The Menstrual Cycle

What are the two main phases of the menstrual cycle and what occurs in each?

1. Follicular phase (Days 1–14): Begins on the first day of menstruation; involves growth of antral follicles and is dominated by oestradiol (E2) production from granulosa cells.

2. Luteal phase (Days 14–28): After ovulation, the follicle transforms into the corpus luteum (CL), which secretes progesterone, maintaining the endometrium and preparing for implantation.

How does oogenesis integrate with the menstrual cycle?

During follicular development, the primary oocyte (arrested in prophase I since birth) resumes meiosis I just before ovulation, forming a secondary oocyte and first polar body.

The secondary oocyte begins meiosis II but arrests again until fertilisation occurs.

What is the significance of folliculogenesis in the cycle?

Folliculogenesis ensures that one dominant follicle (DF) is selected and matured each cycle, leading to ovulation of a haploid oocyte capable of fertilisation

What are the main hormones regulating the menstrual cycle?

GnRH (from hypothalamus) – released in pulses.

FSH and LH (from anterior pituitary) – regulate follicle growth and ovulation.

Oestrogen and progesterone (from ovary) – provide positive and negative feedback to the hypothalamus and pituitary.

Why must GnRH be released in a pulsatile manner?

Continuous GnRH suppresses LH/FSH secretion, while pulsatile release stimulates their production—key for normal cyclicity.

Describe the hormonal feedback during the cycle.

Early follicular phase: Low oestrogen and progesterone → negative feedback removed → FSH rises.

Mid-follicular: Rising E2 → negative feedback on FSH.

Late follicular: Sustained high E2 (> 300 pmol/L for > 48 h) → positive feedback → LH surge.

Luteal phase: High progesterone → negative feedback → suppresses GnRH, LH, FSH.

What triggers ovulation?

A massive LH surge caused by positive feedback from sustained high oestrogen levels triggers ovulation about 36 hours later.

What events occur during ovulation?

Follicular blood flow increases.

Proteolytic enzymes digest the ovarian wall.

The cumulus–oocyte complex (COC) is released and captured by fimbriae of the uterine tube.

What chromosomal events occur in the oocyte during ovulation?

LH surge induces completion of meiosis I, forming the secondary oocyte (haploid, n) and first polar body. The oocyte then arrests in metaphase II until fertilisation.

What happens to the follicle after ovulation?

The ruptured follicle collapses, forming the corpus luteum (CL) composed of luteinised granulosa and theca cells.

What does the corpus luteum secrete?

Progesterone: stabilises and thickens the endometrium, maintains uterine secretions, modulates tubal and cervical mucus, and supports early embryo transport.

Oestradiol: continues to aid endometrial growth.

What maintains the corpus luteum if pregnancy occurs?

hCG, secreted by the embryo’s syncytiotrophoblast, binds to LH receptors to maintain the CL until placental takeover (~week 10).

What happens if fertilisation does not occur?

The CL degenerates after ~14 days due to lack of hCG → progesterone and oestrogen levels fall, removing negative feedback → FSH rises → new follicular phase begins.

What is menstruation physiologically?

Withdrawal of progesterone causes vasoconstriction and shedding of the endometrial functional layer, marking Day 1 of a new cycle.

How can ovulation be detected clinically or at home?

Basal body temperature (BBT): rises by ~0.5–1°C after ovulation (due to progesterone).

Cervical mucus: becomes clear, stretchy (“spinnbarkeit”), and sperm-friendly near ovulation.

Ovulation kits: detect urinary LH (and sometimes oestrone-3-glucuronide, E3G).

Ultrasound: measures follicle diameter for ovulation induction monitoring.

What is the “fertile window”?

≈ 6 days: 5 days before ovulation (sperm survival) + 1 day after (egg lifespan).

Used in natural family planning or fertility awareness methods.

Why does only one follicle become dominant?

Because it develops more FSH receptors, gains LH receptors, and produces more oestradiol, which suppresses FSH and starves competing follicles.

What are E1, E2, and E3?

Forms of oestrogen:

E1 (oestrone): weaker; post-menopause.

E2 (oestradiol): strongest; main reproductive oestrogen.

E3 (oestriol): dominant in pregnancy.

What is “luteinisation”?

Transformation of granulosa and theca cells into luteal cells after ovulation, enabling progesterone production.

Why must GnRH pulses vary?

Frequency and amplitude of pulses determine whether FSH or LH dominates:

Slow pulses → FSH

Rapid pulses → LH

What do clinical drugs target?

GnRH analogues: control LH/FSH release (e.g., fertility treatment, contraception).

Clomiphene (Clomid): blocks oestrogen receptors to increase FSH/LH for ovulation induction.

OCPs: maintain constant oestrogen/progesterone to suppress LH/FSH → no ovulation.