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Le cycle ovarien
Le cycle menstruel comprend un élément essentiel appelé cycle ovarien, qui se déroule en deux phases principales chaque mois : la phase folliculaire e…
Le cycle ovarien se compose principalement des phases folliculaire et lutéale.
Dans la phase folliculaire, un follicule primordial se transforme en follicule primaire. L’ovocyte primaire de ce follicule sécrète une couche extracellulaire riche en glycoprotéines - la zone pellucide - qui l’entoure.
Le follicule primaire développe ensuite plusieurs couches de cellules de la granulosa et se transforme en un follicule secondaire.
Au fur et à mesure que la phase progresse, une couche de tissu conjonctif et de cellules épithéliales – la thèque folliculaire – se développe également autour du follicule secondaire.
Ensuite, le follicule secondaire se transforme en un follicule vésiculaire ou antral, formant un antre - une cavité remplie de liquide.
Cet antre se dilate avec du liquide jusqu’à ce que l’ovocyte secondaire et sa capsule environnante de cellules de la granulosa - la couronne radiée - soient isolés.
Lorsque le follicule atteint environ 1 pouce de diamètre, il se gonfle à partir de la surface externe de l’ovaire.
L’ovulation se produit lorsque le follicule mature se rompt et que l’ovocyte secondaire est libéré dans la cavité péritonéale.
Après l’ovulation, le follicule rompu forme le corps jaune.
Si aucune grossesse ne se produit, le corps jaune dégénère et laisse derrière lui une cicatrice - le corps albican.
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Q1: What are the main phases of the ovarian cycle?
The ovarian cycle consists of two main phases: the follicular phase and the luteal phase. The follicular phase, averaging around 14 days, involves the growth and maturation of ovarian follicles stimulated by follicle-stimulating hormone (FSH). The luteal phase, lasting approximately 14 days, follows ovulation and is characterized by corpus luteum activity. Together, these phases typically complete a 28-day cycle, though cycles can range from 21 to 40 days.
Q2: How does a primordial follicle develop into a mature follicle?
A primordial follicle transforms into a primary follicle when the primary oocyte secretes the zona pellucida, a glycoprotein-rich extracellular layer. The primary follicle then develops multiple layers of granulosa cells and becomes a secondary follicle. As development continues, theca folliculi—connective tissue and epithelial cells—form around the secondary follicle. Finally, the secondary follicle matures into a vesicular or antral follicle containing a fluid-filled cavity called an antrum.
Q3: What triggers ovulation and what happens during the process?
Ovulation is triggered by a surge in luteinizing hormone (LH), which causes the mature follicle to rupture. During ovulation, the secondary oocyte, surrounded by the corona radiata (a capsule of granulosa cells), is released into the peritoneal cavity. Some individuals experience mittelschmerz, or ovulatory pain, during this process, though the precise mechanism remains unclear. In rare cases (1-2%), two oocytes may be released, potentially resulting in fraternal twins.
Q4: What is the corpus luteum and what does it produce?
The corpus luteum forms from the ruptured follicle after ovulation and secretes large amounts of progesterone along with smaller quantities of estrogens, relaxin, and inhibin. Progesterone prepares the endometrium for implantation, while inhibin reduces further FSH secretion. If pregnancy occurs, human chorionic gonadotropin (hCG) from the developing embryo sustains the corpus luteum until the placenta can produce adequate hormones independently.
Q5: How does the dominant follicle emerge during the follicular phase?
During the follicular phase, FSH promotes the growth of multiple follicles while preventing atresia. Among these follicles, a dominant follicle emerges and becomes increasingly sensitive to FSH by up-regulating its FSH receptors. This dominant follicle is also sensitive to LH, allowing it to mature fully at ovulation while its counterparts regress. The primary oocyte within the dominant follicle undergoes meiosis I, forming a secondary oocyte and polar body in preparation for ovulation.
Q6: What happens to the corpus luteum if pregnancy does not occur?
Without pregnancy, the corpus luteum degenerates into the corpus albicans, a scar-like remnant. As the corpus luteum degenerates, progesterone levels decline, triggering endometrial shedding and the onset of menstruation. This hormonal shift marks the transition from the luteal phase back to the follicular phase, restarting the ovarian cycle. The corpus albicans remains as a permanent scar on the ovary.
Q7: Why does cycle length vary among individuals?
While a 28-day cycle is common, ovarian cycles range from 21 to 40 days, with variability largely due to differences in the follicular phase and ovulation timing. The follicular phase is variable and can be influenced by hormonal factors and individual physiology, whereas the luteal phase remains relatively consistent at approximately 14 days. Additionally, some individuals experience variations in oocyte release influenced by factors beyond hormonal signaling, potentially explaining rhythm method contraceptive failures.