Temporal effects of ouabain on in-vitro development of mouse zygotes

Gene expression profiling of the pre-implantation mouse embryo by microarray analysis: comparison of the two-cell stage and two-cell block

To improve our understanding of the molecular mechanisms underlying early embryo development, further characterization of gene activity in oocytes and embryos is urgently required. The transition from the two-cell to four-cell stage is particularly important in pre-implantation embryonic development, as it involves transcriptional reprogramming and cellular differentiation. In this study, we used a 7.4 K cDNA microarray to screen mRNA transcript levels in the pre-implantation mouse embryo. Real-time PCR was used to confirm microarray data. We profiled 7,410 genes and identified 4,562 genes that were differentially expressed in the pre-implantation embryo. We selected a total of 248 genes with significant expression changes that are functionally involved in the two-cell and two-cell block embryo. Of these genes, 114 were down-regulated and the remainder (n=134) were up-regulated in the two-cell embryo. This study provides a developmental map of a large number of genes in the pre-implantation mouse embryo with particular emphasis on gene expression in the two-cell embryo and two-cell block embryo. Further investigations based on this data will provide a better understanding of the effects of various external conditions and may facilitate comparative analysis of pre-implantation development in other mammalian species, including human.

Surface-expressed E-cadherin, and mitochondrial and microtubule distribution in rescue of mouse embryos from 2-cell block by aggregation

E-cadherin (uvomorulin)-mediated cell interactions are essential for preimplantation development in mammals. We observed that E-cadherin is expressed at contact sites between blastomeres of 2-cell mouse embryos of non-blocking genotype (CBA x C57BL F1) explanted at 32 h post human chorionic gonadotrophin (HCG) and cultured in vitro, while blastomere rounding and reduced zones of contact and E-cadherin-staining were observed in embryos of a blocking strain (MF1) arrested at the 2-cell stage. Embryos of MF1 strain can be rescued by aggregation with four 2-cell embryos of the non-blocking genotype. An early event in rescue is E-cadherin expression at contact zones between adjacent embryos of different genotype in aggregation chimeras. E-cadherin-mediated signalling appears important for the rescue (including formation of adherens-like contacts, cell polarization and morphogenetic processes) since there is no rescue when E-cadherin-specific antibodies are present during phytohaemagglutinin-mediated aggregation and subsequent culture. In blocked embryos, the distribution of microtubules is disturbed and concomitantly mitochondria cluster around the nucleus. Rescue by aggregation retains normal mitochondrial distribution in the presence of a dense microtubular lattice in all blastomeres. Therefore, E-cadherin-mediated signalling and its downstream effects on cytoskeletal organization are essential in the rescue of blocking embryos by aggregation. Normal preimplantation development appears to be dependent on the polarized expression of surface E-cadherin and the microtubule-mediated dispersal of mitochondria.

Transmembrane regulation of intracellular calcium by a plasma membrane sodium/calcium exchanger in mouse ova

Regulation of cytoplasmic free calcium concentration ([Ca2+)]i) is a key factor for maintenance of viability of cells, including oocytes. Indeed, during fertilization of an ovum, [Ca2+]i is known to undergo oscillations, but it is unknown how basal [Ca2+]i or calcium oscillations are regulated. In the present study we investigated the role of the plasma membrane in regulating [Ca2+]i of metaphase II-arrested mouse oocytes (ova). Ova were collected from B6C3F1 mice treated with eCG (10 IU) and hCG (5 IU), and intracellular calcium was determined by means of fura-2. Extracellular calcium flux across the zona pellucida was detected noninvasively by a calcium ion-selective, self-referencing microelectrode that was positioned by a computer-controlled micromanipulator. Under basal conditions ova exhibited a calcium net efflux of 20.6 +/- 5.2 fmol/cm2 per sec (n = 69). Treatment of ova with ethanol (7%) or thapsigargin (25 nM-2.5 microM) transiently increased intracellular calcium and stimulated calcium efflux that paralleled levels of [Ca2+]i. The presence of a Na+/Ca2+ exchanger was indicated by experiments employing both bepridil, an inhibitor of Na+/Ca2+ exchange, and sodium-depleted media. In the presence of bepridil, a net influx of calcium was revealed across the zona pellucida, which was reflected by an increase in the [Ca2+]i. In addition, replenishment of extracellular sodium to ova that had been incubated in sodium-depleted media induced a large calcium efflux, consistent with the actions of Na+/Ca2+ exchange. Sodium/calcium exchange in mouse ova may be an important mechanism that regulates [Ca2+]i.