Perivitelline threads in cleavage-stage human embryos: observations using time-lapse imaging.

Time-lapse imaging of the human preimplantation embryo in vitro has revealed a transient phenomenon involving the appearance of perivitelline threads, commonly observed at the two-cell stage. These threads span the perivitelline space, arising at the specific area where the cytoplasmic membrane contacts the zona pellucida, before any perivitelline space is formed. The threads persist as the cytoplasmic membrane retracts from the zona pellucida to form the first cleavage furrow.