Holding bovine oocytes in the absence of maturation inhibitors: kinetics of in vitro maturation and effect on blastocyst development after in vitro fertilization

Holding immature oocytes before maturation simplifies the transport of oocytes and aids in scheduling later manipulations. We examined the effect of holding bovine oocytes in the absence of meiotic inhibitors on their subsequent meiotic and developmental competence. Oocytes were matured immediately after recovery (control) or were held in a mixture of 40% TCM 199 with Earle's salts, 40% TCM 199 with Hanks' salts, and 20% FBS, at room temperature for 16 to 18h (EH-held) and then matured. Chromatin status was determined at 0, 10, 14, 18, and 22h of maturation culture. Oocytes were fertilized in vitro after either 18 or 22-24h maturation. The EH treatment maintained oocytes at the germinal vesicle stage (79.3%, vs. 87.7% for control oocytes at 0h; P>0.05). Upon culture, held oocytes matured more quickly than did control oocytes. The proportions of mature oocytes were not significantly different between groups at 18h (EH-held, 80.6% and control, 79.3%); however, after 22h significantly more EH-held than control oocytes had degenerated (24.1% vs. 4.5%, P<0.0001). Blastocyst development was similar between groups for oocytes fertilized after 18h maturation (EH-held, 29.6% and control, 27.8%). When oocytes were fertilized after 22-24h maturation, EH-held oocytes yielded lower blastocyst development than did control oocytes (16.5% vs. 29.3%, P<0.05). In conclusion, bovine oocytes may be effectively held in the EH treatment before maturation without adversely affecting meiotic or developmental competence. However, holding affects the kinetics of maturation and this must be taken into account when subsequent manipulations are performed.