Tetraploid Mouse Embryos Research Articles

Tetraploid Embryonic Stem Cells Maintain Pluripotency and Differentiation Potency into Three Germ Layers.

Polyploid amphibians and fishes occur naturally in nature, while polyploid mammals do not. For example, tetraploid mouse embryos normally develop into blastocysts, but exhibit abnormalities and die soon after implantation. Thus, polyploidization is thought to be harmful during early mammalian development. However, the mechanisms through which polyploidization disrupts development are still poorly understood. In this study, we aimed to elucidate how genome duplication affects early mammalian development. To this end, we established tetraploid embryonic stem cells (TESCs) produced from the inner cell masses of tetraploid blastocysts using electrofusion of two-cell embryos in mice and studied the developmental potential of TESCs. We demonstrated that TESCs possessed essential pluripotency and differentiation potency to form teratomas, which differentiated into the three germ layers, including diploid embryonic stem cells. TESCs also contributed to the inner cell masses in aggregated chimeric blastocysts, despite the observation that tetraploid embryos fail in normal development soon after implantation in mice. In TESCs, stability after several passages, colony morphology, and alkaline phosphatase activity were similar to those of diploid ESCs. TESCs also exhibited sufficient expression and localization of pluripotent markers and retained the normal epigenetic status of relevant reprogramming factors. TESCs proliferated at a slower rate than ESCs, indicating that the difference in genomic dosage was responsible for the different growth rates. Thus, our findings suggested that mouse ESCs maintained intrinsic pluripotency and differentiation potential despite tetraploidization, providing insights into our understanding of developmental elimination in polyploid mammals.

Production of chimeras by aggregation of embryonic stem cells with diploid or tetraploid mouse embryos

The production of mouse chimeras is a common step in the establishment of genetically modified animal strains. Chimeras also provide a powerful experimental tool for following cell behavior during both prenatal and postnatal development. This protocol outlines a simple and economical technique for the production of large numbers of mouse chimeras using traditional diploid morula<-->diploid embryonic stem (ES) cell aggregations. Additional steps are included to describe the procedures necessary to produce specialized tetraploid chimeras using tetraploid morula<-->diploid ES cell aggregations. This increasingly popular form of chimera produces embryos of nearly complete ES cell derivation that can be used to speed transgenic production or ask developmental questions. Using this protocol, mouse chimeras can be generated and transferred to pseudopregnant surrogate mothers in a 5-d period.

Defective trophoblast function in mice with a targeted mutation of Ets2.

Members of the Ets family of transcription factors mediate transcriptional responses of multiple signaling pathways in diverse cell types and organisms. Targeted deletion of the conserved DNA binding domain of the Ets2 transcription factor results in the retardation and death of homozygous mouse embryos before 8.5 days of embryonic development. Defects in extraembryonic tissue gene expression and function include deficient expression of matrix metalloproteinase-9 (MMP-9, gelatinase B), persistent extracellular matrix, and failure of ectoplacental cone proliferation. Mutant embryos were rescued by aggregation with tetraploid mouse embryos, which complement the developmental defects by providing functional extraembryonic tissues. Rescued Ets2-deficient mice are viable and fertile but have wavy hair, curly whiskers, and abnormal hair follicle shape and arrangement, resembling mice with mutations of the EGF receptor or its ligands. However, these mice are not deficient in the production of TGFalpha or the EGF receptor. Homozygous mutant cell lines respond mitogenically to TGFalpha, EGF, FGF1, and FGF2. However, FGF fails to induce MMP-13 (collagenase-3) and MMP-3 (stromelysin-1) in the Ets2-deficient fibroblasts. Ectopic expression of Ets2 in the deficient fibroblasts restores expression of both matrix metalloproteinases. Therefore, Ets2 is essential for placental function, mediating growth factor signaling to key target genes including MMP-3, MMP-9, and MMP-13 in different cell types, and for regulating hair development.

Effect of insulin on in vitro development of tetraploid mouse embryos.

The effect of insulin on in vitro development of early stage tetraploid mouse embryos was examined. Tetraploid embryos were produced at the 2-cell stage by treatment with cytochalasin B for 12 hr, then they were cultured with M16 medium containing 1 microM insulin. Although insulin had no effect on the embryos until 72 hr after hCG injection, at 96 hr the number of cells of insulin (+) tetraploid blastocysts increased 20.2% more than insulin (-) tetraploid embryos. The increase in the number of cells mainly depended on the inner cell mass (ICM), in which insulin (+) cells increase 76.3% more than insulin (-) cells. These effects of insulin on tetraploid embryos were comparable with those on diploid embryos. The present results suggested that the effect of insulin on the cell proliferation of early stage embryos was not affected by tetraploidy, but the increased number of cells in tetraploid blastocysts was significantly smaller than that in diploid embryos.

Preimplantation development of tetraploid mouse embryo produced by cytochalasin B.

Tetraploid mouse embryos usually cease to develop early after implantation, though they can develop to blastocysts. To characterize the failure of development in detail, tetraploid mouse embryos at the preimplantation period were examined as to both their morphology and number of cells. The tetraploid embryos were produced by 12 hr treatment with cytochalasin B (CB) at the 2-cell stage of backcross of (C57BL/6 x C3H/He) F1 x C3H/He. The tetraploid embryos in the preimplantation period exhibited compaction at 72 hr after hCG injection and blastocyst formation at 96 hr, as well as diploid embryos, but the number of cells composing the embryos was significantly smaller than that in the diploid embryos. At the term 60-96 hr after hCG injection, mean cell cycles were 14.03 hr in the tetraploid embryos, but 12.02 hr in the diploid. When tetraploid embryos were transferred into the oviducts of pseudopregnant recipients immediately after CB treatment, the number of cells in tetraploid blastocysts was increased compared with the embryos cultured in vitro, though the number did not reach that of diploid embryos. These results suggested that compaction and blastocyst formation in preimplantation development of tetraploid embryos depended on the time after hCG injection, irrespective of the number of cells or the length of the cell cycle. The lengthening of the cell cycle in tetraploid embryos may be one of the causes of failure in postimplantation development.

Restricted Distribution of Tetraploid Cells in Mouse Tetraploid ↔ Diploid Chimaeras

Tetraploid mouse embryos were produced by electrofusion at the 2-cell stage, cultured overnight, and aggregated with normal diploid embryos to produce tetraploid↔diploid (4 n ↔ 2 n) chimaeric conceptuses. At 7 1 2 days the 4 n ↔ 2 n chimaeras were usually smaller and developmentally retarded compared to control diploid↔diploid chimaeras. At 12 1/2 days the 4 n ↔ 2 n chimaeras had heavier placentas but there was no significant difference in fetal size. Tetraploid cells showed a restricted tissue distribution at both developmental stages studied: 4 n cells were commonly present in both the primitive endoderm and the trophectoderm lineages but they rarely contributed to the primitive ectoderm lineage. The overall similarity in the distribution of tetraploid cells at 7 1 2 and 12 1 2 days implies that whatever causes the restricted tissue distribution operates largely before 7 1 2 days. There was no evidence for excessive embryonic losses of 4 n ↔ 2 n chimaeras. So, if the restricted distribution of 4 n cells was a result of cell selection, the mechanism is more likely to involve loss of 4 n cells from the primitive ectoderm early in development rather than selective death of conceptuses with tetraploid cells in this lineage. Alternatively, 4 n cells may be preferentially allocated to the trophectoderm and primitive endoderm rather than the primitive ectoderm layer at the blastocyst stage.

Stage-specific gene expression in asynchronous tetraploid mouse embryos formed by fusion of blastomeres and fertilized eggs.

Asynchronous tetraploid mouse embryos were generated by electrofusion of fertilized eggs with blastomeres from different cleavage stages. The majority of the cytoplasm was always contributed by the egg. The best development was observed when eggs were fused with 2-cell blastomeres. Both genomes became active in fusion embryos (at least the genes for glucose phosphate isomerase did). Stage-specific protein synthesis seemed to be more adjusted to the developmental stage of the egg's than of the blastomere's genome, but at the 2-cell stage both contributed slightly differently to the protein patterns. Also, the time range of the first appearance of the stage-specific embryonic antigen SSEA-1 was wider in fusion embryos than in controls. It seems that the two genomes are not completely synchronized in these tetraploid embryos, a further indication that, in the mouse, the cytoplasm of fertilized eggs might not be compatible with older embryonic nuclei.

Relationship between cell size and nuclear volume in nucleated red blood cells of developmentally matched diploid and tetraploid mouse embryos.

Analysis of control diploid and polyploid amphibia indicated that cell and nuclear volumes were closely related to their ploidy, so that an increase in ploidy was generally associated with an increase in cell size. This relationship is also believed to occur in mammalian polyploids. However, since the latter are only rarely encountered spontaneously, or only occasionally following experimental manipulation, no detailed morphometric studies have been carried out to date to confirm whether such a relationship exists. In this study, the cellular and nuclear volume of primitive red blood cells was analyzed in carefully developmentally matched control diploid mouse embryos and tetraploid embryos produced by the technique of electrofusion. All of the cells and/or nuclei studied had a characteristic spherical shape which greatly simplified the morphometric analysis. A defined and predictable relationship between ploidy and cellular and/or nuclear volume was observed in the red blood cells between 8.25 and 14.5 days of gestation. During this period the primitive red blood cells are gradually replaced by the definitive erythrocytes. The ratio of control values to tetraploid values was found to be close to the theoretical value of 1:2 when comparable cells and/or their nuclei were analyzed in carefully developmentally matched material.

Cleavage rates of diploid and tetraploid mouse embryos during the preimplantation period

Despite the fact that spontaneous tetraploidy is a rare phenomenon in mice, such embryos may be produced experimentally by a variety of means, though only a very limited degree of postimplantation development has been achieved. Despite this apparent limitation, much data on the rate of development of preimplantation tetraploid embryos has been published. However, the findings from these studies has often been conflicting. In the light of the recent successful achievement of advanced postimplantation tetraploid development in our laboratory, we decided it was an opportune time to re-evaluate the preimplantation development of these embryos in as near to optimal conditions as we could achieve. Three groups were studied, namely 1) control (diploid) embryos developing in vivo, 2) control (diploid) embryos that had been isolated at the 2-cell stage, briefly retained in culture, then transferred to the oviducts of pseudopregnant recipients, and 3) tetraploid embryos produced by electrofusion of blastomeres at the 2-cell stage, then transferred to the oviducts of pseudopregnant recipients. Embryos were isolated from females from each group at specific times after the HCG injection to induce ovulation. The total cell number of each embryo was established and the log mean values were plotted against time. From the gradients of the lines it was possible to establish that there was a significant difference between the cell doubling time of the transferred controls (group 2) compared to the in vivo controls (group 1) with cell doubling times of 15.86 +/- 1.45 h and 10.27 +/- 0.24 h, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Postimplantation development of tetraploid mouse embryos produced by electrofusion

Despite the fact that a variety of experimental techniques have been devised over the years to induce tetraploid mammalian embryonic development, success rates to date have been limited. Apart from the early study by Snow, who obtained development to term of a limited number of cytochalasin B-induced tetraploid mouse embryos, no other researchers have achieved development of tetraploid embryos beyond the early postimplantation period. We now report advanced postimplantation development of tetraploid mouse embryos following electrofusion of blastomeres at the 2-cell stage, and subsequent transfer of these 1-cell 'fused' embryos to appropriate recipients. Cytogenetic analysis of the extraembryonic membranes of all of the postimplantation embryos encountered in the present study has provided an unequivocal means of confirming their tetraploid chromosome constitution. A preliminary morphological and histological analysis of the tetraploid embryos obtained by this technique has revealed that characteristic craniofacial abnormalities particularly involving the forebrain and eyes were consistently observed, and these features were often associated with abnormalities of the vertebral axis and heart. The most advanced viable embryo in this series was recovered on the 15th day of gestation, and its morphological features suggest that it was developmentally equivalent to a normal embryo of about 13.5-14 days p.c.

Sex-chromosome constitution of postimplantation tetraploid mouse embryos

Tetraploid mouse embryos were produced at the two-cell stage by blastomere fusion induced by inactivated Sendai virus. The embryos were from chromosomally normal female mice that had been fertilised by homozygous Rb(1.3)1Bnr males carrying a pair of large metacentric marker chromosomes in their karyotype. These "reconstructed" one-cell tetraploid embryos were then transferred to the oviducts of pseudopregnant recipients, which were subsequently autopsied early on the 10th day of gestation. Two-cell stage embryos that did not undergo blastomere fusion after 4-5 h were transferred to a second group of recipients, which were also autopsied early on the 10th day of gestation. From a total of 153 tetraploid embryos transferred to females that subsequently became pregnant, 135 implanted. Sixty-eight implantation sites were found to contain resorptions, whereas 67 contained mostly headfold presomite-stage embryos. Four embryos possessed four to six pairs of somites. All 57 embryos that could be analysed cytogenetically were found to be tetraploid. G-banding analysis revealed that 30 of these embryos had an XXYY and 27 and XXXX sex-chromosome constitution. The presence of two marker chromosomes in all mitotic preparations from each of these tetraploid embryos confirmed that they had all been produced by duplication of their original XY or XX diploid chromosome constitution, respectively. The XXYY:XXXX sex ratio observed was not significantly different from unity. In the control series of transfers, all of the embryos recovered were at the forelimb bud stage and had a diploid chromosome constitution. The results reported here differ from human clinical findings, in which the XXYY:XXXX sex ratio of 120 human tetraploid spontaneous abortions recovered over the last 20 years is 45:75. Possible explanations for these differences are briefly discussed.

Manufacture of diploid/tetraploid chimeric mice.

Tetraploid mouse embryos were produced by cytochalasin B treatment. These embryos usually die before completion of embryonic development and are abnormal morphologically and physiologically. The tetraploid embryos can be rescued to develop to maturity by aggregating them with normal diploid embryos to produce diploid/tetraploid chimeric mice. The diploid/tetraploid chimeric embryos are frequently abnormal: the larger the proportion of tetraploid cells, the greater the abnormality. By karyotype analysis and by the use of appropriate pigment cell markers, we have demonstrated that two of our surviving chimeras are in fact diploid/tetraploid chimeras. One surviving chimera is retarded in growth and displays neurological abnormalities. The coat color chimerism suggests that this chimera is about 50% tetraploid. Another chimera with about 10% tetraploid pigment cells in the coat is only slightly retarded in growth and is a fertile male. Tetraploid cells are distributed in many, if not all, tissues of embryos but evidently are physiologically inadequate to support completely normal development and function in the absence of substantial numbers of normal diploid cells.

Tetraploid mouse embryos produced by cytochalasin B during cleavage.

IT has been reported that cytochalasin B can prevent cytokinesis without influencing nuclear division1. Cells undergoing mitosis in the presence of cytochalasin B become binucleate and occasionally, with prolonged incubation, multinucleate1–3. Treated cells generally resume cleavage upon removal from cytochalasin and reculturing. It has been shown that many of the binucleate cells produced in a BHK21 cell line will develop as tetraploids upon reculturing in fresh medium4,5. Here I report the treatment of cleaving mouse eggs with cytochalasin and the establishment of tetraploidy in mouse embryos.