She explained the mechanism that underlies blastocoelic collapse and re-expansion. The fluid in the cavity must be under positive pressure as this pressure is the motive force for expansion of the blastocyst. The trophectoderm maintains the pressure by pumping the fluid into the cavity. I believe that the collapses occur when some of the junctions between the cells fail possibly due to localised cell death, or maybe due to a structural weakness in the junction itself and the blastocoelic fluid leaks out. These collapses occur quite quickly far more quickly than a pump could manage. The magnitude of the collapses is determined by the number of failed junctions. The greater the number of failed junctions, the more severe the collapse. In some cases the embryo cannot re-establish the junctions and the blastocyst is unable to re-expand and thus dies.

Seventeen of the 33 embryos went on to become fully formed blastocysts and 11 either started to hatch or hatched completely from the zona pellucida (the gelatinous protective coating around the blastocyst).

Fifteen embryos degenerated during culture and 11 of them did not re-expand after a collapse and subsequently degenerated. There was no evidence of embryo splitting during the hatching which was one of the theories as to how twins were formed from a single blastocyst. However, two of the 26 embryos (8%) had two distinct ICMs and a third had a possible second ICM. The most common form of monozygotic twinning (identical twins resulting from the dividing of one embryo fertilised by a single sperm) is monochorionic/diamniotic, when two ICMs form before hatching.

Ms Payne said: The second ICM was evident early in blastocyst formation in both embryos, and appeared to be the result of some ICM cells relocating and adhering to the opposite trophectoderm wall, seeded
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Contact: Emma Mason
wordmason@mac.com
European Society for Human Reproduction and Embryology
2-Jul-2007