S)

Acetylation of H3K9 and H4K12 decreased during human oocyte maturation. Residual H3K9 acetylation was found in 37.7% of oocytes matured in vitro, compared with 11.8% in oocytes matured in vivo. Abnormal metaphase spindle was more frequent in oocytes with residual histone acetylation than without (51.6% vs. 25.4%, respectively). Treatment with the HDAC inhibitor trichostatin A increased the incidence of an abnormal metaphase but had no adverse effect on maturation efficiency. Furthermore, expression of HDAC1 transcripts was significantly lower in oocytes matured in vitro versus in vivo.

CONCLUSION(S)

Reduced HDAC1 expression and insufficient histone deacetylation are associated with metaphase defects in human oocytes matured in vitro.

Sonic hedgehog supplementation of oocyte and embryo culture media enhances development of IVF porcine embryos

We investigated the expression of sonic hedgehog (SHH) receptor PTCH1 and its co-receptor smoothened (SMO) in fertilized porcine embryos. Effects of exogenous SHH on embryonic development and expressions of survival- and pluripotency-related genes were also determined. We found that PTCH1 and SMO are expressed from two-cell to blastocyst embryos. When oocytes or fertilized embryos were respectively cultured in the maturation or embryo culture medium supplemented with SHH (0.5 μg/ml), their blastocyst rates and total cell numbers increased (P<0.05) compared with the untreated control. When cultured simultaneously in thein vitromaturation (IVM) andin vitroculture (IVC) media supplemented with SHH, the oocytes gained increased blastocyst rates and total cell numbers in an additive manner, with reduced apoptotic indices (P<0.05). Interestingly, SHH treatment did not affect the expression of theBCL2L1(BCL-XL) gene, yet reducedBAXexpression. Blastocysts cultured with various SHH regimes had similar pluripotency-related gene (POU5F1(OCT-4) andCDX2) expression levels, but blastocysts derived from SHH treatment during IVM had higherZPF42(REX01) expression (P<0.05). The highestZPF42expression was observed in the blastocysts derived from SHH-supplemented IVC and from dual IVM and IVC treatments. The levels of acetylated histone 3 (AcH3K9/K14) increased in the two-cell and the four-cell embryos when IVM and/or IVC media were supplemented with SHH (P<0.05). Our findings indicate that SHH conferred a beneficial effect on preimplantation development of porcine embryos, particularly when both IVM and IVC media were supplemented with SHH, and the effects may be further carried over from IVM to the subsequent embryonic development.

Differential methylation of pluripotency gene promoters in in vitro matured and vitrified, in vivo-matured mouse oocytes

OBJECTIVE

To assess the methylation patterns of four pluripotency gene promoters in mouse oocytes after in vivo maturation, in vitro maturation (IVM), and vitrification followed by IVM.

DESIGN

Experimental study.

SETTING

Research laboratory.

ANIMAL(S)

Three populations of metaphase II mouse oocytes were analyzed after in vivo maturation, IVM, and vitrification followed by IVM (V-IVM). Cumulus cells and blastocyst embryos were controls.

INTERVENTION(S)

The CpG methylation patterns (overall and CpG specific) in the promoters of four pluripotency genes (Oct4, Nanog, Foxd3, and Sox2) were analyzed for each cell type by traditional DNA bisulfite sequencing.

MAIN OUTCOME MEASURE(S)

Differences for overall methylation were evaluated using the Student's t-test and for individual CpG sites by χ2 analysis.

RESULT(S)

Significantly lower levels of overall methylation in promoters of Oct4 (25%) and Sox2 (4.5%) were noted in V-IVM oocytes compared with in vivo-matured oocytes (62.5% and 8.5%, respectively). Cumulus cell promoters were generally hypomethylated at Nanog, Foxd3. and Sox2, but hypermethylated at Oct4.

CONCLUSION(S)

The methylation status of Oct4 and Sox2 promoters of V-IVM mouse oocytes are altered when compared with in vivo-matured oocytes. The biological risk and significance of these changes are unknown and this study indicates caution and that further analyses are warranted.

Prolonged interval between fusion and activation impairs embryonic development by inducing chromosome scattering and nuclear aneuploidy in pig somatic cell nuclear transfer

The aim of the present study was to examine the effect of various intervals between electrofusion and activation (FA interval) on the nuclear remodelling and development of somatic cell nuclear transfer (SCNT) embryos in pigs. Reconstructed oocytes were activated at 0 (simultaneous fusion and activation; SFA), 1, 2 and 3 h (delayed activation) after electrofusion; these groups were designated as DA1, DA2 and DA3, respectively. When oocyte nuclear status was examined at 0.5, 1, 2 and 3 h after electrofusion, the incidence of chromosome scattering was increased (P < 0.01) as the FA interval was extended (0.0%, 12.0%, 77.3% and 78.0%, respectively). Extending the FA interval led to an increase (P < 0.01) in the percentage of oocytes containing multiple (>or=3) pseudopronuclei (PPN) (0.0% of SFA; 5.3% of DA1; 21.7% of DA2; and 33.5% of DA3). The development of SCNT embryos to the blastocyst stage was decreased (P < 0.05) in DA2 (5.7%) and DA3 (5.0%) compared with SFA (18.1%) and DA1 (19.5%). Our results demonstrate that extending the FA interval impairs the development of SCNT pig embryos by inducing chromosome scattering and the formation of multiple PPN, which may result in increased nuclear aneuploidy.

Effects of In Vitro Maturation on Histone Acetylation in Metaphase II Oocytes and Early Cleavage Embryos

In vitro maturation (IVM) of oocyte is an effective procedure for avoiding ovarian hyperstimulation syndrome in patients with polycystic ovaries (PCOS) during in vitro fertilization (IVF). To investigate the influences of IVM on epigenetic reprogramming and to search for the possible reasons for the lower rates of fertilization and cleavage in IVM oocytes, we examined the expression of two enzymes controlling histone acetylation, histone acetyltransferase GCN5 (GCN5) and histone deacetylase 1 (HDAC1), as well as their common target, acetyl-histone H3 (Ac-H3), in mouse metaphase II (MII) oocytes and preimplantation embryos. Results showed that IVM downregulated the protein expression of GCN5 in MII oocytes and two-cell embryos and changed the distribution of GCN5 in two-cell embryos. Expression of HDAC1 mRNA in MII oocytes and two-cell embryos decreased in the IVM group. However, none of these changes persisted after two-cell embryos. Levels of Ac-H3 in both oocytes and embryos remained unchanged after IVM. Our studies indicated that IVM could affect the protein and gene expression related to histone acetylation in oocytes and early cleavage embryos. By function of selection, parts of the changes could be recovered in late embryo development.

Active demethylation of paternal genome in mammalian zygotes

Epigenetic reprogramming in early preimplantation embryos, that refers to erasing and remodeling epigenetic marks such as DNA methylation, is essential for differentiation and development. In many species, paternal genome is subjected to genome-wide active demethylation before the DNA replication commences, while maternal genome maintains its methylation status until being demethylated passively during the subsequent cleavage divisions. The purpose of this manuscript was to review the available knowledge about the paternal genome active demethylation process concerning the possible mechanisms, species variation and the factors affecting the active demethylation dynamics such as in vitro protocols for production of pronuclear-stage zygotes. Better understanding the mechanisms by which the epigenetic reprogramming is occurred may contribute to clarify the biological significance of this process.

Enhancement of histone acetylation by trichostatin A during in vitro fertilization of bovine oocytes affects cell number of the inner cell mass of the resulting blastocysts

Histone acetylation is one of the major mechanisms of epigenetic reprogramming of gamete genomes after fertilization to establish a totipotent state for normal development. In the present study, the effects of trichostatin A (TSA), an inhibitor of histone deacetylase, during in vitro fertilization (IVF) of bovine oocytes on subsequent embryonic development were investigated. Cumulus-enclosed oocytes obtained from slaughterhouse bovine ovaries were matured in vitro and subjected to IVF in a defined medium supplemented with 0 (control), 5, 50, and 500 nM TSA for 18 h. After IVF, presumptive zygotes were cultured in modified synthetic oviductal fluid (mSOF) medium until 168 h postinsemination (hpi). Some oocytes were immunostained using antibody specific for histone H4-acetylated lysine 5 at 10 hpi. Cleavage, blastocyst development and cell number of inner cell mass (ICM) and trophectoderm (TE) of blastocysts were assessed. TSA treatment enhanced histone acetylation that was prominent in decondensed sperm nuclei. TSA did not affect the postfertilization cleavage, blastocyst rates, and TE cell number. However, it significantly enhanced ICM cell number (p < 0.05). These results indicate that TSA treatment during IVF of bovine oocytes does not affect blastocyst development but alters the cell number of ICM, suggesting that overriding epigenetic modification of the genome during fertilization has a carryover effect on cell proliferation and differentiation in preimplantation embryos. Thus, further environmental quality controls in assisted reproductive technologies are needed in terms of factors which affect chromatin remodelling.

Elevated histone H1 (MPF) and mitogen-activated protein kinase activities in pig oocytes following in vitro maturation do not indicate cytoplasmic maturation

Effects of different media (TCM 199 + BSA, TCM 199 + FCS, TCM 199 + NBCS, Whitten's medium + BSA) supplemented with estradiol-17beta and two isolated and everted follicle shells on MPF and MAP kinase activities and the sensitivity to parthenogenetic activation of pig oocytes were examined at the end of culture (48 h). Elevated (P < 0.05) activities of MAP kinase were recorded in metaphase II oocytes following culture in Whitten's medium, whereas MPF levels were lowest (P < 0.05) in MII oocytes matured in TCM 199 supplemented with BSA. Oocytes matured in TCM 199 based media showed higher (P < 0.05) activation rates when compared to oocytes incubated in Whitten's medium. Whitten's medium supplemented with different protein sources (amino acids, FCS, BSA) was used to study the effects of different exposure periods to eCG/hCG stimulation on MPF and MAP kinase activities and in vivo fertilisability following culture for 48 h. MPF and MAP kinase activities were significantly increased by eCG/hCG stimulation of COCs during maturation. Further, the continuous presence of eCG/hCG during culture (48 h) significantly increased the levels of both kinases in comparison to stimulation by gonadotrophins alone during the first 24 h of incubation. In vivo fertilisation of oocytes matured in Whitten's medium supplemented with eCG/hCG for 24 or 48 h led to a significant retardation of early embryonic development compared to ovulated oocytes. In conclusion, media composition and gonadotrophin stimulation affect MPF/MAP kinase activities and the susceptibility to parthenogenetic activation of IVM oocytes. However, elevated kinase levels in pig oocytes following culture do not indicate complete cytoplasmic maturation.

Intracytoplasmic sperm injection in livestock species: an update

Intracytoplasmic sperm injection (ICSI) is a powerful technique in the field of assisted reproduction (ART) and provides exciting opportunities for studying the basic mechanisms of fertilization and early embryo development. Nevertheless, its application in agriculture and conservation biology has been greatly hampered by the low success rate reported for this method in respect of economically important species. Specifically, the rates of blastocyst formation and live newborn are greatly reduced when zygotes are generated by ICSI. Except for humans, ICSI remains a low efficiency technology in comparison with alternatives such as in vitro fertilization (IVF) and its application is less widespread. In this paper, we discuss the present status, applications and factors affecting ICSI in pigs and other species.

Aberrant DNA methylation in porcine in vitro-, parthenogenetic-, and somatic cell nuclear transfer-produced blastocysts

Early embryonic development in the pig requires DNA methylation remodeling of the maternal and paternal genomes. Aberrant remodeling, which can be exasperated by in vitro technologies, is detrimental to development and can result in physiological and anatomic abnormalities in the developing fetus and offspring. Here, we developed and validated a microarray based approach to characterize on a global scale the CpG methylation profiles of porcine gametes and blastocyst stage embryos. The relative methylation in the gamete and blastocyst samples showed that 18.5% (921/4,992) of the DNA clones were found to be significantly different (P < 0.01) in at least one of the samples. Furthermore, for the different blastocyst groups, the methylation profile of the in vitro-produced blastocysts was less similar to the in vivo-produced blastocysts as compared to the parthenogenetic- and somatic cell nuclear transfer (SCNT)-produced blastocysts. The microarray results were validated by using bisulfite sequencing for 12 of the genomic regions in liver, sperm, and in vivo-produced blastocysts. These results suggest that a generalized change in global methylation is not responsible for the low developmental potential of blastocysts produced by using in vitro techniques. Instead, the appropriate methylation of a relatively small number of genomic regions in the early embryo may enable early development to occur.

Chromatin in early mammalian embryos: achieving the pluripotent state

Gametes of both sexes (sperm and oocyte) are highly specialized and differentiated but within a very short time period post-fertilization the embryonic genome, produced by the combination of the two highly specialized parental genomes, is completely converted into a totipotent state. As a result, the one-cell-stage embryo can give rise to all cell types of all three embryonic layers, including the gametes. Thus, it is evident that extensive and efficient reprogramming steps occur soon after fertilization and also probably during early embryogenesis to reverse completely the differentiated state of the gamete and to achieve toti- or later on pluripotency of embryonic cells. However, after the embryo reaches the blastocyst stage, the first two distinct cell lineages can be clearly distinguished--the trophectoderm and the inner cells mass. The de-differentiation of gametes after fertilization, as well as the differentiation that is associated with the formation of blastocysts, are accompanied by changes in the state and properties of chromatin in individual embryonic nuclei at both the whole genome level as well as at the level of individual genes. In this contribution, we focus mainly on those events that take place soon after fertilization and during early embryogenesis in mammals. We will discuss the changes in DNA methylation and covalent histone modifications that were shown to be highly dynamic during this period; moreover, it has also been documented that abnormalities in these processes have a devastating impact on the developmental ability of embryos. Special attention will be paid to somatic cell nuclear transfer as it has been shown that the aberrant and inefficient reprogramming may be responsible for compromised development of cloned embryos.

Gradual development of a genome-wide H3-K9 trimethylation pattern in paternally derived pig pronucleus

The cytoplasm of a mature oocyte contains many protein complexes that are programmed to restructure incoming sperm chromatins on fertilization. Of the complicated biochemical events that these functional machineries control, the most impressive and important is epigenetic reprogramming. Despite its importance in epigenetic resetting, or "de-differentiation," of gamete genomes back to an incipient status, the mechanisms of epigenetic reprogramming do not seem to be conserved among mammals. Here, we report that, unlike in the mouse, the pig sperm-derived pronucleus is markedly trimethylated at lysine 9 of histone H3 (H3-m(3)K9), which might be associated with preservation of paternally derived cytosine methylation in pig zygotes. The male H3-m(3)K9 pattern is gradually established during pronucleus development, and this process occurs independently of DNA replication. Considering these unique epigenetic features, the pig zygote is, we believe, suited to serve as another model of epigenetic reprogramming that is antithetical to the well-characterized mouse model.

Evaluation of oocyte quality: morphological, cellular and molecular predictors

Mounting evidence that oocyte quality profoundly affects fertilisation an d subsequent embryo development drives the continued search for reliable predictors of oocyte developmental competence. In the present review, we provide an overall summary and analysis of potential criteria that can be used to evaluate oocyte quality. These criteria are specifically classified as morphological and cellular/molecular predictors. Traditional methods for the evaluation of oocyte quality are based on morphological classification of thefollicle, cumulus-oocytecomplex, polar body and/or meiotic spindle. Although the use of morphological characteristics as predictors of oocyte quality is controversial, such a grading system can provide valuable information for the preselection of oocytes with higher developmental competence and, therefore, may maximise embryo developmental outcome. Several intrinsic markers (such as mitochondrial status and glucose-6-phosphate dehydrogenase 1 activity) and extrinsic markers (such as apoptosis of follicular cells and levels of the transforming growth factor-beta superfamily in follicular fluid or serum) have also been reported as useful indicators of oocyte competence and embryo quality. Compared with the morphological parameters, these cellular and molecular predictors of oocyte quality may prove to be more precise and objective, although further studies and refinement of techniques are needed.

Region-specific DNA methylation in the preimplantation embryo as a target for genomic plasticity

It has been long known that the unique genetic sequence each embryo inherits is not the sole determinant of phenotype. However, only recently have epigenetic modifications to DNA been implicated in providing potential developmental plasticity to the embryonic and fetal genome, with environmental influences directly altering the epigenetic modifications that contribute to tissue-specific gene regulation. Most is known about the potential environmental regulation of DNA methylation, epigenetic addition of methyl groups to cytosine residues in DNA that acts in the long-term silencing of affected sequences. While most attention has been paid to the methylation of imprinted gene sequences, in terms of developmental plasticity there are many more parts of the genome that are methylated and that could be affected. This review explores the distribution of cytosine methylation in the genome and discusses the potential effects of regional plasticity on subsequent development. Widening our consideration of potentially plastic regions is likely to greatly enhance our understanding of how individuals are shaped not only by DNA sequence, but by the environment in which pluripotent embryonic cells are transformed into the many cell types of the body.

Transmission of Mitochondrial DNA Disorders: Possibilities for The Elimination of Mutated Mitochondria

The recent article by D.T. Brown et al. (2006) discusses the transmission of mitochondrial DNA disorders. In the second part, the authors describe certain techniques that can be potentially used to eliminate the transmission of mutated mtDNA from mother to offspring. It is our opinion that this brief account needs broader explanation. Moreover, some approaches mentioned are not always correctly interpreted.

Genomic DNA methylation patterns in bovine preimplantation embryos derived from in vitro fertilization

By using the approach of immunofluorescence staining with an antibody against 5-methylcytosine (5MeC), the present study detected the DNA methylation patterns of bovine zygotes and preimplantation embryos derived from oocyte in vitro maturation (IVM), in vitro fertilization (IVF) and embryo in vitro culture (IVC). The results showed that: a) paternal-specific demethylation occurred in 61.5% of the examined zygotes, while 34.6% of them showed no demethylation; b) decreased methylation level was observed after the 8-cell stage and persisted through the morula stage, however methylation levels were different between blastomeres within the same embryos; c) at the blastocyst stage, the methylation level was very low in inner cell mass, but high in trophectoderm cells. The present study suggests, at least partly, that IVM/IVF/IVC may have effects on DNA methylation reprogramming of bovine zygotes and early embryos.

Ontogeny of metabolic effects on embryonic development in lactating and weaned primiparous sows

Using an established experimental paradigm, feed restriction during the last week of lactation in primiparous sows reduces embryonic growth and development and produces female-specific embryonic mortality by Day 30 of gestation. Because this gender-specific loss of embryos at Day 30 was associated with changes in the variation of markers of epigenetic imprinting, the present study sought to establish the ontogeny of such epigenetic affects. Leucocyte DNA of restrict-fed sows exhibited decreased global methylation during the last week of lactation and during the return to oestrus (P < 0.05), but no associated changes in plasma folate and vitamin B12. Furthermore, no changes in methylation of blastocyst DNA, embryonic sex ratios or development were evident at Day 6 of gestation that would characterise the underlying defects that reduced female embryo survival by Day 30. However, regardless of treatment, embryo recovery rates and synchrony in embryonic development were associated with the stage of development of the recovered embryos (r = 0.68; P < 0.001). The subset of sows classified as bearing litters with superior embryonic development had lower net energy balance over lactation (P < 0.01) and higher ovulation rates (P < 0.005) compared with sows classified as having poorer embryonic development. Collectively, these data suggest that a subset of litters within restrict-fed sows will be most sensitive to the latent epigenetic mechanisms that ultimately trigger gender-specific loss of embryos by Day 30 of gestation, but that these selective mechanisms are not evident by Day 6 of gestation.

Modifications in chromatin morphology and organization during sheep oogenesis

This research has been designed to study the major events of nuclear remodeling that characterize sheep oocytes during the early stage of folliculogenesis (transition from preantral to antral stage). In particular, the modifications in large-scale chromatin configuration, the global DNA methylation, and the process of telomere elongation have been investigated as crucial events of oocyte nuclear maturity. In addition, the spatio-temporal distribution of the major enzymes involved in DNA methylation, the DNA methyltransferase 1 (Dnmt1), and in telomere elongation, telomerase catalytic subunit (TERT), have been described. To these aims, the nuclei of isolated oocytes were investigated using immunocytochemistry and Q-FISH analyses. In absence of preliminary information, these nuclear determinants were compared with those of fully competent germ cells obtained from medium and preovulatory antral follicles. The nuclei of sheep oocytes acquired a condensed chromatin configuration, stable high levels of global DNA methylation, and a definitive telomere length already in the majority of late growing stage oocytes (110 microm) derived from early antral follicles. In addition, while the process of methylation resulted strictly related to oocyte diameter, the telomeric program appeared to be highly chromatin configuration-dependent. The translocation of Dnmt1 and TERT from the nucleus to the cytoplasm in the oocytes derived from early antral follicles seems to confirm the definitive chromatin asset of these germ cells. In conclusion, changes in large-scale chromatin structure, epigenesis, and telomere size in the sheep are established prior to oocyte acquires the ability to resume meiosis.

The Use of Micromanipulation Methods as a Tool to Prevention of Transmission of Mutated Mitochondrial DNA

The introduction of different micromanipulation techniques into reproductive and developmental biology has helped us not only to answer many essential biological questions but it is now evident that these techniques also have wide practical applications. In human-assisted reproduction, the most commonly used approach is the injection of a donor sperm into the oocyte cytoplasm--intracytoplasmic sperm injection. It is, however, speculated that with these techniques it would be also possible to improve the oocyte developmental potential especially in those cases when the quality of the cytoplasm is rather poor and thus its function is compromised. Another important application would be the elimination of mutated mitochondrial DNA (mtDNA) by transferring the nuclear material from an abnormal oocyte into a healthy donor oocyte cytoplast. Some of these techniques were already successfully tested in experimental animals, but it is evident that before their introduction into human medicine many questions must be answered, and we must be sure that these approaches are absolutely or almost absolutely safe. In our contribution, we will be specifically oriented to the nuclear (nuclear material) replacement approaches that could be potentially used to prevent the transmission of mutated mtDNA from mother to offspring. Because these techniques are very delicate, some training with oocytes from other species other than human is strongly recommended.

Comparison of histone modifications in in vivo and in vitro fertilization mouse embryos

Histone modifications are thought to play important roles in various cellular functions. In this article, the distribution patterns of acetylation on histone H4, methylation on histone H3 lysine 9, and phosphorylation on histone H3 serine 10 were examined in in vivo and in vitro fertilization (IVF) preimplantation mouse embryos by using indirect immunofluorescence and scanning confocal microscopy. We desired to know whether the IVF, which has been widely used as a routine assisted reproductive technology in animal and human, was safe at the epigenetic level. As results, we found that there was no difference in these histone modification patterns in in vivo and IVF mouse embryos from zygote to blastocyst stage. Moreover, these histone modifications had different distributions at all examined stages, but they were consistent with the mouse embryo developmental stages.

Effects of 50 Hz extremely low frequency magnetic field on the morphology and function of boar spermatozoa capacitated in vitro

The aim of this study was to evaluate the effect of an acute exposure to a sinusoidal MF-ELF (50 Hz, 1mT) on the ability of boar mature spermatozoa to acquire the fertilizing competence in vitro. The spermatozoa exposed during the 4h of incubation to the MF-ELF were evaluated for morphological (surface morphology and acrosome integrity) and functional parameters (cell viability, motility, induction of acrosomal reaction, AR, and the ability to in vitro fertilize oocytes). In parallel, the intracellular Ca(2+) levels as well as the major mechanisms of Ca(2+) clearance were assessed: (45)Ca intakes and intracellular Ca(2+) sequestration by analyzing intracellular Ca(2+) elevation induced by thapsigargin or studying mitochondrial function with Mito-Tracker. The MF-ELF exposure did not affect sperm viability and morphology during the first h of incubation when sperm Ca(2+) homeostasis were already compromised. First of all, MF-ELF treated spermatozoa showed resting intracellular Ca(2+) levels significantly lower than those recorded in controls. This result was dependent on a lower extracellular Ca(2+) intake and from the inhibitory role exerted on both intracellular Ca(2+) storages. As a consequence, after 1h of incubation MF-ELF exposed cells displayed a reduced motility, a modest reactivity when coincubated with solubilized zonae pellucidae and a reduction in oocyte penetrating ability. After 2 or 4h of incubation, in addition, signs of morphological damage appeared on plasma membrane and at acrosomal level. In conclusion, MF-ELF influence negatively spermatozoa first by impairing cell Ca(2+) homeostasis then by dramatically affecting sperm morphology and function.

Chromatin remodeling in somatic cells injected into mature pig oocytes

We examined the involvement of histone H3 modifications in the chromosome condensation and decondensation of somatic cell nuclei injected into mature pig oocytes. Nuclei of pig granulosa cells were transferred intoin vitromatured intact pig oocytes, and histone H3 phosphorylation, acetylation, and methylation were examined by immunostaining with specific antibodies in relation to changes in chromosome morphology. In the condensed chromosomes of pig oocytes at metaphase II, histone H3 was phosphorylated at serine 10 (H3–S10) and serine 28 (H3–S28), and methylated at lysine 9 (H3–K9), but was not acetylated at lysine 9, 14 and 18 (H3–K9, H3–K14 and H3–K18). During the first 2 h after nuclear transfer, a series of events were observed in the somatic nuclei: nuclear membrane disassembly; chromosome condensation to form a metaphase-like configuration; an increase in histone H3 phosphorylation levels (H3–S10 and H3–S28). Next, pig oocytes injected with nuclei of somatic cells were electroactivated and the chromosome morphology of oocytes and somatic cells was examined along with histone modifications. Generally, chromosomes of the somatic cells showed a similar progression of cell cycle stage to that of oocytes, through anaphase II- and telophase II-like stages then formed pronucleus-like structures, although the morphology of the spindles differed from that of oocyte spindles. The chromosomes of somatic cells also showed changes in histone H3 dephosphorylation and reacetylation, similar to oocytes. In contrast, histone H3 methylation (H3–K9) of somatic cell nuclei did not show any significant change after injection and electroactivation of the oocytes. These results suggest that nuclear remodeling including histone H3 phosphorylation and acetylation of injected somatic nuclei took place in the oocytes under regulation by the oocyte cytoplasm.

Nutritional restriction in lactating primiparous sows selectively affects female embryo survival and overall litter development

This study explored the possibility of sex-specific effects on embryonic survival in primiparous sows subjected to restricted feed intake during the last week of lactation and bred after weaning (Restrict; n = 16), compared with control sows fed close to ad libitum feed intakes (Control; n = 17). Restrict sows were in a substantial negative net energy balance at weaning, and lost 13% of estimated protein and 17% of fat mass during lactation, yet the weaning-to-oestrous interval and ovulation rate were not different between treatments. However, embryonic survival at Day 30 of gestation was lower (P < 0.05) in Restrict than Control sows, and selectively reduced the proportion of female embryos surviving (P < 0.01). A decrease in weight and crown-rump length of surviving female (P < 0.05) and male (P < 0.05) embryos was seen in Restrict litters. The mechanisms mediating this sex-specific effect on embryonic loss in feed-restricted sows are unclear. The data presented here indicate that feed-restriction during the last week of lactation in primiparous sows causes a selective decrease in survival of female embryos and limits the growth of all surviving embryos.

Generation of transgenic rabbits by the novel technique of chimeric somatic cell cloning

A novel technique of chimeric somatic cell cloning was applied to produce a transgenic rabbit (NT20). Karyoplasts of transgenic adult skin fibroblasts with Tg(Wap-GH1) gene construct as a marker were microsurgically transferred into one, previously enucleated, blastomere of 2-cell non-transgenic embryos, while the second one remained intact. The reconstructed embryos either were cultured in vitro up to the blastocyst stage (Experiment I) or were transferred into recipient-females immediately after the cloning procedure (Experiment II). In Experiment I, 25/102 (24.5%) embryos formed blastocysts from whole embryos and 46/102 (44.12%) embryos developed to the blastocyst stage from single non-operated blastomeres, while the reconstructed blastomeres were damaged and degenerated. Thirteen (12.7%) embryos did not exceed 3- to 4-cell stages and 18 (17.7%) embryos were inhibited at the initial 2-cell stage. Out of 14 blastocysts which were subjected to molecular analysis, the transgene was detected in the cells of 4 blastocysts. In Experiment II, 163/217 (75.0%) embryos were transferred into 9 pseudopregnant recipient-rabbits (an average of 18 embryos per recipient). Four recipient-females (44.4%) became pregnant and delivered a total of 24 (14.7%) pups. Molecular analysis confirmed that two pups (1.2%), one live and one stillborn, showed a positive transgene signal. Live transgenic rabbit NT20 appeared healthy and anatomically as well as physiologically normal. The results of our experiments showed that transgenic adult skin fibroblast cell nuclei, which have been introduced into the cytoplasmic microenvironment of single enucleated blastomeres from 2-cell stage rabbit embryos, are able to direct the development of chimeric embryos not only to the blastocyst stage but also up to term.

DNA methylation pattern in pig in vivo produced embryos

DNA methylation/demethylation pattern, determined by 5-methylcytosine (5-MeC) immunostaining, was evaluated in porcine "in vivo" produced embryos from zygote up to the blastocyst stage. In one-cell stage embryos, only the maternal pronucleus showed a positive labeling whilst the paternal pronucleus showed almost no labeling. The intensity of labeling is high until the late morula stage. Blastocysts containing less than 100 cells showed the same intensity of labeling in both the inner cell mass (ICM) nuclei and the trophectodermal (TE) cell nuclei. Interestingly, with further cell multiplication, cells of the ICM became more intensively labeled when compared to TE cells. This distinct methylation pattern is even more profound in blastocysts containing about 200-300 cells and is not caused by the difference in the cell volume of ICM and TE cells.

Pronuclear stage porcine embryos do not possess a strict asymmetric distribution of lysine 9 dimethylation of histone H3 based solely on parental origin

Epigenetic modification of core histone proteins plays an important role in gene transcription and chromatin structure. It is known that the methylation status of the lysine 9 residue of histone H3 (H3K9) is closely linked with repression of transcription and heterochromatin formation. It has been reported that a strict asymmetric distribution of the dimethylated form of H3K9 exists between the two parental genomes in murine embryos. The aim of this study was to characterize the H3K9 dimethylation pattern in cleavage stage porcine embryos. Indirect immunocytochemical analysis showed that this modification is present throughout the nuclei of pronuclear, 2-cell, 4-cell and blastocyst stage porcine embryos. However, at the pronuclear stage the pronuclei within in vitro-fertilized porcine embryos possessed an asymmetric distribution of the dimethylated form of H3K9; this distribution was not based solely on the parent-of-origin. These results show that dimethylation of H3K9 is differentially regulated in pronuclear stage porcine embryos.

Embryogenesis of chimeras, twins and anterior midline asymmetries

Human spontaneous chimerism, with one body built from cells of both twins of a dizygotic (DZ) pair, is supposed to be extremely rare, arising from the exchange of blood cells through placental anastomoses. Mosaicism is supposed to be far more common, arising from single zygotes by embryonic mutation. Because typical diagnosis of mosaicism can neither identify nor exclude chimerism, 'mosaicism' may often be chimerism undiscovered. Evidence shows chimerism arises primarily from DZ embryo fusion and is not rare, although it has negligible probability under the hypothesis of independent double ovulation and independent embryogenesis. If, instead, DZ twin embryos begin development as a single cell mass, chimerism is likely. This would be consistent with observations that DZ twins develop as differently from singletons as monozygotic twins do with regard to embryogenic establishment of asymmetries of midline neural-crest-driven structures of brain, face and heart. Chimerism is a significant component of human embryonic development that deserves closer attention as a mechanism of developmental variation. The 'common knowledge' understanding of twinning mechanisms is at best inadequate. The importance of the difference lies in what we can learn from chimerism about human embryogenesis and the cellular origins of structures and functions basic to the business of becoming human.