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Calcium signalling is involved in important events in oocytes, such as meiotic competence acquisition. We have previously demonstrated the positive influence of animal age and gonadotropin stimulation in vivo regarding the ability of oocytes recovered from preantral follicles to exhibit calcium spikes. In the present work we determined whether preantral follicle development in vitro also allows oocytes to acquire calcium signalling activity. We also aimed to verify the influence of animal age, FSH + LH and/or insulin on oocyte calcium spike acquisition during preantral follicle culture. Early preantral follicles were isolated from 12-day-old and 1- to 3-month-old F1 hybrid mice and cultured individually for either 2 or 6 days. At the end of the culture period the oocytes were processed for calcium imaging by confocal microscopy. We show that oocytes recovered from cultured preantral follicles exhibit variable calcium spike activity rates, depending on animal age, culture duration and hormonal supplementation. Oocytes recovered from adult animals continue to exhibit calcium spikes, and those recovered from juveniles acquire that activity after culture. Insulin and gonadotropins in combination account for an early and maintained inhibitory effect on calcium signalling acquisition by oocytes. Insulin alone also leads to an early inhibitory effect, which, however, disappears with longer culture periods. Contrary to the complex in vivo situation, the acquisition of calcium signalling by oocytes in a controlled in vitro environment does not seem to be dependent on gonadotropins alone.

The location of the phospholipase C β1-isoform (PLC-β1) in the mouse oocyte and its role in the resumption of meiosis were examined. We used specific monoclonal antibodies to monitor the in vitro dynamics of the subcellular distribution of the enzyme from the release of the oocyte from the follicle until breakdown of the germinal vesicle (GVBD) by Western blotting, electron microscope immunohistochemistry, and confocal microscope immunofluorescence. PLC-β1 became relocated to the oocyte cortex and the nucleoplasm during the G2/M transition, mainly in the hour preceding GVBD. The enzyme was a 150-kDa protein, corresponding to PLC-β1a. Its synthesis in the cytoplasm increased during this period, and it accumulated in the nucleoplasm. GVBD was dramatically inhibited by the microinjection of anti-PLC-β1 monoclonal antibody into the germinal vesicle (GV) only when this accumulation was at its maximum. In contrast, PLC-γ1 was absent from the GV from the time of release from the follicle until 1 h later, and microinjection of anti-PLC-γ1 into the GV did not affect GVBD. Our results demonstrate a relationship between the relocation of PLC-β1 and its role in the first step of meiosis.

The role of calmodulin in fertilisation events was examined in a zona-free mouse system by using a selective calmodulin inhibitor, calmidazolium (1 μM). The effects of this antagonist were studied either on the ooplasmic calcium oscillations induced by fertilisation by using the Ca2+ indicator, fluo-3/AM, or on pronucleus formation 4 h later by using the nucleic acid stain, Syto-15. When the calmidazolium treatment was applied to one or the other gamete before insemination, the fertilisation process was affected only when spermatozoa were treated: most of the oocytes were partially fertilised as demonstrated by the profile of Ca2+ oscillations and the presence of polar bodies with no typical male and female pronuclei. When the treatment was applied during insemination, more than half the oocytes were unfertilised and only a few were partially fertilised. These results demonstrate that: (1) the calmodulin-dependent events taking place in spermatozoa before insemination appear essential at least for regular Ca2+ oscillations and for pronucleus formation; (2) the inhibition of calmodulin by calmidazolium applied to metaphase II oocytes before insemination has no major impact on their fertilising ability; and (3) at the time of gamete fusion calmodulin, either from the oocyte or from the spermatozoon, is essential for fertilisation to occur.

Using confocal laser scanning microscopy and time-lapse reconstruction, we monitored [Ca²⁺]_i in living mouse oocytes loaded with the calcium-sensitive fluorescent dyes fluo-3/AM or NuCa Green (specific for nuclear calcium) at the time of meiosis reinitiation. Our data confirm that spontaneous Ca²⁺ oscillations occurred in most of the immature fully grown mouse oocytes at least during the first half hour after release of the folliele and these oscillating oocytes underwent GVB in a large proportion after 1 h of culture. These spontaneous oscillations were disrupted when GVB was inhibited by dbcAMP and suppressed in the absence of external calcium. They were dependent on intracellular InsP₃-sensitive Ca²⁺ stores since they were inhibited by heparin, an InsP₃-receptor antagonist, or thapsigargin, which depletes InsP₃-sensitive Ca²⁺ stores. A relation appeared between the germinal vesicle chromatin appearance and the oocyte's ability to exhibit calcium oscillations. NuCa Green-loaded oocytes exhibited clear fluorescence oscillations in the nuclear region which lead us to discuss the implication of the nuclear calcium in the meiotic process. Moreover, the observation of a clear polarization of the Ca²⁺ waves in 30% of the oocytes permits us to hypothesize the existence of a predetermined localization for the initial point of the calcium wave. Copyright 1995, 1999 Academic Press

Summary.Analysis of ovaries from 31 women with normal ovarian function permitted study of the diameter of the largest healthy and atretic follicles during the menstrual cycle. The follicle destined to ovulate is selected during the early follicular phase (Days 1–5). Throughout the cycle the diameter of the largest healthy follicles, with the exception of the dominant follicle, did not exceed, on average, 6 mm during the follicular phase and 4 mm during the luteal phase. Consequently, excluding the dominant follicle during the second half of the follicular phase, the largest follicles present in the human ovary are atretic.From these data, it was concluded that a new ovulation could not occur very soon after a spontaneous or experimentally induced premature disappearance of the dominant follicle or the corpus luteum of the cycle.

Summary.Follicular rupture was never observed when preovulatory rabbit follicles were isolated from the ovary before the ovulatory gonadotrophin discharge and placed in hormone-free culture for 10–14 h (n= 48). However, if the follicles were taken 1 h post coitumand cultured under the same conditions, 11/24 (45·8%) ruptured spontaneously. Follicular rupture under these conditions appeared to be chronologically and histologically comparable to that of ovulation in vivo. Culture in the presence of progesterone increased the proportion of follicles ovulating in vitro(17/23 = 73·9%; P< 0·05). Inhibition in vitroof the synthesis of steroids or prostaglandins suppressed ovulation, although meiotic maturation of the oocyte did take place. These findings indicate that the follicle constitutes an independent entity within the ovary from 1 h post coitum.

Summary.Ovulation was never observed, even in the presence of gonadotrophins, when preovulatory follicles isolated from oestrous does were cultured in vitro(n= 112). If the follicles were co-cultured with pieces of ovarian tissue and gonadotrophins, then 50% ovulated (P< 0·01). Attempts to replace the ovarian tissue by various steroid hormones or prostaglandins did not result in follicular rupture except when PGE-2 was used (23·8%, P< 0·05). We suggest that gonadotrophins have an immediate (<1 h) action upon the follicle and ovarian interstitial tissue simultaneously. The response of the ovarian tissue to gonadotrophins is essential for eventual follicular rupture and involves PGE-2.

The meiosis resumption process has been related to spontaneous cytoplasmic InsP3-dependent calcium oscillations in fully grown mouse oocytes. Our purpose was to determine whether the acquisition of meiotic competence during the growth phase of oogenesis was associated with that of Ca2+oscillations and whether these oscillations were dependent on the phosphoinositide cycle. We used confocal laser scanning microscopy to image free calcium ions in fluo-3/AM-loaded oocytes recovered from 12- to 26-day-old mice for 15 min following follicular release. As expected, oocytes isolated from 12-day-old mice were totally incompetent to undergo GVBin vitro,whereas the GVB rate increased progressively with mouse age and oocyte diameter. The percentage of oocytes exhibiting spontaneous calcium oscillations and that of oocytes resuming meiosis were similarly correlated with the female age, with incompetent oocytes failing to exhibit spontaneous Ca2+oscillations. It is noteworthy that regardless of the stage of growth, thapsigargin induced an ooplasmic calcium release from the InsP3-sensitive stores when it was added to the culture medium. However, intracytoplasmic microinjection of InsP3 induced a shorter sequence of Ca2+oscillations in 12-day-old mouse oocytes than in 15-day-old mouse oocytes and, whereas InsP3 increased the GVB rate at 15 days, it was unable to induce GVB at 12 days. These data lead us to conclude that the acquisition of meiotic competence is related to the functionality of the InsP3 pathway and, correspondingly, to the oocyte's ability to generate spontaneous cytoplasmic InsP3-dependent calcium oscillations.

The influence of four factors on the cleavage rate of 705 mature oocytes submitted for in vitro fertilization (IVF) was assessed with the use of a multifactorial method, i.e., logistic regression. The studied factors were (1) the regimen for cycle stimulation, (2) cumulus cell mass appearance, (3) semen quality, and (4) the time of incubation of oocytes before insemination. The logistic function permitted testing the respective influence of each factor on the cleavage rate, with the level of the other factors taken into account. The most important factor was the stimulation treatment, the association of human menopausal gonadotropin with clomiphene citrate (CC) giving better results than CC alone. Time of incubation was demonstrated to have no influence on the cleavage rate, whereas semen quality had an influence. A problem was raised by the existence of an interaction between the stimulation treatment used and the appearance of the cumulus cell mass. It can be concluded that adequate cycle treatment and eventually couple selection with respect to the semen quality of the male partner are to be considered in view of obtaining better results in IVF attempts.

Summary.Detailed histological analysis of ovaries obtained from 25 women by ovariectomy for medical reasons demonstrated that the degeneration of the cyclic corpus luteum is a long process, taking several cycles.The ages of the various regressing corpora lutea were identified to permit classification of the side of ovulation during the preceding 4 or 5 cycles. In the 25 women studied, with regular menstrual cycles of 28 ± 3 days (48 cycles), ovulation occurred most often (87·6%) in an alternating manner.