Cryopreservation of immature and in-vitro matured human oocytes by vitrification

The objective of this study was to determine the efficacy of vitrification of human oocytes before and after in-vitro maturation (IVM). The immature oocytes recovered (n = 472) were divided into two groups: (i) immature oocytes (n = 219) vitrified at the germinal vesicle (GV) stage; and (ii) immature GV-stage oocytes (n = 253) that were firstly matured in vitro (MII-stage oocytes; n = 178), then vitrified (n = 79). The remaining oocytes (n = 99), which were not vitrified, were processed as controls. After warming, the oocyte survival, maturation and fertilization rates, as well as embryonic development, were compared. The results showed no significant difference between the survival rates of the oocytes vitrified at GV stage and those vitrified at MII stage (85.4% versus 86.1%). However, oocyte maturation rates were significantly reduced (P < 0.05) when oocytes were vitrified at immature GV stage followed by IVM (50.8%) in comparison with the control group (70.4%). Following insemination by intracytoplasmic sperm injection, there was no difference in the fertilization (62.1% versus 58.8%), cleavage (69.5% versus 67.5%) and blastocyst development (0.0% versus 0.0%) rates between these two groups. However, these results were significantly lower (P < 0.05) than those achieved in the control group. This suggests that better results can be achieved by vitrifying mature oocytes rather than immature oocytes.