Talk:Carnegie stage 2

Orientation of mitotic spindles during the 8- to 16-cell stage transition in mouse embryos

Dard N, Louvet-Vallée S, Maro B. PLoS One. 2009 Dec 4;4(12):e8171.

BACKGROUND: Asymmetric cell divisions are involved in the divergence of the first two lineages of the pre-implantation mouse embryo. They first take place after cell polarization (during compaction) at the 8-cell stage. It is thought that, in contrast to many species, spindle orientation is random, although there is no direct evidence for this.

METHODOLOGY/PRINCIPAL FINDINGS: Tubulin-GFP and live imaging with a spinning disk confocal microscope were used to directly study spindle orientation in whole embryos undergoing the 8- to 16-cell stage transition. This approach allowed us to determine that there is no predetermined cleavage pattern in 8-cell compacted mouse embryos and that mitotic spindle orientation in live embryo is only modulated by the extent of cell rounding up during mitosis.

CONCLUSIONS: These results clearly demonstrate that spindle orientation is not controlled at the 8- to 16-cell transition, but influenced by cell bulging during mitosis, thus reinforcing the idea that pre-implantation development is highly regulative and not pre-patterned.

PMID: 19997595 http://www.ncbi.nlm.nih.gov/pubmed/19997595

X chromosome inactivation is initiated in human preimplantation embryos

van den Berg IM, Laven JS, Stevens M, Jonkers I, Galjaard RJ, Gribnau J, van Doorninck JH. Am J Hum Genet. 2009 Jun;84(6):771-9. Epub 2009 May 28. PMID: 19481196

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2694969/?tool=pmcentrez

"X chromosome inactivation (XCI) is the mammalian mechanism that compensates for the difference in gene dosage between XX females and XY males. Genetic and epigenetic regulatory mechanisms induce transcriptional silencing of one X chromosome in female cells. In mouse embryos, XCI is initiated at the preimplantation stage following early whole-genome activation. It is widely thought that human embryos do not employ XCI prior to implantation. Here, we show that female preimplantation embryos have a progressive accumulation of XIST RNA on one of the two X chromosomes, starting around the 8-cell stage. XIST RNA accumulates at the morula and blastocyst stages and is associated with transcriptional silencing of the XIST-coated chromosomal region. These findings indicate that XCI is initiated in female human preimplantation-stage embryos and suggest that preimplantation dosage compensation is evolutionarily conserved in placental mammals."

XIST expression from the maternal X chromosome in human male preimplantation embryos at the blastocyst stage. Ray PF, Winston RM, Handyside AH. Hum Mol Genet. 1997 Aug;6(8):1323-7. PMID: 9259279