Timing of development is a critical parameter for predicting successful embryogenesis.

Development of embryos from the 1-cell stage into blastocysts in vitro is generally slower than the time-course for development in vivo. It was the objective of this work to determine whether embryos that reach the 8-cell stage within a normal time-frame have a developmental advantage (both in vitro and post-embryo transfer) over slower embryos. Hamster 1-cell embryos were collected 10 h post-egg activation (PEA) and cultured for 48 h (58 h PEA = t50 for 8-cell embryo development in vivo) in hamster embryo culture medium-6. Embryos were sorted according to stage reached, culture was continued in fresh medium and stage of development was observed at 78, 82 and 86 h PEA. At 58 h PEA, embryos were < 4-cell (4%), 4-cell (19%), 5- to 7-cell (16%) or 8-cell (61%). The 58 h 8-cell embryos had a significantly greater ability to develop to the blastocyst stage than 58 h 4-cell embryos at 78, 82 and 86 h PEA (74 versus 13%, 69 versus 25% and 65 versus 37% respectively). The percentages of 14-day-old fetuses collected after embryo transfer indicated that morulae and blastocysts derived from 58 h 4-cell embryos were, on average, less viable (26% fetuses) than morulae and blastocysts from 58 h 8-cell embryos (51% fetuses). Thus morulae and blastocysts developing in vitro from faster or slower cleaving embryos can be qualitatively as well as quantitatively different. These data indicate that the timing of development in vitro, specifically the timing of completion of the third cell cycle, is a critically important parameter for predicting successful embryogenesis in the hamster.