Bovine blastocyst diameter as a morphological tool to predict embryo cell counts, embryo sex, hatching ability and developmental characteristics after transfer to recipients.

The aim of the present study was to explore whether the blastocyst diameter and the zona thickness at 168 h after fertilisation are useful parameters to predict quality and viability of bovine in-vitro-produced (IVP)-embryos. Although significant (P <l 0.05), the blastocyst diameter at 168 h correlated only poorly with the total number of cells (R(2) = 0.13) and with the number of trophectoderm (TE) cells (R(2) = 0.17). Hatched blastocysts (n = 66) at 216 h had a significantly greater mean diameter at 168 h (194.8 +/- 16.8 microm) compared with either blastocysts that had started but not finished hatching at 216 h (n = 26, 178.4 +/- 16.7 microm) or failed to commence hatching (n = 136, 162.7 +/- 12.9 microm). Transfer of 101 IVP blastocysts to synchronised recipients resulted in the birth of 38 calves (38%). There were significantly more bull calves born than cow calves (P < 0.05), but this was not correlated with blastocyst diameter or zona thickness at 168 h. There was also no correlation between the diameter of blastocysts or the zona thickness at 168 h and parameters of subsequent developmental characteristics, including rates of pregnancy, resorptions and abortions, pregnancy duration, delivery to term and birthweight. Overall, the present results indicate that the blastocyst diameter and the zona thickness at 168 h are good predictors for subsequent hatching ability in vitro, but not for the number of TE cells, inner cell mass cells or total cells and neither for subsequent developmental characteristics after transfer to recipients.