RESPI

Development of Respiratory System

Objectives

  • Discuss development of the respiratory system.

  • Discuss common congenital anomalies resulting from abnormal development of the respiratory system.

  • Enumerate derivatives of the pharyngeal apparatus.

  • Differentiate stages of lung development.

Case Study

  • Case: A prenatal ultrasound revealed polyhydramnios and at birth, the baby had excessive fluids in its mouth.

    • Potential defect: may indicate esophageal atresia or tracheoesophageal fistula, associated with embryological origins linked to failures in division of the foregut.

Key Development Stages

4th Week of Development

  • Drain openings of pharyngeal pouches into primordial pharynx; site of oropharyngeal membrane and mouth.

  • Laryngotracheal diverticulum forms in the ventral wall of the primitive foregut.

  • First sign of respiratory development: formation of the laryngotracheal diverticulum, which is in communication with foregut.

  • Development progresses caudally, and median longitudinal groove forms leading to development of tracheoesophageal folds.

Pharyngeal Arches (Week 4 to Week 5)

  • Pharyngeal arches crucial for the development of laryngeal structures.

  • Each arch gives rise to specific cartilaginous and muscular components related to respiratory functions.

Development of the Larynx

  • Laryngotracheal diverticulum opens into the primitive foregut, becoming the laryngeal orifice.

  • Laryngeal epithelium comes from endoderm; muscles derived from the somitomeric mesoderm from pharyngeal arches 4 and 6.

  • Connective tissues from neural crest cells.

Development of the Trachea

  • Trachea develops from the laryngotracheal diverticulum.

  • Epithelial lining derived from endoderm; C-shaped cartilaginous rings from visceral mesoderm.

  • Tracheoesophageal septum separates the trachea from esophagus.

Congenital Anomalies

Tracheoesophageal Fistula (TEF)

  • Abnormal connection between trachea and esophagus.

  • Associated with esophageal atresia from improper division of the foregut.

  • Clinical features: excessive saliva/mucus accumulation, gagging, cyanosis, abdominal distention, reflux.

Types of TEF

  • Type A: Esophageal atresia with TEF at distal end (most common).

  • Type B: TEF only.

  • Type C: Esophageal atresia with TEF at proximal end.

  • Type D: Esophageal atresia with TEF at both ends.

VACTERL Syndrome

  • A group of congenital anomalies including: vertebral anomalies, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies, limb defects.

Development of the Bronchi

  • Bronchial buds form from lung bud and differentiate into primary bronchi (3 on right, 2 on left).

  • Further subdivision leads to secondary and tertiary bronchi.

Lung Development Stages

Pseudoglandular Period (5-16 weeks)

  • Developing lung resembles branching of a compound gland; gas exchange structures not formed, hence non-viable if born.

Canalicular Period (16-26 weeks)

  • Development of respiratory bronchioles and increased vascularization; potential for survival increases for premature infants born after 20 weeks.

Terminal Sac Period (26 weeks-Birth)

  • Terminal sacs (primitive alveoli) develop; differentiation of pneumocytes begins; survival rates improve.

Alveolar Period (Birth-8 years)

  • Lung development continues postnatally with maturation of alveolar structures.

Clinical Considerations

Aeration at Birth

  • Transition from fluid-filled lungs to air-filled with effective clearance mechanisms.

Respiratory Distress Syndrome (RDS)

  • Caused by surfactant deficiency; commonly affects premature infants; treated with corticosteroids to accelerate surfactant production.

Pulmonary Agenesis and Hypoplasia

  • Pulmonary Agenesis: Complete absence of lung structures.

  • Pulmonary Hypoplasia: Underdeveloped bronchial tree; associated with conditions like renal agenesis.

Effects of Amniotic Fluid

  • Essential for lung development; oligohydramnios can cause severe pulmonary hypoplasia due to restricted lung growth.