CBSE · Class 12 · Biology · Human ReproductionDescribe the menstrual cycle in human females. Explain the various phases of the cycle along with the hormonal controls operating during each phase.

Introduction to the 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. One ovum is released (ovulation) in the middle of each menstrual cycle.

Phases of the Menstrual Cycle\nThe menstrual cycle consists of the following four main phases:

1. Menstrual Phase (Days 1-5):

   • The cycle starts with menstrual flow, which lasts for 3-5 days.
   • This flow results due to the breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through the vagina.
   • Menstruation only occurs if the released ovum is not fertilized, leading to the withdrawal of estrogens and progesterone.
   • Hormonal Control: Low levels of estrogen and progesterone.

2. Follicular or Proliferative Phase (Days 6-14):

   • During this phase, the primary follicles in the ovary grow and become a fully mature Graafian follicle.
   • Concurrently, the endometrium of the uterus regenerates through proliferation.
   • These changes are induced by the increasing levels of pituitary and ovarian hormones.
   • Hormonal Control: Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the anterior pituitary to secrete LH (Luteinizing Hormone) and FSH (Follicle-Stimulating Hormone). FSH stimulates follicular growth and secretion of estrogens by the growing follicles.

3. Ovulatory Phase (Around Day 14):

   • Peak levels of LH and FSH lead to the maximum size of the Graafian follicle.
   • Rapid secretion of LH reaches its peak level called the LH surge during the mid-cycle (around the 14th day).
   • The LH surge induces rupture of the Graafian follicle and thereby the release of ovum (ovulation).

4. Luteal or Secretory Phase (Days 15-28):

   • The remaining parts of the Graafian follicle transform as the corpus luteum.
   • The corpus luteum secretes large amounts of progesterone, which is essential for the maintenance of the endometrium.
   • The endometrium thickens further and gets ready for implantation of the fertilized ovum.
   • If fertilization does not occur, the corpus luteum degenerates into the corpus albicans, leading to the disintegration of the endometrium and initiating a new cycle.
   • Hormonal Control: High progesterone maintains the uterine lining; declining levels trigger menstruation.