Stages of Lung Development

• Murray & Nadel's Textbook of Respiratory Medicine, p. 64-65
• Fishman's Pulmonary Diseases, p. 1856

Stage 1 - Embryonic Stage (PCW 4 to 7)

• The respiratory diverticulum outpouches from the foregut; the common foregut tube divides into the trachea (ventral) and esophagus (dorsal).
• Two primary bronchi form; these divide into secondary (lobar) bronchi - three on the right and two on the left - by 5 weeks.
• The earliest respiratory epithelial marker is Nkx2-1 (also known as TTF-1), expressed in foregut endoderm-derived ventral progenitor cells.
• Left-right asymmetry of the lung buds is already specified at this stage, in concert with visceral organ lateralization.
• The lung buds elongate into surrounding mesenchyme and begin branching morphogenesis.

Stage 2 - Pseudoglandular Stage (5 to 16 weeks)

• The lung continues to grow laterally and caudally on exposure to the splanchnic mesoderm.
• Conducting airways branch progressively until terminal bronchioles (16th to 25th generation) are formed by 16 weeks.
• On cross-section, the lung resembles a developing exocrine gland - hence "pseudoglandular."
• Mesenchymal tissue differentiates into ciliated columnar epithelial cells, goblet cells, and eventually type II pneumocytes.
• This stage marks the transition zone between conducting and respiratory airways.
• No gas exchange is possible at this stage - fetuses born in this period cannot survive.
• Key molecular drivers: FGF10 (from mesenchyme) drives branching; WNT, BMP, and Sprouty signaling networks shape the branching pattern.

Stage 3 - Canalicular Stage (16 to 26 weeks)

• The epithelium differentiates into type I pneumocytes (alveolar cells), the primary structural cells for gas exchange.
• Extensive angiogenesis occurs - capillary networks form surrounding alveolar cells.
• Lamellar bodies begin forming within type II pneumocytes - these are the primary storage sites for surfactant (surfactant production begins around 24 weeks).
• Conducting airways continue to elongate and widen, creating the characteristic "canalicular" appearance.
• The air-blood barrier begins to form as capillaries approximate type I cells.
• Survival becomes marginally possible at the limits (~22-24 weeks) if intensive neonatal care is provided, owing to early surfactant production and minimal gas exchange capacity.

Stage 4 - Saccular Stage (26 weeks to term)

• Progressive maturation of type I and type II pneumocytes; interstitial tissue decreases.
• Air spaces called primary sacculi form - these are the future gas exchange units.
• Exponential growth in primary sacculi with formation of primary septa between each saccular unit (primary septa contain a double capillary loop).
• The terminal saccules are simpler and wider than mature alveoli; further maturation occurs postnatally.
• Surfactant production increases substantially; adequate surfactant is typically present by ~34-36 weeks.
• Premature birth at this stage risks respiratory distress syndrome (RDS) due to surfactant deficiency.

Stage 5 - Alveolar Stage (postnatal - continues through early childhood)

• True alveoli form by ingrowth of secondary crests from primary saccular walls, thinning the double capillary loop into a single loop.
• Secondary myofibroblasts (at the tips of secondary septa) produce elastin and elastin cross-linking enzymes critical for septum formation. They form a "fishnet-like" network that contracts to generate novel septa.
• Alveolar type 2 (AT2) cells produce surfactant and can differentiate into AT1 cells, which cover ~95% of the surface area.
• The conventional view holds that alveolarization is complete by 6-7 years; however, more recent evidence shows new alveoli continue to be added into the 20s.
• There are approximately 480 million alveoli in a mature lung, with a gas-exchange surface area of ~130 m².

Summary Table

Congenital Anomalies of the Lung

• Fishman's Pulmonary Diseases, Figure 105-1

A. Airway Disorders

1. Tracheal Agenesis

2. Bronchial Atresia

3. Congenital Tracheobronchomegaly (Mounier-Kuhn Syndrome - MKS)

4. Tracheobronchial Branching Abnormalities

• Tracheal bronchus: Abnormal bronchus arising directly from the trachea (88% are displaced upper lobe bronchi; 12% are supernumerary). Right-sided prevalence 0.1-2%, left-sided 0.3-1%. Usually asymptomatic; may cause recurrent infections or atelectasis.
• Cardiac bronchus: Accessory bronchus arising from the medial wall of the right main or intermediate bronchus, directed toward the pericardium. Most are blind-ending; may cause recurrent infections.
• Bridging bronchus: Rare; the right lower lobe bronchus anomalously originates from the left main bronchus. Associated with pulmonary artery sling.

5. Bronchogenic Cyst

• A congenital anomaly derived from the lung bud of the foregut during the embryonic stage.
• Characterized by a fluid-filled blind cystic pouch - location depends on timing of the defect:
  • Early defect → mediastinal cysts (paratracheal, subcarinal, paraesophageal, hilar)
  • Later defect → intrapulmonary cysts (predominantly lower lobes)
• Incidence: ~1 in 42,000-68,000 admissions; male predilection.
• Often asymptomatic; may cause superior vena cava syndrome, dyspnea, dysphagia, or recurrent infections (postobstructive pneumonia in 82% of symptomatic cases).
• Histology (diagnostic): respiratory ciliated pseudostratified columnar epithelium with bronchial glands, smooth muscle, and cartilage in the cyst wall.
• CT imaging: well-defined ovoid cyst that does not enhance with contrast; content variable (water to soft-tissue density), may show "milk of calcium."
• Management: symptomatic cysts are surgically resected; asymptomatic cysts managed conservatively.
• Fishman's Pulmonary Diseases, p. 1861

6. Congenital Pulmonary Airway Malformation (CPAM)

• Pathogenesis: abnormal branching of primitive bronchioles with arrested development, primarily during the pseudoglandular stage (but may occur as late as the alveolarization stage).
• Prevalence: ~9 cases per 100,000 total births.
• Up to 70% present as cystic lesions in distal bronchi; a subset presents as solid masses.
• Overexpression of Hoxb5 (beyond pseudoglandular stage), KGF/FGF7, and PDGFB has been implicated.

• Prenatal: Detected on ultrasound as macrocystic (>5 mm) or microcystic (<5 mm); CVR (cyst volume ratio) >1.6 associated with hydrops fetalis risk.
• Large lesions cause mediastinal shift, pulmonary hypoplasia, cardiac compression, and hydrops.
• Most small lesions regress in the 3rd trimester.
• Management: Surgical resection (usually lobectomy) postnatal for symptomatic or large lesions.
• Fishman's Pulmonary Diseases, p. 1862; Miller's Anesthesia, p. 9036

7. Bronchopulmonary Sequestration (BPS)

• Second most common developmental lung disorder; accounts for ~6.4% of all congenital pulmonary malformations.
• A lung mass with no communication with the tracheobronchial tree - does not participate in respiration.
• Pathogenesis: originates from a supernumerary lung bud; supplied by systemic arterial circulation (thoracic aorta 75%, abdominal aorta 25%) - this systemic arterial supply is its hallmark.
• Hypothesized to originate as a supernumerary lung bud; the relationship to pleural formation determines type.

• Imaging: Color Doppler US identifies systemic feeding artery; CT chest angiography is the modality of choice in adults (shows consolidation/mass with feeding artery in left lower hemithorax).
• Management: Asymptomatic small BPS - conservative; larger or symptomatic lesions - surgical resection (VATS) at 6-12 months.
• Fishman's Pulmonary Diseases, p. 1863

B. Parenchymal Disorders

8. Congenital Lobar Emphysema (CLE) / Congenital Lobar Overinflation

• Rare (1 in 20,000 live births); accounts for ~10.5% of developmental lung disorders; more common in males.
• Pathogenic mechanisms:
  • Focal cartilage defect: absent or deficient bronchial cartilage → dynamic airway collapse → air trapping (25% of cases)
  • Polyalveolar lobe: fivefold increase in number of normal alveoli (alveolar giantism) without over-distension - described by Hislop and Reid (1970)
• Sites (in order of frequency): left upper lobe (42-57%) > right middle lobe (27-35%) > right upper lobe (14-30%).
• Associated with cardiac anomalies (VSD, ASD, tetralogy of Fallot) in some cases.
• Imaging: Chest X-ray shows focal lobar hyperlucency with contralateral mediastinal shift; bronchovascular markings are preserved within the hyperlucent area (distinguishes from pneumothorax). CT confirms lobar overdistension with compression of adjacent lung.
• Management: Most cases identified in early neonatal period; symptomatic cases require lobectomy. Asymptomatic adults - observation.
• Fishman's Pulmonary Diseases, p. 1864

9. Pulmonary Hypoplasia

• Underdevelopment of one or both lungs; may result from extrinsic compression (diaphragmatic hernia), oligohydramnios (reducing fetal breathing movements and lung fluid), or intrinsic developmental defects.
• Oligohydramnios impairs lung development since fetal breathing movements and lung fluid distension are required for normal lung growth.
• Unilateral hypoplasia presents with mediastinal shift toward the affected side; CT shows marked volume loss with bronchiectasis in the hypoplastic lung.

10. Pulmonary Cystic Mesenchymal Hamartoma

• Rare condition involving inappropriate mesenchymal tissue in the lung arising from disordered mesoderm during development.
• Slow-growing nodules that cavitate into cystic structures >1 cm; most often diagnosed in adults (chronic cough, hemoptysis, pneumothorax).
• Histology: normal respiratory epithelium surrounding cysts with a layer of primitive mesenchymal cells with dark nuclei (low mitotic activity distinguishes from malignancy).
• Rare malignant transformation to rhabdomyosarcoma reported.

C. Vascular Disorders

11. Pulmonary Artery Sling (PAS)

• The left main pulmonary artery anomalously originates from the right main pulmonary artery and traverses between the trachea and esophagus to reach the left lung.
• Associated with tracheal stenosis due to complete tracheal rings in up to 80% of patients.
• Type I: Sling above the carina - compresses right main bronchus and distal trachea, causing hyperinflation of right lung; often associated with tracheal bronchus.
• Type II: Sling at T5-T6 level, associated with complete tracheal rings and more severe stenosis.

12. Unilateral Absent Pulmonary Artery (UAPA)

• Failure of development of one pulmonary artery, typically the right.
• Presents with recurrent pulmonary infections, hemoptysis, or pulmonary hypertension.
• The affected lung is hypoplastic and supplied by bronchial/systemic collaterals.

13. Scimitar Syndrome (Pulmonary Venolobar Syndrome)

• A complex syndrome where the right pulmonary vein drains anomalously into the inferior vena cava instead of the left atrium, creating a "scimitar sign" on chest X-ray (curved opaque line resembling a Turkish sword).
• Associated with hypoplasia of the right lung, dextroposition of the heart, and systemic arterial supply to the right lower lobe.
• Often presents in infancy with heart failure or recurrently as an incidental finding in adults.

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