Blastocyst development in co-culture: development and morphological aspects.

A retrospective study was undertaken to determine if initial culture conditions and embryo quality had an effect on subsequent blastocyst development in co-culture for cryopreservation. The apparent effects of freeze-thawing on blastocysts at the ultrastructure level were also observed. On day 3 of culture, embryos were categorized into two groups based on their morphological attributes. Results suggest that the initial culture environment of embryos up to day 3 (5- to 8-cell stage) did not affect the subsequent rate of blastocyst formation in co-culture. However, the initial embryo quality had an impact on blastocyst formation and quality. On day 5.5, 90% (60/67) of the optimal quality embryos (six to eight blastomeres with minimal or no fragmentation on day 3) had attained the blastocyst stage, which was greater (P < 0.01) than the 55% (31/56) observed with the sub-optimal embryos (five to eight blastomeres with 30-50% fragmentation on day 3). Furthermore, 66% (44/67) of embryos initially graded as optimal were suitable for cryopreservation, which was greater (P < 0.01) than attained with embryos of lesser quality (22/56; 39%). At the ultrastructural level, the polarized distribution of plasma membrane microvilli was retained, as was the integrity of the nuclear membrane following thawing.