Effect of mono-(2-ethylhexyl) phthalate on bovine oocyte maturation in vitro

The effect of mono(2-ethylhexyl) phthalate (MEHP) on bovine oocyte maturation in vitro was examined. Cumulus-oocyte complexes (COCs) were cultured in maturation medium supplemented with various levels of MEHP for 24h, and then examined for the degree of cumulus expansion and the stage of maturation. A higher percentage of oocytes remained at the germinal vesicle (GV) stage after exposure to 75 and 100 micro M MEHP treatments (13.8 and 44.9% of oocytes, respectively) than the control (2.1% of oocytes). The proportion of oocytes that progressed to the metaphase II (MII) stage was significantly decreased with 25 micro M (59.6% of oocytes), 50 micro M (19.8%), 75 micro M (21.3%), and 100 micro M (3.1%) treatments than the control (77.3%). MEHP did not affect the process of cumulus expansion. For denuded oocytes, MEHP treatment of 50-100 micro M resulted in a significantly higher rate of oocytes remained at the GV stage compared to the control (53.4, 80.2, 88.4, and 5.4%, respectively). The rate of MII formation was significantly decreased with 10 micro M (60.9%) and 25 micro M (22.5%) MEHP treatments compared to control (68.9%). Furthermore, with 50, 75 or 100 micro M MEHP, no oocyte reached the MII stage. When COCs were cultured for 24h with 50 or 100 micro M MEHP and then cultured for an additional 24h in MEHP-free medium, most of the oocytes reached the MII stage (71.1 and 64.5%, respectively).Taken together, these results indicate that MEHP, at doses lower than those reported in blood transfusion patients, could negatively modulate bovine oocyte meiotic maturation in vitro, suggesting possible risks for human and other mammalians reproductive health.