pseudoglandular period

Lung development is divided into five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. The pseudoglandular period occurs from weeks 5 to 17 of gestation and is marked by continued branching of the bronchial tree into terminal bronchioles, forming structures that resemble exocrine glands on histology (hence the name). During this stage, all major conducting airway components form, but there are no respiratory bronchioles, alveolar ducts, or alveoli, and thus no capacity for gas exchange. Key structures that develop include bronchi, bronchioles, and terminal bronchioles, along with the surrounding mesenchyme that will give rise to pulmonary vasculature and connective tissue.

Clinically, this stage is critical because errors in branching morphogenesis during this period can lead to congenital anomalies such as bronchopulmonary sequestration or congenital cystic adenomatoid malformation (CCAM). Additionally, premature infants born before completion of the canalicular period (i.e., before week 25) cannot survive due to lack of alveolar and vascular development necessary for gas exchange, making the pseudoglandular period developmentally too early for extrauterine viability.

This concept is frequently tested on USMLE Step 1 in the context of embryology and respiratory system development, particularly to distinguish the timing and features of each lung developmental stage: Embryonic (weeks 4–7): formation of lung buds. Pseudoglandular (weeks 5–17): branching into terminal bronchioles, glandular appearance. Canalicular (weeks 16–25): formation of respiratory bronchioles, vascularization begins. Saccular (weeks 26–birth): terminal sacs form, surfactant production begins. Alveolar (week 36–8 years): true alveoli form and mature.