Lecture on the Menstrual Cycle

Introduction

• Menstrual Cycle: Regular changes in ovaries and endometrium for reproduction.
• Endometrium: Tissue lining the uterus, consisting of:
  • Functional layer (shed during menstruation)
  • Thin basal layer (supports the functional layer)
• Two synchronized processes: Ovarian Cycle and Uterine/Endometrial Cycle.

Key Phases of the Menstrual Cycle

Ovarian Cycle

• Focuses on the development of ovarian follicles and ovulation.

Uterine/Endometrial Cycle

• Centers on the thickening and shedding of the functional endometrium.

Milestones

• Menarche: Onset of the first menstrual period, occurs during early adolescence.
• Menstrual Cycle Duration: 20-35 days, average 28 days.
• Ovulation: Occurs 14 days before the start of the next cycle.
• Menopause: Cessation of menstrual periods as ovarian function declines.

Ovarian Cycle Breakdown

Follicular Phase (Weeks 1-2)

• Begins on the first day of menstruation.
• Controlled by hypothalamus and pituitary gland (masterminds of reproduction).
• Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH).
• Anterior Pituitary Gland: Releases Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
• Puberty triggers pulsatile release of GnRH, affecting FSH and LH levels.
• FSH and LH involved in maturation of ovarian follicles, containing primary oocytes.
• Follicles develop receptors for LH and FSH, producing hormones and enzymes to convert androgens to estrogens.
• Dominant follicle emerges by having the most receptors and continues to grow.
• Estrogen levels rise, eventually providing positive feedback, causing a surge in FSH and LH leading to ovulation.

Uterine/Endometrial Cycle Breakdown

Menstrual Phase (First Few Days)

• Shedding of the old functional endometrial layer.

Proliferative Phase

• Estrogen thickens the endometrium, stimulates gland growth, and spiral artery emergence.
• Estrogen alters cervical mucus to become more sperm-friendly.

Secretory Phase (Post-Ovulation)

• Luteal phase coincides with the secretory phase in the uterus.
• Corpus luteum (remnant follicle) secretes progesterone more than estrogen.
• Progesterone causes the endometrium to become receptive to implantation.
• Spiral arteries grow and become coiled, uterine glands secrete mucus.
• As optimal fertilization window closes, mucus thickens, corpus luteum degenerates (forming corpus albicans).
• Drop in progesterone causes shedding of the functional layer, marking the start of a new cycle.

Recap

• Menstrual Cycle Phases:
  • Follicular Phase (First 14 days)
  • Ovulation (Day 14)
  • Luteal Phase (Last 14 days)
• Uterine Phases:
  • Menstrual Phase (First few days)
  • Proliferative Phase (First 14 days)
  • Secretory Phase (Last 14 days)