Cooperative inhibitory effect of follicular fluid and cAMP on hamster oocyte maturation

Meiotic state of bovine oocytes is regulated by interactions between cAMP, cumulus, and granulosa

Bovine oocytes are arrested at the prophase of first meiotic cell cycle. Meiosis resumes in oocytes of pre-ovulatory follicles upon LH surge. However, oocytes from secondary follicles spontaneously resume meiosis in the absence of hormones if removed from the follicle and cultured in vitro. The nature of meiotic arrestor in bovine follicles is poorly understood. In this study we investigated the role of cell-cell interactions between granulosa and cumulus cells and the oocyte in mediating maintenance of meiotic arrest by cAMP. We sorted oocytes as granulosa-cumulus oocyte complexes (GCOC) if surrounded with cumulus cells attached to a large granulosa investment or cumulus oocytes complexes (COC) if surrounded with cumulus cells only and investigated the role cAMP in maintenance of meiotic arrest in these oocytes under various conditions. In hormone- and serum-free medium both GCOC and COC enclosed oocytes resumed meiosis. When [cAMP](i) was elevated with addition of invasive adenylate cyclase (iAC) GCOC enclosed oocytes were maintained in the prophase with intact germinal vesicle (GV) while COC enclosed oocytes underwent GV breakdown (GVBD). iAC elevated [cAMP](i) in both types of oocytes to the same level. If oocytes were liberated from the cumulus and granulosa cells, they re-initiated meiosis in serum and hormone free medium, but remained in the GV stage if iAC was added to the medium. Untreated GCOC and COC enclosed oocytes extruded first polar body at the same frequency in hormone-supplemented media. GCOC and COC enclosed oocytes but not denuded oocytes (DO) cultured without somatic cells acquired developmental competence if cultured in hormone-containing medium. It is concluded that maintenance of meiotic arrest is regulated by the interplay of [cAMP](i), and cumulus and granulosa cells.

FSH bioactivity is strongly enhanced by kinetic incubation of pre-antral hamster follicles

Isolation of meiotic arrested oocytes from mammalian ovaries involves a resumption of the meiosis. Studying the effect of FSH on this process it was found that doses up to of 0.1 IU/ml did not maintain the meiotic arrest in hamster oocytes in pre-antral follicles during static incubation conditions. However, when the FSH medium was replaced at 120, 60 and 30 min the resumption of the meiosis gradually decreased. Using a replacement frequency of 30 min a complete log dose-curve was constructed (ED50: 0.5 mU/ml; minimum effective dose: 0.1 mU/ml). The lack of a FSH response during static incubation was not caused by exhaustion of its bioactivity: mixing of the FSH containing medium, obtained from a 4 hours' static incubation, resulted in an inhibition similar to the inhibition that occurred during the replacement procedure.

IN CONCLUSION

FSH is able to maintain the meiotic arrest in pre-antral follicle enclosed oocytes at low doses only when the medium is in circulation.

In vitro and in vivo studies reveal that hamster oocyte meiotic arrest is maintained only transiently by follicular fluid, but persistently by membrana/cumulus granulosa cell contact

Studies were carried out with the golden Syrian hamster to investigate the capacity of follicular fluid to maintain oocyte meiotic arrest and to determine the importance of cumulus-membrana granulosa cell contact in the regulation of meiotic status. The follicular fluid studies were conducted by cytological assessment of meiotic stage up to 6 hr after transferring cumulus-free oocytes into antra of explanted "host" follicles in vitro or into follicles of anesthetized animals prior to the gonadotropin surge at proestrus in vivo. The cumulus-membrana granulosa contact studies were undertaken with explanted follicles in which the oocyte-cumulus complex was dislodged from the underlying membrana granulosa, released into the antrum, and subsequently allowed to reestablish contact during 6 hr of incubation within the follicle. The extent of recontact of the dislodged complex with the underlying membrana granulosa was assessed visually at the end of incubation and was classified as close, moderate, or none. These various degrees of contact typically involved the following number of cumulus cells, as determined by serial sectioning of a representative sample of follicles after dislodgement and subsequent incubation: close, 32.7 +/- 1.78; moderate, 9.0 +/- 2.1; and no contact, 0. After 6 hr of incubation either in vitro or in vivo, few transferred oocytes remained at the germinal vesicle (GV) stage (18.8 +/- 8.7 and 17.3 +/- 4.0% GV, respectively). However, time course experiments revealed that meiotic resumption was significantly delayed in transferred oocytes compared with either liberated oocytes, spontaneously maturing oocytes, or follicle-enclosed oocytes induced to mature by luteinizing hormone in vitro (after 4 hr, transferred, 31.3 +/- 6.0% GV; liberated, 0% GV; follicle-enclosed, 0% GV; after 6 hr, 0% transferred oocytes exhibited a GV). In the dislodgement studies, after 6 hr of incubation, 26% of complexes reestablished close contact with the underlying membrana granulosa, 67% showed moderate contact, while 7% revealed no contact. There was a significant increase in the percentage GV stage oocytes as the extent of recontact increased (no contact, 21.9 +/- 3.6% GV; moderate contact, 56.6 +/- 6.8% GV; close contact, 87.5 +/- 14.4% GV). These data argue in favor of a stringent control of hamster oocyte meiotic status by the follicle cell/oocyte syncytium and against the possibility that follicular fluid is independently responsible for maintaining meiotic arrest.(ABSTRACT TRUNCATED AT 400 WORDS)