| A preimplantation embryo is a fertilized oocyte which is undergoing early post-zygotic cell divisions as it travels down the fallopian tube into the uterus. The embryo implants in the wall of the uterus at the 64-cell blastocyst stage.
Review pictures of the early cell divisions.
There is a very high rate of mosaic aneuploidy in the two to eight cell-stage human embryo. In a study of 250 embryos by Almeida & Bolton (1996) the overall incidence of chromosomal abnormality was 49%. Studies show that there is progressive loss of chromosomally-abnormal embryos during preimplantation development (Almeida & Bolton, 1996) and the preimplantation embryos with a larger proportion of aneuploid cells are the ones less likely to survive to implantation. There is evidence of early cell selection factors which select diploid cells over trisomic cells for survival in the developing embryo. |