Foetal Lung Development

Overview – Foetal Lung Development

Foetal lung development is a complex, staged process that prepares the respiratory system for extrauterine life. During gestation, maternal respiratory physiology is significantly altered to facilitate gas exchange, while the foetal lungs progress through embryonic, pseudoglandular, canalicular, saccular, and alveolar stages. Critical events include airway branching, vascularisation, surfactant production, and alveolar maturation. Understanding these stages is essential for recognising the basis of neonatal respiratory disorders such as infant respiratory distress syndrome (IRDS).

Maternal Respiratory Adaptations in Pregnancy

  • High oestrogen levels:
    • Fluid retention → ↑ blood volume.
    • Oedema of airway mucosa.
    • Stimulates mucous gland proliferation and growth.
  • High progesterone levels (≈6× normal):
    • Hypersensitises central CO₂ chemoreceptors in the medulla.
    • Resets normal PaCO₂ to a lower level → relative hyperventilation (≈40% ↑ tidal volume, frequency unchanged).
    • Facilitates clearance of foetal CO₂.
    • Consequences: ↓ arterial PaCO₂, stable PaO₂, mild alkalosis (↑ pH).
  • Metabolic changes:
  • Mechanical changes:
    • Enlarged uterus compresses lungs and diaphragm → ↓ compliance, residual volume, functional residual capacity, ERV, and IRV.
    • Compensation via increased anteroposterior and transverse chest diameters.
Maternal Respiratory Adaptations in Pregnancy
Yeomans, Edward R., and Larry C. Gilstrap III. “Physiologic changes in pregnancy and their impact on critical care.” Critical care medicine 33.10 (2005): S256-S258.

Stages of Foetal Lung Development

Foetal Lung Development Stages
https://www.embryology.ch/anglais/rrespiratory/phasen01.html

1. Embryonic Stage (Weeks 4–7)

  • Upper airway structures form before lower.
  • Week 4: olfactory placodes (ectodermal) form nasal cavities, which connect to the foregut.
Olfactory Pit
Parexocoetus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Week 5: laryngotracheal bud forms from endoderm, developing into respiratory mucosa (pharyngeal, tracheal, bronchial, bronchiolar, alveolar).
  • By weeks 6–7, the basic upper respiratory tree is laid down.
Foetal Lung Development
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

2. Pseudoglandular Stage (Weeks 8–16)

  • Development of airway smooth muscle, cartilage, blood vessels, and interstitium.
  • Rapid proliferation of endodermal epithelium.
  • By week 16, all airway divisions are complete down to terminal bronchioles.
Foetal Lung Development - pseudoglandular stage
Adapted from: http://www.embryology.ch/dutch/rrespiratory/phasen03.html

3. Canalicular Stage (Weeks 16–26)

  • Rapid angiogenesis → proliferation of pulmonary capillaries.
  • Acinar walls thin due to fibroblast apoptosis → facilitates gas exchange.
  • First appearance of surfactant from type II pneumocytes.
  • Surfactant reduces surface tension, promotes alveolar expansion, and ↑ lung volume.
Foetal Lung Development - Canalicular stage
Adapted from: https://www.embryology.ch/anglais/rrespiratory/phasen04.html

4. Saccular Stage (Weeks 25–35)

  • Airspaces develop into saccules.
  • Contain both type I and type II epithelial cells.
  • By ≈27 weeks: lungs are structurally capable of gas exchange (survival possible in premature infants).
Foetal Lung Development - Saccular stage
Adapted from: https://www.embryology.ch/anglais/rrespiratory/phasen05.html

5. Alveolar Stage (Week 35 – 3 years postnatal)

  • Terminal saccules form primitive alveoli.
  • Type II cells differentiate into thin type I cells, dramatically ↑ surface area.
  • At birth: ~50 million alveoli present.
  • By 3 years: ~300 million alveoli.
Foetal Lung Development - Alveolar stage
Adapted from: https://www.embryology.ch/anglais/rrespiratory/phasen06.html

Additional Foetal Lung Features

Foetal Lung Fluid

  • Secreted by type I epithelial cells into airspaces, flows into amniotic fluid.
  • Maintains positive pressure in airspaces, preventing collapse.
  • Stimulates lung expansion, cell growth, and differentiation of type II → type I cells.

Foetal Respiratory Movements

  • In-utero “practice” breathing occurs from 22–35 weeks.
  • Inhibited in the final week before birth to prevent aspiration of fluid or meconium.

Foetal Haemoglobin (HbF)

  • HbF has a higher affinity for oxygen than adult haemoglobin (left-shifted dissociation curve).
  • Facilitates oxygen transfer from maternal to foetal blood.
  • HbF is replaced by adult haemoglobin within 6 months postnatally.
Foetal Haemoglobin
Creative Commons: Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/

Pulmonary Surfactant

  • Without surfactant:
  • With surfactant:
    • Composed mainly of dipalmitoylphosphatidylcholine (DPPC, 90%) plus proteins/carbohydrates.
    • Secreted by type II alveolar cells.
    • Reduces alveolar pressure, collapse tendency, and work of breathing.
    • Equalises stability: ↓ surface tension more in small alveoli than large ones, preventing collapse into larger alveoli.
Foetal Lung Development - Pulmonary Surfactant
BQUB19-PDonatiu, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Postnatal Lung Growth

  • Lung maturation continues postnatally
  • Increases in alveoli number, diameter, surface area
  • Growth from 50 million to 300 million alveoli

Summary – Foetal Lung Development

Foetal lung development proceeds through embryonic, pseudoglandular, canalicular, saccular, and alveolar stages, with critical events including vascularisation, surfactant production, and alveolar expansion. Maternal respiratory adaptations ensure efficient clearance of foetal CO₂ while supporting oxygen supply. Surfactant production and alveolar maturation are essential for postnatal survival, and deficiencies can lead to infant respiratory distress syndrome. For broader context, see our Respiratory Overview page.

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