In considering the embryology of the fetal lung, it would be assumed that at 20 weeks gestation

• A. Gas-exchanging acinar units are forming
• B. Alveolar sacs are fully mature
• C. Type II pneumocytes predominate
• D. The diaphragm forms

Answer: (A)

Mid-gestation lung development is in the canalicular stage, when the airways divide into respiratory bronchioles and primitive acinar structures begin to form. The key feature of this period, around 16 to 25 weeks, is the development of a closer relationship between the epithelium and the adjacent capillaries, along with the appearance of type II pneumocytes and the start of surfactant production. This stage sets up the first steps toward gas exchange, even though the alveolar units are not fully mature yet. By about 20 weeks, it is reasonable to say that gas-exchanging acinar units are forming, as the primitive gas-exchange apparatus is beginning to establish, though mature alveolar sacs and extensive alveolarization occur later in gestation and after birth. The other options describe processes that are either already established earlier (diaphragm formation) or not yet complete at this gestational age (alveolar sacs fully mature, or a predominance of type II pneumocytes).