Blastocyst formation and cell numbers in human frozen-thawed embryos following extended culture

BACKGROUND: Blastomere loss following human embryo cryopreservation has been shown to be associated with reduced implantation potential. In order to elucidate the underlying mechanism, the present study was designed to investigate the consequences of blastomere loss on subsequent preimplantation development in vitro. METHODS: Cryopreserved embryos destined for disposal were thawed and cultured for 96 h prior to determination of total cell numbers in resultant blastocysts. RESULTS: The proportion of embryos which formed blastocysts in vitro was significantly reduced when blastomere loss was evident in thawed embryos (25 versus 41% in fully intact thawed embryos). In addition, blastocysts from partially intact thawed embryos exhibited a significant reduction in total cell number (45.0 versus 58.4 in blastocysts from fully intact thawed embryos). Development to the blastocyst stage was significantly less frequent in fully intact thawed embryos which had been generated using ICSI (19%) compared with standard IVF (47%), although cell numbers in the resultant blastocysts were similar (57.1 and 58.6 respectively). CONCLUSIONS: Blastomere loss following cryopreservation impairs preimplantation development and results in reduced cell numbers at peri-implantation stages. ICSI is associated with reduced potential for post-thaw development of cryopreserved embryos in vitro.