Development of the Respiratory System

Development of the Respiratory System

• The lower respiratory organs (larynx, trachea, bronchi, and lungs) develop during the fourth week of development.
• The tracheobronchial tree develops caudal to the fourth pair of pharyngeal pouches.
• The endoderm lining the laryngotracheal groove forms the pulmonary epithelium and glands of the larynx, trachea, and bronchi.
• Connective tissue, cartilage, and smooth muscle develop from the splanchnic mesoderm.
• By the end of the fourth week, the laryngotracheal groove evaginates to form a laryngotracheal diverticulum (lung bud).

Laryngotracheal Diverticulum

• The laryngotracheal diverticulum separates from the foregut to form the trachea and two lateral outpouchings (primary bronchial buds).
• The endoderm lining the laryngotracheal tube, distal to the larynx, differentiates into the epithelium and glands of the trachea and the pulmonary epithelium.
• Cartilage, connective tissue, and muscles of the trachea are derived from the surrounding splanchnic mesenchyme.

Development of the Trachea

• The endoderm forms the epithelium and glands of the trachea.
• The surrounding mesenchyme forms the connective tissue, muscle, and cartilage.

Development of the Larynx

• The epithelium lining the larynx is endodermal in origin.
• The larynx's cartilages and muscles arise from the fourth and sixth pharyngeal arches' mesenchyme.

Development of Bronchi and Lungs

• The respiratory bud develops at the caudal end of the laryngotracheal diverticulum in the fourth week.
• The bud divides into two outpouchings (primary bronchial buds), which grow laterally into the pericardioperitoneal canals (primordia of the pleural cavities).
• Secondary and tertiary bronchial buds develop.
• The lungs acquire a visceral pleura from the splanchnic mesoderm and a parietal pleura from the somatic mesoderm.

Development of Respiratory Passages

• The respiratory passages appear as an endodermal bud arising from the caudal end of the laryngotracheal groove.
• The bud becomes covered by mesenchyme that differentiates into blood vessels, cartilage, smooth muscle, and connective tissue.
• The endoderm lines the entire respiratory tract.
• The lung bud grows caudally and undergoes successive divisions that form the main bronchi.
• Subsequent divisions result in finer branches, culminating in small expansions called infundibula, which become definitive alveoli.

Successive Stages of Bronchial Bud Development

• Diagrams illustrate the development of bronchial buds, bronchi, and lungs from 28 days to 56 days.
• The primordia of secondary bronchial buds and main bronchi are shown at various stages.
• Diagrams show lobes of the lungs in different phases.

Maturation of the Lungs

• The lung development goes through three phases: pseudoglandular, canalicular, and terminal sac.
• The repeated divisions of the endodermal tubes create a glandular appearance in the lung.
• Nuclei are variable, and mitosis is common in the earliest stages.
• Epithelial cells are separated from the mesenchyme by a basement membrane.
• Cells become cuboidal, and cilia and goblet cells appear. Clara cells differentiate, and endocrine cells can be found later.

Canalicular Phase

• The canalicular phase extends from week 17 to 26.
• Tubules transform into canaliculi, with epithelial flattening and cytodifferentiation.
• Osmiophilic lamellar bodies associated with surfactant production appear.
• The cells have well-developed endoplasmic reticulum and ribosomes.
• Type II pneumocytes and surfactant appear.
• Intercellular junctions shift to the lower cell half.

Terminal Sac Phase

• The terminal sac phase extends from week 26 until birth.
• Respiratory bronchioles develop into tubes with flattened epithelium on one side and ciliated cuboidal epithelium on the other.
• Clusters of five to six generations of saccules appear and are lined with flattened epithelium associated with capillaries.
• Alveoli are small and shallow at birth, with only a few present in respiratory bronchioles.
• Inflation of alveoli begins after birth, first in the hilar region.
• Distal alveoli are inflated after several hours of breathing.
• The adult number of alveoli is reached by the eighth postnatal year.

Adaptations for Autonomous Gas Exchange

• Surfactant production in alveolar sacs.
• Transformation of lungs from secretory to gas-exchanging organs
• Parallel pulmonary and systemic circulations.

Histological Stages

• Images show the histological stages (pseudoglandular, canalicular, etc.) of lung development at different weeks of gestation.
• Stages include the appearance of bronchi, bronchioles, and alveoli, along with changes in the epithelial cells and surrounding tissue.