Talk:Carnegie stage 2

2009

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

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.

"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."

PMID 19481196

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2694969

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

Carnegie Embryo

No. 8698

• 2-cell specimen
• spherical and surrounded by a transparent zona pellucida.
• Two polar bodies were present.
• Each blastomere was nearly spherical.

• Described by Hertig et al. (1954).
• Tubal Diameter, 178.5 μm (122 x 88 after fixation; 111.6 x 75 after sectioning); blastomeres, 71 (74 x 64 and 80 x 56 after fixation; 68.3 x 61.6 and 70 x 50 after sectioning); polar bodies, 20 x 18 after fixation. A few cells of the corona radiata were present. Thick zona pellucida (18 μm in thickness before fixation).

No. 8904

• 12-cell specimen
• perfectly spherical and surrounded by a clear zona pellucida.
• One blastomere, central in position and larger than the others, was presumed to be embryogenic, whereas the smaller cells were thought to be trophoblastic.

• Described by Hertig et al. (1954).
• Uterine Diameter, 172.8 μm (115.2 after fixation); blastomeres, 38.4 x 19.2. Clear zona pellucida (10 μm in thickness before fixation). Polar bodies not identified. No evidence of a blastocystic cavity, A large, central blastomere was thought to be embryogenic, the other cells trophoblastic. Specimen lost during preparation. Believed to be about 3 days old.