The menstrual cycle includes a critical component known as the ovarian cycle, which undergoes two main phases each month—the follicular phase and the luteal phase. The follicular phase is variable and averaging around 14 days. Ovulation, triggered by a surge in luteinizing hormone (LH), marks the transition between the two phases. The second phase, the luteal phase, is relatively consistent, lasting approximately 14 days, and is marked by the activity of the corpus luteum. While a cycle length of 28 days is common, it can range from 21 to 40 days, with variability largely due to differences in the follicular phase and ovulation timing.
During the follicular phase, follicle-stimulating hormone (FSH) promotes the growth and survival of multiple follicles by preventing atresia. Amid these follicles, a dominant follicle emerges which becomes increasingly sensitive to FSH, up-regulating its FSH receptors. As the follicle is also sensitive to LH, it matures entirely at the time of ovulation while its counterparts regress. The dominant follicle's primary oocyte undergoes meiosis I and forms a secondary oocyte, and a polar body, in preparation for ovulation. However, granulosa cells impede the completion of meiosis until ovulation, which involves the ovarian wall's rupture and expulsion of the secondary oocyte, encapsulated by its corona radiata, into the abdominal cavity. Some individuals might sense this occurrence, known as mittelschmerz or ovulatory pain, although the precise mechanism behind it remains unclear.
Occasionally, during ovulation (in about 1-2% of cases), two oocytes might be released, potentially leading to the birth of fraternal twins. In contrast, identical twins come from a single fertilized oocyte that subsequently divides into two during the early stages of embryo development. There are also instances where oocyte release is not as influenced by hormonal signaling, potentially explaining rhythm method contraceptive failures and varied conception dates in some cases of fraternal twins.
Following ovulation, the ruptured follicle transforms into the corpus luteum, which secretes large amounts of progesterone and smaller amounts 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. Without pregnancy, the corpus luteum degenerates into the corpus albicans, and declining progesterone levels lead to endometrial shedding, marking the onset of menstruation.
Should fertilization and consequent pregnancy ensue, the corpus luteum sustains its operation until the placenta can independently produce the necessary hormones, generally by the end of the first trimester.
The ovarian cycle mainly consists of the follicular and the luteal phases.
In the follicular phase, a primordial follicle is transformed into a primary follicle. The primary oocyte within this follicle secretes a glycoprotein-rich extracellular layer — the zona pellucida — around it.
The primary follicle then develops multiple layers of granulosa cells and transforms into a secondary follicle.
As the phase progresses, a layer of connective tissue and epithelial cells — theca folliculi — also develops around the secondary follicle.
Next, the secondary follicle matures into a vesicular or antral follicle, forming an antrum — a fluid-filled cavity.
This antrum expands with fluid until the secondary oocyte and its surrounding capsule of granulosa cells — the corona radiata — are isolated.
When the follicle reaches about 1 inch in diameter, it bulges from the external ovarian surface.
Ovulation occurs when the mature follicle ruptures and the secondary oocyte is released into the peritoneal cavity.
Post-ovulation, the ruptured follicle forms the corpus luteum.
If no pregnancy occurs, the corpus luteum degenerates and leaves behind a scar — the corpus albicans.
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