Respiratory System Development

Respiratory System Development

• Begins as a median outgrowth, the laryngo-tracheal groove, caudal to the 4th pharyngeal pouches
• Endoderm lining the groove gives rise to epithelium and glands of the respiratory tract
• Surrounding splanchnic mesoderm gives rise to connective tissue, cartilage, and smooth muscles of the respiratory tract

Larynx and Trachea Development

• Laryngo-tracheal diverticulum forms by the end of the 4th week, ventral to the caudal part of the foregut
• Proximal part remains tubular, opens into the pharynx, and forms the larynx and trachea
• Distal end dilates to form a lung bud, which divides to give rise to 2 lateral outgrowths (primary bronchial buds)

Lung Development

• Lung buds divide into primary bronchi, which subdivide into secondary bronchi on each side
• Segmental bronchi begin to form by the 7th week of development
• Respiratory bronchioles develop by the 24th week of development
• Additional development and branching of bronchioles occur after birth

Maturation of Lungs

• Divided into 4 periods: Pseudoglandular, Canalicular, Saccular, and Alveolar
• Pseudoglandular Period: 6th-16th weeks, major elements of lungs have formed, but respiration is not possible
• Canalicular Period: 16th-26th weeks, lumina of bronchi and terminal bronchioles become larger, and respiratory bronchioles are formed