We analyzed images of human blastocysts obtained from an in vitro culture system we have developed for time-lapse cinematography (TLC). Blastocoel collapse and re-expansion were repeated throughout the blastocyst stage because of disturbance of the trophectoderm (TE) wall. We also found an aberrant “strand phenomenon” within the blastocoel cavity in which the inner cell mass (ICM) ectopically adhered to the opposite TE wall. We also identified that this phenomenon was closely associated with the occurrence of monozygotic twin pregnancies. Monozygotic twinning increases in frequency with day-5 embryo transfers compared with day-2 or day-3 embryo transfers in human ART programs. When the blastocysts eventually escaped through the zona pellucida, we observed two types of hatching (inward and outward). Although the former was always accompanied by blastocoel collapse, this did not occur with outward hatching. We believe that the outward process without blastocoel collapse is more likely to represent the in vivo hatching pattern than the inward process. These results suggest that extended in vitro culture to the late blastocyst stage has a negative impact on human embryonic development.
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