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Reproductive Physiology - Complete Topic Guide

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GROUP A: The Hypothalamo-Pituitary-Gonadal Axis

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13. Hypothalamo-Pituitary-Ovarian (HPO) Axis

Key Hormones and Their Sources

Feedback Flowchart - All Three Phases

FOLLICULAR PHASE          MIDCYCLE                 LUTEAL PHASE
─────────────────         ─────────────────         ─────────────────
Hypothalamus              Hypothalamus              Hypothalamus
    │ GnRH                    │ GnRH                    │ GnRH
    ▼ (+)                     ▼ (+)                     ▼ (+)
Ant. Pituitary            Ant. Pituitary            Ant. Pituitary
    │ FSH, LH                 │ FSH, LH ↑↑              │ FSH, LH
    ▼ (+)                     ▼ (+)                     ▼ (+)
  Ovary                      Ovary                     Ovary
    │ Estradiol ↑              │ Estradiol ↑↑↑            │ Progesterone ↑
    └──► (−) feedback          └──► (+) feedback           └──► (−) feedback
         on HPO                  (LH surge!)                   on HPO

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4. Neuroendocrine Reflexes in Reproduction

STIMULUS (sensory input)
        │
        ▼
  Neural pathway to hypothalamus
        │
        ▼
  GnRH secretion altered
        │
        ▼
  FSH/LH altered
        │
        ▼
  Gonadal response

Key Neuroendocrine Reflexes

Stress → ↑CRH → ↑ACTH → ↑Cortisol
                    │
                    └──► ↓GnRH → ↓FSH/LH → ↓Ovulation

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GROUP B: The Menstrual Cycle

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3. Menstrual Cycle

Phase Overview

Menstrual Cycle Flowchart

Day 1-5: MENSTRUATION
  Corpus luteum degenerates → E2 & P4 ↓ → Spiral artery spasm
  → Endometrial necrosis + shedding
        │
        ▼
Day 5-13: FOLLICULAR / PROLIFERATIVE PHASE
  FSH ↑ → Follicle grows → E2 ↑
  → Endometrium proliferates (thickness: 1→10 mm)
  → Cervical mucus: watery, copious, "ferning" pattern, sperm-penetrable
        │
        ▼
Day 13-14: OVULATORY PHASE
  E2 > 200 pg/mL → Positive feedback → LH SURGE
  → Dominant follicle ruptures (day 14) → Secondary oocyte released
        │
        ▼
Day 14-28: LUTEAL / SECRETORY PHASE
  LH → Corpus luteum forms → Progesterone dominates
  → Glands tortuous + glycogen-rich
  → Cervical mucus: thick, non-elastic, non-ferning (blocks sperm)
  → BBT rises ~0.5°C (thermogenic effect of progesterone)
        │
  ┌─────┴──────────────────────┐
  │ If fertilized              │ If NOT fertilized
  ▼                            ▼
hCG maintains             Corpus luteum → Corpus albicans
corpus luteum              E2 & P4 fall → DAY 1 again

Cervical Mucus Changes (Clinical Tool)

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5. Mechanism of Ovulation

Flowchart

Dominant follicle reaches 20 mm (Day 12-13)
        │
        ▼
Estradiol ≥ 200 pg/mL for ≥ 50 hours
        │
        ▼
POSITIVE FEEDBACK → GnRH pulse frequency ↑
        │
        ▼
ANTERIOR PITUITARY: LH SURGE (10-12x rise)
        │
        ▼
LH acts on follicle → ↑ prostaglandins (PGE2, PGF2α)
                    → ↑ proteolytic enzymes (collagenase, plasmin)
                    → Resumption of meiosis I in oocyte
        │
        ▼
Follicular wall thins, stigma forms
        │
        ▼
OVULATION: Follicle ruptures (~38 hrs after LH surge)
Secondary oocyte + cumulus oophorus expelled into peritoneum
        │
        ▼
Fimbriae sweep oocyte into fallopian tube
        │
        ▼
LUTEINIZATION: Granulosa + theca cells → Corpus luteum
Corpus luteum secretes Progesterone + Estradiol

Follicular Development Stages

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1. Indicators (Tests) of Ovulation and Their Clinical Significance

Table of Ovulation Indicators

Clinical Significance

• Infertility workup: Day-21 progesterone is the gold standard for confirming ovulation
• Safe period / rhythm method: Uses BBT + mucus pattern to identify infertile days
• ART timing: LH surge detected by urine kits for optimal intercourse/IUI timing

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GROUP C: Spermatogenesis and Male Physiology

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10. Spermatogenesis

Spermatogenesis Flowchart

SPERMATOGONIUM (2n, diploid) - Seminiferous tubule basal layer
        │ Mitosis (self-renewal + differentiation)
        ▼
TYPE A SPERMATOGONIUM → TYPE B SPERMATOGONIUM
        │ Mitosis
        ▼
PRIMARY SPERMATOCYTE (2n, 46 chromosomes) - largest germ cell
        │ MEIOSIS I (reduction division) - ~22 days
        ▼
2× SECONDARY SPERMATOCYTES (n, 23 chromosomes)
        │ MEIOSIS II - hours
        ▼
4× SPERMATIDS (n, 23 chromosomes - haploid round cells)
        │ SPERMIOGENESIS (~24 days)
        │ [Acrosome formation, tail growth, nucleus condensation,
        │  cytoplasm shedding, mitochondrial sheath development]
        ▼
4× MATURE SPERMATOZOA
        │
        ▼ (enter lumen of seminiferous tubule)
EPIDIDYMIS → functional maturation + storage (~12 days)
        │
        ▼
DUCTUS DEFERENS → VAS DEFERENS → URETHRA

Spermiogenesis - Transformation Diagram

Hormonal Regulation

Hypothalamus → GnRH (pulsatile)
        │
        ▼
Anterior Pituitary
   ├── FSH → Sertoli cells → supports spermatogenesis, secretes Inhibin
   └── LH  → Leydig cells → Testosterone → (paracrine) supports Sertoli cells
        │
        ▼
TESTOSTERONE (+ FSH) → Complete spermatogenesis

Feedback:
  Testosterone → (−) Hypothalamus & Pituitary (↓ GnRH, ↓ LH)
  Inhibin      → (−) Anterior Pituitary (↓ FSH selectively)

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12. Functions of Sertoli Cells

Summary Table

FSH + Testosterone
        │
        ▼
    SERTOLI CELL
    ┌──────────────────────────────────────┐
    │ • Secretes ABP (concentrates T)      │
    │ • Secretes Inhibin → ↓ FSH           │
    │ • Secretes MIF/AMH (fetal)           │
    │ • Secretes transferrin               │
    │ • Phagocytoses residual bodies       │
    │ • Forms Blood-Testis Barrier         │
    │ • Nurtures developing spermatids     │
    └──────────────────────────────────────┘

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GROUP D: Contraception

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2. Methods of Contraception (Overview)

CONTRACEPTION METHODS
├── HORMONAL
│   ├── Combined OCP (estrogen + progestogen)
│   ├── Progestogen-only pill (mini-pill)
│   ├── Injectables (DMPA - Depo-Provera)
│   ├── Implants (subdermal progestogen)
│   └── Emergency contraception (levonorgestrel/ulipristal)
│
├── BARRIER
│   ├── Male condom
│   ├── Female condom
│   ├── Diaphragm + spermicide
│   └── Cervical cap
│
├── INTRAUTERINE
│   ├── Copper IUD (non-hormonal)
│   └── Levonorgestrel IUS (Mirena)
│
├── NATURAL / BEHAVIORAL
│   ├── Rhythm/Calendar method
│   ├── Basal body temperature method
│   ├── Billings (cervical mucus) method
│   └── Lactational amenorrhea method (LAM)
│
└── PERMANENT
    ├── Vasectomy (male)
    └── Tubal ligation / Tubectomy (female)

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11. Combined Oral Contraceptive Pills (COCP) - Mechanism of Action

Components

• Estrogen component: Ethinyl estradiol (20-35 mcg)
• Progestogen component: Levonorgestrel, norethindrone, desogestrel, drospirenone, etc.

Mechanism Flowchart

COMBINED OCP (Estrogen + Progestogen) taken daily
        │
        ├──► Constant hormone levels
        │
        ▼
1. SUPPRESS HPO AXIS (Primary mechanism)
   Estrogen + Progestogen → negative feedback on hypothalamus & pituitary
   → ↓ GnRH pulsatility → ↓ FSH and LH → NO follicular development
   → NO LH surge → NO OVULATION
        │
        ├──► 2. CERVICAL MUCUS THICKENING (Progestogen effect)
        │       Thick, hostile mucus → sperm cannot penetrate cervix
        │
        ├──► 3. ENDOMETRIAL CHANGES (Progestogen effect)
        │       Thin, atrophic, non-receptive endometrium
        │       → Impairs implantation even if fertilization occurs
        │
        └──► 4. TUBAL MOTILITY CHANGES (minor)
                Altered peristalsis → delays sperm/egg transport

Pearl Index (failure rates/100 woman-years)

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15. Hormonal Contraceptives (Full Classification)

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8 & 9. Rhythm Method of Contraception and Safe Period

Calendar/Rhythm Method

Based on a 28-day cycle:
  Ovulation → Day 14
  Sperm survival → up to 5 days
  Egg survival → 12-24 hours

FERTILE WINDOW = Day 9 to Day 15 (avoid unprotected sex)
SAFE PERIOD   = Day 1-8 (post-menstrual) + Day 16-28 (pre-menstrual)

Calculation Rule (for irregular cycles)

Record last 6-12 cycles
  Shortest cycle - 18 = First fertile day
  Longest cycle  - 11 = Last fertile day
  e.g., 26-30 day cycles:
    First fertile: 26 - 18 = Day 8
    Last fertile:  30 - 11 = Day 19

Lactational Amenorrhea Method (LAM)

1. Baby < 6 months old
2. Exclusive breastfeeding (day and night)
3. Amenorrhea (no periods returned)

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GROUP E: Pregnancy, Placenta, and Parturition

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6. Physiological Changes During Pregnancy

System-by-System Table (from Morgan & Mikhail's Clinical Anesthesiology)

Hormone Changes Flowchart

CONCEPTION
     │
     ▼
Trophoblast → hCG secretion begins (Day 8-10)
     │            hCG peaks at 8-10 weeks
     │            hCG maintains corpus luteum
     │               → Corpus luteum → E2 + P4
     ▼
PLACENTA forms (8-10 weeks) → Takes over steroid production
     │
     ├── Progesterone: synthesized entirely by placenta from maternal cholesterol
     │    → Maintains uterine quiescence; prevents preterm labor
     │
     └── Estriol (E3): Requires fetal DHEAS (adrenal) + fetal liver + placenta
          → Used as marker of fetal wellbeing
     │
     ▼
FIRST TRIMESTER: hCG ↑ (morning sickness, thyroid stimulation)
SECOND TRIMESTER: HPL ↑ (insulin resistance, fetal fuel partitioning)
THIRD TRIMESTER: E3 ↑↑, P4 ↑↑, cortisol ↑ (maturation of fetal organs)

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14. Functions of the Placenta

PLACENTA FUNCTIONS
├── TRANSPORT
│   ├── O2 delivery to fetus (diffusion, fetal Hb has higher O2 affinity)
│   ├── CO2 removal from fetus
│   ├── Glucose (facilitated diffusion - most important fetal fuel)
│   ├── Amino acids (active transport)
│   ├── Fatty acids (diffusion)
│   ├── IgG (active transport - passive immunity to fetus)
│   └── Water, electrolytes, vitamins
│
├── ENDOCRINE
│   ├── hCG: Maintains corpus luteum; basis of pregnancy test; TSH-like action
│   ├── hPL (Human Placental Lactogen): Insulin antagonist; promotes lipolysis
│   │        (spares glucose for fetus)
│   ├── Progesterone: Uterine quiescence; breast preparation
│   ├── Estrogens (E1, E2, E3): Uterine growth; ductal breast development
│   └── CRH: Drives fetal cortisol production near term; linked to labor onset
│
└── BARRIER (selective)
    ├── Blocks most maternal antibodies EXCEPT IgG
    ├── Blocks most bacteria (but NOT viruses like CMV, rubella, HIV)
    └── Blocks some drugs (but NOT alcohol, nicotine, warfarin, thalidomide)

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7. Parturition

Hormonal Mechanism of Parturition

NEAR TERM (38-40 weeks)
        │
        ▼
Fetal hypothalamic-pituitary-adrenal axis activated
        │
        ▼
Fetal cortisol ↑
        │
        ▼
↑ Estrogen : Progesterone ratio in uterus
(Cortisol converts P4 → E2 via placental enzymes)
        │
        ├──► E2 stimulates:
        │       • Oxytocin receptors ↑ (up-regulated on uterine muscle)
        │       • PGE2 and PGF2α production ↑
        │       • Gap junction formation between myometrial cells
        │
        └──► P4 inhibition removed → Uterus becomes "primed"
        │
        ▼
OXYTOCIN (Ferguson reflex: cervical stretch → oxytocin ↑)
        │ + PROSTAGLANDINS
        ▼
Uterine contractions (coordinate, synchronized via gap junctions)
        │
        ▼
LABOR - THREE STAGES:
  Stage 1: Cervical dilation + effacement (longest stage)
  Stage 2: Fetal expulsion through birth canal
  Stage 3: Placental delivery (+ uterine contraction to stop bleeding)
        │
        ▼
After delivery: E2, P4 fall rapidly → Prolactin ↑ → LACTATION begins

Key Players in Parturition

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GROUP F: Gametogenesis

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10. Spermatogenesis (Comparison Table with Oogenesis)

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Summary Flowchart: Integration of All Topics

HYPOTHALAMUS
GnRH (pulsatile)
     │
     ▼
ANTERIOR PITUITARY
FSH ──────────────────────────► Follicle growth (Spermatogenesis in males)
LH ───────────────────────────► Ovulation / Corpus luteum (Testosterone in males)
     │                                │
     │                    ┌───────────┘
     │                    │
     ▼                    ▼
OVARY / TESTIS    MENSTRUAL CYCLE (28 days)
E2, P4, Inhibin   [Follicular → Ovulation → Luteal → Menstruation]
     │
     ▼ (if fertilized)
PREGNANCY
  hCG → maintains corpus luteum
  Placenta → E2, P4, hPL, hCG
  Physiological changes in all organ systems
     │
     ▼ (at 40 weeks)
PARTURITION
  Fetal cortisol → ↑ E2/P4 ratio → Prostaglandins + Oxytocin → Labor
     │
     ▼
LACTATION
  Prolactin ↑ (milk production) + Oxytocin (milk ejection)
  → Suckling → ↓ GnRH → Lactational amenorrhea

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Quick Reference: Ovulation Indicators + Safe Period Summary

MENSTRUAL CYCLE DAY:
 1   5   8   9  13  14  15  16  21  28
 │   │   │   │   │   │   │   │   │   │
 MENSES  │   │   │   │   │   │   │   │
         │  FERTILE WINDOW (Days 9-15)│
         │   │───────────│   │   │   │
         │   │  ↑ LH SURGE (day 13)   │
         │   │     │ OVULATION (d14)  │
         │   │                        │
         PROLIFERATIVE PHASE     SECRETORY (LUTEAL) PHASE
         (E2 dominance)          (P4 dominance + BBT ↑)

BBT: Low until ovulation → Rises 0.5°C on day after ovulation
Ferning: Present Day 8-13 → Absent post-ovulation
Progesterone: Checked Day 21 → >3 ng/mL confirms ovulation

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• Costanzo Physiology, 7th Edition (Chapters 9-10: Male and Female Reproduction)
• The Developing Human: Clinically Oriented Embryology (Chapter 2: Spermatogenesis, Oogenesis)
• Morgan & Mikhail's Clinical Anesthesiology, 7th Edition (Chapter 40: Physiological Changes in Pregnancy)
• Ganong's Review of Medical Physiology, 26th Edition (Chapter: Menstrual Cycle, Parturition)