14. Lung Development (and Digestive System)

Question 1

Lung

Answer 1

• Endodermal origin
• Derived from foregut: respiratory diverticulum
  
  • Depend on retinoic acid produced by mesoderm
  • Upregulation of TBX4 (endoderm), induces bud formation
• Laryngotracheal orifice is the opening between trachea and larynx
• Cartilage, muscle, connective tissue from splanchnic/visceral mesoderm or neural crest cells (ectoderm)
• Tracheoesophageal ridge separates trachea and lung buds from esophagus
  
  • Abnormal septation: fistulas and atresias

Question 2

Lung development stages

Answer 2

Embryonic
Pseudoglandular
Canlicular
Saccular
Alveolar
Alveolar cells
Pulmonary surfactant

Question 3

Embryonic

Answer 3

• Bronchiol buds: invagination of foregut
  
  • Primary (26-28 days)
  • Secondary (early week 5): 3 right, 2 left (form lung lobes)
  • Tertiary (week 6): form bronchopulmonary segments

Question 4

Pseudoglandular

Answer 4

• Bronchiol buds expand int he pericardioperitoneal canals
  
  • Week 10: ciliated cells
  • Week 11: cartilage appears
  • By week 16: terminal bronchioles

Question 5

Canlicular

Answer 5

• Until week 28: division into more respiratory bronchioles with stubby branches
• week 13-24: bronchial glands develop
• Epithelium specialization (type II → I)
• Canaliculi compose the proper respiratory part of lungs, pulmonary parenchyma
• Week 24: sufficient differntiation into type I pneumocytes and proliferation of capillaries allow breathing
• Week 25: production of amniotic fluid

Question 6

Saccular

Answer 6

• Bronchioles subdivide to produce terminal sacs
• Development of gas-exchange
• Week 25-26: alveolar-capillary membrane able to sustain extrauterine life
• Week 28-29: terminal sacs line with mature Type II cells
  
  • Surfactant appears
  • Produced by type II alveolar cells
• Week 34-36: mature alveolar structure evident

Question 7

Alveolar

Answer 7

Continues after birth
- New sacculi form and are divided by septa
- Until week 38: alveoli mature
- Terminal sacs continue to form, mostly until age 2, finished at 10
- Gas-exchange only when alveoli are thin-walled (last weeks before birth)

Question 8

Alveolar cells

Answer 8

• Type I: gas exchange
• Type II: surfactant (decreased surface tension)
• 16% present at birth, remainder develop for 10 years
• RDS: respiratory distress syndrome
  
  • Insufficient surfactant
  • Increased surface tension
  • Collapse of alveoli

Question 9

Pulmonary surfectant

Answer 9

• Reduces surface tension (no collapse)
• Prevents inflammatory response
• Phospholipids and protein
• Lecithin: major surfactant
• Sphinomyelin (in amniotic fluid)

Question 10

Delayed and increased pulmonary surfectant

Answer 10

• Delayed
  
  • Acidemia
  • Hypoxia
  • Circulatory shock
  • Over or underinflation
  • Pulmonary oedema
• Increased
  
  • Maternal heroin adiction
  • Premature rupture of membranes
  • Maternal hypertension
  • Maternal infection
  • Placental insufficiency
  • Betamethasone or thyroid hormone