Lecture Notes: The Luteal Phase and Hormonal Regulation in the Menstrual Cycle

Overview

• The menstrual cycle is divided into two main phases:
  • Follicular Phase: Focuses on the growth and maturation of ovarian follicles, led by hormones FSH, LH, and estrogen.
  • Luteal Phase: Follows ovulation, involves the corpus luteum development and increased progesterone production.

Luteal Phase

• Begins after ovulation when the egg is expelled from the follicle.
• Corpus Luteum Formation:
  • Triggered by high levels of LH and FSH.
  • The remaining follicle transforms into the corpus luteum ("yellow body" due to yellow pigments).
• Hormonal Changes:
  • Estrogen Production: Significantly reduced, but still present.
  • Progesterone Production: Increased significantly to prepare the endometrium for potential implantation.

Role of Progesterone

• Prepares the uterine lining (endometrium) for implantation and gestation, called the secretory phase.
• Stimulates development of spiral arteries in the endometrium for increased blood flow.
• Increases uterine secretions for embryo nourishment.
• Reduces uterine muscle contractility to prevent premature expulsion of the embryo.
• Supports the maintenance of a hospitable environment for embryo implantation.

Hormonal Feedback and Corpus Luteum Atrophy

• Negative Feedback:
  • Estrogen and progesterone levels suppress FSH and LH production via the anterior pituitary.
  • Inhibin peaks and further inhibits FSH release.
• Corpus Luteum Atrophy:
  • Needs FSH and LH to survive; their suppression leads to its deterioration.
  • Results in the drop of estrogen and progesterone levels, ending the luteal phase and initiating menstruation.

Menstruation and Cycle Renewal

• Menstruation involves shedding the endometrial lining, indicating no pregnancy.
• Typically lasts 2-7 days and involves the loss of about 40 ml of blood.
• Drop in progesterone and estrogen removes negative feedback, allowing FSH and LH levels to rise again.
• This rise in FSH stimulates new follicular development, restarting the cycle.

Impact of Pregnancy on the Corpus Luteum

• If Pregnancy Occurs:
  • The embryo produces human chorionic gonadotropin (HCG), similar to LH, to keep the corpus luteum alive.
  • Continued production of estrogen and progesterone maintains the endometrial lining.
  • HCG presence is the basis for most pregnancy tests.
• Placenta Formation:
  • After 2-3 months, the placenta takes over progesterone production to sustain the pregnancy.
• Cycle Suspension: No menstruation occurs as the cycle is put on hold during pregnancy.

Summary

• Without pregnancy, the corpus luteum atrophies, and the menstrual cycle repeats every 28 days.
• With pregnancy, HCG rescues the corpus luteum, supports endometrial maintenance, and halts the cycle.