Changes in porcine oocyte germinal vesicle development as follicles approach preovulatory maturity

This study was conducted to determine the distribution of oocytes in meiotic arrest as a function of follicle maturation, atresia status, and follicular fluid steroid concentrations. Oocytes (n = 138) from > or = 3 mm follicles were recovered from gilts (n = 3/d) on Days 1, 3, 5, and 7 of the follicular phase initiated by withdrawal of altrenogest treatment. They were fixed in 4% paraformaldehyde, stained with Hoechst 33342, and examined by laser scanning confocal microscopy using combined bright field Nomarski optics and ultraviolet laser illumination. The number of oocytes in complete meiotic arrest increased (P < 0.05) as a function of the stage of maturation from 29% on Day 1 to 79 and 67% on Days 3 and 5, respectively. Oocytes showing complete germinal vesicle breakdown (GVBD) were found only on Day 7 (24 to 36 h after the preovulatory LH surge). The distribution of GV stages on Days 1 to 5 did not differ between atretic (n = 27) and nonatretic follicles (n = 81). In nonatretic follicles, GV stage was inversely related to the concentration of estradiol on Day 7 and to the concentrations of progesterone and androstenedione (P < 0.05) on Days 5 and 7 indicating that meiotically arrested oocytes were likely to be found in follicles with highest levels of steroidogenesis. In conclusion, a large proportion of oocytes present in 3 to 5 mm follicles had begun GVBD. The follicles in the ovulatory cohort may be recruited or selected from preexisting 3 to 5 mm follicles, or younger population with oocytes that are in complete meiotic arrest.