Chromatin configurations and meiotic competence of oocytes are related to follicular diameter in nonstimulated rhesus monkeys.

Specific aims of this study were to compare relationships between size of intact antral follicles and meiotic competence, diameters, and chromatin configurations of germinal vesicle (GV) oocytes in non-gonadotropin-stimulated rhesus monkeys. Intact antral follicles were dissected from excised ovaries of nine normally cycling monkeys in the follicular phase of the menstrual cycle and of two acyclic monkeys. Follicles were classified according to diameter: I (200-450 mum), II (451-700 mum), III (701-1000 mum) and IV (> 1000 mum). Cumulus-enclosed oocytes were released from follicles and either measured (diameter) and fixed immediately or cultured for 48 h in modified CMRL medium containing 0.5 mug/ml ovine FSH, 10 mug/ml ovine LH, and 10% bovine calf serum. Following Hoechst staining, three distinct patterns of chromatin organization (GV1-3) were identified in GV oocytes according to the degree of association with the nucleolar periphery (encapsulation or "rimming"). In antral follicles > 450 mum in diameter, perinucleolar encapsulation (GV3) of oocytes before culture and meiotic maturation (metaphase II) of oocytes after culture increased (p < 0.01) with antral follicle growth in a graded fashion. While 56.3% of oocytes from large (> 1000 mum) follicles completed maturation, few (9.3% ) oocytes from small (200-450 mum) follicles were competent to mature in vitro. At 0 h of culture, class IV follicles contained a greater (p < 0.01) proportion of GV3 oocytes and a smaller (p < 0.01) proportion of GV1 oocytes than classes I, II, and III follicles. While the percentage of GV3 oocytes decreased (p < 0.05) by 48 h of culture, with a corresponding increase in metaphase II oocytes, GV1 and GV2 oocytes did not progress in GV status by 48 h. In the nonstimulated rhesus monkey, oocytes from antral follicles are not all equally competent to mature, but undergo nucleolar encapsulation and acquire meiotic competence with follicular growth.