Monday, November 3, 2008
A. A normally fertilized egg showing two pronuclei
B. An uneven 6-cell embryo with one multinucleated cell
C. An 8-cell embryo with minor cytoplasmic fragmentation
Following the first division, some blastomeres in human embryos show multiple nuclei rather than the normal single nucleus. Possible causes are the lack of appropriate oxygen levels during follicular development or a rapid response to hormones during ovarian stimulation. Regardless of the cause, implantation and pregnancy rates decrease with increasing proportion of embryos with multinucleated cells replaced in the uterus. These embryos also have a considerably reduced ability to reach the blastocyst stage in extended culture. The selection of such embryos for replacement is avoided if at all possible.