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Respiratory System Development
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Respiratory System Development

Respiratory Development
  • The respiratory system arises from endoderm and mesoderm
    • Endoderm gives rise to the mucosal lining of the bronchial tree and the epithelial cells of the alveoli, which are the sites of gas exchange.
    • Mesoderm gives rise to the supporting muscle and cartilage of the respiratory tract, and to the visceral pleura of the lungs.
Key stages
Embryonic stage
  • Lasts from approximately 26 days to 6 weeks, and is marked by the appearance of the respiratory diverticulum. The diverticulum gives rise to two lungs, comprising 5 lobes and 18-20 broncho-pulmonary segments. Additionally, the stem of the diverticulum gives rise to the trachea, aka, windpipe, and, the larynx, aka, the voice box.
Pseudo-glandular stage
  • Lasts from approximately 6-16 weeks, and is marked by the appearance of terminal bronchioles; histologically, lung tissue from this phase appears glandular.
Canalicular stage
  • Lasts from approximately 16-28 weeks, and is marked by formation of respiratory bronchioles and their associated vasculature, and, by the specialization of lung epithelium.
Saccular stage
  • Lasts from approximately 28-36 weeks, and is marked by the appearance of primitive alveoli, though the formation of mature alveoli continues throughout childhood.
Illustrations
Week 3
Foregut gives rise to the respiratory diverticulum, ventrally.
  • Tracheoesophageal ridge marks where the two structures are continuous.
Respiratory diverticulum is surrounded by, and growing into, the splanchnic mesoderm.
Week 4
The tracehoesophageal ridges meet and fuse at the midline to form a septum.
    • Esophagus and respiratory tract are separate, distinct features.
    • Superior end of the respiratory tract will become the larynx and trachea.
The respiratory diverticulum bifurcates to form the primary bronchial lung buds, which expand into the surrounding splanchnic mesoderm.
Week 5
The primary bronchial buds give rise to secondary bronchial buds, which "sprout" in a set pattern that reflects the future lobes of the lungs:
  • On the right side, superior, middle, and inferior lobes, each of which comprises a secondary bronchial lung bud and the surrounding mesoderm.
  • On the left, superior and inferior lobes; the asymmetry reflects the fact that the mature heart will nestle into ventro-medial aspect of the left lung.
  • The outermost layer of the surrounding mesoderm is the visceral pleura.
Week 6
At the start of the pseudoglandular period, the secondary bronchi give rise to tertiary bronchial buds.
  • The lobes of the lungs expand to accommodate each new round of branching.
Weeks 16 and 36
  • Tertiary bronchus is the foundation of a bronchopulmonary segment.
    • Gives rise to multiple bronchioles, which then undergo additional rounds of branching.
    • During the canalicular stage, terminal bronchioles give rise to respiratory bronchioles, which are enmeshed in pulmonary capillaries.
    • Finally, during the saccular stage, the respiratory bronchioles give rise to alveolar ducts and sacs, which open to the primitive alveoli.

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