Nuclear dynamics of parthenogenesis of bovine oocytes matured in vitro for 20 and 40 hours and activated with combined ethanol and cycloheximide treatment

Abstract

Bovine oocytes matured in vitro (IVM) for 20 hr vs. 40 hr were treated for activation with 7% ethanol in Dulbecco's phosphate-buffered saline for 5 min followed by incubation in M199 + 7.5% fetal calf serum containing cycloheximide (10 μg/ml). TreatedIVM oocytes and the controls (no ethanol and cycloheximide exposures) were fixed after 0, 1, 2, 3, 4, 5, 7, 10, and 20 hr of incubation and stained 24 hr later with 1% acetoorcein to examine nuclear events. Different stages of nuclear development of the activated oocytes were identified on the basis of nuclear and chromosomal morphology. Pronuclear development was classified into four stages (PN I, II, III, and IV) according to pronuclear progression in chromatin decondensation, nucleoplasm appearance, and nuclear size. The results demonstrated that the combined activation treatment effectively drove the IVM oocytes, both young (20 hr) and aging (40 hr), out of metaphase arrest. The activation rates for young oocytes examined immediately after 0, 1, 2, 3, 4, 5, 7, 10, and 20 hr of incubation with cycloheximide were, respectively, 7%, 24%, 77%, 96%, 92%, 97%, 98%, 93%, and 98%. For aging oocytes (40 hr) the corresponding activation values at the same time intervals were 6%, 84%, 100%, 100%, 100%, 100%, 98%, 100%, and 100%, respectively. These values were significantly higher than those for the corresponding controls. The activated aging oocytes achieved peak activation response more rapidly than did young oocytes. In addition, nuclear events in aging oocytes proceeded faster than those in young ones. Spontaneous activation rates of the aging oocytes were also higher (6–57%) than those of the young ones (0–14%).