Embryology — Ectoderm, Mesoderm, Endoderm Derivatives & Limb Development

Recap of Previous Topics

Review: Formation of male & female germ cells, fertilization, cleavage, blastocyst, implantation.
Transition of embryo from bilaminar to trilaminar disc → establishment of $\text{ectoderm}$ , $\text{mesoderm}$ , $\text{endoderm}$ .
Ectodermal derivatives already covered:
- Central nervous system (CNS): neural induction & neurulation.
- Neural crest: migration & derivatives (head, heart, trunk, sacral).
  • Pathologies: Treacher-Collins (mandibulofacial dysostosis), Hirschsprung (aganglionic megacolon).

Cranial Placodes – The “Third Ectodermal Derivative”

Definition: Localized thickenings of surface ectoderm in the head that produce sensory organs & some cranial ganglia.
Two morphogenetic modes:
1. Invagination → Vesicle formation
  • Lens placode → lens.
  • Otic placode → otic vesicle → inner ear (hearing & balance).
  • Olfactory placode → olfactory epithelium.
  • Adenohypophyseal placode (roof of mouth) → Rathke’s pouch → adenohypophysis (anterior pituitary).
2. Delamination from surface thickening
  • Trigeminal placode → CN V ganglion (ophthalmic + maxillo-mandibular lobes).
  • Epibranchial placodes (geniculate, petrosal, nodose) → proximal ganglia of CN VII, IX, X.
Timing & landmarks: $\text{~Day }25$ embryo shows lens & otic placodes adjacent to closing neuropores; heart bulge visible ventrally.

Mesoderm – Regionalization & Derivatives

Gastrulation Origins

Primitive streak/node dictate fate by time + position of ingress:
$\text{Head mesoderm \& notochord} \rightarrow \text{paraxial} \rightarrow \text{intermediate} \rightarrow \text{lateral plate}$ (anterior → posterior sequence).

Four Principal Mesodermal Domains

Axial (Chordamesoderm) – Notochord
• Signals (Noggin, Chordin, Follistatin) → neural induction; Shh → ventral patterning.
Paraxial – Somites + Head mesenchyme
• Somite sub-compartments:
– Sclerotome → vertebrae.
– Myotome → skeletal muscle (primaxial & abaxial).
– Dermatome → dermis.
– Syndetome → tendons.
Intermediate – Urogenital system
• Kidneys (pronephros → mesonephros → metanephros).
• Gonads + genital ducts.
Lateral Plate – Visceral (splanchnic) vs. Parietal (somatic)
• Visceral: covers gut, heart, blood vessels, smooth muscle, CT of GI & respiratory tracts.
• Parietal: body wall, limb skeleton (except muscle), serous membranes.
• Space between layers = $\text{intra-embryonic coelom}$ (thoracic & abdominal cavities).

Intermediate Mesoderm in Detail

Triphasic Kidney Development

Pronephros (non-functional; regresses ~ $\text{Week }4$ ).
Mesonephros (temporary excretory organ; gives mesonephric/Wolffian duct).
Metanephros (definitive kidney):
• Ureteric bud (collecting system) + metanephric blastema (nephrons).
• Ascends from sacral → lumbar region while gonads descend.

Gonad & Duct Fate

Indifferent gonad (Week 4–6): PGCs invade genital ridge.
Dual duct system present:
• Wolffian (mesonephric)
• Müllerian (paramesonephric).
Male (SRY + androgens): Wolffian persists → epididymis, vas deferens, seminal vesicle; Müllerian degenerates.
Female (absence of testicular signals): Müllerian persists → uterine tube, uterus, cervix, upper vagina; Wolffian degenerates.

Paraxial Mesoderm & Somite Patterning

Somite Formation

Presomitic mesoderm → segmental blocks; periodicity via Notch/Wnt “clock.”
Human count: $\approx 1–4$ somites at $\text{Day }20$ ; $13–17$ at $\text{Day }24$ ; final ≈ $\,!!42–44$ pairs.

Subdivision & Molecular Cues

Region	Inducing Signals	Marker Genes	Fate
Ventral (sclerotome)	Shh (notochord)	$\text{PAX1}$	Vertebrae, ribs
Dorsomedial (primaxial myotome)	Wnt1/3a, NT-3 (neural tube)	$\text{MYF5}$	Deep back muscles
Ventrolateral (abaxial myotome)	BMP4, FGF5 (LPM) + Wnt from ectoderm	$\text{PAX3}, \text{MyoD}$	Body wall & limb muscles
Central (dermatome)	Overlap of above	$\text{PAX3}$	Dermis
Syndetome	Shh + FGF	$\text{Scleraxis}$	Tendons

Resegmentation

Each vertebra = caudal half of somite $n$ + cranial half of somite $n+1$ → allows spinal nerves to exit at intervertebral foramina.
Notochord remnant forms $\text{nucleus pulposus}$ ; surrounding annulus fibrosus from sclerotome.

Limb Development

Limb Bud Composition & Axes

Surface ectoderm overlaying apical ectodermal ridge (AER).
Mesenchyme core:
• Lateral plate (somatic) → cartilage & bone.
• Somitic myoblasts → muscle.
Three orthogonal axes:
1. Proximo-distal (shoulder → digits).
2. Antero-posterior (thumb → little finger).
3. Dorso-ventral (extensor vs. flexor surface).

AER–Progress Zone Interaction (Proximo-Distal)

AER secretes $\text{FGF4/8}$ → maintains underlying progress zone (PZ) in proliferation.
“Time-in-zone” model: earliest PZ cells exit first → proximal bones; latest exit → digits.
Teratogen example: Thalidomide blocks PZ cell exit → phocomelia (proximal limb absence).

ZPA & Sonic Hedgehog (A-P Axis)

Posterior mesoderm forms zone of polarizing activity (ZPA) → secretes $\text{Shh}$ gradient.
High $\text{Shh}$ → posterior digit (digit 5); low → anterior (digit 1).
Misexpression → polydactyly.

Digit Separation

Programmed apoptosis in interdigital mesenchyme. Failure → syndactyly.

Lateral Plate Mesoderm Derivatives Recap

Somatic (parietal): limb girdles & long bones, dermis of ventrolateral body wall, parietal serous membranes.
Splanchnic (visceral): cardiac muscle, blood vessels, smooth muscle + CT of GI & respiratory tracts; visceral serous membranes.
Intra-embryonic coelom partitions → pericardial, pleural, peritoneal cavities.

Endoderm: Digestive & Respiratory Systems

Gut Tube Partitioning

Longitudinal folding defines:
• Pharyngeal gut (oral cavity → resp. diverticulum).
• Foregut (esophagus → proximal duodenum, liver, pancreas).
• Midgut (distal duodenum → proximal $2/3$ transverse colon).
• Hindgut (distal $1/3$ transverse colon → anal canal; also bladder, urethra).

Retinoic Acid (RA) Gradient

Posterior → anterior RA gradient specifies SOX2 (foregut), PDX1 (duodenum), CDX2/HOX (mid- & hindgut).

Reciprocal Endoderm–Mesoderm Signalling

Endoderm secretes Shh → patterns adjacent splanchnic mesoderm via differential $\text{HOX}$ activation → mesoderm feeds back with FGF/BMP/Wnt to stabilize gut identities.

Liver Development

Hepatic diverticulum emerges (Week 3) from ventral foregut into septum transversum.
Liver potential is widespread but repressed by ectoderm, notochord, & general mesenchyme; cardiac mesoderm provides FGF & BMP → permits liver fate.

Pancreas Development

Dorsal & ventral pancreatic buds (Week 5–7) merge.
Notochord represses Shh in overlying endoderm → allows pancreatic gene cascade (PDX1, etc.).

Midgut Physiological Herniation

Rapid elongation causes midgut loop to herniate into umbilical cord (Week 6).
Returns & rotates $270^{\circ}$ ccw around SMA by Week 10–12. Failure → omphalocele or gastroschisis.

Respiratory System

Respiratory (laryngo-tracheal) diverticulum from ventral foregut (≈ $\text{Day }22$ ).
RA from mesoderm ↑ → TBX4 up-regulation.
Endoderm → epithelium; splanchnic mesoderm → cartilage, muscle, CT.
Branching morphogenesis: trachea → primary bronchus → $2^{\circ},3^{\circ} bronchi → terminal bronchioles.
Four histological stages:
1. Embryonic (Weeks 4–7): lung buds, primary bronchi.
2. Pseudoglandular (Weeks 5–17): terminal bronchioles; no gas exchange.
3. Canalicular (Weeks 16–25): respiratory bronchioles, type I & II pneumocytes begin.
4. Saccular/Alveolar (Week 24–birth + post-natal): alveolar sacs, surfactant $\uparrow$ .
Pleura: visceral (from visceral LPM), parietal (from parietal LPM); space between = pleural cavity enabling lung inflation.

Key Signalling Molecules & Their Roles

Sonic hedgehog (Shh): dorsal–ventral neural tube, sclerotome induction, ZPA digit pattern, gut endoderm pattern.
BMP4: epidermal fate, abaxial myotome, limb DV axis.
FGF family: AER maintenance (FGF4/8), cardiac mesoderm → liver induction, limb outgrowth.
RA: A-P patterning of gut, limb proximodistal specification (low proximal, high distal concept contested).
HOX cluster: regional identity in gut, vertebral subtype specification.

Clinical Correlations & Pathologies

Treacher-Collins syndrome: neural crest hypoplasia → craniofacial defects.
Hirschsprung disease: absence of enteric ganglia (NC derived) → megacolon.
Polydactyly: excess Shh/ZPA activity.
Syndactyly: failure of interdigital apoptosis.
Phocomelia: thalidomide inhibition of limb proximodistal development.
Omphalocele/Gastroschisis: midgut herniation/re-entry defects.

Developmental Timeline Snapshot (Weeks 3–8)

$\text{Week 3}$ : Gastrulation, notochord, somite initiation, hepatic diverticulum.
$\text{Week 4}$ : Neural tube closure, limb buds, heart prominence, pronephros, lung bud.
$\text{Week 5}$ : Mesonephros function, cranial placodes visible, pseudoglandular lung stage.
$\text{Week 6}$ : Physiological herniation, digital rays, dorsal pancreatic bud.
$\text{Week 7}$ : Ventral pancreas rotates, metanephros begins, syndetome forms.
$\text{Week 8}$ : Major organ primordia present; end of embryonic period → fetal growth phase.

Study Tips

Draw axial cross-sections to visualize ectoderm, mesoderm subdivisions, endoderm.
Map signaling centers (node, notochord, AER, ZPA) and their secreted factors.
Associate timepoints with structural hallmarks (somite #, limb stages, gut herniation).
Use clinical cases to reinforce molecular concepts (e.g., Shh ↔ polydactyly).