Summary. 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, more than 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 first 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.