Cinemicrographic observations on the trophoblast and zona pellucida of the mouse blastocyst

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Abstract
The behaviour of mouse blastocysts during penetration and shedding of the zona pellucida was studied in vitro by time lapse cinemicrography. Blastocysts obtained by super-ovulation were filmed in Waymouth’s medium MB 725/1 in a specially designed disposable plastic chamber with a gas space buffered to maintain 5 % CO 2 in air. Mouse blastocysts undergo cycles of rapid contraction (4–5 min) followed by slower re-expansion (2–3 h), both before and after penetration of the zona. The zona also exhibits slight elasticity unless excessively stretched. The initial penetration of the zona was followed in detail and appears to be the result of specific cellular activity, not non-specific herniation due to increasing blastocoelic pressure. Rotation of the blastocyst was also observed.