Relationship between age of blastocyst formation and interferon-tau secretion by in vitro-derived bovine embryos.

This study was designed to examine the relationship between the speed at which bovine embryos reach the blastocyst stage, their cell number, and interferon-tau production. A total of 800 oocytes were fertilized by frozen-thawed semen. On day 2, 44 hr after exposure to sperm, 78, 320, and 296 embryos were at the two-, four-, and eight-cell stages, respectively, with an overall cleavage rate of 86.8%. Within these three groups 15 (19.2%), 106 (33.1%), and 158 (53.4%) embryos proceeded to the blastocyst stage. Of these 46.7%, 65.1%, and 63.3% hatched in the three groups, respectively. Blastocysts began to appear at day 7, but a few did not form until as late as day 13. Expanded blastocysts (n = 279) were cultured individually for 48 hr in 50-microliter droplets of medium, fixed for cell counts, and the concentration of interferon-tau in the medium was determined. Blastocysts originating from two-cell embryos had significantly fewer cells (46.5 +/- 23.3) than either four-cell- (97.2 +/- 13.5) or eight-cell-derived embryos (113.8 +/- 13.6; P < 0.05). Hatching was accompanied by an increase in cell number (129.8 +/- 15.5 versus 41.9 +/- 14.4; P < 0.01). Blastocysts derived from embryos that had reached the eight- or four-cell stage 44 hr after insemination produced significantly more interferon than embryos derived from two-cell embryos (941.7 +/- 92.1, 930.1 +/- 163.1, versus 232.8 +/- 70.1 pM). In contrast, hatching, ovary batch, the speed of early cleavage, cell number, and quality grade had no effect on interferon-tau secretion. The embryo's age at blastocyst formation was not related to the number of its cells but did have a significant effect (P < 0.001) on interferon-tau production, with mean concentrations in the medium of 294.8 +/- 57.9, 563.3 +/- 82.0, 1126.3 +/- 133.6, 1778.5 +/- 297.2, 512.9 +/- 82.0, 315.0 +/- 157.5, and 157.5 pM among blastocysts appearing from days 7 to 13, respectively. These data suggest that blastocysts that form at days 7 and 8 produce less interferon-tau than those that form on days 9 or 10. Since early-forming blastocysts are generally considered more developmentally competent than those which form late, there may be a negative relationship between early interferon-tau production and competence.