Effects of plasma urea nitrogen levels on the bovine oocyte ability to develop after in vitro fertilization

The overall aim of the present study was to evaluate in vitro development ability of oocytes recovered from 56 Holstein Frisian heifers with low [group 1 (G1): <13 mg /dl], moderate [group 2 (G2): 13-16 mg /dl] and high [group 3 (G3): >16 mg /dl] plasma urea nitrogen (PUN) concentrations, to determine whether PUN concentrations affect the competence of oocytes to progress to blastocysts after in vitro fertilization. In vitro oocyte and embryo development was assessed by blastocyst rates, embryo total cell numbers and apoptosis. Blood samples for the determination of PUN were collected 24 h prior to collection of the ovaries at the slaughter. A total of 112 ovaries were collected at a local abattoir and oocytes (n = 697) were aspirated, in vitro matured and fertilized. On day 8, blastocysts were assigned to the terminal dUTP nick end labelling assay. Cleavage rates were significantly higher (p < 0.001) for groups 1 and 2 than for group 3 (i.e. 72.5% and 72.2% vs 61.7%, respectively). The proportion of fertilized oocytes that developed into blastocysts was higher (p < 0.05) for group 1 than for group 3 (34.0% vs 23.0%, respectively). Day 8 blastocysts showed higher total cell counts (p < 0.05) for group 1 than for group 3 (123.7 vs 76.3), and a higher (p < 0.05) total apoptotic cell rate was found in group 3 (25.9 and 19.0 vs 43.2 for G1, G2 and G3, respectively). In conclusion, the ability of oocytes from heifers with increased levels of PUN to develop to the blastocyst stage was significantly reduced when standard routines for in vitro maturation, fertilization and culture were followed. These detrimental effects can be mediated in part through direct effect of urea and/or by the metabolic products on the process of follicle-enclosed oocyte nuclear and cytoplasmic development.