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Luteinizing hormone–independent rise of progesterone as the physiological trigger of the ovulatory gonadotropins surge in the human

2020; Elsevier BV; Volume: 114; Issue: 2 Linguagem: Inglês

10.1016/j.fertnstert.2020.06.016

1556-5653

Dmitri Dozortsev, Michael P. Diamond,

Reproductive Physiology in Livestock

The current ovarian cycle paradigm postulates that ovulation is triggered by a critically sustained elevation of estradiol. However, an in-depth look into the published data reveals considerable uncertainty about the relative roles of progesterone and estradiol in the ovulation process.This review provides compelling evidences that the role of estradiol in ovulation has been misinterpreted and that the true physiological trigger of ovulation is a luteinizing hormone–independent preovulatory progesterone surge in the circulation to approximately 0.5 ng/mL. Furthermore, the current work reconciles the ability of progesterone to trigger ovulation, with its well-established ability to block ovulation during pregnancy, or when administered in the form of a synthetic progestin in birth control formulations and with experimental data that estradiol benzoate triggers ovulation in the complete absence of progesterone. The current ovarian cycle paradigm postulates that ovulation is triggered by a critically sustained elevation of estradiol. However, an in-depth look into the published data reveals considerable uncertainty about the relative roles of progesterone and estradiol in the ovulation process.This review provides compelling evidences that the role of estradiol in ovulation has been misinterpreted and that the true physiological trigger of ovulation is a luteinizing hormone–independent preovulatory progesterone surge in the circulation to approximately 0.5 ng/mL. Furthermore, the current work reconciles the ability of progesterone to trigger ovulation, with its well-established ability to block ovulation during pregnancy, or when administered in the form of a synthetic progestin in birth control formulations and with experimental data that estradiol benzoate triggers ovulation in the complete absence of progesterone. Discuss: You can discuss this article with its authors and other readers at https://www.fertstertdialog.com/users/16110-fertility-and-sterility/posts/30083 Discuss: You can discuss this article with its authors and other readers at https://www.fertstertdialog.com/users/16110-fertility-and-sterility/posts/30083 The current ovulation paradigm postulates that the rise of estradiol (E2) into the range of above 200 to 300 pg/mL for a minimum of 50 hours is what triggers gonadotropin-releasing hormone (GnRH) to surge. GnRH in turn binds to its receptors in the anterior hypophysis, causing the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) into the circulation and culminating in the rupture of the developing ovarian follicle (Fig. 1) (1Christensen A. Bentley G.E. Cabrera R. Ortega H.H. Perfito N. Wu T.J. et al.Hormonal regulation of female reproduction.Horm Metab Res. 2012; 44: 587-591Crossref PubMed Scopus (82) Google Scholar). However, a recent prospective randomized study has clearly demonstrated that the continued administration of letrozole throughout the follicular phase, with well-documented suppression of estradiol, did not have any effect on the timing of ovulation in normally menstruating women. This led the authors to conclude that the role of estradiol in ovulation has been misinterpreted (2Hurst B.S. Merriam K.S. Elliot M. Matthews M.L. Marshburn P.B. Usadi R.S. A sustained elevated estradiol is not the trigger for the preovulatory luteinizing hormone surge.Women’s Health & Gynecology. 2015; 1: 1-3Google Scholar). Notably, this is by far not the first observation questioning the role of the estradiol rise as a trigger of ovulation. Indeed, it has long been known that during ovarian stimulation, a supraphysiological E2 level is reached very early in the follicular phase, yet does not trigger ovulation. Furthermore, when the LH surge does occur, it is often markedly reduced, inconsistent with E2 being a trigger (3Messinis I.E. Templeton A. Effect of high dose exogenous oestrogen on midcycle luteinizing hormone surge in human spontaneous cycles.Clin Endocrinol (Oxf). 1987; 27: 453-459Crossref PubMed Scopus (10) Google Scholar). Furthermore, increasing the circulating E2 levels 10-fold (to ∼13,000 pmol/L) compared with control by the administration of exogeneous estradiol before ovulation had no impact on inducing an LH surge, or ovulation, in normally menstruating women (3Messinis I.E. Templeton A. Effect of high dose exogenous oestrogen on midcycle luteinizing hormone surge in human spontaneous cycles.Clin Endocrinol (Oxf). 1987; 27: 453-459Crossref PubMed Scopus (10) Google Scholar). Reviewing published data on the role of estradiol in ovulation, Zalanyi (4Zalányi S. Progesterone and ovulation.Eur J Obstet Gynecol Reprod Biol. 2001; 98: 152-159Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar) concluded that the idea that E2 causes ovulation has to be discarded. Another agent long known to cause the LH surge is progesterone (P4). Back in 1964, Buchholtz et al. (5Buchholz R. Nocke L. Nocke W. The influence of gestagens on the urinary excretion of pituitary gonadotropins, estrogens, and pregnanediol in women in the postmenopause and during the menstrual cycle.Int J Fertil. 1964; 9: 231-251PubMed Google Scholar) were the first to show that the intramuscular injection of P4 causes an immediate rise in urinary gonadotropins, similar to the gonadotropin peak seen during ovulation. Consistent with that observation, Odell and Swerdloff (6Odell W.D. Swerdloff R.S. Progestogen-induced luteinizing and follicle-stimulating hormone surge in postmenopausal women: a simulated ovulatory peak.Proc Natl Acad Sci U S A. 1968; 61: 529-536Crossref PubMed Scopus (146) Google Scholar) demonstrated that P4 injection elicits an LH and FSH surge in postmenopausal women after FSH downregulation with an estradiol regimen. Despite those early findings clearly demonstrating the ability of P4 to induce an ovulatory peak of gonadotropins, its role as a physiological ovulation trigger has not been recognized to this day. Likely this is largely because Leyendecker et al. (7Leyendecker G. Wardlaw S. Nocke W. Experimental studies on the endocrine regulations during the periovulatory phase of the human menstrual cycle: the effects of exogenous 17-oestradiol and progesterone on the release of pituitary luteinizing and follicle stimulating hormones.Acta Endocrinol (Copenh). 1972; 71: 160-178Crossref PubMed Google Scholar) demonstrated that estradiol benzoate can induce an LH surge in castrated women who completely lack any ovarian-derived P4. Furthermore, P4 has been historically known as an LH suppressor (4Zalányi S. Progesterone and ovulation.Eur J Obstet Gynecol Reprod Biol. 2001; 98: 152-159Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar), and its artificial derivatives–progestins–are the primary components of birth control regiment. Also, the preovulatory peak of P4 is seemingly small, only approximately 0.5 ng/mL (8Hoff J.D. Quigley M.E. Yen S.S.C. Hormonal dynamics at midcycle: a reevaluation.J Clin Endocrinol Metab. 1983; 57: 792-796Crossref PubMed Scopus (419) Google Scholar) compared with the postovulatory rise, which is about 10 times higher. The relatively low level of the preovulatory peak makes it difficult to recognize its clinical significance. It is also easy to miss because it occurs within hours, whereas virtually all relevant clinical studies sample P4 daily. Only when Hoff et al. (8Hoff J.D. Quigley M.E. Yen S.S.C. Hormonal dynamics at midcycle: a reevaluation.J Clin Endocrinol Metab. 1983; 57: 792-796Crossref PubMed Scopus (419) Google Scholar) sampled blood every 2 hours in the days preceding ovulation was it possible to observe that the P4 surge precedes both the estradiol peak and the gonadotropins flare by approximately 12 hours. Another intrinsic challenge in dissecting the roles of estradiol and P4 is their intimate cooperation, whereby the former is required to induce P4 receptors in the hypothalamus (9Leavitt W.W. Chen T.J. Allen T.C. Regulation of progesterone receptor formation by estrogen action.Ann N Y Acad Sci. 1977; 286: 210-225Crossref PubMed Scopus (149) Google Scholar). This review is narrowly focused on bringing together the evidence for the role of P4 as the trigger for the gonadotropin surge: evidence that we believe is credible. We put forth what we believe is compelling evidence that under physiological conditions, there are two waves of P4. The first wave is an LH-independent, precipitous rise of P4 12 hours before the gonadotropin surge to approximately 0.5 ng/mL, which signals to the hypothalamus that a follicle is ready to rupture. This rise activates the GnRH signaling pathway, with an ensuing LH/FSH surge, which causes the follicle to rupture and its granulosa cells to luteinize. Evolutionarily, luteinization first appeared at least 500 million years ago and has survived to this day as a locally controlled event in lower animals (Fig. 2) (10Stout E.P. La Clair J.J. Snell T.W. Shearer T.L. Kubanek J. Conservation of progesterone hormone function in invertebrate reproduction.Proc Natl Acad Sci U S A. 2010; 107: 11859-11864Crossref Scopus (47) Google Scholar). Although mammals, including humans, have developed a higher level of luteinization control, there is overwhelming evidence that the ability of all granulosa cells, including cumulus cells, to luteinize spontaneously was retained in humans (11Murphy B.D. Models of luteinization.Biol Reprod. 2000; 63: 2-11Crossref PubMed Scopus (147) Google Scholar). In vivo, the granulosa cells are apparently committed to luteinization but are prevented from luteinization through a mechanism that is not fully understood. However, irrespective of the details, the maintenance of the block requires an intact basement membrane of the follicle (12Wehrenberg U. Rune G.M. Spontaneous luteinization of antral marmoset follicles in vitro.Mol Hum Reprod. 2000; 6: 504-509Crossref PubMed Scopus (12) Google Scholar). Once the basement membrane is disrupted, whether through inflammation, by activated collagenase, or by intrafollicular pressure (13Takahashi T. Ohnishi J. Molecular mechanism of follicle rupture during ovulation.Zoolog Sci. 1995; 12: 359-365Crossref Scopus (5) Google Scholar), the granulosa cells are released from the differentiation block and become luteinized (14Tedeschi C. Hazum E. Kokia E. Ricciarelli E. Adashi E.Y. Payne D.W. Endothelin-1 as a luteinization inhibitor: inhibition of rat granulosa cell progesterone accumulation via selective modulation of key steroidogenic steps affecting both P4 formation and degradation.Endocrinology. 1992; 131: 2476-2478Crossref PubMed Google Scholar). Interestingly, the pace of luteinization of the intact follicle removed from the ovary is apparently similar in the presence or absence of LH. This strongly supports the assertion that the release from the block is due to changes in the intrafollicular milieu, rather than to an induction of luteinization by LH (12Wehrenberg U. Rune G.M. Spontaneous luteinization of antral marmoset follicles in vitro.Mol Hum Reprod. 2000; 6: 504-509Crossref PubMed Scopus (12) Google Scholar). Consistent with this concept, in hypophysectomized rats completely lacking LH, the follicles undergo normal ovulation and luteinization after an injection of FSH (15Hubbard G.M. Erickson G.F. Luteinizing hormone-independent luteinization and ovulation in the hypophysectomized rat: a possible role for the oocyte?.Biol Reprod. 1988; 39: 183-194Crossref PubMed Scopus (24) Google Scholar). In a milestone publication, Kubiak (16Kubiak J.Z. Mouse oocytes gradually develop the capacity for activation during the metaphase II arrest.Dev Biol. 1989; 136: 537-545Crossref PubMed Scopus (189) Google Scholar) postulated that oocytes gradually develop the ability to become fertilized after the resumption of meiosis and will become arrested at the so-called MIII stage if fertilized prematurely. Similarly, during follicular development, the oocyte gains the ability to resume meiosis at least several days before ovulation and before it acquires developmental competence (17Goudet G. Bézard J. Duchamp G. Gérard N. Palmer E. Equine oocyte competence for nuclear and cytoplasmic in vitro maturation: effect of follicle size and hormonal environment.Biol Reprod. 1997; 57: 232-245Crossref PubMed Scopus (104) Google Scholar), which we define as the ability to develop to term once fertilized. Under physiological conditions, the LH surge triggers degeneration of the gap junctions formed by cumulus cells with the oolemma and between themselves. On occasion, the degenerated connections can be readily observed as debris under the zona pellucida (Fig. 3). However, any changes in the follicular milieu due to the follicle losing its integrity will lead to spontaneous luteinization and degeneration of the gap junctions with the same result: meiosis will be resumed as long as the oocyte has acquired such ability (18Chian R.C. Buckett W.M. Tulandi T. Tan S.L. Prospective randomized study of human chorionic gonadotrophin priming before immature oocyte retrieval from unstimulated women with polycystic ovarian syndrome.Hum Reprod. 2000; 15: 165-170Crossref PubMed Scopus (295) Google Scholar). The timing of meiosis resumption in this case is less predictable, unlike the physiological disruption of gap junctions, which takes place simultaneously for all granulosa cells after an LH flare (19Norris R.P. Freudzon M. Mehlmann L.M. Cowan A.E. Simon A.M. Paul D.L. et al.Luteinizing hormone causes MAP kinase-dependent phosphorylation and closure of connexin 43 gap junctions in mouse ovarian follicles: one of two paths to meiotic resumption.Development. 2008; 135: 3229-3238Crossref PubMed Scopus (181) Google Scholar). If, for example, a dying follicle does not rupture, and the egg resumes meiosis resulting from the death of cumulus cells, it will have a postmature appearance at the time of retrieval (Dozortsev DI, unpublished data) (Fig. 4). This postmaturity is always a local event, inasmuch as a postmature egg can be retrieved alongside of normally appearing oocytes, with LH well suppressed (Dozortsev DI, unpublished data) (Fig. 4). The ability of the ovarian follicles to rupture without an LH surge has been convincingly demonstrated in hypophysectomized rats (15Hubbard G.M. Erickson G.F. Luteinizing hormone-independent luteinization and ovulation in the hypophysectomized rat: a possible role for the oocyte?.Biol Reprod. 1988; 39: 183-194Crossref PubMed Scopus (24) Google Scholar). In in vitro fertilization, this phenomenon is known as vanishing follicles and occurs more frequently in older patients during stimulated cycles (20Kol S. The vanishing follicle in women aged over forty: premature, mechanical, LH-independent luteinization may reflect oocyte-follicle low quality?.Med Hypotheses. 2008; 70: 1227-1228Crossref PubMed Scopus (3) Google Scholar). The vanishing of the follicle is accompanied by an increase in circulating P4 to the postovulation level, even when the LH surge is suppressed (20Kol S. The vanishing follicle in women aged over forty: premature, mechanical, LH-independent luteinization may reflect oocyte-follicle low quality?.Med Hypotheses. 2008; 70: 1227-1228Crossref PubMed Scopus (3) Google Scholar). It must be noted that mere rupture of the follicle that has not been exposed to LH (or human chorionic gonadotropin [hCG]) cannot be considered ovulation unless it results in an egg being released and becoming available for fertilization by a spermatozoon. Even though FSH can cause follicle rupture, it is not known to affect the degeneration of gap junctions, which could result in trapping the egg inside the ruptured follicle, or ovulation of an immature egg, or an egg surrounded by such tight cumulus cells that sperm penetration is not possible. If rupture of the follicle, resumption of meiosis, and luteinization of granulosa cells can all take place in the absence of LH, what is actually accomplished by an LH surge? Even though ovulatory events can each take place without LH, they have an extremely low probability to result in pregnancy without tight coordination in time, because of a relatively narrow viable fertilization window, that can lead to a term pregnancy (21Dozortsev D. Nagy P. Abdelmassih S. Oliveira F. Brasil A. Abdelmassih V. et al.The optimal time for intracytoplasmic sperm injection in the human is from 37 to 41 hours after administration of human chorionic gonadotropin.Fertil Steril. 2004; 82: 1492-1496Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar). The precise duration of the viable fertilization window in the human is unknown; however, the experience with day 1 intracytoplasmic sperm injection clearly demonstrates that it is no longer than 24 hours after retrieval (22Yuzpe A.A. Liu Z. Fluker M.R. Rescue intracytoplasmic sperm injection (ICSI)-salvaging in vitro fertilization (IVF) cycles after total or near-total fertilization failure.Fertil Steril. 2000; 73: 1115-1119Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar) or approximately 60 hours after hCG. Dozortsev et al. (21Dozortsev D. Nagy P. Abdelmassih S. Oliveira F. Brasil A. Abdelmassih V. et al.The optimal time for intracytoplasmic sperm injection in the human is from 37 to 41 hours after administration of human chorionic gonadotropin.Fertil Steril. 2004; 82: 1492-1496Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar) demonstrated that the highest chance of an embryo developing to term is decreased if fertilization takes place later than 41 hours after hCG, even though the chance of fertilization continues to climb. On the other hand, if the sperm penetration of the oocyte takes place earlier than 37 hours after hCG, both fertilization rates and pregnancy rates drop (21Dozortsev D. Nagy P. Abdelmassih S. Oliveira F. Brasil A. Abdelmassih V. et al.The optimal time for intracytoplasmic sperm injection in the human is from 37 to 41 hours after administration of human chorionic gonadotropin.Fertil Steril. 2004; 82: 1492-1496Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar). This means that the human oocyte has the highest chance for viable fertilization within only 4 to 5 hours after ovulation. Given that ovulation takes place 36 to 37 hours after hCG, it is apparently tightly linked to the optimal fertilization window, which implies that the putative messenger must “witness” the follicle’s impending rupture and relate it to the GnRH to trigger an LH surge. As a side note, the physiological LH surge is always accompanied by the surge of FSH and therefore is often referred to as an LH/FSH surge. It is not known whether FSH is truly needed for the follicle rupture at this stage or is simply a “side effect” of an LH surge, which alone is sufficient to invoke both follicle rupture and ovulation. The possibility of an FSH surge being a side effect is created by colocalization of FSH in the same cells, even in the same area of their cytoplasm (23Bauer-Dantoin A.C. Tabesh B. Norgle J.R. Levine J.E. RU486 administration blocks neuropeptide Y potentiation of luteinizing hormone (LH)-releasing hormone-induced LH surges in proestrous rats.Endocrinology. 1993; 133: 2418-2423Crossref PubMed Scopus (46) Google Scholar). The selectivity of their release is determined not by GnRH but by other messengers, such as neuropeptide Y protein (23Bauer-Dantoin A.C. Tabesh B. Norgle J.R. Levine J.E. RU486 administration blocks neuropeptide Y potentiation of luteinizing hormone (LH)-releasing hormone-induced LH surges in proestrous rats.Endocrinology. 1993; 133: 2418-2423Crossref PubMed Scopus (46) Google Scholar), providing a very plausible explanation for observing some amount of one with another. Thus, these observations suggest that the role of LH in ovulation is a coordination of resumption of meiosis, loosening of the cumulus cells, and rupture of the follicle. The preantral follicle growth, before its graduation into an antral follicle, is reproductive hormone independent and is controlled by the growth factors produced by the oocyte itself. Those growth factors control both the granulosa cells and the follicle enlargement, so that there is a tight coordination of both processes (24Eppig J.J. Wigglesworth K. Pendola F.L. The mammalian oocyte orchestrates the rate of ovarian follicular development.Proc Natl Acad Sci U S A. 2002; 99: 2890-2894Crossref PubMed Scopus (362) Google Scholar, 25Gilchrist R.B. Lane M. Thompson J.G. Oocyte-secreted factors: regulators of cumulus cell function and oocyte quality.Hum Reprod Update. 2008; 14: 159-177Crossref PubMed Scopus (656) Google Scholar, 26Sanfins A. Rodrigues P. Albertini D.F. GDF-9 and BMP-15 direct the follicle symphony.J Assist Reprod Genet. 2018; 35: 1741-1750Crossref Scopus (79) Google Scholar). The oocyte within the follicle is encapsulated by several layers of granulosa cells. The most proximal layer of granulosa cells (the so-called corona radiata) forms tight junctions with the oolemma. At this time, all granulosa cells are attending to the oocyte's needs. Before development into the antral follicle, all granulosa cells are committed to acquire FSH receptors and differentiate into mural granulosa. However, continued production of SMAD2/3 (and perhaps other growth factors, including growth differentiation factor 9 and bone morphogenetic protein 15) by an oocyte prevents granulosa cells from acquiring FSH receptors. The growth factors diffuse freely within the follicle (27Diaz F.J. Wigglesworth K. Eppig J.J. Oocytes determine cumulus cell lineage in mouse ovarian follicles.J Cell Sci. 2007; 120: 1330-1340Crossref PubMed Scopus (189) Google Scholar), creating a gradient with the highest concentration proximal to the egg and the lowest at the follicular wall (Fig. 5). As a cavity is formed within the follicle and its volume increases, oocyte-derived growth factors become diluted until the most distal granulosa cells escape oocyte control and acquire enough FSH receptors. This makes the follicle responsive to FSH at the end of the luteal phase of the previous cycle (28Channing C.P. Schaerf F.W. Anderson L.D. Tsafriri A. Ovarian follicular and luteal physiology.Int Rev Physiol. 1980; 22: 117-201PubMed Google Scholar). The development of the oocyte during the follicular phase will remain under control of the oocyte-produced growth factors. However, enlargement of the follicle, which is the result of mural granulosa proliferation and fluid accumulation, will be controlled initially by FSH and later by both FSH and LH, produced by the anterior pituitary. Full recognition of this dual control helps us see that the processes responsible for the egg acquiring developmental competency, and the process responsible for the ultimate egg release from the follicle (ovulation), are driven by completely different, independent, and uncoordinated mechanisms, which may have implications for oocyte quality (29Dozortsev D. Pellicer A. Diamond M.P. Progesterone is a physiological trigger of ovulatory gonadotropins.Fertil Steril. 2020; 113: 923-924Abstract Full Text Full Text PDF Scopus (11) Google Scholar, 30Dozortsev D. Pellicer A. Diamond M.P. Term oocyte maturation and term ovarian stimulation: impact on oocyte competence.Fertil Steril. 2020; 114: 221-222Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar). As already stated, the commitment of the granulosa cells to acquiring FSH receptors is their intrinsic property and cannot be controlled externally. However, the actual differentiation into cumulus and mural granulosa cells is a fluid process, because it is decided by competing gradients of the oocyte’s growth factors and FSH (27Diaz F.J. Wigglesworth K. Eppig J.J. Oocytes determine cumulus cell lineage in mouse ovarian follicles.J Cell Sci. 2007; 120: 1330-1340Crossref PubMed Scopus (189) Google Scholar). Ultimately, those competing gradients will determine how many cells will remain in the oocyte-cumulus complex and how many will be allocated to the follicular wall and produce estradiol. All existing evidence points to the fact that high FSH levels will recruit more granulosa into mural granulosa, leaving fewer cells to remain with an egg (24Eppig J.J. Wigglesworth K. Pendola F.L. The mammalian oocyte orchestrates the rate of ovarian follicular development.Proc Natl Acad Sci U S A. 2002; 99: 2890-2894Crossref PubMed Scopus (362) Google Scholar, 27Diaz F.J. Wigglesworth K. Eppig J.J. Oocytes determine cumulus cell lineage in mouse ovarian follicles.J Cell Sci. 2007; 120: 1330-1340Crossref PubMed Scopus (189) Google Scholar). It is not known how many cells should remain with the oocyte to adequately provide its support, but a lower threshold probably exists. This creates a strong, albeit theoretical, possibility that excess FSH may reduce the quality of the oocyte. Multiple studies demonstrating improvement of oocyte quality with mild ovarian stimulation support such an assumption (31Zhang J. Chang L. Sone Y. Silber S. Minimal ovarian stimulation (mini-IVF) for IVF utilizing vitrification and cryopreserved embryo transfer.Reprod Biomed Online. 2010; 21: 485-495Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). Another possible unintended consequence of excessive recruitment of the granulosa into mural granulosa, even if an adequate number of cumulus cells is left to nurture the oocyte, is a disproportionally fast-growing follicle, which may rupture prematurely. One of the important consequences of this competing relationship is that oocytes does not control the life cycle of the follicle and may not be able to reach term maturation by the time the follicle ruptures (30Dozortsev D. Pellicer A. Diamond M.P. Term oocyte maturation and term ovarian stimulation: impact on oocyte competence.Fertil Steril. 2020; 114: 221-222Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar). Even in the early days of ovulation research, the role of the E2 as an ovulation trigger was questioned when a sharp increase in circulating P4 was detected as early as 12 hours before any changes in LH or E2 (8Hoff J.D. Quigley M.E. Yen S.S.C. Hormonal dynamics at midcycle: a reevaluation.J Clin Endocrinol Metab. 1983; 57: 792-796Crossref PubMed Scopus (419) Google Scholar). Unlike E2, the preovulatory P4 remains relatively constant throughout the follicular phase, and its changes are directly linked to the disintegration of the follicle’s basement membrane, which is a sign of impending rupture of the follicle. Therefore, its rise can serve as an effective positive feedback signaling to the hypothalamus the follicle’s readiness to ovulation (Fig. 6). Buchholz et al. (5Buchholz R. Nocke L. Nocke W. The influence of gestagens on the urinary excretion of pituitary gonadotropins, estrogens, and pregnanediol in women in the postmenopause and during the menstrual cycle.Int J Fertil. 1964; 9: 231-251PubMed Google Scholar) demonstrated that intramuscular injection of P4 induces an immediate rise in gonadotropins similar to the rise observed with ovulation at mid-cycle. Later, the results were confirmed by Odel and Swerdloff (6Odell W.D. Swerdloff R.S. Progestogen-induced luteinizing and follicle-stimulating hormone surge in postmenopausal women: a simulated ovulatory peak.Proc Natl Acad Sci U S A. 1968; 61: 529-536Crossref PubMed Scopus (146) Google Scholar) and by Leyendecker et al. (7Leyendecker G. Wardlaw S. Nocke W. Experimental studies on the endocrine regulations during the periovulatory phase of the human menstrual cycle: the effects of exogenous 17-oestradiol and progesterone on the release of pituitary luteinizing and follicle stimulating hormones.Acta Endocrinol (Copenh). 1972; 71: 160-178Crossref PubMed Google Scholar). Liu and Yen (32Liu J.H. Yen S.S. Induction of midcycle gonadotropin surge by ovarian steroids in women: a critical evaluation.J Clin Endocrinol Metab. 1983; 57: 797-802Crossref PubMed Scopus (187) Google Scholar) showed that P4 is essential to establish the “normal dimensions of the LH surge,” even though the surge itself requires priming by estradiol. In 1992 Batista et al. (33Batista M.C. Cartledge T.P. Zellmer A.W. Nieman L.K. Merriam G.R. Loriaux D.L. Evidence for a critical role of progesterone in the regulation of the midcycle gonadotropin surge and ovulation.J Clin Endocrinol Metab. 1992; 74: 565-570Crossref PubMed Scopus (0) Google Scholar) demonstrated in direct observations that injection of P4 removes the ovulation block created by administration of RU486 in a dose-dependent manner, whereas administration of RU486 prevents a premature LH surge (34Escudero E.L. Boerrigter P.J. Bennink H.J. Epifanio R. Horcajadas J.A. Olivennes F. et al.Mifepristone is an effective oral alternative for the prevention of premature luteinizing hormone surges and/or premature luteinization in women undergoing controlled ovarian hyperstimulation for in vitro fertilization.J Clin Endocrinol Metab. 2005; 90: 2081-2088Crossref PubMed Scopus (19) Google Scholar). Consistent with P4 being an ovulation trigger, P4 gene knockout mice fail to ovulate despite normally growing follicles. There is ample other evidence pointing to the crucial role of P4 as the most upstream ovulation agent (4Zalányi S. Progesterone and ovulation.Eur J Obstet Gynecol Reprod Biol. 2001; 98: 152-159Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar, 35Lydon J.P. DeMayo F.J. Funk C.R. Mani S.K. Hughes A.R. Montgomery Jr., C.A. et al.Mice lacking progesterone receptor exhibit pleiotropic reproductive abnormalities.Genes Dev. 1995; 9: 2266-2278Crossref PubMed Scopus (1487) Google Scholar, 36Conneely O.M. Mulac-Jericevic B. Lydon J.P. De Mayo F.J. Reproductive functions of the progesterone receptor isoforms: lessons from knock-out mice.Mol Cell Endocrinol. 2001; 179: 97-103Crossref PubMed Scopus (200) Google Scholar). The significance of the preovulatory rise in P4 level can be difficult to accept because it is dec