Embryology L9: Respiratory System

Embryology L9: Respiratory System

• The respiratory diverticulum, the lung bud, emerges from the ventral wall of the foregut during the fourth week of embryonic development.
• The lung bud's formation is linked to rising retinoic acid levels prompted by adjacent mesoderm.
• The epithelium lining the larynx, trachea, and bronchi, as well as the lungs, are entirely derived from endoderm.
• Cartilage, muscle, and connective tissue components of the trachea and lungs arise from splanchnic mesoderm surrounding the foregut.
• Initially, the lung bud is connected to the foregut. Later, tracheoesophageal ridges separate it from the foregut, eventually forming the trachea and esophagus.
• The respiratory primordium maintains communication with the pharynx through the laryngeal orifice.

Formation of the Lungs Buds

• The respiratory diverticulum (lung bud) originates from the ventral wall of the foregut.
• The epithelium lining the larynx, trachea, and bronchi, as well as the lungs, is purely endodermal.
• Lung bud growth, shaped by surrounding mesoderm, is crucial for proper development.
• Cartilage, muscle, and connective tissues that support the lungs and tracheas are derived from the splanchnic mesoderm that is closely associated with the foregut.
• Initially, the lung buds are directly connected to the foregut. The lung buds detach and separate from the foregut as they develop, forming the trachea and esophagus.
• A crucial step in formation involves the tracheoesophageal septum development, separating the trachea from the esophagus.
• The respiratory primordium maintains its connection to the pharynx through the laryngeal orifice.

Larynx

• The larynx's inner lining stems from endoderm.
• Cartilages and muscles originate from the fourth and sixth pharyngeal arches' mesenchyme.
• Rapid mesenchyme growth transforms the laryngeal opening from a slit to a T-shape.
• Mesenchyme change into the thyroid, cricoid, and arytenoid cartilages.
• Temporarily, the larynx's lumen gets blocked by rapidly proliferating laryngeal epithelium
• The tenth cranial nerve (vagus nerve) innervates all the laryngeal muscles.

Trachea, Bronchi, and Lungs

• The lung bud divides to form the trachea and two bronchial buds as it detaches from the foregut.
• Two bronchial buds (right and left primary bronchi) branch after the fifth week.
• The right branches into three secondary bronchi; the left branches into two.
• The lung buds project into the pericardioperitoneal space, effectively filling the area.
• The mesoderm that wraps around the lungs develops into visceral pleura.
• Somatic mesoderm, which covers the body wall, differentiates into parietal pleura.
• The pleural cavity forms between visceral and parietal pleura.

Maturation of the Lungs

• Bronchioles continuously divide into even smaller bronchial canals throughout the prenatal period until the seventh month.
• Some cells lining the respiratory bronchioles transform from cuboidal to flat, facilitating gas exchange.
• Terminal sacs, also called primitive alveoli, develop in the respiratory bronchioles.
• Adequate capillary networks are built in the terminal sacs by the seventh month to ensure efficient gas exchange.
• The number of terminal sacs steadily increases during the final two months of fetal development.
• Type I alveolar epithelial cells become thinner for efficient gas exchange, and capillaries penetrate the alveolar sacs.
• Alveoli are fully formed only postpartum.
• Type II alveolar cells emerge by the sixth month, producing surfactant, a fluid that reduces surface tension within the alveoli, preventing collapse.

Summary

• The summary of the respiratory development in embryo.
• Information on the various stages of lung growth and development.