Menstruation is the cyclic shedding of the uterine lining (endometrium) in response to the hormonal fluctuations that occur during a woman's menstrual cycle.
The cycle is typically divided into four phases:
1. Menstrual phase (Days 1-5):
The menstrual phase marks the beginning of the cycle.
Hormonal changes: Decreasing levels of estrogen and progesterone from the previous cycle cause the endometrial lining to shed.
Menstrual bleeding: This shedding results in menstrual bleeding, which typically lasts 3-7 days.
2. Proliferative (Follicular) phase (Days 6-14):
This phase is characterized by the growth of follicles in the ovaries and the repair and thickening of the endometrium.
A. Follicle-stimulating hormone (FSH) secretion:
The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to secrete FSH.
FSH acts on the ovaries to stimulate the growth and development of ovarian follicles, each containing an immature egg (oocyte).
B. Estrogen production:
As the follicles develop, they secrete estrogen.
Estrogen promotes the repair and thickening of the endometrial lining.
Rising estrogen levels inhibit the further release of FSH, allowing a single dominant follicle to mature.
C. Luteinizing hormone (LH) surge:
High estrogen levels eventually trigger a surge in LH secretion from the anterior pituitary gland.
This LH surge is responsible for initiating the process of ovulation.
3. Ovulation (Day 14):
Ovulation is the release of a mature egg from the dominant ovarian follicle into the fallopian tube.
The LH surge triggers a series of enzymatic reactions that cause the follicle to rupture and release the egg.
The egg is now available for fertilization by sperm.
4. Luteal phase (Days 15-28):
This phase begins after ovulation and is characterized by the following events:
A. Corpus luteum formation:
After the follicle ruptures and releases the egg, the remnants of the follicle transform into the corpus luteum.
The corpus luteum is a temporary endocrine structure that secretes both progesterone and estrogen.
B. Progesterone production:
The corpus luteum primarily secretes progesterone, which stabilizes and thickens the endometrial lining, making it more receptive to a fertilized egg.
Progesterone also inhibits further release of FSH and LH to prevent additional follicle development.
C. End of the luteal phase:
If fertilization does not occur, the corpus luteum degenerates around day 22-24.
The drop in progesterone and estrogen levels results in the breakdown of the endometrial lining.
This breakdown leads to menstruation, marking the start of a new cycle.
