The menstrual cycle

The menstrual cycle involves regular changes in the ovaries and the endometrium, the lining of the uterus, for reproduction. It consists of the ovarian cycle (follicle development and ovulation) and the uterine/endometrial cycle (thickening and shedding of the endometrium). Monarchy marks the first period in adolescence, and cycles continue until menopause. A typical cycle lasts 20-35 days, averaging 28 days, starting on the first day of menstruation. Ovulation usually occurs 14 days before the next menstruation.

The menstrual cycle is divided into a pre-ovulatory and a post-ovulatory phase. The ovarian follicular phase (pre-ovulatory) corresponds to the menstrual and proliferative phases of the endometrium. The ovarian luteal phase (post-ovulatory) corresponds to the secretory phase of the endometrium.

The follicular phase begins on day one of menstruation. The hypothalamus secretes gonadotropin-releasing hormone (GnRH) in pulses, which stimulates the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones control the maturation of ovarian follicles, which contain an oocyte surrounded by theca and granulosa cells. Theca cells produce androstenedione, and granulosa cells convert it to 17-beta estradiol (estrogen) using aromatase.

As follicles grow, increasing estrogen levels negatively feedback on the pituitary, reducing FSH and causing most follicles to regress. The follicle with the most FSH receptors becomes dominant. This dominant follicle continues to secrete estrogen, which then acts as a positive feedback signal, causing a surge in FSH and LH from the pituitary one to two days before ovulation, triggering the release of the oocyte.

During the ovarian follicular phase, the uterus undergoes the menstrual phase, where the old endometrial lining is shed. This is followed by the proliferative phase, where high estrogen levels stimulate endometrial thickening, gland growth, and the development of spiral arteries. Estrogen also thins cervical mucus to aid sperm, optimizing fertilization between days 11 and 15 of an average 28-day cycle.

After ovulation, the ruptured follicle transforms into the corpus luteum, composed of luteinized theca and granulosa cells. Luteinized granulosa cells, now exposed to LH, increase activity of p450scc, converting cholesterol to pregnenolone and thus secreting more progesterone than estrogen. Progesterone and inhibin, also secreted by granulosa cells, negatively feedback on the pituitary, reducing FSH and LH, leading to lower estrogen levels.

Dominant progesterone levels during the luteal phase signal ovulation and prepare the endometrium for implantation (secretory phase). Spiral arteries coil, and uterine glands secrete more mucus. The cervical mucus thickens, becoming less hospitable to sperm after day 15. If pregnancy does not occur, the corpus luteum degenerates into the corpus albicans, causing estrogen and progesterone levels to drop. When progesterone is lowest, spiral arteries collapse, and the functional layer sheds, marking the start of a new menstrual cycle.

The menstrual cycle begins on day one of menstruation. For an average 28-day cycle, the first 14 days are the ovarian follicular phase, with ovulation around day 14 due to the LH surge. The last 14 days are the ovarian luteal phase, dominated by progesterone. The follicular phase duration can vary, but the luteal phase consistently precedes menses by 14 days. Concurrently, the uterus goes through the menstrual and proliferative phases in the first 14 days, followed by the secretory phase in the last 14 days.