Respiratory Embryology

• Germ layers of trilaminar germ disc

  • Ectoderm forms CNS, PNS, eye, ear, skin epithelium
  • Mesoderm forms body connective tissues, blood, bone, muscle, skin connective tissue, GI and respiratory tracts
  • Endoderm forms GI tract organs, GI and respiratory tract epithelium
• Development of Lungs

  1. Embryonic period (w4-8)

  2. Projection from gut tube where lungs start to form

  3. Lung buds form away from gut tube - become lungs and primary bronchi

  4. Evagination of future trachea out of foregut

  5. Separation between respiratory and GI tract
• Fistula

  Abnormal pathway tract between two structures
• Tracheoesophageal fistulas


  • Type C - blind-ended pouch between upper oesophagus and lower oesophagus causes abnormal tract into trachea
  • Type A - lowest risk, still have the separation
• In utero, baby does swallowing movements (helps to mature GI system). If problem with oesophageal integrity baby cannot swallow fluid and it builds up around the baby - polyhydramnios.
• Pseudoglandular Period (w5-17)

  1. Conducting airways

  2. Branching

  3. Epithelium glandular appearance - tall columnar and cuboidal

  4. Bronchi forming

  5. By week 8 all segmental bronchi formed

  6. ~14 more generations to reach terminal bronchioles
• Canalicular Period (w16-26)


  1. Differentiation of epithelium

  2. Forming respiratory bronchioles

  3. Canalization of lung parenchyma by capillaries

  4. Formation of future air-blood barrier

  5. Distinguish future gas exchange regions and future conducting airways
• Saccular Period (w24-38)


  1. Terminal sacs (primitive alveoli) form

  2. Cuboidal cells flatten (Type 1 pneumocytes)

  3. ↑surface area gas exchange region

  4. Intimately associated with blood vessels

  5. Vascular tree ↑length and diameter - large surface area

  6. Surfactant production

  7. Type 2 pneumocytes
• Babies born at w24 vulnerability in relation to maturation of airways because got primitive alveoli and need a lot of intensive care
• Infant respiratory distress syndrome (RDS)


  • Affects ~50% infants born at 26-28 weeks
  • Inadequate production of surfactant
  • Causes air sacs to collapse on expiration
  • Increases energy required for breathing
• Exogenous surfactant


  • Reduce mortality by 30%
  • Reduce pulmonary air leaks by 50%
  • Remove intubation as soon as possible because of risk of pneumothorax and provide additional respiratory support e.g. with Continuous Positive Airways Pressure
• Alveolar Period (w36-8y)


  1. Terminal saccules replaced by mature alveoli

  2. Only 16% of alveolar cells are present at birth

  3. Process continues after birth
• May be reason why diseases in children such as asthma may improve
• Maturation


  • Foetus makes respiratory 'breathing' motions
  • Role in muscle development
  • Lung maturation
  • At birth lung fluid rapidly resorbed
  • Babies are obligate nasal breathers - only breathe through nasal cavity - allows to suck on breast or bottle without having to break off
  • At birth amniotic fluid in lungs is rapidly reabsorbed with cry when babies are born
• Development of Pleural Cavities and Diaphragm


  • Pleural Cavities derived from mesoderm
  • Separation of single body space into 3 cavities: Pleural, Pericardial, Peritoneal
  • Pleuropericardial fold develops around lung buds and separates heart and lungs
  • Fusion in midline of pleuro-pericardial membrane on each side, and aorta forms in midline
  • Lungs curve round and surround heart which is surrounded by pericardium and lungs by visceral pleura and pleural cavities
• Early diaphragm


  1. Called septotransversum - septum that is in a transverse plane separating thorax from abdomen

  2. Anterior part becomes central tendon and pleuroperitoneal folds on either side

  3. Inferior vena cava T8 oesophagus T10 and aorta T12 - oesophageal mesentry coming up

  4. Complete separation of thorax from abdomen

  5. Muscular ingrowth from body wall forms crura to give muscular structure
• Congenital diaphragmatic hernia


  • 95% postero-lateral (Bochdalek), of which 85% on left
  • 2-3% retrosternal (Morgangi's), 1% anterior
  • 1% central
  • Baby more likely to present with respiratory signs than GI signs as defect in diaphragm causes small intestine to move up into thoracic cavity, restricting breathing and causing respiratory arrest
  • Baby in intensive care - endotracheal tube pushed to side by abdominal contents
  • Nasogastric tube also herniated into thorax
  • Requires supportive measures to manage oxygenation and to give fluids and food. Needs surgery to repair damage to diaphragm