MP Board · Class 12 · Biology · Human ReproductionExplain the menstrual cycle in human females in detail. Discuss the various phases of the cycle along with the hormonal fluctuations and changes in the ovary and uterus.

Introduction to Menstrual Cycle\nThe reproductive cycle in female primates (such as monkeys, apes, and human beings) is called the menstrual cycle. The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. The cycle ceases around 50 years of age, a phase known as menopause.

Phases of the Menstrual Cycle\nThe menstrual cycle consists of four major phases based on the ovarian and uterine changes:

1. Menstrual Phase (Bleeding Phase):

   • Duration: Days 1 to 5 (approximate).
   • Events: If fertilization does not occur, the corpus luteum degenerates. This causes a sharp decline in progesterone and estrogen levels.
   • Result: The withdrawal of these hormones leads to the breakdown of the thick uterine endometrium and its blood vessels, resulting in menstrual flow (bleeding), which consists of blood, tissue fluid, and unfertilized ovum. If pregnancy occurs, menstruation does not happen.

2. Follicular Phase (Proliferative Phase):

   • Duration: Days 6 to 14 (approximate).
   • Events: During this phase, the primary follicles in the ovary grow and mature into Graafian follicles under the stimulation of gonadotropins (FSH and LH).
   • Hormonal Changes: Secretion of estrogens by the growing ovarian follicles increases.
   • Uterine Changes: Estrogen stimulates the proliferation, regeneration, and thickening of the uterine endometrium through the formation of new blood vessels and uterine glands.

3. Ovulatory Phase:

   • Duration: Around the 14th day of the cycle.
   • Events: Rapid secretion of LH (known as the LH surge) reaches its peak during the mid-cycle (around day 14), inducing the rupture of the mature Graafian follicle and releasing the secondary oocyte (ovum) into the pelvic cavity. This release is called ovulation.

4. Luteal Phase (Secretory Phase):

   • Duration: Days 15 to 28 (approximate).
   • Events: The remaining parts of the Graafian follicle transform into a temporary endocrine structure called the corpus luteum.
   • Hormonal Changes: The corpus luteum secretes large amounts of progesterone, which is essential for maintaining and thickening the endometrium for potential implantation of a fertilized ovum.
   • Result: If fertilization does not take place, the corpus luteum degenerates into the corpus albicans, progesterone production drops, and a new menstrual cycle begins.