Understanding The Menstrual Cycle: Phases, Hormones, And Physiology

Understanding The Menstrual Cycle: Phases, Hormones, And Physiology

Describe uterine and ovarian changes occurring during normal menstrual cycle. Give physiological basis of these changes.
Answer:

Menstrual cycle:

• The cyclic events that take place in a rhythmic fashion during the reproductive menstrual cycle.

1. Uterine changes occurring during menstrual cycle:

• The changes in uterus take place in three phases.
  • Menstrual phase:
    • If the ovum shed at ovulation is not fertilized, menstruation occurs, which last for about 3-5 days.
    • Changes that occur during this phase.
    • On 26th or 27th
    • There is sudden reduction in the release of estrogen and progesterone.
    • It leads to sudden desquamation of the endometrium.
    • During the next 24 hours, endomentrial blood vessels undergo vasoconstriction.
    • It leads to hypoxia which results in necrosis of the endometrium.
    • This necrotic endometrium is expelled out through vagina alongwith some blood and tissue fluid.
    • This process of shedding and exit of uterine lining along with blood and fluid is called menstrual bleeding.
    • Uterine contractions expel this blood to the exterior through vagina.
  • Proliferative phase:
    • Extends from 5th and 14th day of menstruation.
      Changes:
    • During this phase, regeneration of endometrium starts under the influence of estrogen.
    • the uterine glands start developing
    • Vascularity also increases
    • Endometrium reaches the thickness of 3-4 mm at the end of this phase
      On 14th day, ovulation occurs under the influence of LH.
  • Secretory phase:
    • It extends between 15th and 28th day of the menstrual cycle.
    • Following ovulation corpus luteum develops.
      Changes:
    • Corpus luteum secretes.
      • Progresterone.
        • Causes enlargement of endometrial stroma.
        • Glands increase in size and starts secreting mucus.
      • Oestrogen.
        • Causes further proliferation of cells in uterus.
        • Thickens endometrium.
        • Further vascularity increases
        • This phase represents the preparation of the endometrium to receive the fertilized ovum.

2. Ovarian changes occurring during menstrual cycle:

• Ovarian changes occur in two phases.
  • Follicular phase.
    • It extends from the 5th day of menstrual cycle till 14th day i.e., ovulation.
    • During this phase, maturation of ovum occurs.
    • During each cycle; some 10 – 15 follicles enlarge to become secondary follicles, under the influence of FSH.
    • The matured ovarian follicle alongwith matured ovum is called Graffian follicle.
    • There is increase in the size of graffian follicle upto it surrounds the whole thickness of ovarian cortex.
    • Some epithelial cells start secreting the female sex hormones.
    • On 14th day, graffian follicle is prepared for ovulation.
      Ovulation:
    • It is the process in which there is rupture of graffian follicle with subsequent discharge of ovum into abdominal cavity.
    • The ovum gets surrounded by zona pellucida and granulosa cells.
    • This is shed into the abdominal cavity and is picked up by the fimbriated end of the fallopian tube.
  • Luteal phase:
    • This extends between 15th and 28th day of menstrual cycle.
    • During this, corpus luteum is developed.
    • After ovulation, the clotted blood is replaced with yellowish, lipid rich luteal cells, forming corpus luteum.
    • It enlarges from 8-9 days during which the luteal cells secrete estrogen and progesterone.