RELATIONSHIP BETWEEN TIMING OF DEVELOPMENT, MORULA MORPHOLOGY, AND CELL ALLOCATION TO INNER CELL MASS AND TROPHECTODERM IN IN VITRO-PRODUCED BOVINE EMBRYOS

Noninvasive measurements of bovine embryo quality, such as timing of c leavage, morula morphology, blastocyst formation, and hatching ability , were linked with the number of inner cell mass (ICM) cells and troph ectoderm (TE) cells of the resulting embryos. First, it was confirmed that fast-cleaving embryos proved to have significantly higher chances to reach advanced developmental stages vs. intermediate and slow clea vers (P = 0.01). They also showed significantly less fragmentation at the morula stage, implying the presence of more excellent morulae amon g fast-cleaving embryos (P < 0.05). Second, the quality of hatched bla stocysts, resulting from morulae of different morphological grades, wa s examined by differential staining. The total cell and ICM cell numbe rs were significantly lower for hatched blastocysts developed from poo r morulae compared to hatched blastocysts developed from excellent, go od, or fair morulae. However, hatched blastocysts with < 10 ICM cells were seen in embryos belonging to all four morphological scores. Final ly, it was found that timing of first cleavage was not significantly c orrelated with timing of blastocyst formation or with cell number of b lastocysts. Timing of blastocyst formation, however, was significantly correlated with cell number: day 8 blastocysts had significantly lowe r total cell and ICM cell numbers than day 6 and day 7 blastocysts (P < 0.001). These results suggest that the quality of in vitro-produced bovine embryos is very variable and cannot be linked with a single cri terion such as embryo morphology and/or hatching ability. Timing of bl astocyst formation was the most valuable criterion with regard to embr yonic differentiation. (C) 1997 Wiley-Liss, Inc.