Menstrual Cycle

Learning Objectives:

  • Describe the phases of the cycle and integrate folliculogenesis and oogenesis with these phases

  • Understand the cyclical secretion of steroid hormones and how they control hypothalamic and pituitary hormone secretion by positive and negative feedback feedback

  • Explain ovulation of the dominant follicle and reduction in and reduction in chromosome numbers to form a haploid egg ready for fertilization

  • Discuss the formation of the corpus luteum and how its secretions prepare the uterus and tubes to receive a fertilized egg.

  • Understand the demise of the corpus luteum and reinstatement of the next menstrual cycle.

  • Analyse how the physiological features of the menstrual cycle can be used to predict ovulation for either family planning or contraception

Phases of the Menstrual Cycle and Integration with Folliculogenesis and Oogenesis

Aims of the Menstrual Cycle:

•       selection of a single oocyte

  • •       regular spontaneous ovulation

    •       correct number of chromosomes in eggs

    •       cyclical changes in the vagina, cervix and Fallopian tube

    •       preparation of the uterus

    •       support of the fertilised dividing egg

Overview of the Phases:

  1. Follicular Phase (Day 1 to Ovulation):

    • Begins with menstruation (shedding of the endometrial lining).

    • FSH stimulates the growth of a cohort of antral follicles; one becomes the dominant follicle.

    • Dominant follicle produces increasing oestradiol, which prepares the endometrium for potential implantation.

    • Oogenesis: The dominant follicle's oocyte resumes meiosis I in preparation for ovulation.

  2. Ovulation (Mid-Cycle, ~Day 14):

    • Triggered by an LH surge following sustained high oestradiol levels.

    • The oocyte completes meiosis I and arrests in meiosis II, forming a haploid secondary oocyte.

    • Released oocyte is captured by the fimbriae of the fallopian tube.

  3. Luteal Phase (Post-Ovulation, Days 15–28):

    • The follicle remnants form the corpus luteum (CL), which secretes progesterone and some oestradiol.

    • Progesterone prepares the uterus for implantation by stabilizing the endometrium.

    • If fertilization does not occur, the CL degenerates, hormone levels drop, and the cycle restarts.

Cyclical Steroid Secretion and Hormonal Feedback

  1. Follicular Phase:

    • Oestradiol is produced by granulosa cells in response to FSH.

    • Initially, oestradiol exerts negative feedback on the hypothalamus and pituitary, reducing FSH.

    • Sustained high oestradiol levels (>300 pmol/L for ~42 hours) trigger a positive feedback loop, leading to an LH surge.

  2. Ovulation:

    • The LH surge:

      • Causes resumption of meiosis in the oocyte.

      • Induces the rupture of the follicle, releasing the oocyte.

      • Promotes luteinization of granulosa and theca cells into the corpus luteum.

  3. Luteal Phase:

    • Progesterone from the corpus luteum exerts negative feedback on the hypothalamus and pituitary, suppressing FSH and LH.

    • Without fertilization, the CL regresses, leading to a drop in progesterone and oestradiol, lifting the negative feedback and restarting FSH secretion.

Ovulation and Chromosome Reduction

  • Chromosomal Reduction:

    • The oocyte, arrested in prophase I since fetal development, completes meiosis I in response to the LH surge.

    • Unequal cytokinesis results in:

      • A large secondary oocyte (haploid).

      • A small polar body (non-functional).

    • The secondary oocyte arrests in metaphase II, completing meiosis only upon fertilization.

  • Ovulation:

    • LH triggers follicular rupture.

    • The oocyte, surrounded by cumulus cells, is extruded and captured by the fallopian tube's fimbriae.

Formation and Function of the Corpus Luteum

  1. Formation:

    • After ovulation, granulosa and theca cells luteinize, forming the CL (yellow body).

    • Increased vascularization supports hormone production.

  2. Secretions:

    • Progesterone (primary secretion):

      • Stabilizes and prepares the endometrium for implantation.

      • Reduces uterine contractility to support a potential pregnancy.

    • Oestradiol (secondary secretion):

      • Maintains the endometrium and enhances progesterone’s effects.

  3. Function During Pregnancy:

    • If fertilization occurs, the embryo secretes hCG, which maintains the CL until the placenta takes over progesterone production.

Demise of the Corpus Luteum and Restart of the Cycle

  1. Without Fertilization:

    • The CL degenerates into the corpus albicans after ~14 days.

    • Progesterone and oestradiol levels fall, leading to:

      • Endometrial shedding (menstruation).

      • Increased FSH secretion, restarting follicular recruitment.

  2. With Fertilization:

    • hCG from the embryo sustains the CL, maintaining progesterone levels and pregnancy support.

Predicting Ovulation for Family Planning or Contraception

  1. Physiological Indicators:

    • Cervical Mucus:

      • Becomes thin and stretchy ("egg-white consistency") at ovulation.

    • Basal Body Temperature:

      • Slight increase (~0.3–0.5°C) after ovulation due to progesterone.

    • LH Surge Detection:

      • Measured via home ovulation kits to predict ovulation within 12–36 hours.

  2. Calendar Method:

    • For a regular 28-day cycle:

      • Ovulation occurs ~day 14.

      • Fertile window: ~days 10–16.

    • Less reliable for irregular cycles.

  3. Contraceptive Considerations:

    • Hormonal contraceptives suppress ovulation by maintaining negative feedback on FSH and LH.

    • Natural methods rely on cycle tracking but require consistent monitoring.

This integration of follicular dynamics, hormonal feedback, and physiological indicators highlights the menstrual cycle's complex regulation and its practical applications. Let me know if you'd like further clarification on any aspect!