Ovarian cycle

The ovarian cycle is the biological process through which the ovary recruits, selects and brings to maturity a follicle destined for ovulation, then transforms the residual structure into the corpus luteum and, in the absence of pregnancy, undergoes luteal regression with the initiation of a new gonadotropin-dependent window. This sequence does not correspond to a simple chronological alternation of phases, but to an integrated physiology in which the hypothalamic-pituitary-ovarian axis translates progressive changes in the follicular population into hormonal signals. Fluctuations in FSH and LH do not act on a static ovary, but on an endocrine organ undergoing continuous cellular, vascular and steroidogenic reorganization, in which different follicles coexist at different stages of development and compete for survival within an environment of variable feedback.

From a physiological perspective, the ovarian cycle results from cooperation at multiple levels. The hypothalamus provides the pulsatile GnRH signal that establishes the pituitary secretory profile; the pituitary releases gonadotropins with an amplitude and frequency that change throughout the cycle; the ovary responds with follicular growth, steroid production and secretion of regulatory peptides such as inhibins, activins and AMH; these signals, in turn, reshape the behavior of the central axis. The result is a feedback circuit in which the status of the follicular reserve, the quality of the antral cohort, the receptor sensitivity of granulosa and theca cells and the capacity to produce estradiol and progesterone determine whether the cycle will progress toward ovulation or be interrupted by anovulation.

Understanding the ovarian cycle therefore requires more than describing the follicular phase, ovulation and the luteal phase. It requires following the biological continuity linking activation of the reserve, preantral growth, antral recruitment, dominant follicle selection, acquisition of ovulatory competence and subsequent luteinization. Only from this perspective does it become clear that the cycle is not simply a “menstrual clock”, but the periodic terminal manifestation of a much longer and more complex follicular physiology.

Ovarian cyclicity is based on an essential principle: follicular growth is a continuous process, whereas what appears cyclic is primarily the final phase in which a cohort of antral follicles becomes dependent on the variable profile of FSH and LH. Primordial follicles enter the growth program continuously over time, independently of the day of the clinically observed cycle, and pass through a prolonged preantral phase regulated mainly by intraovarian signals. Only when they reach the small antral stage do they become fully responsive to the systemic endocrine environment and eligible for recruitment into the functional window that, under normal conditions, will lead to the selection of a single dominant follicle.

This means that the ovarian cycle does not begin anew with menstruation. The follicle that will eventually ovulate has already spent weeks or months undergoing growth and differentiation, during which the oocyte, granulosa cells, theca cells and surrounding stroma have progressively developed the organization required for the subsequent gonadotropin response. When progesterone, estradiol and inhibin A decline at the end of the luteal phase, the relative increase in FSH does not “create” the follicles of the next cycle, but allows an already existing cohort to enter the competitive FSH-dependent growth phase.

The gonadotropin-dependent window is therefore the portion of the process in which selection becomes possible. During this phase, the FSH level must be sufficient to support granulosa cell proliferation, aromatization of theca-derived androgens and antral growth, but not so high as to allow several follicles to overcome selection simultaneously. The normal cycle is based precisely on this balance: an initial rise in FSH that recruits a cohort, followed by a progressive decline induced by estradiol and inhibin B, which imposes selective pressure and permits the survival of only the most biologically competent follicle.

Ovarian cyclicity emerges from the combination of three physiological properties:

• continuous folliculogenesis, which proceeds even outside the clinically recognizable phase of the cycle;
• late gonadotropin dependence, which makes only the cohort of antral follicles responsive to FSH and LH eligible for selection;
• progressive feedback mechanisms involving ovarian steroids and peptides, which transform follicular growth into selection, ovulation and luteal function.

This organization explains why cycle length primarily reflects variation in the follicular phase, whereas the luteal phase tends to be more stable. Selection of the dominant follicle and the time required to generate an estrogen signal sufficient to induce positive feedback represent the most variable component of the entire endocrine sequence.

The ovarian reserve consists of primordial follicles, each formed by an oocyte arrested in meiosis and surrounded by flattened pregranulosa cells. The transition from quiescence to active growth represents the first critical step in follicular dynamics and occurs through a network of local signals involving the oocyte, somatic cells and stroma. Intracellular pathways such as PI3K-AKT-FOXO, factors belonging to the BMP and GDF families, interactions with KIT ligand and numerous regulators of the ovarian microenvironment play important roles during this phase. Follicular activation does not yet depend on cyclic fluctuations in FSH and LH, and it is precisely this initial relative autonomy that makes folliculogenesis a continuous process.

AMH, produced by granulosa cells in growing preantral and small antral follicles, exerts a physiological restraining effect on this system. Its function is not to completely block follicular activation, but to reduce the likelihood that an excessive number of primordial follicles will enter the growing pool simultaneously. In this way, AMH protects the ovarian reserve and modulates the rate at which the follicular pool is depleted throughout reproductive life.

As the follicle progresses from the primary to the secondary stage, granulosa cells proliferate and the surrounding stroma organizes into the future theca. The oocyte grows, intensifies its bidirectional communication with somatic cells and itself contributes to regulating follicular fate through paracrine factors. This preantral phase is decisive because it establishes the structural and functional foundation of the future follicle by determining the number of granulosa cells, the capacity to support the oocyte, the competence to form an antrum and the potential to respond to gonadotropins once the follicle reaches the stage at which it becomes sensitive to systemic control.

From an endocrinological perspective, preantral growth demonstrates that the future cyclic history of a follicle is conditioned long before selection becomes visible on ultrasound. The follicle that later becomes dominant is not inherently dominant, but reaches the gonadotropin-dependent window with a biological profile built during a prolonged phase of local maturation and interaction with the ovarian microenvironment.

With formation of the antrum, the follicle enters the phase in which gonadotropin regulation becomes central. The internal theca, under stimulation by LH, acquires full steroidogenic capacity and synthesizes androgens, particularly androstenedione and testosterone, from cholesterol. Granulosa cells, in contrast, express aromatase under stimulation by FSH and convert these androgens into estrogens, primarily estradiol. This functional organization is known as the two-cell, two-gonadotropin model and represents one of the fundamental principles of ovarian physiology.

The biological significance of this model extends beyond the simple distribution of enzymatic steps. It creates reciprocal dependence between the theca and granulosa compartments: the theca provides the androgen substrate, the granulosa converts it into estrogens, and the increase in estradiol modifies both the local follicular environment and feedback on the central axis. Follicular growth therefore becomes an integrated phenomenon in which the success of a follicle depends on the quality of cooperation between distinct cellular compartments.

During the early antral phase, FSH also stimulates granulosa cell proliferation, receptor synthesis, follicular fluid production and secretion of inhibin B. Inhibin B enters the circulation and, together with estradiol, contributes to the progressive reduction in FSH. Antral recruitment is therefore only apparently paradoxical: the same hormone that permits growth of the cohort induces, through follicular products, the conditions that will ultimately restrict that cohort to a single dominant follicle.

At this point ovarian physiology assumes a competitive logic. Antral follicles do not continue growing in parallel until ovulation, but are progressively exposed to an increasingly restrictive environment in which only the follicle that has developed the greatest efficiency of response to FSH and the most robust estrogen-producing capacity can continue.

Selection of the dominant follicle represents the physiological core of the follicular phase. After the initial recruitment of a cohort of antral follicles, their combined growth increases the secretion of estradiol and inhibin B. The systemic result is a progressive decline in circulating FSH. This decline is not a secondary effect, but the selection mechanism itself, because it imposes a higher survival threshold on the follicular cohort.

The follicle that becomes dominant possesses a complex biological advantage. It generally has a larger granulosa cell mass, better thecal vascularization, more efficient androgen production by the theca, greater aromatase activity, higher expression of FSH receptors and, progressively, acquisition of LH receptors in granulosa cells. Owing to these characteristics, the dominant follicle continues growing even when the FSH level is no longer sufficient to support the subordinate follicles.

Follicles that cannot maintain growth in an environment of declining FSH undergo atresia. Atresia is not simply cessation of growth, but an active process of apoptosis and involution that primarily involves granulosa cells and interrupts the steroidogenic capacity of the follicle. In endocrine terms, selection of the dominant follicle is therefore the result of differences in biological resilience within the same hormonal environment.

As dominance emerges, estradiol production becomes increasingly sustained and progressively less dependent on high FSH levels. This transition marks the transformation of the follicle from a growing structure into the main generator of the signal that will alter the physiology of the axis. The dominant follicle not only survives selection, but also establishes the preovulatory positive feedback that will trigger the LH surge.

When the dominant follicle reaches full functional maturity, estradiol is secreted at concentrations sufficiently high and for a sufficiently prolonged period to reverse the nature of estrogen feedback on the hypothalamic-pituitary-ovarian axis. During most of the cycle, estrogens exert an inhibitory effect, but during the preovulatory phase sustained high estrogen exposure sensitizes the central system and promotes the onset of positive feedback, culminating in the LH surge.

This transition depends not only on the absolute concentration of estradiol, but also on the duration of exposure and the neuroendocrine preparation of the central circuit, in which modulation of kisspeptin neurons, GnRH dynamics and increased responsiveness of pituitary gonadotroph cells play important roles. The dominant follicle therefore does more than produce estrogens for the endometrium or the clinical manifestations of the cycle, it compels the axis to shift to a different functional state.

At the same time, the follicle itself prepares for ovulation. Granulosa cells acquire LH receptors, a crucial event that enables the surge to act directly on the compartment that until then had been predominantly FSH-dependent. At the enzymatic and transcriptional levels, there is a gradual shift from a strongly estrogenic program toward a more luteinizing program, with increased expression of proteins involved in progestogenic steroidogenesis and in pathways responding to the ovulatory signal.

The preovulatory phase is therefore the point at which the physiology of growth gives way to the physiology of the event. The dominant follicle ceases to behave as a merely selected follicle and becomes an endocrine structure programmed to translate the LH signal into meiotic resumption, cumulus expansion, remodeling of the follicular wall and subsequent formation of the corpus luteum.

Ovulation is the result of the LH surge, which triggers a local cascade of events in granulosa cells, theca cells, the cumulus and the oocyte. Intracellular signaling pathways involving cAMP, EGF-like factors, MAPK and ERK proteins, prostaglandins, cytokines and proteases responsible for controlled remodeling of the follicular wall are activated. Follicular rupture is not a purely mechanical event, but a biological process resembling, in some respects, a highly coordinated sterile inflammatory response.

At the same time, the oocyte resumes meiosis, completes the first meiotic division and extrudes the first polar body, while the cumulus oophorus expands its extracellular matrix rich in hyaluronic acid. This expansion facilitates detachment of the cumulus-oocyte complex and its subsequent capture by the fallopian tube. In the somatic compartment, granulosa and theca cells begin to lose their classical follicular organization and progressively acquire luteal characteristics.

Immediately after rupture, the collapsed follicle undergoes intense vascular and cellular reorganization. Formation of the corpus luteum begins, creating a highly vascularized transient endocrine gland specialized in progesterone production. The close coupling between ovulation and luteinization has decisive physiological significance: the same structure that enabled release of the oocyte must be transformed without interruption into the organ that prepares for possible implantation.

The luteal phase is dominated by the endocrine function of the corpus luteum, which secretes large amounts of progesterone, together with estradiol and inhibin A. Progesterone profoundly modifies feedback on the axis by reducing the pulsatile frequency of GnRH and helping to restrain gonadotropin secretion. This endocrine environment prevents the development of another ovulatory window within the same cycle and gives the luteal phase relative temporal stability compared with the follicular phase.

In the uterus, progesterone transforms the proliferative endometrium into a secretory endometrium, but from an ovarian perspective its principal function is to maintain a postovulatory environment compatible with possible implantation. Inhibin A secretion further reinforces negative feedback on FSH. In the absence of a pregnancy signal, however, the corpus luteum has a limited lifespan: it progressively reduces steroidogenesis, loses trophic support, undergoes decreased vascularization and enters structural involution, with formation of the corpus albicans.

Luteal regression is the event that allows the follicular phase to restart. The decline in progesterone, estradiol and inhibin A removes pituitary inhibition and permits a new rise in FSH. This increase opens a new window of functional recruitment for the existing antral cohort. For this reason, the cycle must be understood as a closed physiological loop: the end of the luteal phase is not only the conclusion of the current cycle, but also the endocrine trigger for the next one.

The physiology of the ovarian cycle demonstrates that no single hormone is sufficient to describe the functional state of the ovary. The significance of an FSH value depends on the phase of the cycle and the context of ovarian reserve; estradiol reflects follicular steroidogenic mass but does not automatically correspond to ovulatory competence; progesterone is the principal indicator of luteal function, but its interpretation depends on the timing of measurement; AMH, inhibin B and other peptides provide information about the follicular population but do not encompass the complexity of oocyte quality or the probability of a normal ovulatory sequence.

Cycle duration, apparent regularity and the magnitude of hormonal fluctuations also depend on the integration of many variables: the size of the ovarian reserve, the rate of follicular recruitment, granulosa cell sensitivity to FSH, the quality of cooperation between theca and granulosa cells, the efficiency of positive estrogen feedback and the adequacy of luteal function. Two cycles of equal duration may therefore conceal different biological dynamics, and apparently similar hormonal profiles may correspond to different functional states when the timing or follicular context differs.

In conclusion, the ovarian cycle is the periodic expression of continuous follicular physiology. Follicular growth, dominant follicle selection, establishment of positive estrogen feedback, ovulation and luteal function are segments of a single biological chain. Only by recognizing this continuity is it possible to correctly understand female reproductive physiology, normal variation in menstrual cycles and the significance of ovarian endocrine disorders.