Development of the Respiratory System

Introduction

• Overview of respiratory system development.
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Germ Layer Origin

• Respiratory system develops from two germ layers:
  • Endoderm
  • Mesoderm

Transition from Bilaminar to Trilaminar Disc

• Week 2: Bilaminar disc consists of:
  • Epiblast layer (top)
  • Hypoblast layer (bottom)
  • Yolk sac layer (below hypoblast)
• Formation of primitive streak in epiblast:
  • Epiblast cells migrate through to form endoderm.
  • Additional epiblast cells form mesoderm.
• Notochord and neural tube formation through neurulation.

Gastrulation Process

• Converts bilaminar disc to trilaminar disc.
• Involves formation of:
  • Ectoderm
  • Mesoderm
  • Endoderm
• Also involves notogenesis (notochord development).

Folding Processes

Lateral Folding

• Folding brings layers together in a three-dimensional structure.

Cranial-Caudal Folding

• Further anatomical development, aiding in formation of respiratory structures.

Key Structures from Germ Layers

• Endoderm: Forms epithelial lining of the respiratory tract (larynx, trachea, lungs).
• Mesoderm: Forms cartilage, muscle, and connective tissue around respiratory structures.
  • Paraxial mesoderm (closest to neural tube)
  • Intermediate mesoderm (lateral)
  • Somatic layer
  • Splanchnic layer (next to endoderm; crucial for respiratory system).

Development of the Larynx

• Larynx formation through:
  • Pharyngeal pouches (epithelial lining from endoderm).
  • Pharyngeal arches (muscle/connective tissue from mesoderm).
  • Important structures include:
    • Epiglottic swelling (above laryngeal orifice)
    • Arytenoid swelling (below laryngeal orifice).
• Week 12: Developed larynx structure:
  • Derived from fourth and sixth pharyngeal arches.
  • Vagus nerve supplies laryngeal muscles and provides sensory information.

Development of Trachea and Lungs

Lung Bud Formation

• Week 4: Formation of lung bud from foregut.
• Development of tracheoesophageal septum separates trachea from esophagus.

Stages of Lung Development

1. Pseudoglandular Stage (Week 5-16):
   • Development of primary, secondary, and tertiary bronchi.
   • Terminal bronchioles form.
2. Canalicular Stage (Week 16-26):
   • Forms respiratory bronchioles and alveolar ducts.
   • Development of primitive alveoli.
   • Immature alveoli (cuboidal cells) with poor gas exchange.
3. Saccular Stage (Week 26-Birth):
   • Increased number of alveoli and respiratory bronchioles.
   • Differentiation of alveoli into type 1 and type 2 pneumocytes.
   • Type 2 pneumocytes produce surfactant.
4. Alveolar Stage (Week 36-8 years):
   • Formation of septation in alveoli to increase surface area.
   • Continued maturation of type 1 and type 2 cells.
   • Increase in pulmonary capillaries.

Importance of Surfactant

• Surfactant reduces surface tension, preventing alveolar collapse.
• Premature births may result in insufficient surfactant, leading to infant respiratory distress syndrome (IRDS).

Conclusion

• Summary of respiratory system development discussed.
• Encouragement for ongoing support and engagement.