Lung Development

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Embryology • 5 Stages

🧠 Memory Hook

Every Pulmonologist Can See Alveoli

Embryonic • Pseudoglandular • Canalicular • Saccular • Alveolar

Lung development occurs in five stages, starting from the lung bud (distal respiratory diverticulum) in week 4.

1. Embryonic

Weeks 4 – 7

Lung Bud (Foregut) → Trachea → Bronchial Buds → Primary → Secondary → Tertiary Bronchi

• • Mesenchyme forms: cartilage, smooth muscle, connective tissue, vasculature.
• • Large airways → NOT endodermal tubules.
• • Clinical: Errors → tracheoesophageal fistula, esophageal atresia.

2. Pseudoglandular

Weeks 5 – 17

Tertiary Bronchi → Endodermal Tubules → Terminal Bronchioles

• • Endodermal tubules: small distal epithelial tubes → precursors of terminal bronchioles.
• • Surrounded by modest capillary network. Respiration impossible; incompatible with life.
• • Lungs resemble glandular tissue (“pseudoglandular”).
• • Clinical: Congenital malformations → CPAM, bronchial atresia.

3. Canalicular

Weeks 16 – 25

Terminal Bronchioles → Respiratory Bronchioles → Alveolar Ducts

• • Airways increase in diameter. Pneumocytes begin developing (~week 20).
• • Surrounded by prominent capillary network → gas exchange possible.
• • Respiration capable (~week 25).
• • Clinical: Preterm infants ≥24 weeks may survive; errors → pulmonary hypoplasia.

4. Saccular

Wk 24 – Birth

Alveolar Ducts → Terminal Sacs (Primitive Alveoli) → Primary Septae

• • Surfactant production begins (~week 24–28).
• • Capillaries closely apposed → ready for gas exchange at birth.
• • Clinical: Preterm infants → respiratory distress syndrome (RDS).

5. Alveolar

Wk 36 – 8 Yrs

Terminal Sacs → Secondary Septation → Adult Alveoli (~300M)

• • In utero: “breathing” amniotic fluid → gradually reduces pulmonary vascular resistance.
• • At birth: Air replaces fluid → resistance drops → efficient gas exchange.
• • Postnatal alveolar growth continues, especially first 2 years.

✅ Summary Table – High-Yield

Stage	Weeks	Structures Formed	Key Notes & Clinical
Embryonic	4–7	Lung bud → Trachea → Primary, Secondary, Tertiary Bronchi	Large airways; errors → TE fistula, esophageal atresia.
Pseudoglandular	5–17	Endodermal Tubules → Terminal Bronchioles	Gas exchange impossible; CPAM, bronchial atresia.
Canalicular	16–25	Terminal Bronchioles → Resp. Bronchioles → Alveolar Ducts	Pneumocytes develop; capillary network expands; preterm survival possible.
Saccular	24–Birth	Terminal Sacs (Primitive Alveoli), Primary Septae	Surfactant production; preterm RDS risk.
Alveolar	36–8 yrs	Secondary Septation → Mature Alveoli (~300M)	In utero breathing of amniotic fluid; postnatal growth continues.

✅ Terminology Clarification

Bronchi vs. Tubules Bronchi (primary, secondary, tertiary): Embryonic stage; large airways.
Endodermal Tubules: Pseudoglandular stage; small distal tubes → terminal bronchioles.

Maturation Path Terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli: Progressive maturation to gas-exchanging units.

Surfactant Appears in Saccular stage → critical for postnatal respiration.

CPAM Note A hamartomatous overgrowth of terminal bronchioles occurring during the Pseudoglandular stage.

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