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Overview of the Menstrual Cycle

Apr 23, 2025

Menstrual Cycle Overview

Key Concepts

  • The menstrual cycle lasts roughly 28 days.
  • It is divided into two main cycles:
    • Ovarian Cycle (occurs in the ovary)
    • Uterine Cycle (occurs in the uterus)
  • The actions in the ovary affect the uterus.

Phases of the Menstrual Cycle

  1. Menstrual Phase
    • Shedding of the uterine lining (endometrium).
  2. Proliferative Phase
    • Growth of the primary follicle.
  3. Ovulation
    • Release of an egg cell into the fallopian tube.
  4. Secretory Phase
    • Maintenance of the uterine lining; if no pregnancy, the lining will shed.

Hormones Involved

Pituitary Gland Hormones

  • Follicle Stimulating Hormone (FSH)
    • Stimulates follicle growth in the ovary.
  • Luteinizing Hormone (LH)
    • Matures the follicle and causes ovulation.

Ovarian Hormones

  • Estrogen
    • Secreted by the follicle; repairs and thickens the endometrium.
  • Progesterone
    • Secreted by the corpus luteum; makes the endometrium more vascular and glandular.

Hormonal Interactions

  • Negative Feedback
    • FSH and LH influence ovarian hormones (estrogen and progesterone) and vice versa.
    • High levels of estrogen and progesterone inhibit FSH and LH secretion.

Graph Interpretation

  1. FSH starts to rise at the beginning of the cycle and peaks before ovulation.
  2. LH also has a dramatic peak just before ovulation (day 14).
  3. Estrogen has two peaks:
    • First peak just before ovulation
    • Second peak around day 21, signaling the preparation for menstruation.
  4. Progesterone increases significantly after ovulation and drops if pregnancy does not occur.

Cycle Summary Steps

  1. Pituitary gland secretes FSH.
  2. FSH stimulates the growth of a follicle in the ovary.
  3. The growing follicle produces estrogen.
  4. Estrogen stimulates the pituitary gland to secrete LH.
  5. LH and estrogen cause ovulation (release of the egg).
  6. After ovulation, the follicle transforms into the corpus luteum.
  7. Corpus luteum secretes progesterone.
  8. Progesterone maintains the thickened endometrium.
  9. High levels of estrogen and progesterone inhibit further secretion of FSH and LH.
  10. If no pregnancy, the corpus luteum degenerates, leading to decreased levels of estrogen and progesterone.
  11. The endometrium sheds, marking the start of menstruation.

Important Notes

  • Day 0 of the menstrual cycle is the first day of menstruation, not the end.
  • Understanding the interactions of hormones is crucial for interpreting graphs and exam questions related to the menstrual cycle.

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