The primordium of the lower respiratory system develops in about the fourth week. The laryngotracheal diverticulum arises from endoderm on the ventral wall of the foregut. Tracheoesophageal folds develop on either side and join to form a tracheoesophageal septum that separates it from the rest of the foregut. This divides the foregut into the laryngotracheal tube (ventral) and the esophagus (dorsal). The caudal end of the laryngotracheal diverticulum enlarges to form the lung bud, which is surrounded by splanchnic mesenchyme.
The opening of the laryngotracheal tube becomes the inlet of the larynx. The laryngeal cartilages are derived from the fourth and sixth pharyngeal arches.
The epithelium and glands of the trachea develop from the endoderm of the laryngotracheal tube. The cartilage, connective tissue, and smooth muscle are derived from the surrounding splanchnic mesenchyme.
At the end of the fourth week the lung bud divides into two bronchial buds, which enlarge to form the primary bronchi. The right bronchus is larger and more vertically oriented than the left one, and this relationship persists throughout life. In the fifth week, each bronchial bud divides into secondary bronchi. In the eighth week the secondary bronchi divide to form the segmental bronchi (tertiary bronchi), ten in the right lung and eight in the left. Each segmental bronchus becomes a bronchopulmonary segment (segment in a lung). The smooth muscle, connective tissue, and cartilaginous plates in the bronchi are derived from splanchnic mesenchyme.
Table 14 - Stages of Lung Development
Figure 13 - Development of Lungs
An abnormal communication between the trachea and esophagus due to incomplete separation of the trachea and esophagus during the fourth week of development. It is commonly associated with esophageal atresia. Newborn infants with these malformations cough and choke during eating due to the aspiration of food and saliva into the lungs.
Respiratory Distress Syndrome
Respiratory Distress Syndrome is common in premature infants and is due to a deficiency of surfactant. It is commonly associated with hyaline membrane disease in which the alveolar surfaces of the lungs are coated with a glassy hyaline membrane. Treatment with thyroxin and cortisol can increase production of surfactant.
Pulmonary HypoplasiaPulmonary hypoplasia results when lungs are compressed by abnormally positioned abdominal viscera and cannot develop normally or expand at birth. It is commonly caused by congenital posterolateral diaphragmatic hernia.