Oocyte vitrification induces loss of DNA methylation and histone acetylation in the resulting embryos derived using ICSI in dromedary camel

Zygote ◽  
2021 ◽  
pp. 1-10
Author(s):  
F. Moulavi ◽  
I.M. Saadeldin ◽  
A.A. Swelum ◽  
F. Tasdighi ◽  
H. Hosseini-Fahraji ◽  
...  
Keyword(s):  
Dna Methylation ◽  
Blastocyst Stage ◽  
Epigenetic Reprogramming ◽  
Developmental Competence ◽  
Oocyte Cryopreservation ◽  
Dromedary Camel ◽  
Cleavage Rate ◽  
Oocyte Vitrification ◽  
Preimplantation Embryo Development ◽  
H3k9 Acetylation

Summary Oocyte cryopreservation has become an important component of assisted reproductive technology with increasing implication in female fertility preservation and animal reproduction. However, the possible adverse effects of oocyte cryopreservation on epigenetic status of the resulting embryos is still an open question. This study evaluated the effects of MII-oocyte vitrification on gene transcripts linked to epigenetic reprogramming in association with the developmental competence and epigenetic status of the resulting embryos at 2-cell and blastocyst stages in dromedary camel. The cleavage rate of vitrified oocytes following intracytoplasmic sperm injection was significantly increased compared with the control (98.2 ± 2 vs. 72.7 ± 4.1%, respectively), possibly due to the higher susceptibility of vitrified oocytes to spontaneous activation. Nonetheless, the competence of cleaved embryos derived from vitrified oocytes for development to the blastocyst and hatched blastocyst was significantly reduced compared with the control (7.7 ± 1.2 and 11.1 ± 11.1 compared with 28.1 ± 2.6 and 52.4 ± 9.9%, respectively). The relative transcript abundances of epigenetic reprogramming genes DNMT1, DNMT3B, HDAC1, and SUV39H1 were all significantly reduced in vitrified oocytes relative to the control. Evaluation of the epigenetic marks showed significant reductions in the levels of DNA methylation (6.1 ± 0.3 vs. 9.9 ± 0.5, respectively) and H3K9 acetylation (7.8 ± 0.2 vs. 10.7 ± 0.3, respectively) in 2-cell embryos in the vitrification group relative to the control. Development to the blastocyst stage partially adjusted the effects that oocyte vitrification had on the epigenetic status of embryos (DNA methylation: 4.9 ± 0.4 vs. 6.2 ± 0.6; H3K9 acetylation: 5.8 ± 0.3 vs. 8 ± 0.9, respectively). To conclude, oocyte vitrification may interfere with the critical stages of epigenetic reprogramming during preimplantation embryo development.

Generation of parthenogenetic goat blastocysts: effects of different activation methods and culture media

Zygote ◽  
2014 ◽  
Vol 23 (3) ◽  
pp. 327-335 ◽  
Author(s):  
Hruda Nanda Malik ◽  
Dinesh Kumar Singhal ◽  
Shrabani Saugandhika ◽  
Amit Dubey ◽  
Ayan Mukherjee ◽  
...  
Keyword(s):  
Culture Media ◽  
Combined Treatment ◽  
Blastocyst Stage ◽  
Developmental Competence ◽  
Cleavage Rate ◽  
Cell Stage ◽  
Quantitative Expression ◽  
Pattern Of Variation ◽  
Better Than

SummaryThe present study was carried out to investigate the effects of different activation methods and culture media on the in vitro development of parthenogenetic goat blastocysts. Calcium (Ca2+) ionophore, ethanol or a combination of the two, used as activating reagents, and embryo development medium (EDM), modified Charles Rosenkrans (mCR2a) medium and research vitro cleave (RVCL) medium were used to evaluate the developmental competence of goat blastocysts. Quantitative expression of apoptosis, stress and developmental competence-related genes were analysed in different stages of embryos. In RVCL medium, the cleavage rate of Ca2+ ionophore-treated oocytes (79.61 ± 0.86) was significantly (P < 0.05) higher than in ethanol (74.90 ± 1.51) or in the combination of both Ca2+ ionophore and ethanol. In mCR2a or EDM, hatched blastocyst production rate of Ca2+ ionophore-treated oocytes (8.33 ± 1.44) was significantly higher than in ethanol (6.46 ± 0.11) or in the combined treatment (6.70 ± 0.24). In ethanol, the cleavage, blastocyst and hatched blastocyst production rates in RVCL medium (74.90 ± 1.51, 18.30 ± 1.52 and 8.24 ± 0.15, respectively) were significantly higher than in EDM (67.81 ± 3.21, 14.59 ± 0.27 and 5.59 ± 0.42) or mCR2a medium (65.09 ± 1.57, 15.36 ± 0.52 and 6.46 ± 0.11). The expression of BAX, Oct-4 and GlUT1 transcripts increased gradually from 2-cell stage to blastocyst-stage embryos, whereas the transcript levels of Bcl-2 and MnSOD were significantly lower in blastocysts. In addition, different activation methods and culture media had little effect on the pattern of variation and relative abundance of the above genes in different stages of parthenogenetic activated goat embryos. In conclusion, Ca2+ ionophore as the activating agent, and RVCL as the culture medium are better than other tested options for development of parthenogenetic activated goat blastocysts.

133 VITRIFICATION CAUSES DAMAGE OF THE ANTIOXIDANT DEFENSE SYSTEM IN IVM PORCINE OOCYTES

2007 ◽  
Vol 19 (1) ◽  
pp. 184 ◽  
Author(s):  
T. Somfai ◽  
M. Ozawa ◽  
J. Noguchi ◽  
H. Kaneko ◽  
K. Ohnuma ◽  
...  
Keyword(s):  
Polar Body ◽  
Antioxidant Defense System ◽  
Blastocyst Stage ◽  
Developmental Competence ◽  
Control Group ◽  
Cleavage Rate ◽  
Sperm Penetration ◽  
Second Polar Body ◽  
Polar Body Extrusion

The present study investigated the ability of in vitro-matured (IVM) porcine oocytes to be fertilized in vitro after vitrification. Oocytes matured in vitro for 46 h according to Kikuchi et al. (2002 Biol. Reprod. 66, 1033–1041) were cryopreserved by solid surface vitrification (SSV; Dinnyes et al. 2000 Biol. Reprod. 63, 513–518) or subjected to the steps of SSV without cooling (toxicity control, TC). Oocyte viability was assessed 2 h after treatment by morphology and fluorescein diacetate staining. Live oocytes were in vitro-fertilized (IVF) and cultured (IVC) for 6 days according to Kikuchi et al. (2002). Fertilization and pronuclear development of oocytes were assessed 10 h after IVF by aceto-orcein staining. Cleavage and blastocyst rates were recorded during IVC. Glutathione (GSH) and hydrogen peroxide levels in oocytes were analyzed by DTNB-glutathione disulfide reductase recycling assay and 20,70-dichlorofluorescein fluorescence assay, respectively. Data were analyzed by ANOVA and paired t-test. The rate of live oocytes after SSV was lower compared to the control and the TC groups (54.4%, 100%, and 100%, respectively; P &lt; 0.05). Sperm penetration rates of SSV oocytes were lower than those of the control group (51.9% and 67.8%, respectively; P &lt; 0.05). Significantly fewer penetrated oocytes in the SSV group formed male pronuclei than those in the control and the TC groups (66.7%, 96.5%, and 98.5%, respectively; P &lt; 0.05). There were no differences in second polar body extrusion and monospermy rates between the treatment groups. The cleavage rate of SSV oocytes was significantly lower than that of the control and the TC groups (13.3%, 46.6%, and 47.7%, respectively; P &lt; 0.05). Blastocyst rates of control and TC oocytes were similar (20.7% and 23.6%, respectively), whereas only a single embryo developed to the blastocyst stage in the SSV group. GSH content of SSV oocytes was significantly lower than that of the control oocytes (7.3 pM and 10.5 pM, respectively), whereas the peroxide level was higher in SSV oocytes than in the control oocytes (59.0 and 50.5 FIU, respectively; P &lt; 0.05). Our results reveal a cryopreservation-related drop of intracellular GSH level in oocytes, which may cause their decreased ability to form a male pronucleus and their increased sensitivity to oxidative stress. These factors might contribute to the low developmental competence of vitrified oocytes. This work was supported by a grant-in-aid for the Japanese Society for the Promotion of Science Postdoctoral Fellowship for Foreign Researchers (P05648) and the Bilateral Scientific and Technological Collaboration Grant between Hungary and Japan (TET, no. JAP-11/02).

Tolerance of lamb and mouse oocytes to cryoprotectants during vitrification

Zygote ◽  
2018 ◽  
Vol 27 (1) ◽  
pp. 36-45
Author(s):  
Jaqueline Sudiman ◽  
Alice Lee ◽  
Kheng Ling Ong ◽  
Wu Zi Yuan ◽  
Sarah Jansen ◽  
...  
Keyword(s):  
Cell Number ◽  
Blastocyst Stage ◽  
Developmental Competence ◽  
Cortical Granule ◽  
Blastocyst Development ◽  
Cleavage Rate ◽  
Parthenogenetic Activation ◽  
Mouse Oocytes ◽  
Different Response

SummaryMouse and lamb oocytes were vitrified with, or exposed to, different cryoprotectants and evaluated for their effects on their survival and developmental competence after in vitro fertilization (IVF) and activation treatments. Control oocytes remained untreated, whilst the remainder were exposed to three different combinations of vitrification solutions [dimethyl sulfoxide (DMSO) + ethylene glycol (EG), EG only, or propanediol (PROH) + EG] and either vitrified or left unfrozen (exposed groups). Oocytes in the control and vitrified groups underwent IVF and developmental competence was assessed to the blastocyst stage. In lambs, survival rate in vitrified oocytes was significantly lower than for oocytes in the exposed groups (P <0.05). Blastocyst development was low in vitrified oocytes compared with controls (<6% vs 38.9%, P <0.01). Parthenogenetic activation was more prevalent in vitrified lamb oocytes compared with controls (P <0.05). No evidence of zona pellucida hardening or cortical granule exocytosis could account for reduced fertilization rates in vitrified lamb oocytes. Mouse oocytes demonstrated a completely different response to lamb oocytes, with survival and parthenogenetic activation rates unaffected by the vitrification process. Treatment of mouse oocytes with DMSO + EG yielded significantly higher survival and cleavage rates than treatment with PROH + EG (87.8% and 51.7% vs 32.7% and 16.7% respectively, P <0.01), however cleavage rate for vitrified oocytes remained lower than for the controls (51.7% vs 91.7%, P <0.01) as did mean blastocyst cell number (33 ± 3.1 vs 42 ± 1.5, P <0.05). From this study, it is clear that lamb and mouse show different tolerances to cryoprotectants commonly used in vitrification procedures, and careful selection and testing of species-compatible cryoprotectants is required when vitrifying oocytes to optimize survival and embryo development.

Exposure to mono-n-butyl phthalate disrupts the development of preimplantation embryos

2013 ◽  
Vol 25 (8) ◽  
pp. 1174 ◽  
Author(s):  
Da-Peng Chu ◽  
Shi Tian ◽  
Da-Guang Sun ◽  
Chan-Juan Hao ◽  
Hong-Fei Xia ◽  
...  
Keyword(s):  
Reactive Oxygen Species ◽  
Dna Methylation ◽  
Dibutyl Phthalate ◽  
Preimplantation Embryo ◽  
Blastocyst Stage ◽  
Preimplantation Embryos ◽  
Oxygen Species ◽  
Preimplantation Embryo Development ◽  
Immunofluorescent Analysis ◽  
Butyl Phthalate

Dibutyl phthalate (DBP), a widely used phthalate, is known to cause many serious diseases, especially in the reproductive system. However, little is known about the effects of its metabolite, mono-n-butyl phthalate (MBP), on preimplantation embryo development. In the present study, we found that treatment of embryos with 10–3 M MBP impaired developmental competency, whereas exposure to 10–4 M MBP delayed the progression of preimplantation embryos to the blastocyst stage. Furthermore, reactive oxygen species (ROS) levels in embryos were significantly increased following treatment with 10–3 M MBP. In addition, 10–3 M MBP increased apoptosis via the release of cytochrome c, whereas immunofluorescent analysis revealed that exposure of preimplantation embryos to MBP concentration-dependently (10–5, 10–4 and 10–3 M) decreased DNA methylation. Together, the results indicate a possible relationship between MBP exposure and developmental failure in preimplantation embryos.

scholarly journals Epigenetic alteration of the donor cells does not recapitulate the reprogramming of DNA methylation in cloned embryos

Reproduction ◽  
2007 ◽  
Vol 134 (6) ◽  
pp. 781-787 ◽  
Author(s):  
Gabbine Wee ◽  
Jung-Jae Shim ◽  
Deog-Bon Koo ◽  
Jung-Il Chae ◽  
Kyung-Kwang Lee ◽  
...  
Keyword(s):  
Dna Methylation ◽  
Trichostatin A ◽  
Dna Methyltransferases ◽  
Epigenetic Reprogramming ◽  
Preimplantation Development ◽  
Developmental Competence ◽  
Epigenetic Mark ◽  
Satellite Sequence ◽  
Donor Cells

Epigenetic reprogramming is a prerequisite process during mammalian development that is aberrant in cloned embryos. However, mechanisms that evolve abnormal epigenetic reprogramming during preimplantation development are unclear. To trace the molecular event of an epigenetic mark such as DNA methylation, bovine fibroblasts were epigeneticallyaltered by treatment with trichostatin A (TSA) and then individually transferred into enucleated bovine oocytes. In the TSA-treated cells, expression levels of histone deacetylases and DNA methyltransferases were reduced, but the expression level of histone acetyltransferases such as Tip60 and histone acetyltransferase 1 (HAT1) did not change compared with normal cells. DNA methylation levels of non-treated (normal) and TSA-treated cells were 64.0 and 48.9% in the satellite I sequence (P < 0.05) respectively, and 71.6 and 61.9% in the α-satellite sequence respectively. DNA methylation levels of nuclear transfer (NT) and TSA-NT blastocysts in the satellite I sequence were 67.2 and 42.2% (P < 0.05) respectively, which was approximately similar to those of normal and TSA-treated cells. In the α-satellite sequence, NT and TSA-NT embryos were substantially demethylated at the blastocyst stage as IVF-derived embryos were demethylated. The in vitro developmental rate (46.6%) of TSA-NT embryos that were individually transferred with TSA-treated cells was higher than that (31.7%) of NT embryos with non-treated cells (P < 0.05). Our findings suggest that the chromatin of a donor cell is unyielding to the reprogramming of DNA methylation during preimplantation development, and that alteration of the epigenetic state of donor cells may improve in vitro developmental competence of cloned embryos.

scholarly journals Development to Blastocyst Stage of Pig Oocytes Matured, Fertilized and Electroactivated In Vitro

2002 ◽  
Vol 45 (6) ◽  
pp. 547-556
Author(s):  
N. R. Mtango ◽  
M. D. Varisanga ◽  
D. Y. Juan ◽  
P. Wongrisekeao ◽  
T. Suzuki
Keyword(s):  
In Vitro Maturation ◽  
Blastocyst Stage ◽  
Developmental Competence ◽  
Cleavage Rate ◽  
Vitro Fertilization ◽  
Nuclear Maturation ◽  
Significant Difference ◽  
Activation Rates ◽  
Oviduct Fluid

Abstract. This study was designed 1) to determine the effectiveness of two in vitro maturation (IVM) media (tissue culture medium [TCM] and modified synthetic oviduct fluid supplemented with amino acids [mSOFaa]), 2) to compare the effects of two in vitro fertilization (IVF) media (modified Tris-buffered medium [mTBM] and mSOFaa) on the developmental competence of pig oocytes, and 3) to test the activation ability of IVM pig oocytes matured in TCM or mSOFaa, electroactivated and cultured in mSOFaa. The nuclear maturation rates were similar between IVM media (91.0 % vs. 89.0 %). A similar result was obtained when the activation rates were 54.2 % in TCM and 56.0 % in mSOFaa, and the blastocyst rates were 7.9 % and 6.1 %, respectively. There was no significant difference between mSOFaa and mTBM in the percentage of embryos with two pronuclei 33.2 % vs. 13.8 % or polypronuclei 5.3 % vs. 13.4 %. The cleavage rate was the same in both media. The medium mSOFaa gave a significantly higher (P< 0.05) blastocyst rate than mTBM (12.7 % vs. 3.9 %). We concluded that mSOFaa can enhance in vitro maturation, fertilization and culture of pig oocytes.

Corrigendum to: Exposure to mono-n-butyl phthalate disrupts the development of preimplantation embryos

2014 ◽  
Vol 26 (3) ◽  
pp. 491 ◽  
Author(s):  
Da-Peng Chu ◽  
Shi Tian ◽  
Da-Guang Sun ◽  
Chan-Juan Hao ◽  
Hong-Fei Xia ◽  
...  
Keyword(s):  
Reactive Oxygen Species ◽  
Dna Methylation ◽  
Dibutyl Phthalate ◽  
Preimplantation Embryo ◽  
Blastocyst Stage ◽  
Preimplantation Embryos ◽  
Oxygen Species ◽  
Preimplantation Embryo Development ◽  
Immunofluorescent Analysis ◽  
Butyl Phthalate

Dibutyl phthalate (DBP), a widely used phthalate, is known to cause many serious diseases, especially in the reproductive system. However, little is known about the effects of its metabolite, mono-n-butyl phthalate (MBP), on preimplantation embryo development. In the present study, we found that treatment of embryos with 10–3M MBP impaired developmental competency, whereas exposure to 10–4M MBP delayed the progression of preimplantation embryos to the blastocyst stage. Furthermore, reactive oxygen species (ROS) levels in embryos were significantly increased following treatment with 10–3M MBP. In addition, 10–3M MBP increased apoptosis via the release of cytochrome c, whereas immunofluorescent analysis revealed that exposure of preimplantation embryos to MBP concentration-dependently (10–5, 10–4 and 10–3M) decreased DNA methylation. Together, the results indicate a possible relationship between MBP exposure and developmental failure in preimplantation embryos.

130 PREDICTION OF PARTHENOGENETIC DEVELOPMENTAL POTENTIAL BY POLAR BODY EXTRUSION AND FIRST CLEAVAGE ON IN VITRO MATURATION AND DEVELOPMENT OF PORCINE FOLLICULAR OOCYTES

2008 ◽  
Vol 20 (1) ◽  
pp. 145
Author(s):  
H. J. Kim ◽  
S. R. Cho ◽  
C. Y. Choe ◽  
S. H. Choi ◽  
D. S. Son ◽  
...  
Keyword(s):  
In Vitro Maturation ◽  
Polar Body ◽  
Blastocyst Stage ◽  
Developmental Competence ◽  
Cleavage Rate ◽  
Developmental Potential ◽  
Non Invasive ◽  
First Polar Body ◽  
Polar Body Extrusion

The objective of this study was to examine the selection effects of in vitro matured porcine follicular oocytes with polar body extrusion and early cleavage as a non-invasive marker to know the developmental competence in advance. Porcine oocytes matured for 48 h and then examined for polar body extrusion. The examined oocytes were matured for an additional 16–18 h, activated with 7% ethanol, and cultured in 5 µg mL–1 cytochalasin B for 5 h for diploid formation. The treated oocytes were examined for cleavage after 48 h and continued culturing for 5 days. Each treatment was replicated by 3–4 times. Oocytes of 21.9% (70/320) were discarded in morphological selection, and 32.1% (167/520) oocytes were discarded by failure of first polar body extrusion. The selected oocytes were matured and activated, and after 48 h, the cleavage rate was examined. In morphologically selected oocytes, 15.8% (30/190) were not cleaved, 52.6% (100/190) were normally cleaved (consisted of 2–7 cells), and 31.6% (60/190) were hyper-cleaved (consisted of 8 cells or more) at 48 h after activation. However, in the first polar body extruded oocytes, 7.1% (18/253) were not cleaved, 73.1% (185/253) were normally cleaved, and 19.8% (50/253) were hyper-cleaved. From the morphologically selected oocytes, 16.7% (10/60) were developed up to blastocyst stage from those in which cleavage selection was not performed and 31.7% (19/60) from those in which cleavage selection was performed. From the polar body extruded oocytes, 39.0% (39/100) were developed up to blastocyst stage from those in which cleavage selection was not performed and 49.0% (49/100) from those in which cleavage selection was performed. Cleavage was examined within 12 h interval after activation (0 = time of activation) up to 48 h. At 0–12, 12–24, 24–36, and 36–48 h intervals, 4.1% (9/220), 68.6% (151/220), 19.1% (42/220), and 2.3% (5/220) oocytes were cleaved, respectively, and 5.9% (13/220) oocytes were not cleaved at 48 h after activation. The cleaved embryos in each interval were cultured and developed up to blastocyst with 0 (0/9), 39.1 (59/151), 9.5 (4/42), and 0% (0/5), respectively. This result suggests that the polar body extruded and cleaved at 12–36 h embryo has higher developmental potential than the others.

310 COBALAMIN SUPPLEMENTATION DURING IN VITRO MATURATION IMPROVES PREIMPLANTATION DEVELOPMENT OF SHEEP EMBRYOS

2015 ◽  
Vol 27 (1) ◽  
pp. 244 ◽  
Author(s):  
F. Zacchini ◽  
P. Toschi ◽  
P. Loi ◽  
G. E. Ptak
Keyword(s):  
Blastocyst Stage ◽  
Developmental Competence ◽  
Pcr Analysis ◽  
Fisher Exact Test ◽  
Placental Development ◽  
Cleavage Rate ◽  
Fetal Survival ◽  
Qrt Pcr

Oocyte maturation process includes the establishment of proper epigenetics marks, fundamental to ensure successful pregnancy. Epigenetic maturation of the oocyte depends on one-carbon-metabolism (OCM), whose key elements (cobalamin, folate) are crucial cofactors for the transfer of methyl groups onto chromatin. Commercially available IVM-media only partially supports oocyte metabolic requirements, and thus may negatively affect epigenetic maturation. Of relevance, cobalamin, one of the OCM cofactors normally present in follicular fluid, is missing in IVM-media. We investigated if cobalamin supplementation of IVM media affects sheep oocyte developmental competence in term of subsequent embryo development, epigenetic pattern, and fetal survival. Briefly, sheep oocytes were isolated from ovaries and divided into 2 groups: an untreated control (in vitro CTR group) and oocytes in vitro matured in medium containing cobalamin at 200 p.m. (Cobalamin group). Following maturation, MII oocytes were in vitro fertilized and cultured until blastocyst stage. A cohort of blastocysts was surgically transferred to recipient ewes and then conceptuses were collected at 20 days of pregnancy. Naturally mated conceptuses were also collected (in vivo CTR group). In vitro-matured oocytes and -derived blastocysts were analysed (i) by immunofluorescence anti-5-methylcitydine and (ii) by qRT-PCR for the expression of a panel of genes (MATb, ACHY, CBS, MTHFR, DNMT1) involved in OCM pathway. Immunofluorescence results were analysed by Image J software. Decimal variables were analysed using the Mann-Whitney test, while variables were expressed as percentage with the Fisher exact test. Cobalamin exposure during IVM significantly increased (i) cleavage rate (cobalamin 130/220 v. in vitro CTR 134/191, P < 0.02) following in vitro fertilization and (ii) global DNA methylation in blastocyst-stage embryos (P < 0.05). Then, qRT-PCR analysis of a select panel of OCM genes following IVM supplemented with cobalamin revealed a downregulation of MATb, ACHY, and DNMT1 in cobalamin-treated oocytes v. in vitro CTR group (P < 0.05), while no differences were observed at blastocyst stage. Finally, we found that the survival rate at 20 days of pregnancy was comparable in in vitro-produced (IVP) embryos from cobalamin and in vitro CTR oocytes, but reduced compared to in vivo CTR (cobalamin 60%, in vitro CTR 72%, and in vivo CTR 100%). Altogether, our results showed that cobalamin supplementation in IVM medium positively affects competence of oocytes and methylation profile at the blastocyst stage, presumably through the OCM pathway. Moreover, postimplantation survival of IVP conceptus, derived from untreated and treated oocytes, was reduced compared to naturally mated ones. Further investigation will clarify whether cobalamin supplementation influences fetal and placental development of IVP conceptus.

scholarly journals Epigenetic reprogramming during early development in mammals

Reproduction ◽  
2004 ◽  
Vol 127 (6) ◽  
pp. 643-651 ◽  
Author(s):  
Fátima Santos ◽  
Wendy Dean
Keyword(s):  
Dna Methylation ◽  
Inner Cell Mass ◽  
Cell Mass ◽  
Blastocyst Stage ◽  
Epigenetic Reprogramming ◽  
Epigenetic Alterations ◽  
Chromatin Organisation ◽  
Genomic Imprints ◽  
Genomic Regions ◽  
Derivatives Of

Epigenetic modifications serve as an extension of the information content by which the underlying genetic code may be interpreted. These modifications mark genomic regions and act as heritable and stable instructions for the specification of chromatin organisation and structure that dictate transcriptional states. In mammals, DNA methylation and the modification of histones account for the major epigenetic alterations. Two cycles of DNA methylation reprogramming have been characterised. During germ cell development, epigenetic reprogramming of DNA methylation resets parent-of-origin based genomic imprints and restores totipotency to gametes. On fertilisation, the second cycle is triggered resulting in an asymmetric difference between parental genomes. Further epigenetic asymmetry is evident in the establishment of the first two lineages at the blastocyst stage. This differentiative event sets the epigenetic characteristics of the lineages as derivatives of the inner cell mass (somatic) and trophectoderm (extra-embryonic). It is the erasure and subsequent re-tracing of the epigenetic checkpoints that pose the most serious obstacles to somatic nuclear transfer. Elaboration of the mechanisms of these interactions will be invaluable in our fundamental understanding of biological processes and in achieving substantial therapeutic advances.

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