Timing of Nuclear Maturation and Postovulatory Aging in Oocytes of In Vitro-Grown Mouse Follicles with or Without Oil Overlay1

Abstract

Meiotic maturation of the oocyte is a timed sequence of events induced by the ovulatory LH surge. In vitro maturation of oocytes is known to alter the meiotic time course. This study documented the timing of meiosis in oocytes grown in vitro for 12 days, from the preantral follicle stage onward, and the influence of an oil overlay. In the oil-free culture, the stability of the metaphase II spindle was further explored to determine the postovulatory aging events. After the maturation stimulus, in vitro-grown oocytes were collected at 2-h intervals spanning the period of meiosis (0-18 h) and at 3-h intervals during early postovulatory aging (18-27 h). Stage of maturation was assessed both morphologically and by detailed spindle analysis and chromosome alignment. Results revealed that oil overlay did not impair the competence of cultured oocytes to proceed to meiosis II, but delayed meiosis I progression. Oil overlay during culture causes a different hormonal exposure of the follicles by a differential segregation into the oil overlay. The use of a progesterone receptor antagonist, however, did not induce a delay in meiotic progression. Aging effects in oil-free cultured follicles were detected 5 h after the establishment of the metaphase II spindle, comparable to their in vivo grown counterparts. The predominant effect of aging was an interphase-like appearance of the cytoskeleton. So an optimal time window for fertilization after in vitro follicular growth was determined to be 16-21 h after maturation induction.

Unaltered imprinting establishment of key imprinted genes in mouse oocytes after in vitro follicle culture under variable follicle-stimulating hormone exposure

“…However, 28h of in vitro maturation is excessively long, considering the fact that at 16h already, a maximum proportion of oocytes reach maturation, and that at 21h post hCG aging effects are visible on the spindle apparatus (Segers et al, 2008).…”

The cumulus cell gene expression profile of oocytes with different nuclear maturity and potential for blastocyst formation

“…Within 6 h after ovulation, oocytes are expected to be fertilized [4]. Outside this fertilization window, post-ovulatory oocyte ageing sets in as demonstrated in in vivo [4,5] and in vitro maturation settings [6].…”

Live Offspring from Cryopreserved Embryos Following In Vitro Growth, Maturation and Fertilization of Oocytes Derived from Preantral Follicles in Mice

“…Preantral follicle culture requires the use of normal oxygen (about 20% O 2 ) tension [36], but OGC culture demonstrates high preimplantation developmental competence under reduced oxygen (5% O 2 ) tension [4]. The oil overlay during IVM reduces progesterone levels in the medium and delays the timing of nuclear maturation [37,38], but in long-term follicle culture, the oil overlay did not affect follicle survival, the MII rate or the oocyte diameter and did not alter the methylation status of differentially methylated regions of imprinted genes [39].…”