This study evaluated the effects of mineral oil (MO) overlay during maturation, fertilization, and embryo culture on the timing of nuclear maturation, the progesterone concentrations in the maturation medium, and the subsequent developmental competence of the oocyte. The results from experiment 1 showed that under the typical humidity of laboratory incubators (95%-97%), the culture media osmolality increased in the absence of oil overlay. For this reason, in experiment 2, maturation, fertilization, and embryo culture media were incubated with either an oil cover (MO group) or a microenvironment system for maximum humidity (HM group). Under these conditions, the media osmolality was maintained below 300 mOsm/kg. A portion of oocytes (n = 1414; four replicates) was removed from the maturation medium at 4- to 6-hour intervals to evaluate the nuclear maturation stage. The corresponding medium was used for progesterone measurement. The remaining oocytes were inseminated with frozen-thawed ejaculated sperm and cultured for 12 hours (n = 305) or 7 days (n = 619) to assess fertilization and embryo development parameters, respectively. The progesterone concentration of the maturation medium of the MO group was lower than 1.5 ng/mL at each time point evaluated. The values obtained at 12 hours of maturation and at the end of maturation were 20 and 55 times lower than those of the HM group, respectively. However, compared with the HM group, oil overlay did not delay oocyte progression to metaphase I and II and did not influence normal fertilization, cleavage, blastocyst formation, and total cell number in blastocysts. In conclusion, despite its pronounced impact on progesterone concentration, the use of MO did not affect the time course of oocyte maturation or oocyte developmental competence.
Keywords: Blastocyst; In vitro production of embryo; Oil overlay; Osmolality; Porcine; Progesterone.
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