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

Abstract

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.

Figure 1

Triploid↔diploid chimeras. Histological sections and DNA in situ hybridisation of (A and B) E9.5 2n↔2n control chimera TrC-21, (C and D) E9.5 3n↔2n chimera TrC-16 and (E and F) E9.5 3n↔2n chimera TrB-3. Arrows show (B) examples of 2n (Tg/−) nuclei with single hybridisation signals and (D) examples of 3n (Tg/Tg/−) nuclei with two hybridisation signals. (F) An abnormal allantoic vesicle in E9.5 3n↔2n chimera TrB-3 (ch, chorion; al, allantoic vesicle; vys, visceral yolk sac). Scale bars=200 μm (A, C, E and F) and 20 μm (B and D).

Figure 2

Histogram showing percentage Tg-positive cell contributions to three fetal tissues of seven E9.5 3n↔2n chimeras from series TrB and TrC (identified as B- and C- respectively) and mean contributions (±s.e.m.) for five 2n↔2n control chimeras.

Figure 3

Diagrammatic summary of the composition of the fetus and three extraembryonic lineages in (A) E12.5 3n↔2n chimeras, (B) E9.5 3n↔2n chimeras and (C) E9.5 Ts3↔2n chimeras. Shaded regions represent the percentage of Tg-positive cells in the different lineages (shading is black for individual 3n↔2n and Ts3↔2n chimeras and grey for means of 2n↔2n control chimeras). The extraembryonic epiblast values are means of amnion and visceral yolk sac mesoderm; primitive endoderm values are means of visceral yolk sac endoderm and parietal endoderm; E12.5 trophectoderm values are means of the placenta (predominantly placental trophoblast) and the trophoblast overlying Reichert's membrane; E9.5 trophectoderm values are means of separate estimates for the placental trophoblast, placental trophoblast giant cells and trophoblast giant cells overlying Reichert's membrane (Tables 3–5). Triploid↔2n chimera TrC-25 is not included because it was damaged and so not analysed. Abbreviation: ExEm, extraembryonic.

Figure 4

Trisomy-3↔diploid chimeras. Histological sections and DNA in situ hybridisation of (A and B) E9.5 2n↔2n control chimera TS-42, (C and D) E9.5 Ts3↔2n chimera TS-46 and (E and F) E9.5 Ts3↔2n chimera TS-10. Arrows show (B) examples of 2n (Tg/−) nuclei with single hybridisation signals and (D and E) examples of Ts3 (Tg/Tg/−) nuclei with two hybridisation signals. Inset in F shows trophoblast giant cell at the same magnification. Two hybridisation signals are not seen in all sections of Ts3 nuclei. Scale bars=200 μm (A, C and E) and 20 μm (B, D and F).