Development of the Respiratory System

Development of the Respiratory System

Schematic presentation of the lung development, which is comprised of five developmental stages that progressively occur during gestation.

The embryonic stage. Mouse E9-12, human 3-7 weeks. The period of organogenesis; at this stage, the lung buds appear and mesenchyme-driven branching is evident.

The pseudoglandular stage. Mouse E12–15, human 5-17 weeks. The conducting airways are formed. In humans, 16 generations of branching are completed by 16 weeks of gestation. In this stage, the respiratory zone of the lung is formed; it contains the respiratory bronchioles, the alveolar ducts and the alveolar sacs. In addition, during this stage, cellular differentiation of the conducting airways in a proximal to distal direction.

The canalicular stage. Mouse E15–17, human 16-26 weeks. Characterized by completion of the airway branching pattern, during the respiratory bronchioles develop.

The saccular or terminal sac stage. Mouse E17 to PD5, human 26-36 weeks. Characterized by pulmonary parenchyma growth and maturation of the surfactant system, resulting in terminal sac formation and vasculogenesis. At birth, the functional lung is still structurally immature, since the gas-exchange units of the adult lung, called alveoli, are virtually non-existent.

The alveolar or postnatal stage. Mouse PD5 to PD30, human 36 weeks to postnatal 3 years. The terminal respiratory sacs mature into the functioning alveoli, which are connected to alveolar ducts.