In Vitro Culture of Single Bovine Embryos with Microwell Plates Made of Poly(dimethylsiloxane) Cured under Low Pressure.

Single embryo culture is useful for assessing the developmental competence of an embryo in detail. Recently, a device made of poly(dimethylsiloxane) (PDMS), which is biocompatible and nontoxic, has been widely used for culture various types of cells. However, PDMS plates are porous, causing the serious osmolality increment of the medium (over 600 mOsm/kg from Day 4 to Day 7). Here, we report that curing the PDMS under low pressure (LP-PDMS) greatly reduced the porosity, resulting in a constant osmolality of the medium. The blastocyst rate of single bovine embryos cultured with LP-PDMS microwell (MW) plates was the same as that of group-cultured embryos (25 embryos/50 µl droplet; control, P>0.05). These results indicate that MWs on a plate made of PDMS cured under low pressure can be successfully used for individual embryo culture.

Early germinal vesicle breakdown is a predictor of high preimplantation developmental competent oocytes in mice.

The preselection of highly developmentally competent oocytes for in vitro maturation (IVM) is crucial for improving assisted reproductive technology. Although several intrinsic markers of oocyte quality are known to be closely related to the onset of nuclear maturation (germinal vesicle break down, GVBD), a direct comparison between GVBD timing and oocyte quality has never been reported. In this study, we established a non-invasive oocyte evaluation method based on GVBD timing for preselecting more developmental competent oocytes in mice. Because the O2 concentration during IVM may affect the nuclear kinetics, all experiments were performed under two distinct O2 concentrations: 20% and 5% O2. First, we determined the time course of changes in nuclear maturation and preimplantation developmental competence of in vitro-matured oocytes to estimate GVBD timing in high developmental competent oocytes. Two-thirds of oocytes that underwent GVBD in early IVM seemed to mainly contribute to the blastocyst yield. To confirm this result, we compared the preimplantation developmental competence of the early and late GVBD oocytes. Cleavage and blastocyst formation rates of early GVBD oocytes (80.2% and 52.7% under 20% O2, respectively, and 67.6% and 47.3% under 5% O2, respectively) were almost double those of late GVBD oocytes (44.8% and 26.0% under 20% O2, respectively, and 40.4% and 17.9% under 5% O2, respectively). With no observable alterations by checking the timing of GVBD in preimplantation developmental competence, oocyte evaluation based on GVBD timing can be used as an efficient and non-invasive preselection method for high developmental competent oocytes.

Early development of cloned bovine embryos produced from oocytes enucleated by fluorescence metaphase II imaging using a conventional halogen-lamp microscope.

Enucleation of a recipient oocyte is one of the key processes in the procedure of somatic cell nuclear transfer (SCNT). However, especially in bovine species, lipid droplets spreading in the ooplasm hamper identification and enucleation of metaphase II (MII) chromosomes, and thereby the success rate of the cloning remains low. In this study we used a new experimental system that enables fluorescent observation of chromosomes in living oocytes without any damage. We succeeded in visualizing and removing the MII chromosome in matured bovine oocytes. This experimental system consists of injecting fluorescence-labeled antibody conjugates that bind to chromosomes and fluorescent observation using a conventional halogen-lamp microscope. The cleavage rates and blastocyst rates of bovine embryos following in vitro fertilization (IVF) decreased as the concentration of the antibody increased (p<0.05). The enucleation rate of the conventional method (blind enucleation) was 86%, whereas all oocytes injected with the antibody conjugates were enucleated successfully. Fusion rates and developmental rates of SCNT embryos produced with the enucleated oocytes were the same as those of the blind enucleation group (p>0.05). For the production of SCNT embryos, the new system can be used as a reliable predictor of the location of metaphase plates in opaque oocytes, such as those in ruminant animals.

Expression analysis of circadian genes in oocytes and preimplantation embryos of cattle and rabbits.

We previously showed that circadian genes clock, bmal1, cry1, cry2, per1, and per2 are expressed and function as maternal mRNA regulating events in the oocytes and preimplantation embryos of mice. Recent evidence indicates however that either or both expression profiles of circadian genes in some tissues, and transcript sequences of circadian genes, differ to generate the physiological differences between diurnal and nocturnal species. We therefore investigated the expression profiles of circadian genes in oocytes and preimplantation embryos of species other than mice, namely cattle and rabbits, representing diurnal and nocturnal species, respectively, and determined the protein sequences of circadian genes in these species. Quantitative real-time PCR revealed that all circadian genes considered in this study were present in the oocytes and preimplantation embryos of both species, and the transcript amounts of clock, cry1 and per1 contained in oocytes were significantly higher than in preimplantation embryos of both species. The transcripts of clock, cry1, and per1 of cattle and rabbits were determined by primer walking, and functional domains in the estimated amino acid sequences were compared between cattle and rabbits and with those of humans and mice. The sequences of clock, cry1, and per1 in cattle and rabbits closely resembled those in mice (85-100% homologies), and no difference based on diurnality or nocturnality was observed. These findings suggest that circadian genes in the oocytes and preimplantation embryos of mammals fulfill the same functions across species as maternal mRNA.

Effect of polyethylene glycol and dimethyl sulfoxide on the fusion of bovine nuclear transfer using mammary gland epithelial cells.

The effects of polyethylene glycol and dimethyl sulfoxide (PEG/DMSO) treatment of donor cells on the fusion and subsequent development of bovine nuclear transfer embryos using mammary gland epithelial (MGE) cells before electrofusion (fresh MGE cells) was studied. The same study was conducted on those cells that were frozen and stored in liquid nitrogen, and then thawed (frozen-thawed MGE cells). Experiment 1 showed that the exposure time and pH of PEG/DMSO solution affected the fusion of nuclear transfer, and that a higher fusion rate was obtained when fresh MGE cells were exposed to PEG/DMSO solution at pH 8.0 for 5 min. In Experiment 2, the proportion of fused oocytes with fresh PEG/DMSO-treated cells (70 +/- 6%) was significantly higher than that with non-treated cells (50 +/- 13%, p < 0.05). The same tendency was observed when frozen-thawed cells as donor nuclei were used (48 +/- 6% vs. 34 +/- 12%, p < 0.05). In addition, PEG/DMSO treatment has neither harmful nor beneficial effects on the cleavage and development of the blastocyst stage of reconstructed embryos (p > 0.05). The fusion and cleavage rates of frozen-thawed cells were significantly lower than those of fresh cells (p < 0.05). After 10 blastocysts, derived from fresh PEG/DMSO-treated cells, were transferred to five recipient heifers, one live female calf was obtained. Experiment 3 showed that PEG/DMSO treatment reduced the viability of both fresh and frozen-thawed MGE cells (p < 0.05). We conclude that the PEG/DMSO treatment of fresh MGE cells, as well as the frozen-thawed cells, before electrofusion has a positive effect on the fusion of nuclear transfer without decreasing the in vitro development of reconstructed embryos.

Remarkable differences in telomere lengths among cloned cattle derived from different cell types.

Regarding cloned animals, interesting questions have been raised as to whether cloning restores cellular senescence undergone by their donor cells and how long cloned animals will be able to live. Focusing our attention on differences in telomere lengths depending on the tissue, we had produced 14 cloned cattle by using nuclei of donor cells derived from muscle, oviduct, mammary, and ear skin. Here, we show remarkable variation in telomere lengths among them using Southern blot analysis with telomere-specific probe. Telomere lengths in cloned cattle derived from muscle cells of an old bull were longer than those of a donor animal but were within the variation in normal calves. On the other hand, those derived from oviductal and mammary epithelial cells of an equally old cow were surprisingly shorter than any found in control cattle. The telomere lengths of cloned cattle derived from fibroblasts and oviductal epithelial cells of younger cattle showed the former and the latter results, respectively. In both cases, however, less telomere erosion or telomere extension from nuclear transfer to birth in most cloned cattle was observed in comparison with telomere erosion from fertilization to birth in control cattle. Embryonic cell-cloned cattle and their offspring calves were also shown to have telomeres longer than those in age-matched controls. These observations indicate that cloning does not necessarily restore the telomere clock but, rather, that nuclear transfer itself may commonly trigger an elongation of telomeres, probably more or less according to donor cell type. Remarkable variations among cloned cattle are suggested to be caused by variation in telomere length among donor cells and more or less elongation of telomere lengths induced by cloning.