Female Reproductive Cycle Lecture Notes

Overview

• Covers the female reproductive cycle, focusing on:
  • Menstrual Cycle
  • Ovarian Cycle
• Key aspects include:
  • Hormonal control and regulation
  • Development of the egg (oogenesis)
  • The distinction between repeating the cycle or transitioning into pregnancy

The Cycles

Ovarian Cycle

• Phases:
  1. Follicular Phase: Day 1 to ~Day 13
  2. Ovulation Phase: Typically Day 14
  3. Luteal Phase: Post-ovulation, where the corpus luteum forms

Menstrual Cycle

• Phases:
  1. Menstrual Phase: Day 1 to Day 4-7 (shedding of uterine lining)
  2. Proliferative Phase: Day 5 to Day 14 (regrowth of uterine lining)
  3. Secretory Phase: Day 15 to Day 28

Oogenesis

• Prenatal Development:
  • Primary oocytes form between 18-22 weeks, arrested in prophase 1 of meiosis.
  • Born with a fixed number of primary oocytes.
• Folliculogenesis:
  • Development of follicles post-puberty
  • Primordial follicles develop into primary, secondary, and antral follicles.
  • Selection of Graafian follicles, responsive to FSH and LH.
• Key Transformation:
  • At ovulation, secondary oocyte (haploid) is released.

Hormonal Regulation

• Hypothalamic-Pituitary-Gonadal (HPG) Axis:
  • Involves GnRH, LH, FSH, estrogen, and progesterone.
• Cycle Day 1:
  • Menstruation begins due to low estrogen and progesterone.
  • Low levels release negative feedback on the hypothalamus; increase in GnRH, LH, FSH.
• Follicular Development:
  • FSH stimulates follicle development, releasing estrogen.
  • Estrogen initially provides negative feedback, stabilizing LH and FSH levels.
  • High estrogen threshold leads to positive feedback, causing an LH spike.

Ovulation and Luteal Phase

• Ovulation:
  • Triggered by LH spike, releasing the oocyte.
• Luteal Phase:
  • Corpus luteum forms, producing progesterone.
  • Progesterone inhibits multiple ovulations by negative feedback.
• Cycle Restart:
  • Progesterone levels eventually drop, restarting the cycle unless fertilization occurs.

Pregnancy and Fertilization

• Post-Fertilization:
  • If fertilization occurs, embryo releases HCG, maintaining progesterone levels.
  • HCG maintains early pregnancy by mimicking progesterone effects.
  • Placenta takes over hormone production in the second trimester.

Important Concepts

• Steroid Hormone Mechanism:
  • Estrogen, a steroid hormone, enters cells, binds cytosolic receptors, influences gene expression.
• Feedback Mechanisms:
  • Both negative and positive feedback loops are critical in hormonal regulation.
• MCAT Relevance:
  • Understanding these processes is vital for MCAT questions, especially those integrating biochemistry and genetics.

Tips for MCAT

• Relate reproductive cycle processes to genetic and cell biology concepts.
• Understand hormone pathways and feedback mechanisms thoroughly.
• Explore additional resources on the endocrine system and genetics for comprehensive understanding.