Evidence of a second gamete fusion after the first cleavage of the zygote in a 47,XX,+18/70,XXX,+18 mosaic. A remarkable diploid-triploid discrepancy after CVS

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.

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.

Triploid mosaicism in a 45,X/69,XXY infant

We report on an infant referred for chromosome analysis during the neonatal period due to ambiguous genitalia. The genitalia appeared male with bilaterally palpable testes, penoscrotal hypospadias, chordee, and a bifid scrotum. Chromosome analysis and interphase FISH analysis of lymphocytes showed a 45,X karyotype and no evidence for SRY in 200 nuclei examined, respectively. Subsequent chromosome analysis of fibroblasts revealed a 69,XXY karyotype. Molecular studies were carried out to determine the etiology of the chromosome findings. Results indicated that the two cell lines are mosaic rather than chimeric and that the triploidy resulted from delayed dispermy rather than delayed polar body inclusion. To our knowledge this is the first reported living individual with (near) diploid/triploid mosaicism for 45,X/69,XXY.

Chromosome mosaicism in cleavage-stage human embryos: evidence of a maternal age effect

The present study evaluated mosaicism in a large series of cleavage-stage human embryos analysed by fluorescence in-situ hybridization. Only embryos with at least three cells analysed were included (n = 1235), of which 556 were mosaics. The most common types of mosaicism were chaotic (48%), diploid/polyploid (26%), and those caused by mitotic non-disjunction (25%). The number of abnormal cells per embryo ranged from 44% in diploid/polyploid to 84% in chaotic mosaics. Chromosome 16 was most commonly involved in mitotic non-disjunction mosaics. While overall mosaicism did not increase with maternal age, the average maternal age of the embryos that had mosaics caused by mitotic non-disjunction was significantly higher than that for normal or other mosaic embryos (P < 0.001). During the cleavage stage, the embryonic genome is not yet fully activated and consequently the mRNA and protein pools are still similar to those found in the oocyte. We therefore propose that the malfunctioning of the meiosis apparatus, which is similar to the mitotic one, may cause either meiotic errors or mitotic non-disjunction at cleavage-stage embryo development.

Some causes of genotypic and phenotypic discordance in monozygotic twin pairs

The use of the adjective "identical" rather than monozygotic leads to misunderstandings about the biology of monozygotic twinning. Most monozygotic twin pairs are not identical; there may be major discordance for birth weight, genetic disease, and congenital anomalies. These indicate that postzygotic events may lead to the formation of two or more cell clones in the inner cell mass and early embryo that actually stimulate the monozygotic twinning event. There is also evidence that there may be unequal allocation of numbers of cells to the monozygotic twins; this may have widespread implications for the cascade of developmental events during embryogenesis, formation, and vascularization of the placenta. Large-scale zygosity testing at birth could be the template for analysis of twin outcomes and their biologic causes.