At what stage of lung development can respiration potentially occur?

The Cannalicular Stage is a crucial period in lung development where the basic structures necessary for gas exchange begin to form, and the potential for respiration emerges. During this stage, which occurs roughly between 16 to 26 weeks of gestation, the terminal bronchioles continue to divide and convert into respiratory bronchioles leading to the formation of alveolar ducts. Alveolar epithelial cells start differentiating, and the production of surfactant begins, which is essential for reducing surface tension in the alveoli and facilitates the expansion of the lungs at birth.

This stage marks the beginning of the functional capability of the lungs because it is when the architecture necessary for respiration is established. The presence of a developing capillary network also plays a key role, allowing future gas exchange. Consequently, while true alveolar structures and full lung function are not completely developed until later stages, including the Saccular Stage, the Cannalicular Stage is the earliest point where the lungs have the structural components that can initiate respiration if the fetus were to be born during this period.