Respiratory System Development

• The respiratory system's development stems from the endoderm and mesoderm germ layers.
• Development of this system starts around the fourth week of gestation.

Formation of the Lung Bud

• The respiratory diverticulum (lung bud) is a median outgrowth from the ventral wall of the foregut.
• Influenced by the transcription factor TBX4, the bud's location initiates respiratory system development.
• TBX4 encourages lung bud formation, along with continued differentiation and lung growth.

Separation of Esophagus and Trachea

• The tracheoesophageal septum divides the respiratory diverticulum from the foregut.
• The foregut divides to form the esophagus and trachea.
• From the respiratory diverticulum comes the trachea and two lateral primary bronchial buds.
• Primary bronchial buds elongate into the right and left main bronchi.

Development of Bronchi and Lungs

• The main bronchi divide further into secondary bronchi.
  • The left side forms two secondary bronchi.
  • The right side forms three secondary bronchi.
• These secondary bronchial buds develop into lobar bronchi.
• Lobar bronchi are what form the lobes of the lungs.
• Each lobar bronchus continues to divide.
• Approximately 17 generations of subdivisions have formed by the end of the sixth month.
• Seven additional generations of airways form before birth.
• Postnatally, the number of airway generations continues to increase into early adulthood.

Maturation of the Lungs

• The lungs mature through four stages.
  • Pseudoglandular period (6–16 weeks)
    • The developing lungs resemble an exocrine gland.
    • By week 16, all major lung elements have formed, except those for gas exchange.
  • Canalicular period (16–26 weeks)
    • Lung tissue becomes highly vascularized.
    • The lumina of bronchi and terminal bronchioles enlarge.
    • By 24 weeks, terminal bronchioles divide to form respiratory bronchioles.
    • Type II pneumocytes start secreting surfactant.
    • Fetuses born at the end of this period may survive with intensive care.
  • Terminal saccular period (26 weeks to birth)
    • Terminal saccules (primitive alveoli) develop.
    • The alveolar epithelium thins.
    • Capillaries closely contact the alveolar epithelium.
    • Adequate gas exchange becomes possible.
  • Alveolar period (late fetal period to childhood)
    • Mature alveoli develop.
    • Approximately 95% of mature alveoli develop after birth.
    • Postnatally, primitive alveoli continue to expand.
    • New alveoli form through the division of existing alveoli.

Pleural Cavity Formation

• Mesoderm around the lung bud differentiates into the visceral and parietal pleura.
• The space located between the visceral and parietal pleura forms the pleural cavity.

Fetal Breathing Movements

• Breathing movements take place before birth
• These movements support lung development and ready the respiratory muscles.

Lung Development Post-Birth

• The number of alveoli continues to increase until about 8 years old.
• Lung maturation is completed during late adolescence.

Clinical Correlations

• Tracheoesophageal Fistula
  • Abnormal connection exists between the trachea and esophagus.
  • The result of incomplete division of the foregut as it becomes the trachea and esophagus.
  • The most common type involves the lower esophagus connecting to the trachea.
• Esophageal Atresia
  • The esophagus ends in a blind pouch.
  • Often occurs alongside a tracheoesophageal fistula.
• Respiratory Distress Syndrome (RDS)
  • Premature infants experience a surfactant deficiency.
  • Leads to alveolar collapse and difficult breathing.
  • Managed through artificial surfactant and ventilation support.
• Congenital Diaphragmatic Hernia
  • A defect in the diaphragm allows abdominal organs into the thoracic cavity.
  • Lung development is inhibited.
  • Associated with a high mortality rate.
• Lung hypoplasia
  • Underdevelopment of the lungs
  • Congenital diaphragmatic hernia and oligohydramnios (low amniotic fluid) are associated.
• Congenital lung cysts
  • Terminal bronchioles dilate and form cysts.
  • Typically located at the periphery of the lung.
  • Can be asymptomatic or result in respiratory problems.
• Ectopic Lung Lobes
  • Lung tissue develops in an abnormal location.
  • Can occur in the trachea or esophagus.