The influence of ploidy on the distribution of cells in chimaeric mouse blastocysts

Re-evaluation of the causes of variation among mouse aggregation chimaeras

The composition of adult mouse aggregation chimaeras is much more variable than X-inactivation mosaics. An early theoretical model proposed that almost all the extra variation in chimaeras arises, before X-inactivation occurs, by spatially constrained, geometrical allocation of inner cell mass (ICM) cells to the epiblast and primitive endoderm (PrE). However, this is inconsistent with more recent embryological evidence. Analysis of published results for chimaeric blastocysts and mid-gestation chimaeras suggested that some variation exists among chimaeric morulae and more variation arises both when morula cells are allocated to the ICM versus the trophectoderm (TE) and when ICM cells are allocated to the epiblast versus the PrE. Computer simulation results were also consistent with the conclusion that stochastic allocation of cells to blastocyst lineages in two steps, without the type of geometrical sampling that was originally proposed, could cause a wide variation in chimaeric epiblast composition. Later allocation events will cause additional variation among both chimaeras and X-inactivation mosaics. We also suggest that previously published U-shaped frequency distributions for chimaeric placenta composition might be explained by how TE cells are allocated to the polar TE and/or the subsequent movement of cells from polar TE to mural TE.

Developmental potential of 2n/3n mixoploid mouse embryos produced by fusion of individual second polar bodies and blastomeres of 2-cell embryos

Using 2n/3n mixoploid mouse embryos produced by fusion of individual second polar bodies (PB2s) with individual blastomeres of 2-cell embryos, the dynamics of PB2 nuclei in the host blastomeres during mitosis were examined and the fate of the 3n cell line in the mixoploid embryos was followed. Most of the PB2 nuclei were synchronised with the cell cycle of the host blastomeres and all chromosomes were incorporated into a single mitotic spindle. The majority of the mixoploid embryos developed to blastocysts with 3n cells. In conceptuses at Day 11.5 and Day 18.5 of gestation, 3n cells were recognised in both of the embryonic/fetal and placental tissues. When green fluorescent protein (GFP)-transgenic mice were used as a donor of PB2, GFP-positive 3n cells were found in more than 40% of morulae and blastocysts, indicating that the PB2 genome can be reactivated during the pre-implantation stage. GFP-positive 3n cells were non-randomly allocated in trophectoderm in blastocysts. These findings may explain the production mechanism of 2n/3n mixoploid human embryos, that is, a PB2 is incorporated into one daughter blastomere during the early cleavage period.

Nonviable human pre-implantation embryos as a source of stem cells for research and potential therapy

Human embryonic stem cells are derived from the inner cell mass of the human blastocyst. Presumably normal (frozen/thawed) human preimplantation embryos that remain unused following assisted reproduction procedures have provided the main source of blastocysts for stem cell derivation. Alternatively, embryos have been generated from gametes donated for the unique purpose of in vitro fertilization, blastocyst culture, and stem cell isolation. This article describes two previously published methods--and the background to those methods--that allow the use of nonviable embryos excluded from transfer and cryopreservation as a source of stem cells. The first method is based on the observation that some blastomeres from embryos with abnormal division during the first 3-5 d in culture can continue very limited development in isolation. When aggregated in a chimaeric form, some of these blastomeres can contribute to the formation of normally organized blastocysts. Blastocysts so obtained provide a route to embryonic stem cells from otherwise nonviable embryos. Thus the inner cell masses of blastocysts obtained from trisomic embryos were placed on feeder cells and cultured for seven additional days, following which the resulting cell colonies were examined for chromosome content. The second method concerns embryos diagnosed with specific chromosome abnormalities many of which are incompatible with life. Some of these aneuploidies do not preclude development to the blastocyst stage in culture. A proportion of these cells were found to be disomic and the cultures were shown to express OCT-4, a molecular marker for pluripotent cells. This apparent correction of the trisomic state in some cells within the colonies suggests that embryos with cromosomal abnormalities incompatible with life may be another source of human embryonic stem cells.

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.

Evaluation of the developmental competence and chromosomal compliment of mouse oocytes derived from in-vitro growth and maturation of preantral follicles

PURPOSE

To evaluate the developmental potential and aneuploidy rates of in-vitro versus in-vivo grown and matured mouse oocytes.

METHODS

Mice were superovulated to obtain in-vivo matured oocytes. Mouse preantral follicles were also mechanically isolated and cultured in-vitro. In-vitro fertilization (IVF) was performed and fertilization, cleavage, and morula/blastocyst formation rates were compared between groups. Cytogenetic analysis was used to compare oocyte aneuploidy rates and aneuploidy characteristics in the developing embryos.

RESULTS

In-vivo oocyte maturation resulted in higher IVF fertilization, cleavage, and morula/blastocyst formation rates versus in-vitro follicle culture (96.4% versus 78.5%, p<0.001; 95.3% versus 77.4%, p<0.001; 94.1% versus 76.9%, p<0.001). Total aneuploidy rates were higher in embryos derived from in-vitro matured oocytes versus those grown in-vivo (4.0% versus 1.3%, p<0.05).

CONCLUSIONS

Results indicate a reduced developmental competency of in-vitro matured oocytes. The data also highlight an increased susceptibility to meiotic errors in early stage follicles undergoing in vitro culture.

Transgenic RNA interference in mice

The discovery that small interfering RNA duplexes (siRNA) can silence gene expression in mammalian cells has revolutionized biomedical research. The most successful application of the discovery has been to study gene function in cultured human or mouse cells. However, the knockdown effect of siRNA is only transient. To achieve a more sustained gene-silencing effect, shRNA (small hairpin RNA) expressed from a vector is preferred. An additional benefit of shRNA is that RNA interference (RNAi) can now be applied in vivo through delivering shRNA-expressing vectors by transgenic technology. Transgenic RNAi not only allows the study of biological processes not present in cultured cells but also offers chronic therapeutic potentials. In this review, we will summarize the developments in the generation of transgenic RNAi mice.

In vitro development and cell allocation of porcine blastocysts derived by aggregation of in vitro fertilized embryos

In pigs, the morphology and cell number of in vitro-produced blastocysts are inferior to those of their in vivo counterparts. The objective of this study was to increase developmental competence and to gain an understanding of cell allocation in blastocysts derived from the aggregation of four-cell stage porcine embryos produced in vitro. After removal of the zona pellucida, two (2x) and three (3x) four-cell stage embryos were aggregated by co-culturing them in aggregation plates. Five days after aggregation, the developmental ability and the number of cells in the aggregated embryos were determined. The percentage of blastocysts was higher (P < 0.05) in both the 2x and 3x aggregated embryos (66.6% and 72.0%, respectively) compared to that of the 1x embryos and the intact controls (43.1% and 36.4%, respectively). The total cell number of blastocysts also increased in aggregated embryos compared to that of intact controls (2.6-fold for 2x and 3.4-fold for 3x) (P < 0.05). The cells of two differentially stained embryos were started to mix at 72 hr after aggregation. In vitro-fertilized porcine aggregates (2x) were developed to blastocyst with a random distribution of cells from each embryo. The mRNA levels for the oct-4, bcl-xL and connexin 43 genes were higher (P < 0.05) and bak gene were lower (P < 0.05) in both the 2x and 3x aggregated embryos than the intact controls. Therefore, the aggregation of the four-cell stage embryos could be used to improve the quality of porcine preimplantation stage embryos produced in vitro.

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.

Few polyploid blastomeres in morphologically superior bovine embryos produced in vitro

Morphologically inferior bovine embryos developed in vivo have been shown by karyotyping to have a higher rate of chromosomally abnormal cells than morphologically normal embryos. The objective of this study was to re-examine this finding using interphase cytogenetics. A total of 155 IVP Day 8 bovine blastocysts were graded by their morphology (excellent, good, or poor) and timing of development (hatched, expanded, or non-expanded), and afterwards analysed for chromosome abnormalities by fluorescence in situ hybridization using differentially labelled probes for chromosomes 6 and 7. The overall frequency of diploid embryos was 7%, and did not differ according to grading. Although the frequency of mixoploidy was not correlated to the morphological grading, the blastocysts with excellent morphology displayed fewer polyploid nuclei in comparison to blastocysts with good (P=0.05) or poor morphology (P=0.01). There were however also prominent exceptions showing that a blastocyst with an excellent morphology can display a high degree of polyploidy. The results further demonstrate that the morphologically normal embryos contain a higher number of cells and develop more rapidly than the morphologically inferior embryos.

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.

Experimentally produced diploid〈-〉triploid mouse chimaeras develop up to adulthood

Spontaneous diploid-triploid chimaeras occur sporadically in various mammalian species including man, but so far have never been produced experimentally. In order to get a deeper insight into the developmental consequences of this anomaly, we have developed two procedures that enabled for the first time to produce routinely diploid-triploid embryos, foetuses, and animals in the mouse. These procedures are: (1) aggregation of cleaving diploid embryos with triploid embryos produced by suppression of the second polar body in zygotes, and (2) fusion of a haploid karyoplast with one blastomere of the two-cell diploid embryos. The first procedure yielded 23 living and 6 dead postimplantation embryos and foetuses (age: 8th-19th day) out of which 22 were chimaeric. In addition, three chimaeric neonates reached adulthood. Two animals were fertile, and one--an overt chimaera--was an infertile male. The rate of postimplantation development of aggregation chimaeras was normal or only slightly retarded, and with one exception the foetuses were morphologically normal. Generally, the highest contribution of the 3n component in extra-embryonic structures was noted in the yolk sac, and usually it was higher than its contribution to the organs of the body. Chimaerism was most often noted in the liver, the heart, the intestine, and the lungs. Participation of triploid cells to all tissues studied, both in the body and in extra-embryonic structures, appeared to decrease slightly as development progressed. The second procedure yielded 10 foetuses and 6 adults. Three foetuses were chimaeric. Six fertile adults were probably non-chimaeras: the triploid component was absent in the coat and in the blood.

Hand-made cloning approach: potentials and limitations

Two major drawbacks hamper the advancement of somatic cell nuclear transfer in domestic animals. The first is a biological problem that has been studied extensively by many scientists and from many viewpoints, including the cell, molecular and developmental biology, morphology, biochemistry and tissue culture. The second is a technical problem that may be responsible for 50% or more of quantitative and/or qualitative failures of routine cloning experiments and is partially the result of the demanding and complicated procedure. However, even the relatively rare documented efforts focusing on technique are usually restricted to details and accept the principles of the micromanipulator-based approach, with its inherent limitations. Over the past decade, a small alternative group of procedures, called hand-made cloning (HMC), has emerged that has the common feature of removal of the zona pellucida prior to enucleation and fusion, resulting in a limited (or no) requirement for micromanipulators. The benefits of HMC are low equipment costs, a simple and rapid procedure and an in vitro efficiency comparable with or higher than that of traditional nuclear transfer. Embryos created by the zona-free techniques can be cryopreserved and, although data are still sparse, are capable of establishing pregnancies and resulting in the birth of calves. Hand-made cloning may also open the way to partial or full automation of somatic cell nuclear transfer. Consequently, the zona- and micromanipulator-free approach may become a useful alternative to traditional cloning, either in special situations or generally for the standardisation and widespread application of somatic cell nuclear transfer.

In Vitro Development and Cell Allocation Following Aggregation of Split Embryos with Tetraploid or Developmentally Asynchronous Blastomeres in Rhesus Monkeys

Production of genetically identical pairs of monkeys would have tremendous implications for biomedical research, particularly immunological studies and vaccine trials. Specific aims of this study were to (1) determine whether aggregation of embryos split into halves or quarters with equal numbers of either developmentally asynchronous or tetraploid blastomeres would enhance their developmental potential in vitro and increase total cell numbers in resulting blastocysts, and (2) determine the allocation of tetraploid and developmentally asynchronous blastomeres in resulting blastocysts. Results demonstrated that development into blastocysts was greater (p < 0.05) for embryos split into pairs (39.8%) than for those split into quadruplet sets (17.4%) and similar (p > 0.05) to that of nonmanipulated controls (59.6%). Creation of chimeras from aggregation of a single 4-cell and four 16-cell stage blastomeres resulted in blastocyst formation (69.2%) similar to that of nonmanipulated control embryos (66.9%). However, neither development nor total cell numbers in resulting blastocysts differed between aggregate chimeras and those split into quadruplet sets at the 16-cell stage. Blastocysts resulting from the aggregate chimeras were derived strictly from the 16-cell stage blastomeres, with complete exclusion of the 4-cell stage blastomeres. Aggregation of split embryos with equal numbers of tetraploid blastomeres doubled (p < 0.05) both the proportion developing into blastocysts and the total cell numbers in resulting blastocysts. Tetraploid blastomeres were allocated to both the inner cell mass and trophectoderm of resulting blastocysts. In conclusion, due to exclusion of the less advanced cells, aggregation of developmentally asynchronous blastomeres did not improve the developmental competence or cell numbers of split rhesus embryos. Reconstitution of split embryos with equal numbers of tetraploid blastomeres enhanced their developmental potential and cell numbers in resulting blastocysts. However, tetraploid blastomeres were allocated to both the inner cell mass and trophectoderm.

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.

Strategies for the production of genetically identical monkeys by embryo splitting

Genetically identical rhesus monkeys would have tremendous utility as models for the study of human disease and would be particularly valuable for vaccine trials and tissue transplantation studies where immune function is important. While advances in nuclear transfer technology may someday enable monkeys to be cloned with some efficiency, embryo splitting may be a more realistic approach to creating pairs of genetically identical monkeys. Although several different approaches to embryo splitting, including blastocyst bisection and blastomere separation, have been used successfully in rodents and domestic species for production of pairs and sets of identical offspring, efforts to create monozygotic twins in rhesus monkeys using these approaches have not met with similar success. Aggregation of split embryos with other types of blastomeres, such as tetraploid and developmentally asynchronous blastomeres, that could potentially increase their cell numbers and developmental competence without contributing to term development has been investigated as an alternative approach to creating monozygotic twin monkeys. The major challenges encountered with respect to the efficient production of monozygotic twins in rhesus monkeys and potential strategies to overcome these challenges are discussed.

Bone marrow cells adopt the phenotype of other cells by spontaneous cell fusion

Recent studies have demonstrated that transplanted bone marrow cells can turn into unexpected lineages including myocytes, hepatocytes, neurons and many others. A potential problem, however, is that reports discussing such 'transdifferentiation' in vivo tend to conclude donor origin of transdifferentiated cells on the basis of the existence of donor-specific genes such as Y-chromosome markers. Here we demonstrate that mouse bone marrow cells can fuse spontaneously with embryonic stem cells in culture in vitro that contains interleukin-3. Moreover, spontaneously fused bone marrow cells can subsequently adopt the phenotype of the recipient cells, which, without detailed genetic analysis, might be interpreted as 'dedifferentiation' or transdifferentiation.

In vitro formation of tetraploid rat blastocysts after fusion of two-cell embryos

Gene targeting technology is not available in the rat which is an animal model of major importance, e.g., in cardiovascular research. This is due to the fact that the rat embryonic stem cell (ESC)-like cells established by several groups do not form germ-line chimeras when injected into blastocysts. In the mouse, the aggregation of ESC with tetraploid embryos has allowed the generation of animals completely derived from these cells. However, aggregation of rat ESC-like cells with tetraploid rat embryos has not yet been attempted to evaluate their developmental capacity. Therefore, we established a method to produce tetraploid rat embryos by fusion at the two-cell stage. Chemical fusion by polyethylene glycol (PEG) was shown to be less efficient (56.3% fused embryos) than electrofusion (96.1% fused embryos). The rate of development of fused embryos to blastocysts was independent of the fusion method and similar to the rate of control embryos. However, this rate was lower when the embryos had been cultured from the zygote state before fusion (14-20%) compared to freshly isolated two-cell embryos (41-63%). Alike for the mouse, blastocysts derived from fused two-cell rat embryos contained about half the number of cells as control blastocysts and were homogeneously tetraploid with no evidence of mosaicism. This method may be useful for the establishment of gene-targeting technology in the rat.

Ploidy of Bovine Nuclear Transfer Blastocysts Reconstructed Using In Vitro Produced Blastomere Donors

The higher rate of embryonic loss in nuclear transfer compared to in vitro produced embryos may be due to chromosome abnormalities that occur during preimplantation in vitro development. Because little is known about ploidy errors in nuclear transfer embryos, this was examined using embryos reconstructed from in vitro produced embryo donors. In vitro matured oocytes were enucleated and then activated using calcium ionophore A23187 followed by 6-dimethylaminopurine (6-DMAP). Subsequently, embryos were reconstructed using blastomeres from day 4-5 in vitro produced donors. The embryos were cultured until day 7 at which time blastocyst nuclei were extracted and chromosome abnormalities were evaluated by fluorescent in situ hybridization using two probes that bind to the subcentromeric regions on chromosomes 6 and 7. In 16 nuclear transfer blastocysts generated from 5 donor embryos, 53.8 +/- 20.2 (mean % +/- SD) nuclei/embryo were examined. Of these 16, 7 embryos (43.8%) were potentially abnormal because in these, 1.1%, 1.4%, 5.3%, 7.5%, 26.3%, 30.4%, and 66.2% % of the nuclei had a chromosome composition deviating from the diploid condition, indicating a wide degree of variation between embryos. These errors comprised mainly triploid (8.2 +/- 10.3 [0-26.3]: % +/- SD [range]) and tetraploid (10.6 +/- 19.9 [0-54.9]) nuclei with other ploidy combinations accounting for only 0.9 +/- 2.1 [0-2.1]% of deviant nuclei. The proportion of completely normal nuclear transfer embryos was no less than those produced by in vitro fertilization but the distribution of chromosome abnormalities was different (p = 0.0002). In conclusion, nuclear transfer embryos reconstructed using blastomere cells can produce over 50% blastocysts with a diploid chromosome complement. However, the contribution of chromosome abnormalities to embryonic loss in the remaining embryos deserves further investigation.

Electrofusion of two-cell bovine embryos for the production of tetraploid blastocysts in vitro

Tetraploid bovine blastocysts were produced experimentally by electrofusion of in vitro matured and fertilized, zona-enclosed two-cell embryos (33-35 hr after initiation of sperm-egg incubation) using three fusion protocols. Field strengths of 1.0, 1.4, and 2.4 kV/cm were tested and the rate of fusion, subsequent cleavage, and blastocyst development were measured for each. High rates of fusion (76.5% +/- 2.8%), cleavage (72.5% +/- 7.4%) and blastocyst development (56.1% +/- 6.4%) were achieved with the application of 1. 4 kV/cm as a single 100-microseconds pulse. Embryos were scored 30 and 60 min after stimulation for fusion. No time effect for fusion, cleavage, or blastocyst development was observed. Chromosome preparations of day 7 blastocysts revealed 12.5% of fused embryos were tetraploid. This is a significant increase from that found in nonfused embryos where spontaneous tetraploidy did not occur. An electrical stimulus of 1.0 kV/cm applied as two 50-microseconds pulses produced significantly less one-cell embryos (64.2% +/- 3.0%) compared to 1.4 kV/cm while cleavage (79.9% +/- 3.4) and blastocyst development (44.6% +/- 4.0%) were not different from that for unexposed control embryos (89.5% +/- 2.3% and 57.2% +/- 3.2%, respectively). Embryos fused at 2.4 kV/cm applied as a single 30-microseconds pulse (69.7% +/- 5.7%) showed significantly lower cleavage (72.1% +/- 3.7%) and blastocyst rates (40.2% +/- 4.6%) compared to the unexposed control.

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

Studies of tetraploid<-->diploid (4n<-->2n) mouse chimaeras have demonstrated unequal contributions of 4n cells to different tissues of the midgestation conceptus. Such a pattern has also been reported in chimaeras as early as E3.5d, which show an enhanced contribution of 4n cells to the mural trophectoderm (Everett & West, 1996). In this study, sectioned 4n<-->2n and 2n<-->2n control chimaeric blastocysts were digitised and reconstructed in 3 dimensions (3-D). The 3-D images revealed only limited mixing of cells from the 2 contributing embryos of individual blastocysts in both chimaera groups. Consequently, the distribution pattern of the 2 cell types was dependent on the spatial relationship between the orientation of the blastocyst and the boundary between the 2 clusters of cells. The distribution patterns observed were not strikingly different for 4n<-->2n and 2n<-->2n chimaeras, each showing some transgenic positive cell contribution in all 3 identifiable developmental lineages. It was notable, however, that in all 4n<-->2n blastocysts at least some 4n cells were located adjacent to the blastocyst cavity. Such a consistent pattern was not evident in 2n<-->2n chimaeras. This study has demonstrated the value of 3-D reconstructions for the analysis of spatial relationships of 2 cell populations in chimaeric mouse blastocysts.

Blastomeres from Somatic Cell Nuclear Transfer Embryos Are Not Allocated Randomly in Chimeric Blastocysts

A marker has been developed to allow detection of blastomeres that originate from embryos produced by nuclear transfer (NT) of genetically engineered fetal fibroblasts. A plasmid (phEFnGFP) was constructed with a G418 resistance cassette for selection in fibroblasts and a nuclear localized green fluorescent protein (nGFP) expression cassette that expresses in every cell of day-6, -7, and -8 bovine embryos. This construct was utilized to follow the blastomere distribution in aggregation chimeras produced from fertilized embryos (in vitro produced, IVP) or parthenotes and NT embryos. Fluorescent and nonfluorescent NT embryos were aggregated early on day 4 and evaluated on day 8. Nuclei of blastomeres that carried the transgene were fluorescent under both UV epifluorescence (Hoechst 33342) and blue epifluorescence (nGFP). There was no bias in the distribution of green fluorescent blastomeres in the inner cell mass (ICM) or trophectoderm in NT<>NT chimeras. However, there was a strong bias for NT blastomeres to populate the ICM when aggregated with IVP embryos or parthenotes. There was also a strong bias against NT blastomeres in the trophectoderm when aggregated to IVP embryos. However, the bias against NT blastomeres in the trophectoderm was significantly less (p < 0.05) when aggregated with parthenotes as compared to aggregation with IVP embryos. In NT<>NT aggregates, no chimeric embryos were produced that had an ICM composed of blastomeres from a single origin. However, in NT<>Parthenote aggregates, 67% of the blastocysts had an ICM composed exclusively of NT origin. The remaining blastocysts ranged from 0% to 83% of the ICM that expressed nGFP. Similarly, in NT<>IVP aggregates 50% of the blastocysts had an ICM composed exclusively of NT origin. The remaining blastocysts ranged from 19% to 71% of the ICM being of NT origin. We conclude that production of divaricated chimeras from NT origin is feasible. Other applications of this technology are discussed.

Genetic control of extraembryonic cell lineages studied with tetraploid<–>diploid chimeric concepti

The first differentiation event during mammalian embryogenesis is the commitment of blastomeres to the trophectoderm cell lineage. Much remains to be learned about the genetic control of this first cell lineage commitment and the subsequent events underlying the differentiation of all extraembryonic cell lineages. Because of the unique features of intrauterine embryonic development, the study of embryogenesis in lower organisms has shed little light on mammalian extraembryonic lineage differentiation. Rather, two major methods in developmental genetics have contributed to our understanding of genetic control of extraembryonic cell lineages. First, abnormalities in extraembryonic tissues have been described in many genetically engineered mutant mouse lines. However, the histological description of these abnormalities does not demonstrate whether the observed defect is the primary cause of embryonic lethality. Second, tetraploid<–>diploid aggregation experiments have been used to generate chimeric concepti with distinct genotypes in the extraembryonic tissues and the embryo proper. This experimental approach has provided the definitive demonstration of the crucial role of several transcription factors, growth factors and cytoskeleton proteins in extraembryonic tissue formation. The present review summarizes the origin of tetraploid<–>diploid aggregation experiments and it usefulness for the study the genetic control of extraembryonic cell lineages.Key words: tetraploid, aggregation, chimera, extraembryonic cell lineages, placenta.

Skewed X-chromosome inactivation is common in fetuses or newborns associated with confined placental mosaicism

The inactivation of one X chromosome in females is normally random with regard to which X is inactivated. However, exclusive or almost-exclusive inactivation of one X may be observed in association with some X-autosomal rearrangements, mutations of the XIST gene, certain X-linked diseases, and MZ twinning. In the present study, a methylation difference near a polymorphism in the X-linked androgen-receptor gene was used to investigate the possibility that nonrandom X inactivation is increases in fetuses and newborns that are associated with confined placental mosaicism (CPM) involving an autosomal trisomy. Extreme skewing was observed in 7 (58%) of 12 cases with a meiotic origin of the trisomy, but in none of 10 cases examined with a somatic origin of the trisomy, and in only 1 (4%) of 27 control adult females. In addition, an extremely skewed X-inactivation pattern was observed in 3 of 10 informative cases of female uniparental disomy (UPD) of chromosome 15. This may reflect the fact that a proportion of UPD cases arise by "rescue" of a chromosomally abnormal conceptus and are therefore associated with CPM. A skewed pattern of X inactivation in CPM cases is hypothesized to result from a reduction in the size of the early-embryonic cell pool, because of either poor early growth or subsequent selection against the trisomic cells. Since approximately 2% of pregnancies detected by chorionic villus sampling are associated with CPM, this is likely a significant contributor to both skewed X inactivation observed in the newborn population and the expression of recessive X-linked diseases in females.

Multiple functions for Pax6 in mouse eye and nasal development

Mouse embryos, homozygous for the small eye (Sey) mutation die soon after birth with severe facial abnormalities that result from the failure of the eyes and nasal cavities to develop. Mutations in the Pax6 gene are responsible for the Sey phenotype. As a general disruption of eye and nasal development occurs in the homozygous Sey embryos, it is unclear, from the mutant phenotype alone, which tissues require functional Psx6. To examine the roles for Pax6 in eye and nasal development we produced chimeric mouse embryos composed of wild-type and Sey mutant cells. In these embryos we found that mutant cells were excluded from both the lens and nasal epithelium. Both of these tissues were smaller, and in some cases absent, in chimeras with high proportions of mutant cells. The morphology of the optic cup was also severely affected in these chimeras; mutant cells were excluded from the retinal pigmented epithelium and did not intermix with wild-type cells in other regions. The evidence shows that Pax6 has distinct roles in the nasal epithelium and the principal tissue components of the embryonic eye, acting directly and cell autonomously in the optic cup and lens. We suggest that Pax6 may promote cell surface changes in the optic cup and control the fate of the ectoderm from which the lens and nasal epithelia are derived.