U01.16.001 Lung development

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Learning Objectives

Master the five stages of Lung Development. Memorize the high-yield timeline for surfactant production, the specific structures formed in each stage, and the transition from fetal to neonatal circulation for the USMLE Step 1.

1. The 5 Stages of Development

Lung development begins at week 4 with the respiratory diverticulum. Use the mnemonic “Every Pulmonologist Can See Alveoli” to remember the order: Embryonic, Pseudoglandular, Canalicular, Saccular, Alveolar.

Stage Timeline Key Structural Developments
Embryonic Weeks 4–7 Formation of the lung bud, trachea, and bronchial tree (up to tertiary bronchi).
Pseudoglandular Weeks 5–17 Formation of terminal bronchioles. Respiration is impossible.
Canalicular Weeks 16–25 Respiratory bronchioles and alveolar ducts. Surfactant begins at week 20.
Saccular Week 24–Birth Terminal sacs formed; separated by 1° septae.
Alveolar Wk 36–8 Years Adult alveoli form via 2° septation.

2. Clinical Correlates & Critical Milestones

Specific errors during these stages lead to congenital anomalies often tested in vignettes.

Concept Clinical Significance
Embryonic Errors Tracheoesophageal fistula (TEF), tracheal atresia, or pulmonary agenesis.
Viability Respiration is generally capable by week 25 (end of Canalicular stage).
Pneumocytes Type II pneumocytes begin producing surfactant at week 20.

3. Physiological Transition at Birth

The transition from water to air causes a massive shift in pulmonary hemodynamics.

Period Pulmonary Vascular Resistance (PVR) Mechanism
In Utero High PVR Lungs are filled with fluid; very little blood flow.
At Birth Low PVR Air replaces fluid, causing vasodilation and increased blood flow.

Activity:

High-Yield Clinical Pearls:

  • The Canalicular Window: This is the most tested stage because it marks the transition to viability. No survival is possible before the canalicular stage.
  • Postnatal Growth: Remember that most alveoli develop after birth (up to age 8). A newborn’s lung is structurally different from an adult’s.
  • Type II Pneumocytes: These are the “factory” cells. They proliferate during the canalicular stage and produce surfactant, which reduces surface tension.

Activity: