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.