33 ENUCLEATION OF PRE-ACTIVATED MOUSE OOCYTES INDUCED BY DEMECOLCINE, NOCODAZOLE, AND VINBLASTINE

2007 ◽  
Vol 19 (1) ◽  
pp. 135
Author(s):  
N. Costa-Borges ◽  
J. Santaló ◽  
E. Ibàñez
Keyword(s):  
Nuclear Transfer ◽  
Polar Body ◽  
Significant Proportion ◽  
Chi Square ◽  
Microtubule Stabilization ◽  
Mouse Oocytes ◽  
Antimitotic Drugs ◽  
Ultraviolet Illumination ◽  
Second Polar Body ◽  
Activation Rates

Demecolcine-induced enucleation has been previously used to prepare developmentally competent enucleated mouse and bovine cytoplasts for nuclear transfer (Gasparrini et al. 2003 Biol. Reprod. 68, 1259–1266; Fischer-Russell et al. 2005 Mol. Reprod. Dev. 72, 161–170). The approach is technically simple, but the proportion of pre-activated oocytes that extrude all of the chromatin within the second polar body (PB) after exposure to demecolcine is relatively low, especially in the mouse (20%). This study was designed to explore the potential of other antimitotic drugs (nocodazole and vinblastine), besides demecolcine, to induce enucleation of mouse oocytes and to characterize the morphological progression of the treated oocytes after drug removal. Metaphase II (MII) oocytes were collected from cytochalasin D-1 (CD-1) females (6–12 weeks old) at 16 h post-hCG, activated in 7% ethanol (for a fast release from MII arrest) for 5 min and immediately treated for 15, 30, or 60 min with demecolcine (DEM, 0.4 �g mL-1), nocodazole (NOC, 0.3 �g mL-1), or vinblastine (VIN, 0.1 �g mL-1), prepared in calcium-free KSOM containing 10 mM strontium. Then, the oocytes were cultured in drug-free medium for up to 2 h, 6 h, or 20 h post-activation (p.a.) and fixed in a microtubule stabilization buffer-extraction fixative. A triple-labelling protocol for microtubules, microfilaments, and chromatin was used to analyze oocytes (approximately 60 per treatment) by fluorescence microscopy. Results were statistically analyzed by chi-square. At 2 h p.a., the highest rates of enucleation were achieved when pre-activated oocytes were treated with VIN (63.8%) or NOC (41.9%) for 15 min or with DEM (66.1%) for 30 min. Although antimitotic treatments did not affect activation rates (91.8–100%), a significant proportion of DEM- (19.6%) and of VIN-treated (15.5%) oocytes failed to complete second PB extrusion when compared to control (0%) or NOC-treated (4.8%) oocytes. From the total of the enucleated oocytes, 11.5%, 24.3%, and 29.7% had an incomplete second PB extrusion in NOC, VIN, and DEM groups, respectively, and therefore were classified as partially enucleated. Further culture of oocytes after drug withdrawal resulted in 100% of activated oocytes having a completely extruded second PB in all groups by 6 h p.a. and resulted in a significant and similar decrease in enucleation rates for all treatments by 6 h (20.3–34.9%) and 20 h p.a. (10.2–16.1%). This decrease might be caused by the reintegration of the chromosomes into the oocyte after incomplete second PB extrusion, or by re-fusion of second PBs to enucleated oocytes. Thus, our results show that both VIN and NOC, in addition to DEM, can be successfully applied to produce enucleated mouse cytoplasts, omitting the potentially harmful step (staining and ultraviolet illumination) of the traditional enucleation method. However, removal of the second PB at 2 h p.a. is recommended in order to achieve an irreversible oocyte enucleation. It remains to be demonstrated if the cytoplasts prepared with VIN or NOC are as competent as those prepared by DEM to support embryo development to term after being reconstructed by nuclear transfer.

scholarly journals Cell cycle modification during the transitions between meiotic M-phases in mouse oocytes

1992 ◽  
Vol 102 (3) ◽  
pp. 457-467 ◽  
Author(s):  
J.Z. Kubiak ◽  
M. Weber ◽  
G. Geraud ◽  
B. Maro
Keyword(s):  
Kinase Activity ◽  
Polar Body ◽  
Metaphase Arrest ◽  
Phosphorylated Proteins ◽  
Mouse Oocytes ◽  
Sequence Of Events ◽  
M Phase ◽  
Second Polar Body ◽  
Polar Body Extrusion ◽  
Centrosomal Proteins

When metaphase II-arrested mouse oocytes (M II) are activated very soon after ovulation, they respond abortively by second polar body extrusion followed by another metaphase arrest (metaphase III, M III; Kubiak, 1989). The M II/M III transition resembles the natural transition between the first and second meiotic metaphases (M I/M II). We observed that a similar sequence of events takes place during these two transitions: after anaphase, a polar body is extruded, the microtubules of the midbody disappear rapidly and a new metaphase spindle forms. The MPM-2 monoclonal antibody (which reacts with phosphorylated proteins associated with the centrosome during M-phase) stains discrete foci of peri-centriolar material only in metaphase arrested oocytes; during both transitional periods, a diffuse staining is observed, suggesting that these centrosomal proteins are dephosphorylated, as in a normal interphase. However, the chromosomes always remain condensed and an interphase network of microtubules is never observed during the transitional periods. Incorporation of 32P into proteins increases specifically during the transitional periods. Pulse-chase experiments, after labeling of the oocytes in M phase with 32P, showed that a 62 kDa phosphoprotein band disappears at the time of polar body extrusion. Histone H1 kinase activity (which reflects the activity of the maturation promoting factor) drops during both transitional periods to the level characteristic of interphase and then increases when the new spindle forms. Both the M I/M II and M II/M III transitions require protein synthesis as demonstrated by the effect of puromycin. These results suggest that the two M-phase/M-phase transitions are probably driven by the same molecular mechanism.

scholarly journals 271 ASSESSMENT OF NUCLEAR STATUS OF ACTIVATED BOVINE OOCYTES MATURED IN DIFFERENT MATURATION CONDITIONS IN VITRO

2005 ◽  
Vol 17 (2) ◽  
pp. 285
Author(s):  
J.I. Park ◽  
Y. Jang
Keyword(s):  
Oocyte Maturation ◽  
Polar Body ◽  
Calf Serum ◽  
Chi Square ◽  
Cell Stage ◽  
Maturation Medium ◽  
Second Polar Body ◽  
Humidified Air ◽  
Bovine Oocytes

This study was carried out to assess the nuclear status after parthenogenetic activation in in vitro matured oocytes under different conditions. Bovine ovaries were collected from slaughtered cows at a local abattoir. Oocytes were aspirated from follicles of 3–8 mm in diameter and transferred to maturation medium: tissue culture medium (TCM)-199 supplemented with 10% (v/v) fetal calf serum, 100 mg/mL l-cysteine, 20 mg/mL sodium pyruvate, gonadotropins (each 250 IU of eCG and hCG/mL), and 10 mg/mL epidermal growth factor, with or without 5 mM hypotaurine and taurine. Oocytes were cultured at 38.9°C in 5% CO2 in humidified air. After 24 h of culture, oocytes with polar body were selected and submitted to activation treatments. Oocytes were exposed to calcium ionomycin (5 μM for 5 min) followed by incubation with 6-DMAP (2 mM), roscovitine (50 μM), or 6-DMAP + roscovitine for 3.5 h. After activation, oocytes were cultured in mSOF medium containing 0.8% BSA at 38.9°C in 5% CO2, 5% O2 in humidified air for 16 h and stained with Hoechst 33342 or aceto-orcein for assessment of nuclear status. Nuclear status was recorded as follows: 1PB (polar body) + 1PN (pronucleus), 2PB + 1PN and others. Data were analyzed using chi-square test. The maturation rate of bovine oocytes cultured in maturation medium containing hypotaurine/taurine (89.3%, n = 84) was higher (P < 0.05) than those cultured without hypotaurine/taurine (72%, n = 93). In the oocytes matured with hypotaurine/taurine, the rates of diploid activation (1PB + 1PN) were 84% (n = 50) in oocytes treated with 6-DMAP + roscovitine, 78.6% (n = 56) with 6-DMAP, and 52% (n = 50) with roscovitine. In the oocytes matured without hypotaurine/taurine, the rates of diploid activation were 80% (n = 60) in oocytes treated with 6-DMAP + roscovitine, 72% (n = 50) with 6-DMAP, and 54% (n = 50) with roscovitine. The rates of diploid activation were not different in oocytes matured with or without hypotaurine/taurine and among activation treatments. The oocytes treated with roscovitine showed a lower rate (P < 0.05) of diploid activation and higher rate (39.3–40%) of second polar body extrusion (1PN + 2PB) than the other activation groups in both maturation conditions. Cleavage rates to 2-cell stage were 40–45% in all groups. Development rate of blastocysts were 7–10% in all the groups treated with 6-DMAP and 6-DMAP + roscovitine and no blastocysts were obtained from the groups treated with roscovitine alone. Hypotaurine/taurine are known to be stable and potent antioxidants, and have shown the properties of supporting oocyte maturation and further embryonic development (Guerin and Menezo 1995 Zygote 3, 333–43; Mizushima and Fukui 2001 Theriogenology 55, 1432–45). In this study, although the effectiveness of hypotaurine/taurine on promoting oocyte maturation was observed, there were no significant improvements in the rate of diploid activation in oocytes matured with hypotaurine/taurine. These results suggest that the nuclear status of activated oocytes may not have a direct relationship with the enhanced maturation condition. This work was supported by BioGreen 21 Program(#1000520030100000-1), Republic of Korea.

Mouse oocytes injected with testicular spermatozoa or round spermatids can develop into normal offspring

Development ◽  
1995 ◽  
Vol 121 (8) ◽  
pp. 2397-2405 ◽  
Author(s):  
Y. Kimura ◽  
R. Yanagimachi
Keyword(s):  
Polar Body ◽  
Experimental Conditions ◽  
Male And Female ◽  
Testicular Spermatozoa ◽  
Mouse Oocytes ◽  
Foster Mothers ◽  
Sperm Nuclei ◽  
Second Polar Body ◽  
Normal Offspring ◽  
Gametic Imprinting

Genomic imprinting occurs in both male and female gametes during gametogenesis, but the exact time when imprinting begins and ends is unknown. In the present study we injected nuclei of testicular spermatozoa and round spermatids into mature mouse oocytes to see whether these nuclei are able to participate in syngamy and normal embryonic development. If the injected oocytes develop into normal fertile offspring, imprinting in the male germ cells used must have been completed by the time of injection. Ninety-two percent of mouse oocytes injected with testicular spermatozoa survived and 94% of these were fertilized normally (extrusion of the second polar body and formation of male and female pronuclei). When 44 two-cell embryos so created were transferred to 5 foster mothers, 24 (54.5%) developed into normal offspring. Unlike testicular spermatozoa, round spermatids could not activate the oocytes, and therefore the oocytes had to be activated artificially either before or after spermatid injection. The highest rate (77%) of normal fertilization was obtained when the oocytes were first activated by electric current, then injected individually with a single spermatid nucleus. When 131 two-cell embryos were transferred to 15 foster mothers, 37 (28.2%) reached full term. All but two grew into healthy adults. Thus, it would appear that gametic imprinting in mouse spermatogenic cells is completed before spermiogenesis begins. Under the experimental conditions employed, spermatid nuclei were less efficient than testicular sperm nuclei in producing normal offspring, but perhaps this was due to technical rather than inherent problems.

Activation of p90rsk during meiotic maturation and first mitosis in mouse oocytes and eggs: MAP kinase-independent and -dependent activation

Development ◽  
1996 ◽  
Vol 122 (6) ◽  
pp. 1957-1964 ◽  
Author(s):  
P. Kalab ◽  
J.Z. Kubiak ◽  
M.H. Verlhac ◽  
W.H. Colledge ◽  
B. Maro
Keyword(s):  
Kinase Activity ◽  
Polar Body ◽  
Ribosomal Subunit ◽  
Germinal Vesicle ◽  
Maximum Level ◽  
Meiotic Maturation ◽  
S6 Kinase ◽  
Mouse Oocytes ◽  
M Phase ◽  
Second Polar Body

Mitogen-activated protein kinases (MAPK) become activated during the meiotic maturation of oocytes from many species; however, their molecular targets remain unknown. This led us to characterize the activation of the ribosomal subunit S6 kinase of Mr 82 X 10(3) - 92 X 10(3) (p90rsk; a major substrate of MAPK in somatic cells) in maturing mouse oocytes and during the first cell cycle of the mouse embryo. We assessed the phosphorylation state of p90rsk by examining the electrophoretic mobility shifts on immunoblots and measured the kinase activity of immunoprecipitated p90rsk on a S6-derived peptide. Germinal vesicle stage (GV) oocytes contained a doublet of Mr 82 × 10(3) and 84 × 10(3) with a low S6 peptide kinase activity (12% of the maximum level found in metaphase II oocytes). A band of Mr 86 × 10(3) was first observed 30 minutes after GV breakdown (GVBD) and became prominent within 2 to 3 hours. MAPK was not phosphorylated 1 hour after GVBD, when the p90rsk-specific S6 kinase activity reached 37 % of the M II level. 2 hours after GVBD, MAPK became phosphorylated and p90rsk kinase activity reached 86% of the maximum level. The p90rsk band of Mr 88 × 10(3), present in mature M II oocytes when S6 peptide kinase activity is maximum, appeared when MAPK phosphorylation was nearly complete (2.5 hours after GVBD). In activated eggs, the dephosphorylation of p90rsk to Mr 86 X 10(3) starts about 1 hour after the onset of pronuclei formation and continues very slowly until the beginning of mitosis, when the doublet of Mr 82 X 10(3) and 84 X 10(3) reappears. A role for a M-phase activated kinase (like p34cdc2) in p90rsk activation was suggested by the reappearance of the Mr 86 X 10(3) band during first mitosis and in 1-cell embryos arrested in M phase by nocodazole. The requirement of MAPK for the full activation of p90rsk during meiosis was demonstrated by the absence of the fully active Mr 88 X 10(3) band in maturing c-mos −/− oocytes, where MAPK is not activated. The inhibition of kinase activity in activated eggs by 6-DMAP after second polar body extrusion provided evidence that both MAPK- and p90rsk-specific phosphatases are activated at approximately the same time prior to pronuclei formation.

A combination of calcium ionophore and puromycin effectively produces human parthenogenones with one haploid pronucleus

Zygote ◽  
2001 ◽  
Vol 9 (1) ◽  
pp. 83-88 ◽  
Author(s):  
Koji Nakagawa ◽  
Shuji Yamano ◽  
Hisayo Nakasaka ◽  
Kenji Hinokio ◽  
Midori Yoshizawa ◽  
...  
Keyword(s):  
Polar Body ◽  
Calcium Ionophore ◽  
Calcium Ionophore A23187 ◽  
Control Group ◽  
Ionophore A23187 ◽  
Cleavage Rate ◽  
Parthenogenetic Activation ◽  
Second Polar Body ◽  
Activation Rates ◽  
Pronuclear Formation

Parthenogenetic activation with various combinations of the calcium ionophore A23187 and protein synthesis or phosphorylation inhibitors was investigated as a means of producing human parthenogenones with one haploid pronucleus. Unfertilised human aged oocytes exposed to 5 μM A23187 for 5 min were treated with 10 μg/ml puromycin (puromycin group, 46 oocytes) or 2 mM 6-dimethylaminopurine (DMAP group, 42 oocytes) for 5 h. Oocytes treated only with A23187 served as a control (control group, 40 oocytes). After washing the oocytes, they were incubated for up to 37 h. Evidence of activation (pronuclear formation) and cleavage was observed 18 h and 42 h after A23187 treatment, respectively. Activation rates in the puromycin and DMAP groups were significantly higher than in the control group (91% (42/46) and 77% (34/44) vs 20% (8/40), p < 0.05, respectively). In the puromycin group, 81% (34/42) of the activated oocytes showed one pronucleus with the second polar body (2ndPB), whereas none (0/34) of the activated oocytes in the DMAP group extruded the 2ndPB. The cleavage rate in the puromycin group was significantly lower than in the DMAP group (38% vs 68%, p < 0.05). The activated oocytes which had one pronucleus with the 2ndPB in the puromycin group showed a haploid set of chromosomes (10/13). In conclusion, the combination of A23187 and puromycin is effective for producing human parthenogenones with one haploid pronucleus.

Effective activation method with A23187 and puromycin to produce haploid parthenogenones from freshly ovulated mouse oocytes

Zygote ◽  
2000 ◽  
Vol 8 (3) ◽  
pp. 203-208 ◽  
Author(s):  
Hisayo Nakasaka ◽  
Shuji Yamano ◽  
Kenji Hinokio ◽  
Koji Nakagawa ◽  
Midori Yoshizawa ◽  
...  
Keyword(s):  
Polar Body ◽  
Calcium Ionophore ◽  
Calcium Ionophore A23187 ◽  
Ionophore A23187 ◽  
Effective Activation ◽  
Control Groups ◽  
Mouse Oocytes ◽  
Activation Rate ◽  
Second Polar Body ◽  
And Control

Freshly ovulated mouse oocytes exposed to 5 mM calcium ionophore A23187 for 5 min and controls (not exposed) were cultured in TYH medium with 10 μg/ml puromycin (the puromycin group) or 2 mM 6-dimethylaminopurine (DMAP; the DMAP group) for 4 h. Among the controls, few oocytes were activated even if they were treated with DMAP or puromycin. In the oocytes exposed to A23187, in contrast, the activation rate, i.e. the rate of oocytes showing at least one pronucleus (PN) after the treatment, was 46.2% (48/104) in the DMAP group and 90.0% (118/131) in the puromycin group. Activation rate in the puromycin group was significantly higher than in the DMAP and control groups (p < 0.0001, respectively). Furthermore, 82.4% (108/131) of the activated oocytes in the puromycin group showed one PN with extrusion of the second polar body (PB). In the puromycin group, the DNA content of the PN of parthenogenones with 1PN2PB was half that of a set of metaphase II chromosomes. Chromosomal analysis was possible in 14 parthenogenones with 1PN2PB in the puromycin group. The parthenogenones possessed a normal set (n = 20) of haploid chromosomes. The combination of A23187 and puromycin proved to be an effective method of producing haploid parthenogenones.

scholarly journals Effect of genetic background and activating stimulus on the timing of meiotic cell cycle progression in parthenogenetically activated mouse oocytes

Reproduction ◽  
2005 ◽  
Vol 129 (1) ◽  
pp. 27-38 ◽  
Author(s):  
Elena Ibáñez ◽  
David F Albertini ◽  
Eric W Overström
Keyword(s):  
Cell Cycle Progression ◽  
Polar Body ◽  
Ethanol Exposure ◽  
Meiotic Progression ◽  
Spindle Rotation ◽  
Activation Efficiency ◽  
Metaphase Stage ◽  
Nuclear Events ◽  
Second Polar Body ◽  
Activation Rates

With the aim of investigating the effects of oocyte genotype and activating stimulus on the timing of nuclear events after activation, oocytes collected from hybrid B6D2F1, inbred C57BL/6 and outbred CF-1 and immunodeficient nude (NU/+) females were activated using ethanol or strontium and fixed at various time-points. Meiotic status, spindle rotation and second polar body (PB2) extrusion were monitored by fluorescence microscopy using DNA-, microtubule- and microfilament-selective probes. Although activation efficiency was similar in all groups of oocytes, a significant percentage of CF-1 and NU/+ oocytes treated with ethanol and of C57BL/6 oocytes treated either with ethanol or strontium failed to complete activation and became arrested at a new metaphase stage (MIII) after PB2 extrusion. C57BL/6 oocytes also showed slower release from MII arrest but faster progression to telophase (TII) after ethanol exposure, and they exhibited the most rapid exit from TII under both activation treatments. Strontium caused delayed meiotic resumption, spindle rotation and PB2 extrusion, but rapid TII exit, in B6D2F1, CF-1 and NU/+ oocytes when compared with ethanol. Compared with all other strains, NU/+ oocytes were significantly slower in completing spindle rotation and PB2 extrusion, irrespective of the activating stimulus, and a significant decrease in activation rates and pace of meiotic progression was observed after strontium exposure. Thus, our findings demonstrated that the kinetics of meiosis resumption and completion, spindle rotation and PB2 extrusion following parthenogenetic activation depends on both genotype-specific factors and on the activation treatment applied.

Intracytoplasmic sperm injection in the rat

Zygote ◽  
1998 ◽  
Vol 6 (2) ◽  
pp. 143-147 ◽  
Author(s):  
D. Dozortsev ◽  
T. Wakaiama ◽  
A. Ermilov ◽  
R. Yanagimachi
Keyword(s):  
Intracytoplasmic Sperm Injection ◽  
Polar Body ◽  
Survival Rates ◽  
Experimental Conditions ◽  
Injection Techniques ◽  
Partial Zona Dissection ◽  
Second Polar Body ◽  
Polar Body Extrusion ◽  
Activation Rates ◽  
Pronuclear Formation

We applied intracytoplasmic sperm injection (ICSI) to the rat comparing three different sperm injection techniques: conventional setup with a sharp needle bearing a spike (method 1), combination of partial zona dissection (PZD) needle and blunt pipette (method 2) and piezo-injection using a blunt pipette (method 3). We also investigated the timing of sperm pronuclear formation after injection. Survival rates after injection were 8%, 24% and 71% for the methods 1, 2 and 3, respectively. All surviving oocytes formed pronuclei by about 6 h after injection. Although the survival and activation rates following sperm injection using piezo-injection were high, the incidence of normal fertilisation, as evidenced by second polar body extrusion and formation of two pronuclei, was only 10%. The vast majority of the zygotes were multinucleated, although most of them subsequently underwent cleavage. Fixation and staining of injected oocytes at different times after injection revealed that replacement of sperm nuclear protamines by histones takes place by 15 min after injection, sperm head swelling occurs within 0.5–1 h after injection and pronuclei become fully developed by 7 h after injection. Although the rate of normal fertilisation in the rat following ICSI was low under the present experimental conditions, the results indicated that direct ICSI using a piezo-driven pipette would be a potentially valuable method of producing rat offspring.

Highly effective method of human oocyte activation

Zygote ◽  
1995 ◽  
Vol 3 (2) ◽  
pp. 157-161 ◽  
Author(s):  
Jacob Levron ◽  
Jacques Cohen ◽  
Steen Willadsen
Keyword(s):  
Polar Body ◽  
Human Oocyte ◽  
Isotonic Solution ◽  
Oocyte Activation ◽  
Effective Activation ◽  
Low Concentrations ◽  
Activation Efficiency ◽  
Calcium Magnesium ◽  
Second Polar Body ◽  
Activation Rates

SummaryFresh and aged unfertilised human oocytes were activated by electroporation and by exposure to isotonic solution of mannitol supplemented with low concentrations of calcium magnesium and chloride. Over 95% of the fresh oocytes were activated, all showing formation of one pronucleus and extrusion of the second polar body. Oocytes activated 1 and 2 days post-collection showed activation rates of 66.6% and 64.1%, respectively; however, the proportion of one-pronucleate oocytes in these groups was significantly lower (61.6% and 23.5%, respectively). There was no difference in the activation efficiency between the two activation modes. Twelve activated oocytes from the freshly collected group cleaved when left in culture. It is concluded that, in the human, a brief exposure to isotonic solution of mannitol with low concentrations of calcium, magnesium and chloride is a very effective activation stimulus.

34 PRE-IMPLANTATION DEVELOPMENT OF HORSE SOMATIC CELL NUCLEAR TRANSFER EMBRYOS ORIGINATED FROM METAPHASE I OOCYTES

2013 ◽  
Vol 25 (1) ◽  
pp. 165
Author(s):  
I. Lagutina ◽  
S. Colleoni ◽  
G. Lazzari ◽  
C. Galli
Keyword(s):  
Growth Factor ◽  
Nuclear Transfer ◽  
Polar Body ◽  
Metaphase Plate ◽  
Developmental Competence ◽  
Blastocyst Formation ◽  
Chi Square ◽  
Serum Replacement ◽  
Chi Square Test ◽  
Metaphase I

Because of the limited availability of horse oocytes and their wide maturation window that leads to the existence of a subpopulation of oocytes with significantly longer maturation period, horse cloning would benefit from the possibility of also using metaphase I oocytes (Choi et al. 2009 Cloning Stem Cells). The scope of this work was to compare the developmental ability of cloned horse embryos constructed using oocytes in metaphase I (MI) and II (MII). Oocytes of slaughtered mares were matured for 27 h in DMEM/F12 with 10% FCS, 1 µL mL–1 of insulin-transferrin-selenium (ITS), 1 mM sodium pyruvate, 50 ng mL–1 of long-epidermal growth factor, 100 ng mL–1 of long-insulin-like growth factor-I, and 0.1 IU mL–1 each of FSH and LH, denuded of cumulus and enucleated using a zona-free method (Lagutina et al. 2005 Reproduction). Oocytes with a visible polar body were classified as MII and those with no polar body as potential MI. During enucleation, oocytes having a metaphase plate only were confirmed as MI; oocytes in anaphase and telophase were classified as MII. Adult skin fibroblasts of passages 2 to 10 were cultured in TCM 199/DMEM with 10% FCS and serum starved for 1 to 2 days before NT. Nuclear-transfer embryos were constructed after washing of zona-free oocytes in 400 µg mL–1 of phytohemagglutinin P and attachment of each to a single cell in HEPES-SOF by fusion with 2 direct-current (DC) pulses of 1.2 kV cm–1 applied for 30 µs in fusion medium. One hour after fusion, embryos were activated by 5 µM ionomycin for 4 min, followed by culture in 5 µg mL–1 of cycloheximide and 1 mM DMAP in SOFaa for 3 h. Embryos were cultured in SOFaa in 5% CO2 and 5% O2 at 38.5°C. Half of the medium was renewed on Day 3 and replaced on Day 5 with DMEM/F12 with 5% FCS and 5% serum replacement. Cleavage was assessed 48 h after activation and the rate of blastocyst formation was recorded at Day 8. The data were compared by chi-square test. The development ability of MI embryos assessed by cleavage and blastocyst formation was significantly lower than of MII embryos (Table 1). The obtained MI blastocysts were smaller than their MII counterparts. These data demonstrate that MI oocytes account for 20% of the total oocytes after 27 h of maturation, have a lower developmental competence to form blastocysts after NT, and the blastocysts obtained are of smaller size and likely less viable. Therefore, the use of MI oocytes can only marginally improve the outcome of horse cloning. Table 1.Embryo development after SCNT using oocytes in metaphase I or II

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