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G. Vajta, P. Holm, T. Greve and H. Callesen

The purpose of this study was to establish an efficient combination of assisted hatching and cryopreservation procedures for producing bovine embryos in vitro. A total of 1312 day 7 blastocysts were subjected randomly to 14 different combinations of three factors: osmotic stress, assisted hatching and vitrification. Re-expansion, initiation and completion of the hatching process, as well as attachment to the culture dish, were analysed by SAS Genmod procedure. Incubation with sucrose was found to decrease survival rates; among the assisted hatching procedures used, zona fenestration resulted in higher survival rates compared with partial zona dissection and controls; and vitrification decreased survival and further development. The combined effect of sucrose incubation and vitrification decreased further development markedly, as did partial zona dissection followed by vitrification. Partial zona dissection performed in medium containing sucrose severely lowered embryo survival. Zona fenestration without sucrose incubation followed by vitrification did not compromise further embryo development: 86%, 84% and 79% of the blastocysts initiated, completed hatching and attached to the bottom, respectively. These data were not different from the controls (80%, 76% and 63%, respectively; P > 0.05). Cell count analysis revealed a decrease in the total number of cells as a result of the assisted hatching and vitrification compared with controls (135 versus 202, respectively; P < 0.0001). Although embryo transfer results (36% pregnancy rate and 30% calving rate) require further improvement, this combination of methods may prove useful in the commercial production of bovine embryos in vitro.

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Z Macháty, A Páldi, T Csáki, Z Varga, I Kiss, Z Bárándi and G Vajta

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T. T. Peura, M. W. Lane, G. Vajta and A. O. Trounson

The use of cryopreserved in vitro produced bovine embryos as nuclear transfer donors was assessed. Day 4 or 5 morulae were vitrified and warmed using the open pulled straw method and used as donors for nuclear transfer. Although the proportion of morulae and blastocysts that developed from nuclear transfer embryos derived from day 5 vitrified embryos did not differ from that derived from fresh embryos (16.7 and 24.3%, respectively), development to blastocysts was reduced when vitrified donor cells were used (8.3 and 19.1%, respectively). Likewise, development to morulae and blastocysts was not different between nuclear transfer embryos derived from day 4 vitrified embryos allowed to recover for 24 h, and day 5 vitrified embryos allowed to recover for 1–2 h (27.7 and 15.6%, respectively), but the development to blastocysts was reduced when day 5 vitrified donor cells were used (23.2 and 10.0%, respectively). However, in nuclear transfer embryos derived from either day 4 vitrified or day 5 fresh donors, no differences were observed in development rates to morulae and blastocysts (34.3 and 36.3%, respectively) or to blastocysts alone (20.2 and 18.1%, respectively). Nor were there differences in the development rates of fresh or day 4 or day 5 vitrified in vitro produced (non-nuclear transfer) embryos (47.9, 51.0 and 35.5% developing to blastocysts at day 7, respectively). In vitro produced embryos and nuclear transfer embryos derived from day 4 vitrified or day 5 fresh donors were transferred to recipients at morula or blastocyst stage at day 6 or 7. The pregnancy rates were similar in both groups of nuclear transfer embryos, but higher in the control group consisting of in vitro produced embryos (47, 42 and 67%, respectively). In conclusion, if vitrified donor embryos are allowed to recover for 24 h after warming, their use in nuclear transfer results in similar efficiencies to those achieved with fresh embryos.

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Z. Macháty, A. Páldi, T. Csáki, Z. Varga, I. Kiss, Z. Bárándi and G. Vajta

Sex of early bovine embryos was determined by polymerase chain reaction (PCR) using a single blastomere removed at the 16–32 cell stage. Embryos were produced in vitro and biopsied on the fifth day after in vitro fertilization. Biopsied embryos were cultured on a cumulus cell monolayer until embryo transfer. For the PCR, one pair of bovine-specific and one pair of Y-chromosome-specific primers were used. Definite signals following PCR amplification were obtained in 95.4% of cases indicating that one blastomere from a preimplantation bovine embryo is sufficient for sex determination by PCR. Nineteen biopsied embryos of predetermined sex were transferred into synchronized recipient females to examine their developmental potential in vivo. Ten of the recipients (52.6%) were found to be pregnant by ultrasonography 25 days after transfer. This result did not differ significantly from that achieved with the use of the control non-manipulated IVF embryos (54.1%; P > 0.1).

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P Maddox-Hyttel, NI Alexopoulos, G Vajta, I Lewis, P Rogers, L Cann, H Callesen, P Tveden-Nyborg and A Trounson

The problems of sustaining placenta formation in embryos produced by nuclear transfer have emphasized the need for basic knowledge about epiblast formation and gastrulation in bovine embryos. The aims of this study were to define stages of bovine post-hatching embryonic development and to analyse functional mechanisms of germ-layer formation. Embryos developed in vivo were collected after slaughter from superovulated cows on days 9, 11, 14 and 21 after insemination and processed for transmission electron microscopy (n = 26) or immunohistochemistry (n = 27) for potential germ-layer characterization (cytokeratin 8 for potential ectoderm; alpha-1-fetoprotein for potential endoderm; and vimentin for potential mesoderm). On day 9, the embryos were devoid of zona pellucida and presented a well-defined inner cell mass (ICM), which was covered by a thin layer of trophoblast cells (the Rauber's layer). Formation of the hypoblast from the inside of the ICM was ongoing. On day 11, the Rauber's layer was focally interrupted and adjacent underlying ICM cells formed tight junctions. The hypoblast, which formed a thin confluent cell layer, was separated from the ICM and the tropho-blast by intercellular matrix. The embryos were ovoid to tubular and displayed a confluent hypoblast on day 14. The epiblast was inserted into the trophoblast epithelium and tight junctions and desmosomes were present between adjacent epiblast cells as well as between peripheral epiblast and trophoblast cells. In some embryos, the epiblast was more or less covered by foldings of trophoblast in the process of forming the amniotic cavity. Cytokeratin 8 was localized to the trophoblast and the hypoblast underlying the epiblast; alpha-1-fetoprotein was localized to most hypoblast cells underlying the trophoblast; and vimentin was localized to most epiblast cells. On day 21, the smallest embryos displayed a primitive streak and formation of the neural groove, whereas the largest embryos presented a neural tube, up to 14 somites and allantois development. These embryos depicted the gradual formation of the endoderm, mesoderm and ectoderm as well as differentiation of paraxial, intermediate and lateral plate mesoderm. Cytokeratin 8 was localized to the trophoblast, the hypoblast and the surface and neural ectoderm; and alpha-1-fetoprotein was localized to the hypoblast, but not the definitive endoderm, the intensity increasing with development. Vimentin was initially localized to some, but not all, cells positioned particularly in the ventral region of the primitive streak, to presumptive definitive endoderm cells inserted into the hypoblast, and to mesoderm. In conclusion, within 2 weeks of hatching, bovine embryos complete formation of the hypoblast and the epiblast, establishment of the amniotic cavity, ingression of epiblast cells for primitive streak formation, involution of cells through the node and the streak for endoderm and mesoderm fomation, neurulation and differentiation of the mesoderm. The recruitment of cells from the epiblast to form the primitive streak as well as the endoderm and mesoderm is associated with expression of the intermediate filament vimentin.

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Y Du, C S Pribenszky, M Molnár, X Zhang, H Yang, M Kuwayama, A M Pedersen, K Villemoes, L Bolund and G Vajta

The purpose of the present study was to improve cryotolerance using high hydrostatic pressure (HHP) pretreatment of porcine in vitro matured (IVM) oocytes, to facilitate their further developmental competence after parthenogenetic activation. A total of 1668 porcine IVM oocytes were used in our present study. The pressure tolerance and optimal duration of recovery after HHP treatment were determined. Oocytes were treated with either 20 or 40 MPa (200 and 400 times greater than atmospheric pressure) for 60 min, with an interval of 10, 70, and 130 min between pressure treatment and subsequent vitrification under each pressure parameter. Oocytes from all vitrification groups had much lower developmental competence than fresh oocytes (P<0.01) measured as cleavage and blastocyst rates. However, significantly higher blastocyst rates (P<0.01) were obtained in the groups of 20 MPa pressure, with either 70 (11.4±2.4%) or 130 (13.1±3.2%) min recovery, when compared with the vitrification control group without HHP treatment where no blastocysts were obtained. The influence of temperature at HHP treatment on further embryo development was also investigated. Treatments of 20 MPa with 70 min recovery were performed at 37 °C or 25 °C. Oocytes pressurized at 37 °C had a significantly higher blastocyst (14.1±1.4%) rate than those treated at 25 °C (5.3±1.1%; P<0.01). Our results demonstrate that HHP pretreatment could considerably improve the developmental competence of vitrified pig in vitro matured (IVM) oocytes. The HHP pretreatment will be tested as a means to improve survival and developmental competence at different developmental stages in different species including humans.