78 NONINVASIVE CELL LINEAGE TRACING IN BOVINE EMBRYOS FROM 2-CELL STAGE UP TO BLASTOCYST STAGE

Abstract

The first lineage specification occurs during pre-implantation mammalian development. At the blastocyst stage, 2 cell lineages can be distinguished: the inner cell mass (ICM) and the trophectoderm (TE). The exact timing when embryo cells are skewed to these lineages is not clearly determined in mammalian species. In murine embryos, it has been suggested that the first cleavage plane might be related to the embryonic-abembryonic (Em-Ab) axis at blastocyst stage. Thus, the daughter cells of the 2-cell embryo might already be predisposed to a specific cell lineage further on development. The objective of the present study was to observe how the first cleavage in bovine embryos may be related to cell lineage allocation at the blastocyst stage, using a noninvasive tracing approach. Bovine oocytes were harvested, in vitro matured, and fertilised. At the 2-cell stage, embryos were injected in one blastomere with the membrane tracer DiI. At the blastocyst stage, embryos (n = 346) were classified as orthogonal when the Em-Ab axis was orthogonally divided by the borderline between labelled and non-labelled cells; as deviant if the borderline was overlapping the Em-Ab axis; and as random when the labelled and non-labelled cells were randomly distributed. Total cell count (TCC) and the ICM/TE ratio was allowed by DNA staining with 4′,6-diamidino-2-phenylindole (DAPI) and by immunostaining of the ICM with Sox2 antibody. Analysis of variance was performed by one-way ANOVA employing IBM SPSS v21 (SPSS Inc., Chicago, IL, USA) to determine any difference between the cell lineage allocation patterns, TCC, and the ICM/TE ratio. P-values = 0.05 were considered significant. All values are reported as mean ± standard error of mean. Within 40 repetitions, the blastocyst classification was as follows: orthogonal 14.9% (±2.32, n = 56), deviant 22.2% (±2.58, n = 80), and random 62.9% (±2.64, n = 210). A significant difference was found in the incidence between the random group against the orthogonal and deviant, but not between the latter two. Regarding TCC, a significant difference was observed only between the orthogonal (99.6 ± 11.7 cells, n = 15) and deviant (135 ± 7.3 cells, n = 25) groups, but not with random embryos (116 ± 5.5 cells, n = 42). Finally, no significant difference was found among the groups concerning the ICM/TE ratio (0.43 ± 0.07 for orthogonal, n = 7; 0.54 ± 0.06 for deviant, n = 14; and 0.40 ± 0.03 for random embryos, n = 26). In conclusion, bovine embryos present a marked tendency for a random distribution of the daughter cells derived from the 2-cell blastomeres. However, around 37% of the blastocysts present a patterned cell division, where the daughter cells remain together through pre-implantation development. The effect of these cell lineage allocation patterns on implantation and further embryo development needs to be addressed.

The authors acknowledge Laboratoire d'Excellence Revive (Investissement d'Avenir, ANR-10-LABX-73) and CONACyT Mexico for funding.