Three-dimensional reconstruction of tetraploid<-->diploid chimaeric mouse blastocysts

Preimplantation chromosomal mosaics, chimaeras and confined placental mosaicism

Some human preimplantation embryos are chromosomally mosaic. For technical reasons, estimates of the overall frequency vary widely from <15 to >90% and the true frequency remains unknown. Aneuploid/diploid and aneuploid/aneuploid mosaics typically arise during early cleavage stages before the embryonic genome is fully activated and when cell cycle checkpoints are not operating normally. Other mosaics include chaotic aneuploid mosaics and mixoploids, some of which arise by abnormal chromosome segregation at the first cleavage division. Chimaeras are similar to mosaics, in having two genetically distinct cell populations, but they arise from more than one zygote and occur less often. After implantation, the frequency of mosaic embryos declines to about 2% and most are trisomic/diploid mosaics, with trisomic cells confined to the placenta. Thus, few babies are born with chromosomal mosaicism. This review discusses the origin of different types of chromosomal mosaics and chimaeras; their fate and the relationship between preimplantation chromosomal mosaicism and confined placental mosaicism in human conceptuses and animal models. Abnormal cells in mosaic embryos may be depleted by cell death, other types of cell selection or cell correction but the most severely affected mosaic embryos probably die. Trisomic cells could become restricted to placental lineages if cell selection or correction is less effective in placental lineages and/or they are preferentially allocated to a placental lineage. However, the relationship between preimplantation mosaicism and confined placental mosaicism may be complex because the specific chromosome(s) involved will influence whether chromosomally abnormal cells survive predominately in the placental trophoblast and/or placental mesenchyme.

Lay summary

Human cells normally have 23 pairs of chromosomes, which carry the genes. During the first few days of development, some human embryos are chromosomal mosaics. These mosaic embryos have both normal cells and cells with an abnormal number of chromosomes, which arise from the same fertilised egg. (More rarely, the different cell populations arise from more than one fertilised egg and these embryos are called chimaeras.) If chromosomally abnormal cells survive to term, they could cause birth defects. However, few abnormal cells survive and those that do are usually confined to the placenta, where they are less likely to cause harm. It is not yet understood how this restriction occurs but the type of chromosomal abnormality influences which placental tissues are affected. This review discusses the origin of different types of chromosomally abnormal cells, their fate and how they might become confined to the placenta in humans and animal models.

Evaluation of triploid<-->diploid and trisomy-3<-->diploid mouse chimeras as models for investigating how lineage restriction occurs in confined placental mosaicism

Human confined placental mosaicism (CPM), where the placental trophoblast is mosaic for a chromosome abnormality but the fetus is chromosomally normal, can cause problems for prenatal diagnosis, but its causes are poorly understood. Tetraploid↔diploid chimeras provide a model for the development of one type of CPM, but animal models for other types of restricted mosaicism are needed. The objective of the present study was to evaluate triploid↔diploid and trisomy-3↔diploid chimeric mouse conceptuses as new models for investigating the development of restricted mosaicism. Novel stocks of mice were generated to produce triploid and trisomy-3 embryos that could be identified by DNA in situ hybridisation to a chromosome 3 transgenic marker. Triploid↔diploid and trisomy-3↔diploid mouse chimeras were produced by embryo aggregation, and the contribution of triploid or trisomy-3 cells was analysed in the fetus and extraembryonic tissues. Only two trisomy-3↔diploid chimeras were analysed but trisomy-3 cells contributed well to all lineages, so these chimeras did not show restricted mosaicism. In contrast, triploid cells usually contributed poorly to all lineages in the ten 3n↔2n chimeras analysed. They contributed more to the primitive endoderm derivatives than other lineages and were present in the primitive endoderm derivatives of all ten chimeras, but excluded from fetuses and trophectoderm derivatives in some cases. This pattern of restricted mosaicism differs from that reported for tetraploid cells in tetraploid↔diploid chimeras, and triploid↔diploid chimeras may provide a useful model for the development of some types of restricted mosaicism in human conceptuses.

Production of identical mouse twins and a triplet with predicted gender

The aim of this study was to develop a method to generate identical twins and triplets with predicted gender. As a first step toward that aim, single blastomeres obtained from EGFP expressing eight-cell stage embryos and either diploid or tetraploid host embryos were used to compose chimera. We could follow the fate of EGFP expressing diploid blastomere derived cells in 3.5- and 4.5-day-old chimera embryos in vitro. We found that the diploid blastomere-derived cells had significantly higher chance to contribute to the inner cell mass if tetraploid host embryos were applied. After that, we developed a quick and reliable multiplex PCR strategy for sex diagnosis from single blastomeres by simultaneous amplification of the homologous ZFX and ZFY genes. By composed chimeras using single blastomeres, derived from sexed eight-cell stage embryos and a tetraploid host embryo, we could get preplanned sex newborns, wholly derived from these blastomeres. Among these mice, identical twins and a triplet were identified by microsatellite analysis. Unlike clones produced by nuclear transfer, these mice are identical at both the nuclear as well as mitochondrial DNA level. Therefore, the tetraploid embryo complementation method to produce monozygotic twins and triplets could be a valuable tool both in biomedical and agricultural applications.

Development of rat tetraploid and chimeric embryos aggregated with diploid cells

In the present study, we examined the preimplantation and postimplantation development of rat tetraploid embryos produced by electrofusion of 2-cell-stage embryos. Developmental rate of tetraploid embryos to morula or blastocyst stage was 93% (56/60) and similar to that found in diploid embryos (95%, 55/58). After embryo transfer, rat tetraploid embryos showed implantation and survived until day 8 of pregnancy, however the conceptuses were aberrant on day 9. In mouse, tetraploid embryos have the ability to support the development of blastomeres that cannot develop independently. As shown in the present study, a pair of diploid blastomeres from the rat 8-cell-stage embryo degenerated immediately after implantation. Therefore, we examined whether rat tetraploid embryos have the ability to support the development of 2/8 blastomeres. We produced chimeric rat embryos in which a pair of diploid blastomeres from an 8-cell-stage green fluorescent protein negative (GFP−) embryo was aggregated with three tetraploid blastomeres from 4-cell GFP-positive (GFP+) embryos. The developmental rate of rat 2n(GFP−) ↔ 4n(GFP+) embryos to the morula or blastocyst stages was 93% (109/117) and was similar to that found for 2n(GFP−) ↔ 2n(GFP+) embryos (100%, 51/51). After embryo transfer, 2n(GFP−) ↔ 4n(GFP+) conceptuses were examined on day 14 of pregnancy, the developmental rate to fetus was quite low (4%, 4/109) and they were all aberrant and smaller than 2n(GFP−) ↔ 2n(GFP+) conceptuses, whereas immunohistochemical analysis showed no staining for GFP in fetuses. Our results suggest that rat tetraploid embryos are able to prolong the development of diploid blastomeres that cannot develop independently, although postimplantation development was incomplete.

Fate of tetraploid cells in 4n<-->2n chimeric mouse blastocysts

Previous studies have shown that tetraploid (4n) cells rarely contribute to the derivatives of the epiblast lineage of mid-gestation 4n<-->2n mouse chimeras. The aim of the present study was to determine when and how 4n cells were excluded from the epiblast lineage of such chimeras. The contributions of GFP-positive cells to different tissues of 4n<-->2n chimeric blastocysts labelled with tauGFP were analysed at E3.5 and E4.5 using confocal microscopy. More advanced E5.5 and E7.5 chimeric blastocysts were analysed after a period of diapause to allow further growth without implantation. Tetraploid cells were not initially excluded from the epiblast in 4n<-->2n chimeric blastocysts and they contributed to all four blastocyst tissues at all of the blastocyst stages examined. Four steps affected the allocation and fate of 4n cells in chimeras, resulting in their exclusion from the epiblast lineage by mid-gestation. (1) Fewer 4n cells were allocated to the inner cell mass than trophectoderm. (2) The blastocyst cavity tended to form among the 4n cells, causing more 4n cells to be allocated to the hypoblast and mural trophectoderm than the epiblast and polar trophectoderm, respectively. (3) 4n cells were depleted from the hypoblast and mural trophectoderm, where initially they were relatively enriched. (4) After implantation 4n cells must be lost preferentially from the epiblast lineage. Relevance of these results to the aetiology of human confined placental mosaicism and possible implications for the interpretation of mouse tetraploid complementation studies of the site of gene action are discussed.

Developmental potential and behavior of tetraploid cells in the mouse embryo

Tetraploid (4n) mouse embryos die at variable developmental stages. By examining 4n embryos from F2 hybrid and outbred mice, we show that 4n developmental potential is influenced by genetic background. The imprinted inactivation of an X chromosome-linked eGFP transgene in extraembryonic tissues occurred correctly in 4n embryos. A decrease of the cleavage rate in 4n preimplantation embryos compared to diploid (2n) embryos was revealed by real-time imaging, using a histone H2b:eGFP reporter. It has previously been known that mouse chimeras produced by the combination of diploid (2n) embryos with embryonic stem (ES) cells result in mixtures of the two components in epiblast-derived tissues. In contrast, the use of 4n host embryos with ES cells restricts 4n cells from the embryonic regions of chimeras, resulting in mice that are believed to be completely ES-derived. Using H2b:eGFP transgenic mice and ES cells, the behavior of 4n cells was determined at single cell resolution in 4n:2n injection and aggregation chimeras. We found a significant contribution of 4n cells to the embryonic ectoderm at gastrulation in every chimera analyzed. We show that the transition of the embryonic regions from a chimeric tissue to a predominantly 2n tissue occurs after gastrulation and that tetraploid cells may persist to midgestation. These findings suggest that the results of previously published tetraploid complementation assays may be influenced by the presence of tetraploid cells in the otherwise diploid embryonic regions.

Tetraploid cells of enhanced green fluorescent protein transgenic mice in tetraploid/diploid-chimeric embryos

We succeeded in noninvasively analyzing the distribution of tetraploid (4n) cells in tetraploid<-->diploid (4n<-->2n) chimeric embryos by using enhanced green fluorescent protein (EGFP) transgenic (Tg) mouse embryos. We also evaluated whether this technique of analyzing 4n-cells in EGFP Tg 4n<-->2n chimeric embryos could be used to determine which characteristics of 4n-cells cause the death of 4n-embryos and restricted distribution of 4n-cells in 4n<-->2n-chimeric embryos after implantation. In our experiments, the distribution of 4n-cells in 4n<-->2n-embryos was normal until an embryonic age of 3.5 days (E3.5). With respect to morphological development, there were no differences between 4n-, diploid (2n), 4n<-->2n-, and diploid/diploid (2n<-->2n) chimeric embryos, but the number of cells in the tetraploid (4n) blastocyst was smaller than expected. This decrease in the number of cells may have caused cell death or reduced the rate of cell division in 4n-cells, and may have restricted the distribution of 4n-cells in 4n<-->2n-chimeric embryos. This study demonstrated the utility of EGFP transgenic mouse embryos for relatively easy and noninvasive study of the sequential distribution of cells in chimeric embryos.

Paraffin-embedded manipulated blastocysts: a tool to demonstrate stem cell plasticity?

One of the big question marks in current stem cell research is whether there is true plasticity of adult progenitor cells (APC) or if cell fusion is the principle source of the supposed plasticity. The generation of chimeras by injecting adult progenitor cells into blastocysts is not new. This paper describes an efficient embedding technique for murine blastocysts injected with human APC. This method could help in establishing a novel tool to analyse the process of plasticity, if it truly exists. If this is the case, this technology could be of great help to characterize surface markers of stem cells in great detail. On the other hand, fusion of cells could also be investigated. A system of embedding blastocysts was set up using paraffin for further analysis by means of light microscopy and immunohistochemistry. The embedding of the chimaeras consists of fixing them first with paraformaldehyde in phosphate-buffered saline (PFA/PBS), embedding them in gelatine, fixing the gelatine block with PFA/PBS and finally fixing the gelatine block in a Petri dish by embedding it in paraffin. Using this protocol, the morphology of the blastocysts is well preserved.

Evaluation of the mouse TgTP6.3 tauGFP transgene as a lineage marker in chimeras

The mouse TgTP6.3 transgene, encoding a tauGFP fusion protein, is becoming widely used but has yet to be fully characterized and evaluated as suitable lineage marker. The aim of the present study was to investigate the phenotype of TgTP6.3(+/+) homozygotes and TgTP6.3(+/-) hemizygotes, characterize the expression of the TgTP6.3 transgene in different tissues and critically evaluate its use as a lineage marker. TgTP6.3(+/+) homozygotes died between embryonic day 14.5 and weaning, whereas TgTP6.3(+/-) hemizygotes were mostly viable and fertile but smaller than non-transgenic siblings. TgTP6.3 expression began in the late two-cell stage, persisted in most fetal and adult tissues and was uniformly expressed in many (but not all) tissues. TgTP6.3(+/-) cells were readily identified in many chimeric tissues and their contribution appeared to be quantitatively and spatially normal. Overall, tauGFP expression in hemizygous TgTP6.3(+/-) cells fulfils the main criteria of a good lineage marker for many tissues. It provides a useful lineage marker, which should be particularly suitable for axons, blood vessels and pre-implantation embryos.

The case for prepatterning in the mouse

In studies from several laboratories using a variety of different techniques, features of the zygote and two-cell conceptus have been found to map nonrandomly on the blastocyst with respect to both its axis of polarity and bilateral plane. This is not what would be expected if, as is widely believed, early patterning depends entirely on positional relationships and interactions among the progeny of blastomeres that are equipotential until at least the eight-cell stage. Rather, the implication of these findings is that prepatterning is a normal facet of development in mammals, just as it is in most other metazoa. Nevertheless, there is still no general consensus regarding the extent to which such prepatterning depends on intrinsic organization of the oocyte, as opposed to events that are contingent on fertilization.

Tetraploid development in the mouse

Spontaneous duplication of the mammalian genome occurs in approximately 1% of fertilizations. Although one or more whole genome duplications are believed to have influenced vertebrate evolution, polyploidy of contemporary mammals is generally incompatible with normal development and function of all but a few tissues. The production of tetraploid (4n) embryos has become a common experimental manipulation in the mouse. Although development of tetraploid mice has generally not been observed beyond midgestation, tetraploid:diploid (4n:2n) chimeras are widely used as a method for rescuing extraembryonic defects. The tolerance of tissues to polyploidy appears to be dependent on genetic background. Indeed, the recent discovery of a naturally tetraploid rodent species suggests that, in rare genetic backgrounds, mammalian genome duplications may be compatible with the development of viable and fertile adults. Thus, the range of developmental potentials of tetraploid embryos remains in large part unexplored. Here, we review the biological consequences and experimental utility of tetraploid mammals, in particular the mouse.