Time sequence of germinal vesicle breakdown in pig oocytes after cycloheximide and p‐aminobenzamidine block

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Abstract
All porcine oocytes cultured 20 hr in medium with 10 μg/ml cycloheximide rested in the germinal vesicle (GV) stage but with the highly condensed bivalents in nucleoplasm. When these oocytes were washed and cultured in the control medium for 2, 4, and 6 hr, germinal vesicle breakdown (GVBD) was completed in 0, 86, and 100% of them, respectively. When similarly inhibited oocytes cultured successively only 2.5 hr in the control medium were given again in cycloheximide enriched medium (3.5 hr), nearly all of them reached late diakinesis stage again. It means that oocytes cultured for 20 hr and washed free of this inhibitor of protein synthesis completed GVBD rapidly (4 hr) and protein synthesis crucial for nuclear membrane disintegration occurred already during the first 2 hr after washing of inhibitor. All oocytes cultured for 20 hr in medium with 1 mM p‐aminobenzamidine rested in GV with chromatin around the compact nucleolus. The successive culture in cycloheximide (20 hr) and p‐aminobenzamidine (10 hr) prevented GVBD in all oocytes, too. In contrast, when the oocytes washed after cycloheximide block (20 hr) were cultured in p‐aminobenzamidine enriched medium 2 and 3 hr and again for 6 hr in cycloheximide medium, the nuclear membrane dissolved in 62 and 68% of oocytes, respectively. These data suggest that inhibition of protein synthesis in pig oocytes does not prevent the high condensation of bivalents in GV. However, nuclear membrane breakdown requires the successive protein synthesis and proteolysis.