Cryotop and development of vitrified immature bovine oocytes

The effectiveness of different cryodevices (open-pulled straw (OPS), electron microscopy grid (EMG), and Cryotop was evaluated for vitrification of immature bovine oocytes. Polar body, metaphase II stage (MII), survivability, and subsequent developmental rates were determined. Only oocytes with four or five layers of cumulus cells were used. Oocytes were equilibrated in two vitrification solutions – 1: 10% DMSO + 10% ethylene glycol (EG) for 30-45sec and 2: 20% DMSO + 20% EG +0.5M sucrose for 25sec –, mounted on one of the cryodevices and directly plunged into liquid nitrogen for 10 days. Immature vitrified oocytes using Cryotop showed the highest rates of polar body extrusion (PB) and nuclear maturity (MII); 41 and 58% respectively. Vitrified oocytes using OPS and EMG showed 26 and 32%; and 35 and 46% of PB and MII rates, respectively. The highest survivability resulted from Cryotop and EMG groups and no significant difference was found between them. Vitrified oocytes using Cryotop had the highest cleavage and blastocyst rates. All of the mean rates for vitrified immature oocytes were significantly lower than that of control group (P<0.05). The results of this study showed the superiority of Cryotop device for vitrification of immature bovine oocytes.