Mnemonic for Fetal Lung Development Stages
The five stages of fetal lung development can be remembered using the mnemonic "Every Person Can See Air" (Embryonic, Pseudoglandular, Canalicular, Saccular, Alveolar).
The Five Stages with Timeline
E - Embryonic Stage (0-7 weeks gestation)
- The lung primordium forms as a diverticulum of the foregut during the first seven weeks of gestation 1
- This represents the initial laying down of the basic lung structure 1
P - Pseudoglandular Stage (5-17 weeks gestation)
- The lung resembles a tubulo-acinar gland with epithelial tubes sprouting and branching into surrounding mesenchyme 1
- By the end of this stage (week 17), all prospective conducting airways have been formed and acinar limits can be recognized 1, 2
- This stage is critical for establishing the bronchial tree architecture 3
C - Canalicular Stage (16-26 weeks gestation)
- Peripheral tubules widen and the cuboidal epithelium differentiates into type I and type II pneumocytes 1
- The first thin air-blood barriers form during this period 1
- Surfactant production begins, marking a critical transition toward potential viability 1, 2
- This stage represents when gas exchange first becomes theoretically possible 4
S - Saccular Stage (26 weeks to birth)
- Pulmonary parenchyma grows substantially with thinning of connective tissue between airspaces 1
- Further maturation of the surfactant system occurs, which is essential for postnatal lung function 1, 2
- The lung becomes functional but remains structurally immature at birth 1
- Smooth-walled transitory ducts and saccules with primitive thick septa containing double capillary networks characterize this stage 1
A - Alveolar Stage (36 weeks gestation through 1-3 years postnatal)
- True alveoli are practically absent at birth and form postnatally through septation 1
- This process dramatically increases gas exchange surface area during the first 1-3 years of life 1, 3
- Primitive septa undergo complete remodeling to achieve the mature slender morphology of adult lungs 1
Clinical Relevance
Premature birth interrupts normal lung development, with consequences directly related to which stage is interrupted 3:
- Infants born during the canalicular stage (before 26 weeks) face the highest risk of respiratory distress syndrome due to surfactant deficiency and inadequate air-blood barrier formation 5
- Very preterm infants (<30 weeks, <1,000g) commonly develop bronchopulmonary dysplasia characterized by alveolar simplification from interrupted development 5
- The timing of premature birth determines both immediate viability and long-term pulmonary outcomes 6, 5
Common Pitfall
The boundaries between these stages overlap considerably between individuals and are not sharply demarcated 2. Understanding this overlap is essential when counseling families about prematurity risks at borderline gestational ages.