canalicular period
Summary
The canalicular period (16-26 weeks gestation) is the second stage of lung development characterized by formation of respiratory bronchioles and terminal sacs. This period is critical for establishing the basic framework for gas exchange and marks the earliest point of potential extrauterine survival.
Detail
The canalicular period represents weeks 16-26 of gestation during lung embryogenesis, following the pseudoglandular period. During this phase, the respiratory tree undergoes significant branching with formation of respiratory bronchioles and terminal sacs (primitive alveoli). The lung parenchyma becomes highly vascularized as capillaries begin to grow in close proximity to the developing airways. Crucially, pneumocytes begin to differentiate: type I pneumocytes form the gas exchange surface while type II pneumocytes start producing surfactant around 24-28 weeks. The blood-gas barrier becomes sufficiently thin to potentially support gas exchange. This period is clinically significant because fetuses born at the end of this period (around 24-26 weeks) represent the threshold of viability, though they require intensive respiratory support due to immature surfactant production and underdeveloped alveolar structure. Disruption during this period can lead to congenital pulmonary malformations or contribute to bronchopulmonary dysplasia in premature infants.
Sources
- Langman's Medical Embryology
- Moore's The Developing Human
- Robbins Basic Pathology
- First Aid for the USMLE Step 1
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