Early Embryonic Development & Implantation

The first three weeks of human development — from a single fertilised oocyte to a trilaminar germ disc — are among the most heavily tested zones in NEET PG anatomy. Master the day-by-day timeline, the trophoblast layers, and the clinical correlations (ectopic pregnancy, hydatidiform mole, sacrococcygeal teratoma) and you cover the bulk of embryology MCQs.

Orientation: the developmental clock

Development is dated from fertilisation (ovulation age / embryonic age), which is about 2 weeks behind the obstetric last menstrual period (LMP) dating used clinically. Keep these two clocks separate in your head — examiners exploit the confusion.

Time (post-fertilisation)	Key event	Location
Day 0	Fertilisation	Ampulla of uterine tube
Day 1	Zygote (2-cell)	Ampulla
Day 2–3	Cleavage (4–16 cell)	Tube
Day 3–4	Morula (12–16 cells) enters uterus	Uterine cavity
Day 4–5	Blastocyst forms (cavitation)	Uterine cavity
Day 5–6	Zona pellucida shed ("hatching")	Uterine cavity
Day 6–7	Implantation begins	Endometrium (posterosuperior wall)
Day 8	Bilaminar disc; amniotic cavity appears	Endometrium
Day 12–13	Lacunar stage; primary villi; uteroplacental circulation	—
Day 14	Prochordal plate; primitive streak appears	—
Day 15–16	Gastrulation → trilaminar disc	—

High-yield: Fertilisation occurs in the ampulla of the fallopian tube. The morula enters the uterus on day 3–4, and implantation is normally complete by the end of the first week to day 10–11.

Fertilisation

Fertilisation is the fusion of a haploid sperm with a secondary oocyte (arrested in metaphase II) to restore the diploid number and trigger development. It must occur within ~24 hours of ovulation; sperm remain viable ~48–72 hours.

Stepwise sequence:

1. Capacitation — 7-hour conditioning of sperm in the female tract; removes glycoprotein coat & seminal plasma proteins.
2. Acrosome reaction — sperm contacts the zona pellucida, binds glycoprotein ZP3, releasing acrosin/hyaluronidase to penetrate.
3. Penetration of corona radiata → zona pellucida → oocyte membrane.
4. Cortical reaction (zona reaction) — cortical granules harden the zona, the slow block to polyspermy; the fast block is membrane depolarisation.
5. Completion of meiosis II by the oocyte → second polar body extruded.
6. Male & female pronuclei fuse → zygote (restores 46 chromosomes; sex determined by sperm).

High-yield: ZP3 is the primary sperm-binding receptor on the zona pellucida. The cortical (zona) reaction is the slow block to polyspermy.

Results of fertilisation: restoration of diploid number, determination of chromosomal sex (XY sperm → male), and initiation of cleavage.

Cleavage, morula and blastocyst

After fertilisation, the zygote undergoes cleavage — rapid mitotic divisions without cell growth, so total embryo size stays constant within the zona pellucida. Daughter cells are blastomeres.

• Compaction at the 8-cell stage maximises cell-cell contact.
• 16-cell morula (Latin morus = mulberry) forms by ~day 3.
• Fluid enters the morula → blastocyst (~day 4–5) with:
  • Inner cell mass (embryoblast) → embryo proper.
  • Outer cell layer (trophoblast) → placenta & membranes.
  • Blastocoele (blastocyst cavity).
• The embryonic pole is the side bearing the inner cell mass; the blastocyst implants by this pole.

High-yield: Identical (monozygotic) twins arise from one zygote. Timing of splitting determines membranes — splitting at morula/blastocyst before the inner cell mass forms gives dichorionic-diamniotic; splitting after the inner cell mass but before the amnion (most common, days 4–8) gives monochorionic-diamniotic; very late splitting (>day 8) gives monochorionic-monoamniotic; splitting after ~day 13 → conjoined twins.

Mnemonic for monozygotic twin membranes — "Di-Di, Mono-Di, Mono-Mono with time": earlier the split, more separate the membranes.

Implantation

The zona pellucida is shed ("hatching") around day 5–6, allowing the blastocyst to adhere to and invade the endometrium, usually on the posterosuperior wall of the uterine body during the secretory (luteal) phase.

Trophoblast differentiates into two layers:

Layer	Position	Features	Function
Cytotrophoblast	Inner	Mononucleated, mitotically active; "stem" layer	Source of new cells
Syncytiotrophoblast	Outer	Multinucleated syncytium, no mitoses, erosive/invasive	Invades endometrium; secretes hCG; forms lacunae

High-yield: The syncytiotrophoblast secretes human chorionic gonadotrophin (hCG), detectable by the end of the 2nd week; it maintains the corpus luteum (basis of the pregnancy test). It is the layer that erodes maternal vessels and is invasive, but it is non-mitotic — it grows by fusion of cytotrophoblast cells.

Decidual reaction

In response to progesterone and the invading conceptus, endometrial stromal cells become large, glycogen- and lipid-rich decidual cells. The decidua is named by location:

• Decidua basalis — deep to the conceptus; forms the maternal part of the placenta.
• Decidua capsularis — overlies the conceptus toward the lumen.
• Decidua parietalis (vera) — lines the rest of the cavity; capsularis fuses with parietalis as the sac enlarges.

Lacunar stage & uteroplacental circulation

By days 9–13, vacuoles in the syncytiotrophoblast coalesce into lacunae; eroded maternal sinusoids fill them with maternal blood, establishing the primitive uteroplacental circulation.

Bilaminar germ disc (Week 2 — "the week of twos")

The inner cell mass organises into two layers:

• Epiblast (columnar) — faces the amniotic cavity; gives rise to all three definitive germ layers.
• Hypoblast (cuboidal) — faces the blastocyst cavity; lines the primitive yolk sac.

Two cavities and two layers form:

1. Amniotic cavity appears within the epiblast (amnioblasts line it).
2. Primary yolk sac (exocoelomic / Heuser's membrane) lines the cavity below the hypoblast.
3. Extraembryonic mesoderm fills between the trophoblast and the cavities; splits to form the chorionic cavity (extraembryonic coelom).
4. Connecting (body) stalk suspends the embryo — future umbilical cord.
5. Prochordal (prechordal) plate marks the future cranial/head end and the site of the buccopharyngeal membrane.

High-yield: Week 2 = rule of twos — two germ layers (epi/hypoblast), two cavities (amniotic + yolk sac), two trophoblast layers (cyto/syncytio), and the bilaminar disc.

Gastrulation & the trilaminar disc (Week 3 — "the week of threes")

Gastrulation converts the bilaminar disc into a trilaminar disc (ectoderm, mesoderm, endoderm) and establishes body axes.

Flow of gastrulation:

Primitive streak appears (day 15) → primitive node + pit at cranial end → epiblast cells migrate, invaginate → first wave displaces hypoblast to form definitive endoderm → second wave between layers forms intraembryonic mesoderm → cells remaining in epiblast become ectoderm.

• The notochord develops from the primitive node (notochordal process), defining the body axis and inducing the neural plate (neurulation begins late week 3).
• Buccopharyngeal membrane (cranial) and cloacal membrane (caudal) remain bilaminar (no mesoderm) — future mouth and anus.
• Allantois outpouches from the yolk sac into the connecting stalk.

High-yield: Gastrulation is the most critical, teratogen-sensitive period for axis formation. Persistence of the primitive streak in the sacrococcygeal region → sacrococcygeal teratoma, the commonest tumour in the newborn, more frequent in females.

Placental membrane (barrier) layers

The placental (materno-fetal) barrier separates maternal blood in the intervillous space from fetal blood in the villous capillaries.

Early placenta (up to ~20 weeks) — 4 layers	Term placenta — effectively thinner
1. Syncytiotrophoblast	Syncytiotrophoblast (thinned)
2. Cytotrophoblast (+ basement membrane)	Cytotrophoblast becomes discontinuous/lost
3. Extraembryonic (villous) mesoderm/connective tissue	Reduced
4. Fetal capillary endothelium (+ basement membrane)	Fetal capillary endothelium

High-yield: As pregnancy advances, the cytotrophoblast layer disappears, thinning the barrier to facilitate exchange. The placenta is haemochorial — maternal blood directly bathes the chorionic (syncytiotrophoblast) surface.

Villus evolution: primary villi (cytotrophoblast core) → secondary villi (mesoderm core) → tertiary villi (blood vessels) — vessels appear by end of week 3.

Ectopic implantation

Ectopic pregnancy = implantation outside the normal endometrial cavity. Incidence ~1–2% of pregnancies.

Sites in order of frequency:

1. Ampulla of uterine tube — commonest overall (~70%+ of tubal/ectopics).
2. Isthmus of tube.
3. Infundibulum/fimbrial.
4. Interstitial (cornual) — dangerous, late catastrophic rupture.
5. Ovary, abdominal cavity (e.g., rectouterine pouch of Douglas — commonest abdominal site), cervix.

High-yield: Ampullary is the most common ectopic site; the commonest abdominal/peritoneal site is the rectouterine pouch (of Douglas). Risk factors: prior pelvic inflammatory disease (PID), tubal surgery, IUCD, assisted reproduction, smoking.

Clinical clue: 6–8 weeks amenorrhoea + lower abdominal pain + bleeding; β-hCG positive but lower than expected with no rise of the normal doubling; transvaginal USG shows empty uterus. Rupture → haemoperitoneum, shock. Methotrexate (medical) for unruptured stable cases; surgery (salpingectomy/salpingostomy) for ruptured.

Placenta praevia (low implantation near/over internal os) and placenta accreta spectrum (absent decidua basalis → villi invade myometrium) are implantation-site abnormalities worth recalling.

Abnormalities of trophoblast & fertilisation

• Hydatidiform mole — abnormal trophoblastic proliferation with markedly raised hCG.
  • Complete mole: 46,XX (rarely 46,XY), entirely paternal (androgenetic — empty ovum fertilised by sperm that duplicates / two sperm); no fetal tissue; higher malignant (choriocarcinoma) potential.
  • Partial mole: triploid (69,XXX/XXY) — two paternal + one maternal set; some fetal tissue present; low malignant potential.

High-yield: Complete mole = diploid, all paternal, no embryo, "snowstorm/bunch of grapes" on USG, highest choriocarcinoma risk. Partial mole = triploid with fetal parts.

Key differentials & easily confused pairs

Concept A	Concept B	Discriminator
Cytotrophoblast	Syncytiotrophoblast	Cyto = mononuclear, dividing; Syncytio = multinuclear, invasive, secretes hCG
Epiblast	Hypoblast	Epiblast → all 3 germ layers; hypoblast → yolk sac lining only
Morula	Blastocyst	Morula = solid 16-cell; blastocyst = cavity + ICM + trophoblast
Complete mole	Partial mole	Complete = diploid paternal, no fetus; Partial = triploid, fetal parts
Decidua basalis	Decidua capsularis	Basalis = maternal placenta; capsularis overlies embryo

Recently asked / exam angle

• Site of fertilisation = ampulla (perennial single-best-answer).
• Which layer secretes hCG / is invasive? → Syncytiotrophoblast.
• Day of implantation → about day 6–7 begins, ~day 10–11 complete.
• Most common site of ectopic pregnancy → ampulla; most common ectopic to rupture late / most dangerous → interstitial (cornual).
• Sacrococcygeal teratoma ← persistent primitive streak; commonest neonatal tumour.
• ZP3 as the sperm receptor; acrosome reaction trigger.
• Genetics of complete vs partial mole (paternal diploid vs triploid).
• Twin chorionicity by timing of zygote splitting — image-based questions.
• Number of layers in the early placental barrier = 4, and which layer disappears at term (cytotrophoblast).
• Trilaminar disc and gastrulation timing = week 3; first cavity to form = amniotic cavity.

Rapid revision

1. Fertilisation site = ampulla; oocyte arrested in metaphase II until sperm entry.
2. ZP3 binds sperm; cortical/zona reaction = slow block to polyspermy.
3. Cleavage = division without growth; morula at ~day 3, blastocyst at ~day 5.
4. Blastocyst = inner cell mass (embryoblast) + trophoblast + blastocoele.
5. Implantation site = posterosuperior wall, secretory endometrium, begins day 6–7.
6. Syncytiotrophoblast = multinucleated, invasive, secretes hCG, non-mitotic.
7. Cytotrophoblast = mononuclear, mitotic stem layer.
8. Week 2 = bilaminar disc (epiblast + hypoblast), amniotic cavity + yolk sac (rule of twos).
9. Week 3 = gastrulation → trilaminar disc; primitive streak day 15; notochord from primitive node.
10. Persistent primitive streak → sacrococcygeal teratoma (commonest newborn tumour).
11. Early placental barrier = 4 layers; cytotrophoblast disappears at term; placenta is haemochorial.
12. Complete mole = diploid, all-paternal, no fetus, highest choriocarcinoma risk; partial mole = triploid with fetal parts; commonest ectopic = ampullary, commonest abdominal = pouch of Douglas.