🧫 Week 1 · fertilization → blastocyst
ampulla · zona pellucida · hCG
- Fertilization in ampulla of uterine tube; secondary oocyte completes meiosis II.
- Capacitation (≈7h) → acrosome reaction → cortical reaction (prevents polyspermy).
- Cleavage → morula (day 3–4) → blastocyst (day 5): inner cell mass (embryo) & trophoblast (placenta).
- Zona pellucida degenerates (hatching) for implantation in posterior uterine wall.
- Syncytiotrophoblast produces hCG → maintains corpus luteum → progesterone.
📌 high yield: hCG detectable in serum/urine early pregnancy; ectopic pregnancy → lower hCG.
🥯 Week 2 · bilaminar embryo
epiblast · hypoblast · prechordal plate
- Epiblast (columnar) → amniotic cavity, gives rise to all germ layers.
- Hypoblast (cuboidal) → primary yolk sac.
- Prechordal plate – fusion of epiblast/hypoblast; marks future cranial end & mouth.
- Extraembryonic mesoderm splits into somatic (chorion) & visceral (yolk sac).
⚠️ Hematopoiesis: first in yolk sac mesoderm (weeks 3–6), then liver, spleen, bone marrow.
🌊 Week 3 · gastrulation & germ layers
primitive streak · notochord
- Gastrulation – epiblast cells ingress through primitive streak → endoderm, mesoderm, ectoderm.
- Notochord – induces neural plate; remnant = nucleus pulposus of IV discs.
Ectoderm
- surface ectoderm
- neuroectoderm (neural tube)
- neural crest
Mesoderm
- paraxial (somites)
- intermediate
- lateral plate
Endoderm
- gut tube & associated organs
Sacrococcygeal teratoma – most common congenital tumor; arises from primitive streak remnants (all 3 germ layers).
🧠 Neurulation & folding
neural tube · neuropores · cephalocaudal & lateral folding
- Neural plate induced by notochord (day 18) → folds fuse → neural tube (cervical zippering).
- Rostral neuropore closes day 25; caudal day 27–28. Failure → anencephaly (rostral) or spina bifida (caudal).
- Neural crest → PNS ganglia, Schwann cells, melanocytes, adrenal medulla, odontoblasts, endocardial cushions.
- Folding (week 4): cephalocaudal → head & tail folds; lateral → primitive gut tube; amnion envelops embryo.
🧬 Patterning: SHH (ventral) / BMP4 (dorsal). Folate, valproate, carbamazepine affect neural tube closure.
🩸 Placenta · membranes · cord
chorion · amnion · yolk sac · allantois
- Yolk sac – primordial germ cells, early hematopoiesis (blood islands).
- Allantois → urachus (median umbilical ligament); patent → urine leak from umbilicus.
- Placenta: maternal decidua basalis + fetal chorion frondosum.
- Umbilical cord: 2 arteries (deoxygenated) + 1 vein (oxygenated) + Wharton's jelly.
Single umbilical artery – associated with cardiac/renal anomalies.
👯 Twinning & teratogens
Monozygotic twinning (timing)
- 0–4 days → di/di
- 4–8 days → mono/di
- 8–12 days → mono/mono
- >13 days → conjoined
monochorionic = TTTS risk
Teratogens & critical periods
- Weeks 1–2: all-or-none
- Weeks 3–8: major structural defects
- Week 9–birth: growth & functional
ACEi · alcohol · isotretinoin · lithium · tetracycline · thalidomide · valproate · warfarin · DM
🫘 Indifferent gonad → testis / ovary
SRY · MIF · testosterone · DHT · WNT4
Testis (SRY+)
- Sertoli → MIF (regresses Müllerian)
- Leydig → testosterone (Wolffian ducts)
- DHT → external genitalia
Ovary (SRY−)
- WNT4 active
- No MIF → Müllerian ducts → fallopian tubes, uterus, cervix, upper vagina
💡 Clinical: SRY translocation → XX male; SRY mutation → Swyer syndrome (46,XY female).
🧬 Meiosis · spermatogenesis · oogenesis
Spermatogenesis
- continuous from puberty
- ~74 days, seminiferous tubules
- 4 functional sperm per meiosis
Oogenesis
- begins before birth (prophase I arrest)
- 1 ovum + polar bodies
- meiosis II completed after fertilization
📊 Comparison: Spermatogenesis – continuous, 4 gametes; Oogenesis – cyclic, 1 gamete. Nondisjunction → trisomy (advanced maternal age).
- Meiosis I: reductional (homologs separate), crossing over in prophase I.
- Meiosis II: equational (sister chromatids separate).
🗂️ Germ layer derivatives
| Ectoderm | Mesoderm | Endoderm |
|---|
| epidermis, hair, nails, lens, cornea, enamel, anterior pituitary, CNS, neural crest | muscle, bone, cartilage, blood, cardiovascular, kidneys, gonads, adrenal cortex, notochord | GI epithelium, respiratory, bladder, thyroid, parathyroids, liver, pancreas |
yolk sac → primordial germ cells & early blood
💡 Pearls & full timeline
- Fertilization – ampulla; cortical reaction prevents polyspermy.
- Implantation – day 6–7, posterior uterine wall; syncytiotrophoblast → hCG.
- Week 2 – bilaminar disk (epiblast/hypoblast).
- Gastrulation – primitive streak, 3 germ layers by week 3.
- Notochord – induces neurulation; remnant = nucleus pulposus.
- Molar pregnancy – complete mole 46,XX (paternal); partial 69,XXY; high hCG.
⏳ Timeline: weeks 1–2 (all-or-none), weeks 3–8 (organogenesis, most teratogenic), week 9–birth (growth).
⚡ Exam nugget: Testosterone maintains Wolffian ducts; DHT for external virilisation. MIF only from Sertoli cells.