The Menstrual Cycle: Comprehensive Study Notes

I. Objectives

• Review the basic anatomic, histologic, and biochemical concepts governing the hypothalamic-pituitary-ovarian (HPO) axis.
• Discuss cyclic changes in ovarian hormones, pituitary hormones, and growth factors in relation to the menstrual cycle.
• Explain how these hormones affect the ovary, pituitary, and hypothalamus in regulating the menstrual cycle.

II. Menstrual Cycle

A. Prerequisites for Normal Menstrual Function

• Maturation of the hypothalamus ( GnRH ) and pituitary ( FSH, LH ).

• Presence of genetically hormonally responsive uterus and ovaries.

• Intact genital/outflow tract.

• The HPO axis: GnRH from the hypothalamus stimulates the anterior pituitary to release FSH and LH, which act on the ovaries to regulate estrogen and progesterone production.

• Hypothalamic dysfunction -> reduced FSH/LH -> disrupted ovarian steroid production.

• Immature HPO axis can cause irregular menses or amenorrhea in very young girls; irregular cycles in those >16 years are usually due to hormonal imbalances or secondary amenorrhea.

• Figure reference: GnRH stimulates FSH/LH release; feedback from ovarian steroids modulates hypothalamic output (positive/negative feedback).

B. Hormonal Regulation of Oogenesis and Ovulation

• GnRH from the hypothalamus stimulates LH and FSH release from the anterior pituitary, which act on the ovaries.
• Ovarian steroids (estrogen, progesterone) provide feedback to the hypothalamus and pituitary.
• Positive feedback example: when ovarian steroid levels are low, the hypothalamus increases GnRH to stimulate more FSH/LH; as ovarian hormones rise, feedback can transiently amplify GnRH release to promote further gonadotropin release depending on the phase.
• The hypothalamus–pituitary–ovarian axis coordinates follicular growth, ovulation, and luteal function.

C. Modulatory Influences on GnRH Neurotransmitters

• Stimulatory neurohormones/neurotransmitters: Norepinephrine (tonal), Glutamate (puberty), Neuropeptide Y (pulsatile), Prostaglandin (direct GnRH stimulator).
• Inhibitory factors: Estrogen (can decrease amplitude of GnRH), Progesterone (decreases frequency of GnRH pulses), GABA (tonal, puberty), Dopamine, Prolactin interaction (high dopamine reduces GnRH; dopamine agonists can restore GnRH when prolactin is high), β-endorphin, Corticotropin-Releasing Hormone (CRH) – stress-related suppression of GnRH.
• Implication: emotional state, metabolic status, and systemic signals influence GnRH pulsatility and thus menstrual function.
• Practical note: conditions like sadness/depression, obesity, or weight changes can impact GnRH regulation and cycle regularity.

D. Normal Menstrual Cycle

• Mean age at menarche: $12\,\text{years}$.

• Mean age at menopause: $42-55\,\text{years}$ (example: Filipina mean ~ $47\,\text{years}$).

• Mean cycle length: $28\pm7\,\text{days}$; acceptable range $21-35\,\text{days}$.

• Mean duration of menses: $4\,\text{days}$ (range $3-8\,\text{days}$).

• Normal estimated blood loss: $5-80\,\text{mL}$ (blood loss >80 mL is abnormal).

• MIDAS mnemonic for history-taking:

  • M: Menarche age (avg ~ $12\,\text{years}$)
  • I: Interval between menses (normal $21-35\,\text{days}$; average ~ $28\pm7$)
  • D: Duration of menses (avg ~ $4\,\text{days}$; range 3–8)
  • A: Amount of bleeding (pads/day; avg ~ $5-80\,\text{mL}$)
  • S: Symptoms-related (e.g., hypogastric pain, dysmenorrhea, headaches)

• The menstrual cycle comprises ovarian and endometrial changes that occur in tandem or sequentially: Follicular phase, Ovulation, Luteal phase; Endometrium shows Proliferative and Secretory phases, followed by Menstrual phase if pregnancy does not occur.

III. The Ovarian Cycle

A. Follicular Phase (cycle recruitment)

• Begins on day 1 of menses and ends with ovulation.
• Follicular recruitment is variable: recruitment of multiple follicles, selection of a dominant follicle (DF), growth of DF; only one reaches maturity and ovulates.
• Dominant hormone: Estrogen predominates during the follicular phase.
• After ovulation, progesterone becomes dominant (Progesterone is the hormone of pregnancy).
• Primordial follicle development:
  • From germ cells.
  • 5-6 weeks: migration to genital ridge.
  • 6-8 weeks: mitotic multiplication.
  • 16-20 weeks: maximum number of oocytes (≈ $6-7\,\text{million}$ in both ovaries).
  • Just before birth: $1-2\,\text{million}$.
  • Puberty: $3\times10^5-5\times10^5$ oocytes.
  • Approximately $400-500$ will ovulate over reproductive lifetime.
• LH surge: Initiates germinal vesicle disruption; oocyte resumes meiosis; second meiotic division completes at fertilization.
• Post-ovulation: Progesterone rises and peaks approximately 8 days after ovulation.
• LH surge timing: onset of LH surge to ovulation is about $14\,\text{days}$; LH surge lasts about $24-36\,\text{hours}$; meiosis resumes with completion at fertilization.
• Follicular phase details can be visualized in Fig. 3 (normal menstrual cycle) and Fig. 5 (Follicular Phase).

B. Ovulation

• Estradiol peak occurs $24-36\,\text{hours}$ before ovulation.
• Ovulation occurs $10-12\,\text{hours}$ after the LH peak.
• LH surge occurs $24-36\,\text{hours}$ before follicle rupture and must be maintained for $24-36\,\text{hours}$ for full maturation; usually lasts $48-50\,\text{hours}$.
• Estradiol begins rising about 2–3 days before ovulation; ovulation typically occurs around day $13-14$ after menses onset; approximately $13-14\text{ days}$ from day 1 of the cycle is a common estimate for ovulation and egg release.
• Events during ovulation:
  1) LH surge initiates continued meiosis, progesterone and prostaglandin synthesis within the follicle.
  2) Progesterone enhances proteolytic enzymes with prostaglandins to rupture the follicular wall.
  3) Midcycle rise in FSH (influenced by progesterone) frees the oocyte and ensures adequate LH receptor expression for a normal luteal phase.
  4) Regression of the corpus luteum may involve luteolytic action of its own estrogen production.
  5) In early pregnancy, hCG rescues the corpus luteum, maintaining luteal function until placental steroidogenesis is established.
• Relationship between ovarian and menstrual cycles: LH and FSH surges trigger ovulation and luteal maintenance; progesterone and prostaglandins facilitate release; hCG can sustain luteal function in early pregnancy.

C. Luteal Phase

• Post-ovulation, granulosa cells increase in size and acquire a vacuolated, luteinized appearance.
• Angiogenesis occurs as vessels penetrate granulosa cells.
• Corpus luteum function: Produces progesterone (and estrogen) to support the endometrium for possible implantation. It provides nutrients to the mature oocyte during the luteal phase waiting for fertilization.
• Corpus luteum lifespan: about $14\,\text{days}$ if pregnancy does not occur.
• Luteolysis: Transition of the corpus luteum to corpus albicans; occurs around days post-ovulation $D23-D25$ (roughly $+9\text{ to }+11\text{ days}$ after ovulation).
• If fertilization occurs: hCG from the fertilized ovum supports the corpus luteum until placental steroidogenesis is well established.
• Hormone patterns: Corpus luteum function sustains the secretory endometrium; estrogen and progesterone dominate in the luteal phase relative to the follicular phase.

IV. Endometrial Phases

A. Endometrium

• Endometrium consists of two layers:
  • Basal layer (Stratum basale): contains primordial glands and dense cellular stroma; does not desquamate during menstruation; remains intact.
  • Functional layer (Stratum functionale): responds to hormonal changes; subdivided into:
  • Stratum compactum (gland necks and dense stroma)
  • Stratum spongiosum (primary glands and less stroma)
• Endometrium is dynamic across the cycle, reflecting ovarian hormone influence.
• Basal vs functional layers are visible histologically (refer to Figs. 18–20).
• Note on clinical relevance: concept of stable progesterone-only regimens and their effects on endometrium; misbeliefs about causes of endometrial pathology with hormonal contraception.

B. Proliferative Phase

• Occurs after menstruation as the endometrium regrows under estrogen influence.
• Endometrium is about $1-2\text{ mm}$ just after menses and thickens to $2-8\text{ mm}$ during proliferation (via glandular and stromal proliferation).
• Estrogen stimulates proliferation of glands and stroma; mitotic activity increases in glands and stroma.
• If proliferative endometrium thickens beyond $8\,\text{mm}$, consider endometrial hyperplasia and increased cancer risk.
• Menstruation lasting >$8\,\text{days}$ is abnormal; cycle interval <$21\,\text{days}$ is abnormal.
• The hallmark histologic effect of progesterone action in the endometrium is the formation of subnuclear vacuoles in glands; this is an early histologic indicator of progesterone action.
• Proliferative endometrium features are shown in Fig. 21 (Proliferative Endometrium).

C. Secretory Phase

• Progesterone antagonizes mitotic action of estrogen; maintains secretory endometrium.
• Secretory phase features a trilaminar/ triple-line endometrium (Trilaminar) detectable via transvaginal sonography (Fig. 22).
• Endometrium measurement: thickness assessed from the first to the last echogenic line (thickness varies with cycle stage).
• If implantation does not occur, glands collapse and fragment as the luteal phase ends (luteolysis if no pregnancy).
• Visual references: Fig. 24 (ovarian and menstrual cycle) and Fig. 25 (endometrial pathology).

D. Menstrual Phase

• Periodic desquamation of the functional endometrium is the hallmark of the menstrual cycle.

• Prostaglandins reach maximal activity in the endometrium just before menses; prostaglandins contribute to constriction and subsequent ischemia leading to shedding.

• Proteolytic enzymes, including matrix metalloproteinases (MMPs), participate in tissue breakdown and shedding.

• Practical clinical notes:

  • Endometrial symptoms and pathologies are often evaluated using menstrual history (MIDAS), ultrasound assessment of endometrial thickness, and hormonal profiles.
  • In women using hormonal contraception (e.g., Depo-Provera or combined oral contraceptives) who do not menstruate, there is no accumulation of menstrual blood; the endometrium may remain in a stable phase due to ongoing hormonal suppression. Progesterone is used therapeutically in endometriosis or related conditions; estrogen remains important for breast development and other estrogen-dependent processes.
  • Post-coital bleeding should be evaluated to rule out pathology; pathologies may include traumatic causes or other gynecologic conditions.

• Summary of hormonal regulation across phases:

  • Follicular phase: Estrogen predominates; endometrium proliferates; dominant follicle grows.
  • Ovulation: LH surge triggers ovulation; estrogen peaks precede ovulation; progesterone rises after ovulation.
  • Luteal phase: Progesterone dominates; endometrium becomes secretory; corpus luteum supports early pregnancy unless pregnancy occurs.
  • If pregnancy does not occur: corpus luteum regresses, progesterone and estrogen fall, endometrium sheds (menstruation) and the cycle restarts.

• Connections to broader principles:

  • Feedback loops between the hypothalamus, pituitary, and ovaries ensure coordinated timing of follicular development, ovulation, uterine preparation, and shedding.
  • Hormonal regulation integrates neuroendocrine signals with endocrine outputs to regulate reproduction and resource allocation in the body.

• Key numerical references (LaTeX-formatted):

  • Age at menarche: $12\,\text{years}$
  • Menopause age range: $42-55\,\text{years}$
  • Mean cycle length: $28\pm7\,\text{days}$; normal range $21-35\,\text{days}$
  • Menstrual duration: $4\,\text{days}$ (range $3-8\,\text{days}$)
  • Normal blood loss: $5-80\,\text{mL}$
  • Ovulation timing: $\text{ovulation} \,\approx 13-14\,\text{days after the start of menses}$; estradiol peak $24-36\,\text{hours}$ before ovulation; LH peak to ovulation $10-12\,\text{hours}$ after estradiol peak; LH surge duration $24-36\,\text{hours}$; the luteal phase lasts about $14\,\text{days}$ in the absence of pregnancy.
  • Corpus luteum lifespan: $\approx 14\,\text{days}$; luteolysis around $D23-D25$ post-ovulation (≈ +9 to +11 days).
  • Endometrial thickness: proliferative phase $2-8\,\text{mm}$; triple-line secretory endometrium detectable on ultrasound.

• This set of notes consolidates the major and minor points from the transcript, with explicit connections between hormones, phases, histology, and clinical implications for normal and abnormal cycles.