Embryology of the Respiratory System: Development Stages

Embryology of the Respiratory System

From the fourth week of development, the lung appears as an evagination or outline of the foregut. The laryngotracheal groove appears on the ventral surface of the pharynx; this groove deepens to form the laryngotracheal diverticulum, which grows in a ventrocaudal direction.

Through a process of separation, the tracheoesophageal septum develops, effectively separating the esophagus from the laryngotracheal tube. From this laryngotracheal tube, the larynx, trachea, bronchi, and lungs will eventually develop. The endoderm forms the lining epithelium and glands, while the connective tissue, cartilage, and muscle develop from the splanchnic mesenchyme.

Development of the Larynx

The mesenchyme corresponding to the 4th and 6th pharyngeal arches proliferates, and arytenoid swellings occur, which give rise to the larynx. Within these swellings, the laryngeal cartilages develop. The epiglottis develops from the caudal part of the hypobranchial eminence.

The epithelial proliferation of the organ lumen closes it until the 3rd month, when it recanalizes. This process forms the lateral laryngeal diverticula, which give rise to the vocal cords.

Trachea, Bronchi, and Lung Development

In the laryngotracheal tube, two bronchopulmonary buds (or lung sketches) develop at 5 weeks. The right bud divides into three secondary bronchi, and the left bud divides into two; both then continue to divide dichotomously.

The buds grow in a caudal and lateral direction, dipping into the coelomic cavity and forming the primitive pleural cavity. The visceral pleura is derived from the splanchnic mesoderm, while the parietal pleura originates from the somatic mesoderm. The maturation process includes the pseudoglandular, canalicular, terminal sac, and alveolar periods.

Stages of Lung Maturation

• Pseudoglandular Period: During this stage, the air conduction system from the bronchi to the bronchioles is established. These structures are covered with cuboidal epithelium until the 4th month of pregnancy.
• Canalicular Period: Occurring from the 4th to the 6th month, the size of the bronchi and bronchioles increases, and significant vascularization of the lung tissue occurs. Each terminal bronchiole originates two or more respiratory bronchioles, which then form three to six alveolar ducts.
• Terminal Sac Period: Within the alveolar ducts, the proliferation of alveoli occurs. During this stage, there is critical synthesis and secretion of surfactant.
• Alveolar Period: This stage occurs from birth until approximately age 8. The epithelium becomes flattened, and there is a massive increase in the total number of alveoli.