Meiotic competence in vitro of pig oocytes isolated from early antral follicles

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Abstract
Pig oocytes were isolated from early antral follicles of different sizes and their abilities to resume and complete meiotic maturation in vitro were compared. After 24 h of culture, > 80% of the oocytes from follicles 0.3-0.7 mm in diameter remained at the germinal vesicle stage, while 66, 94.3 and 100% oocytes from follicles 0.8-1.6, 1.7-2.2 and 3-5 mm in diameter, respectively, completed germinal vesicle breakdown. After 48 h of culture, 35% of the oocytes in the smallest follicle class progressed to prometaphase and only 4% to metaphase I. Of the oocytes from follicles 0.8-1.6 mm in diameter, 23% reached metaphase I and 17.3% metaphase II. About 50 and 76% of the oocytes from follicles 1.8-2.2 mm and 3-5 mm in diameter, respectively, extruded the 1st polar body. The ability to resume meiosis (i.e., to undergo germinal vesicle breakdown) is reached by porcine oocytes when they approach their full size in antral follicles > 0.8 mm in diameter and before they are capable of completing it (i.e., reaching metaphase II). The ability to complete meiotic maturation acquired in antral follicles of about 2 mm in diameter coincided with a significant decrease in the nucleolar transcriptional activity of the oocytes.

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