Home›A&P II›Reproductive System
🧬 A&P II · Reproductive System

Memory tricks for reproductive anatomy, hormones, and development

Male and female reproductive anatomy, the HPG axis, gametogenesis, the menstrual cycle, fertilization, pregnancy hormones, labor, and lactation — these memory tricks tie together the anatomy of reproductive structures with the physiology of the hormones that drive reproduction.

Community
📋
Reproductive System Forum
Ask questions · Share techniques
💬
Reproductive Chatroom
Live · Study together now
🧬 Reproductive System

Memory Tricks

Proven Mnemonics & Acronyms — fast to learn, hard to forget.

HPG Axis
GnRH → FSH + LH → Gonads → Sex hormones → Negative feedback
Hypothalamus → Pituitary → Gonads — three-level hormonal control of reproduction
The HPG axis — how the brain controls all reproductive function
GnRH (gonadotropin-releasing hormone) from hypothalamus is released in PULSES — pulsatile release is essential. Continuous GnRH paradoxically suppresses the axis by downregulating receptors — used clinically (GnRH agonists) to treat endometriosis, prostate cancer, precocious puberty. GnRH → anterior pituitary releases FSH and LH. FSH: follicle development (female) and spermatogenesis/Sertoli cells (male). LH: ovulation trigger + corpus luteum (female) and testosterone production via Leydig cells (male). Sex hormones feed back NEGATIVELY to suppress GnRH/FSH/LH — except at mid-cycle when high estrogen creates positive feedback → LH surge → ovulation. Inhibin (from Sertoli cells/granulosa cells) selectively inhibits FSH.
Pulsatile GnRH
Required for normal reproduction. Continuous → downregulation → suppression. Basis of GnRH agonist therapy.
FSH
Follicle development + Sertoli cells (spermatogenesis support). Inhibin selectively inhibits FSH.
LH surge
Day 14 — high estrogen switches to positive feedback → LH spike → ovulation within 36 hours.
LH in males
Stimulates Leydig cells → testosterone production. Testosterone → negative feedback.
Difficulty: Intermediate
Male Reproductive Anatomy
SEVEN UP — Seminiferous tubules · Epididymis · Vas deferens · Ejaculatory duct · Nothing · Urethra · Penis
Sperm pathway from production to ejaculation — seven structures in order
Male reproductive anatomy — sperm pathway and accessory glands
Sperm produced in seminiferous tubules (testes) → mature in epididymis (20 days, gain motility) → vas deferens → ejaculatory duct → urethra → outside. Accessory glands add to semen: seminal vesicles (60% of semen volume — fructose for energy, prostaglandins, alkaline fluid), prostate gland (25% — citric acid, enzymes, PSA, alkaline — neutralizes vaginal acidity), bulbourethral glands (Cowper's glands — pre-ejaculatory mucus, lubrication, neutralizes urethral acidity). Testes: in scrotum 2-4°C cooler than body temperature — essential for spermatogenesis (heat sensitive). Cryptorchidism (undescended testes) → impaired spermatogenesis → infertility.
Epididymis
Sperm storage and maturation. 20 days. Gain motility and fertilizing capacity here.
Seminal vesicles
60% of semen. Fructose (energy for sperm), prostaglandins, alkaline. Joins vas → ejaculatory duct.
Prostate
Surrounds urethra. PSA (marker for cancer), citric acid, enzymes. BPH compresses urethra.
Scrotal temperature
2-4°C below body temp — required for spermatogenesis. Cremasteric reflex regulates temp.
Difficulty: Intermediate
Spermatogenesis
Stem cell → Spermatogonia → Primary spermatocyte → Secondary → Spermatids → Spermatozoa
Mitosis then two rounds of meiosis → 4 haploid sperm from each primary spermatocyte
Spermatogenesis — from stem cell to mature sperm in 64 days
Spermatogenesis occurs continuously in seminiferous tubules from puberty. Takes ~64 days total. Spermatogonia (2n): mitosis → maintain stem cell pool AND produce primary spermatocytes. Primary spermatocyte (2n): Meiosis I → two secondary spermatocytes (n). Secondary spermatocyte (n): Meiosis II → four spermatids (n). Spermatids: spermiogenesis — differentiation into spermatozoa (develop head, midpiece, tail). Final maturation in epididymis. Sertoli cells: essential support — blood-testis barrier (immune privilege), nutrient supply, phagocytose excess cytoplasm, secrete inhibin and androgen-binding protein. Testosterone from Leydig cells maintains spermatogenesis locally at very high concentrations.
Sertoli cells
Nurse cells — blood-testis barrier, nutrients, inhibin. FSH stimulates Sertoli function.
Leydig cells
Interstitial — testosterone production. LH stimulates. High local T essential for spermatogenesis.
Sperm structure
Head (nucleus + acrosome with enzymes) + midpiece (mitochondria — ATP for swimming) + tail (flagellum).
64 days
Total spermatogenesis. Males produce ~300 million sperm/day continuously from puberty.
Difficulty: Intermediate
Female Reproductive Anatomy
Ovary → Fallopian tube → Uterus → Cervix → Vagina — egg pathway
Ovaries produce eggs and hormones · Fallopian tubes transport · Uterus nurtures
Female reproductive anatomy — structures and their roles in reproduction
Ovaries: produce oocytes and hormones (estrogen, progesterone). Ovarian cycle occurs here. Fallopian (uterine) tubes: fimbriae sweep oocyte in after ovulation. Site of fertilization (usually ampulla — outer third). Cilia and peristalsis transport zygote to uterus (~3-4 days). Uterus: three layers — perimetrium (serosal outer), myometrium (thick smooth muscle — contracts during labor), endometrium (functional layer shed in menstruation, basal layer regenerates). Cervix: lower uterus → mucus plug (progesterone-thickened), dilates during labor. Vagina: birth canal, copulatory organ. Mammary glands: modified sweat glands — produce milk. Alveoli (milk production) → lactiferous ducts → nipple. Developed under estrogen (duct growth) and progesterone (alveolar development).
Fallopian ampulla
Most common fertilization site. Ectopic pregnancy if zygote implants here → emergency.
Endometrium
Functional layer (shed in menses) + basal layer (regenerates). Estrogen proliferates, progesterone secretory.
Myometrium
Smooth muscle — contracts in labor (oxytocin receptor-mediated). Fibroids = benign myometrial tumors.
Cervix
Normal — firm, closed. Labor — softens (effacement) and dilates (0-10 cm). Pap smear checks for CIN/cancer.
Difficulty: Intermediate
Menstrual Cycle
Follicular: FSH → follicle → estrogen rises · Ovulation day 14 · Luteal: LH → corpus luteum → progesterone
28-day cycle — follicular phase (days 1-13) + ovulation (day 14) + luteal phase (days 15-28)
The menstrual cycle — hormone changes phase by phase
Follicular phase (days 1-13): FSH rises → recruits primordial follicles → dominant follicle emerges → estrogen rises → endometrium proliferates. Estrogen peak → positive feedback → LH surge. Ovulation (day 14): LH surge → dominant follicle ruptures → secondary oocyte released (completes meiosis II only if fertilized). Luteal phase (days 15-28): empty follicle → corpus luteum → progesterone dominant (+ estrogen) → endometrium secretory (prepares for implantation). No fertilization: corpus luteum degenerates → progesterone/estrogen fall → endometrium shed = menstruation. Fertilization: trophoblast secretes hCG → maintains corpus luteum → progesterone continues → no menstruation.
Days 1-5
Menstruation — functional endometrium shed. Low estrogen and progesterone. FSH begins rising.
Days 6-13
Proliferative — estrogen rising, endometrium rebuilding, dominant follicle maturing.
Day 14
LH surge → ovulation. Most fertile 12-24 hrs after LH surge. Ovulation predictor kits detect LH.
Days 15-28
Secretory — progesterone dominant. Corpus luteum. Endometrium thick and glandular for implantation.
Difficulty: Intermediate
Oogenesis
Born with all eggs arrested at Meiosis I · LH surge resumes · Fertilization completes Meiosis II
Females born with ~2 million oocytes — only ~400 ever ovulated — a fundamentally different process from spermatogenesis
Oogenesis — how it differs from spermatogenesis in timing and output
Oogenesis begins in fetal life. Oogonia (2n) undergo mitosis in fetus → become primary oocytes → arrested in Prophase I of Meiosis I — remain arrested from fetal life until puberty. At puberty, each cycle: one primary oocyte completes Meiosis I just before ovulation → secondary oocyte (n) + first polar body. The secondary oocyte is arrested in Metaphase II of Meiosis II at ovulation. Meiosis II completed ONLY if fertilization occurs → ovum + second polar body. Unequal division: one large egg gets most cytoplasm, small polar bodies get discarded. Contrast with spermatogenesis: males produce 300 million/day continuously, females ovulate ~1/month, and females are born with all eggs they will ever have.
Primary oocyte
Arrested Prophase I — from fetal life until puberty (up to 50 years!). Aging increases meiotic errors.
Secondary oocyte
Released at ovulation — arrested Metaphase II. Awaits fertilization. Viable ~12-24 hrs.
Meiosis II completion
Triggered by sperm entry → ovum + 2nd polar body → pronuclei fuse → zygote (2n).
Down syndrome risk
↑ maternal age → ↑ nondisjunction in Meiosis I (long arrest time → spindle errors) → trisomy 21.
Difficulty: Advanced
Fertilization and Implantation
Sperm meets egg in ampulla → zygote → morula → blastocyst → implants day 6-10
Acrosome reaction → cortical reaction prevents polyspermy → cleavage → implantation
From fertilization to implantation — the first two weeks of development
Fertilization: sperm capacitated in female tract (6-8 hours) → acrosome reaction releases enzymes → penetrates zona pellucida → sperm fuses with secondary oocyte → cortical reaction (cortical granules released → zona hardens → polyspermy prevented) → Meiosis II completed → pronuclei fuse → zygote (2n). Cleavage: mitotic divisions without growth → morula (16-cell solid ball, day 3). Blastocyst forms (day 4-5): inner cell mass (embryoblast → embryo) + trophoblast (→ placenta). Implantation (days 6-10): blastocyst implants in endometrium (usually posterior wall of uterine body) → trophoblast invades → hCG secreted immediately → detectable in urine by day 8-10.
Acrosome reaction
Sperm releases hyaluronidase and acrosin → penetrates corona radiata and zona pellucida.
Cortical reaction
Ca2+ wave triggers cortical granule exocytosis → zona pellucida hardens → blocks polyspermy.
Blastocyst
Inner cell mass (embryo) + trophoblast (placenta + membranes). Hatches from zona before implantation.
hCG
From trophoblast immediately after implantation. Maintains corpus luteum. Basis of pregnancy tests.
Difficulty: Advanced
Pregnancy Hormones
hCG peaks week 10 · Placenta takes over at week 12 · Estrogen + Progesterone rise throughout
Luteoplacental shift — corpus luteum to placenta as progesterone source at week 10-12
Hormone changes during pregnancy — what maintains the pregnancy at each stage
First trimester: hCG (from trophoblast) → maintains corpus luteum → corpus luteum makes progesterone + estrogen → maintains endometrium + suppresses new cycles. hCG peaks week 8-10 (morning sickness correlates). Luteoplacental shift (week 10-12): placenta takes over progesterone and estrogen production → corpus luteum can degenerate. Second and third trimester: placenta makes estrogen (from fetal adrenal DHEA via placental aromatase) + progesterone → levels rise throughout. Progesterone: suppresses myometrial contractions (essential for pregnancy). Relaxin: loosens pelvic ligaments and softens cervix (from corpus luteum and placenta). Human placental lactogen (hPL): from placenta → promotes fetal growth + prepares mammary glands + insulin resistance (diabetogenic).
hCG
Trophoblast → maintains CL → progesterone. Peaks wk 8-10. Morning sickness link. Detectable in urine early.
Luteoplacental shift
Week 10-12 → placenta takes over steroid production. Removing CL before this causes miscarriage.
Progesterone
Rises throughout pregnancy. Suppresses myometrial contractions. Cervical mucus plug. Immunosuppressive.
hPL
Human placental lactogen — GH-like. Promotes fetal growth. Causes maternal insulin resistance → GDM risk.
Difficulty: Advanced
Labor and Lactation
Labor: oxytocin positive feedback (Ferguson reflex) · Lactation: prolactin makes · oxytocin ejects
Labor = positive feedback loop · Lactation = prolactin synthesis + oxytocin letdown
Labor and lactation — the physiology of birth and breastfeeding
Labor initiation: late pregnancy → ↓ progesterone (relative), ↑ estrogen → uterine oxytocin receptors upregulate → fetal cortisol triggers labor. Ferguson reflex (positive feedback): cervical stretch → oxytocin release → contractions → more stretch → more oxytocin — escalates until delivery. Three stages: dilation (0-10 cm), expulsion (delivery), placental (delivery of placenta). Lactation: during pregnancy — prolactin rises but milk suppressed by high estrogen/progesterone. After delivery — estrogen/progesterone fall → prolactin uninhibited → colostrum then milk. Suckling → inhibits dopamine → ↑ prolactin (synthesis) + ↑ oxytocin (letdown). High prolactin → suppresses GnRH → lactational amenorrhea. Breastfeeding provides passive immunity via IgA in milk.
Ferguson reflex
Cervical stretch → oxytocin → contractions → more stretch → positive feedback → delivery.
Prolactin
Anterior pituitary → milk SYNTHESIS. Suckling → ↓ dopamine → ↑ prolactin. Suppresses GnRH → amenorrhea.
Oxytocin
Posterior pituitary → milk EJECTION (letdown). Suckling → oxytocin → myoepithelial contraction.
Colostrum
First milk — high IgA, protein, and immune factors. Transitions to mature milk by days 3-5.
Difficulty: Intermediate
🎓 Common Exam Questions
Q: Describe the HPG axis and how it regulates male and female reproduction.
A: Hypothalamic-Pituitary-Gonadal axis: GnRH (hypothalamus, pulsatile) → FSH and LH (anterior pituitary) → gonads. Male: FSH stimulates Sertoli cells (spermatogenesis support); LH stimulates Leydig cells (testosterone). Testosterone provides negative feedback on hypothalamus and pituitary. Inhibin B from Sertoli cells selectively inhibits FSH. Female: FSH stimulates follicle development; LH triggers ovulation and maintains corpus luteum. Estrogen: low levels = negative feedback; high levels (pre-ovulatory peak) = positive feedback → LH surge. Progesterone: negative feedback during luteal phase. Exogenous GnRH (pulsatile): stimulates FSH/LH. GnRH agonists (continuous): initially stimulate then suppress (downregulate receptors) → used for prostate cancer, endometriosis, precocious puberty.
Q: What is the menstrual cycle and what characterizes each phase?
A: Day 1-5 Menstrual: progesterone/estrogen drop → endometrial shedding. Prostaglandins cause uterine contractions (dysmenorrhea). Day 1-13 Follicular/Proliferative: FSH rises → follicle development → estrogen rises → endometrium proliferates (1→10mm). Day 13-14: estrogen peak → positive feedback → LH surge → ovulation (Graafian follicle ruptures). Day 15-28 Luteal/Secretory: corpus luteum secretes progesterone (dominant) + estrogen → endometrium becomes secretory (tortuous glands, glycogen). If no pregnancy: corpus luteum degenerates day 25-26 → progesterone/estrogen drop → spiral arteries spasm → ischemia → menstruation. Normal cycle 21-35 days; luteal phase always ~14 days; variation is in follicular phase.
Q: Compare spermatogenesis and oogenesis.
A: Spermatogenesis: begins puberty, continuous throughout life, ~64 days in seminiferous tubules. 1 spermatogonium → 4 equal haploid spermatids → spermatozoa. Sertoli cells nourish; blood-testis barrier protects. Oogenesis: begins fetal life (oogonia divide mitotically → primary oocytes). At birth: all ~2 million primary oocytes arrested in meiosis I prophase. Puberty: one primary oocyte completes meiosis I per cycle → secondary oocyte + first polar body. Meiosis II completes ONLY after fertilization → mature egg + second polar body. Produces 1 functional egg + 3 polar bodies (vs 4 sperm). Unequal cytoplasmic division — egg keeps most cytoplasm. Menopause: ~age 50, follicles depleted, FSH rises (no inhibin/estrogen feedback), LH rises, estrogen drops.
Q: What are the hormones of pregnancy and what does each do?
A: hCG (human chorionic gonadotropin): produced by syncytiotrophoblast from implantation. Peaks week 10, declines by week 20. Mimics LH → maintains corpus luteum → progesterone maintained → prevents menstruation. Basis of pregnancy test. High in multiple pregnancy, molar pregnancy; low in ectopic, miscarriage. Progesterone: corpus luteum until week 10, then placenta. Maintains endometrium, suppresses uterine contractions, promotes decidualization, prevents maternal immune rejection of fetus. Estrogen (estriol): rises throughout pregnancy → promotes uterine growth, breast development, cervical ripening. hPL (human placental lactogen): from placenta → promotes fetal nutrition (anti-insulin effect → maternal glucose available for fetus → gestational diabetes risk). Relaxin: loosens pelvic ligaments. Oxytocin: labor (positive feedback loop) and milk letdown.
Q: What is labor and what triggers its onset?
A: Exact trigger unclear but involves: fetal cortisol maturation signal, prostaglandin release, decline in progesterone influence, oxytocin receptor upregulation. Three stages: Stage 1 (dilation): cervical effacement and dilation from 0→10cm. Latent phase (0-6cm, slow). Active phase (6-10cm, faster). Contractions become regular, longer, stronger. Transition (8-10cm) most intense. Stage 2 (expulsion): full dilation → delivery of baby. Pushing with contractions. Stage 3 (placental): delivery of placenta (5-30 min). Oxytocin feedback loop: uterine contractions → fetal head presses on cervix → stretch receptors → oxytocin from posterior pituitary → more contractions → positive feedback (Ferguson reflex). Continues until baby delivered. Pitocin (synthetic oxytocin) used to induce or augment labor. Misoprostol (prostaglandin E1) ripens cervix.