Menstruation is the cyclic shedding of the uterine lining (endometrium) in response to the hormonal fluctuations that occur during a woman's menstrual cycle.
1. Menstrual phase (Days 1-5):
The endometrial lining is shed due to decreasing levels of Estrogen and progesterone from the previous cycle.
This results in menstrual bleeding, which usually lasts for 3-7 days.
2. Proliferative (Follicular) phase (Days 6-14):
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:
The developing follicles secrete estrogen, which helps repair and thicken the endometrial lining.
As estrogen levels rise, they inhibit the release of FSH, resulting in the selection of a single dominant follicle that will continue 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, which is then available for fertilization.
4. Luteal phase (Days 15-28):
This phase begins after ovulation and is characterized by the following events:
A. Corpus luteum formation:
The ruptured follicle transforms into the corpus luteum, a temporary endocrine structure that secretes progesterone and estrogen.
These hormones maintain and further thicken the endometrial lining in preparation for a potential pregnancy.
B. Progesterone production:
The corpus luteum primarily secretes progesterone, which inhibits further release of LH and FSH to prevent additional follicle development during the current cycle.
C. End of the luteal phase:
If fertilization does not occur, the corpus luteum degenerates around day 22-24, leading to a drop in progesterone and estrogen levels.
This hormonal decrease causes the endometrial lining to break down, leading to the onset of menstruation and the beginning of a new menstrual cycle.
