Classification of morphological changes based on the number of cleavage divisions in bovine embryos

Quantification based on cleavage division (CD) of bovine preimplantation embryos facilitates quantitative analyses of embryonic developmental processes because CD occurs roughly once each day for all blastomeres for up to at least 9 days after ovulation. Therefore, embryonic morphological changes during this period were classified according to CD number. In this study, embryos collected from superovulated donors 0-9 days after ovulation were first classified morphologically into 14 conventional developmental stages. The total cell numbers (TCN) of embryos were measured using the air-dry method. The respective CD numbers of the embryos were then determined using logarithmic transformation of the TCN. The CD numbers of embryos were increased 0-10th with 11 stages. The 0th CD corresponded to 1-cell stage embryos; the 1st CD corresponded to 2-cell stage embryos; the 2nd CD corresponded to 3-4-cell stage embryos; the 3rd CD corresponded to 5-8-cell stage embryos; the 4th CD corresponded to 9-16-cell stage embryos, the 5th CD corresponded to morulae (17-32-cell stage embryos); and the 6th CD corresponded to the compact morulae. Furthermore, the 7th CD included early blastocysts to blastocysts. The 8th CD included expanded, collapsed and hatching blastocysts. The 9th CD included hatched blastocysts. The 10th CD included expanding-hatched blastocysts. The relationship between the CD number and the morphological characteristics of the bovine embryos 0-9 days after ovulation was expressed using a linear equation, and this revealed a high degree of correlation (y=0.98x-0.96, r=0.99). These results suggest that morphological changes of bovine embryos can be classified accurately using an 11-stage classification system based on the number of cleavages.