Reversible changes in dissolution of the zona pellucida of immature bovine oocytes

The influence of holding immature bovine oocytes in in vitro bovine oviducts on the dissolution of zona pellucida in 0.1% pronase, the role of cumulus cells in this process and the possibility of reversing the process were examined. For the study, 1,045 oocytes were obtained from 2 to 6 mm ovarian follicles. Cumulus-free oocytes were placed in isolated bovine oviducts at 37 degrees C. The average dissolution time of the zona pellucida increased in proportion to the holding time of oocytes in oviducts: 9.9, 13.8, 48.3, 239.3 and 788.3 min after 5, 20, 40, 80 and 120 min in the oviduct, respectively. For the control group, only 4.6 min were required for dissolution of the zona. When cumulus-free and cumulus enclosed oocytes were held for 120 min, no differences were seen in the average lytic time of the zona pellucida of cumulus enclosed oocytes compared with the control group. When cumulus-free oocytes were held in vitro for 120 min and then immersed in follicular fluid from 30 min to 18 h, there was a significant reversal in the sensitivity of the zona pellucida to proteolysis.