Type I pneumocytes cover the surface, type II make surfactant, macrophages clean up
Mnemonic
Two pneumocytes with completely different jobs:
- Type I - 97 per cent of the surface area, but only about 40 per cent of the cells. Thin, squamous, built for gas exchange. Cannot divide
- Type II - about 60 per cent of the cells but a tiny area. Cuboidal, containing lamellar bodies. Produce surfactant, and act as the stem cell that regenerates type I cells after injury
“Type I is the surface, type II is the factory and the repair kit.”
Type II cells appear at about 24 weeks and surfactant production becomes adequate around 34 to 36 weeks, which is the basis of antenatal corticosteroids to accelerate maturity in threatened preterm delivery.
Alveolar macrophages (dust cells) complete the population, and their haemosiderin laden form is the heart failure cell seen in chronic pulmonary congestion.
Expansion
- Type I pneumocyte - flat, squamous, forming about 95 per cent of the surface area. Thin enough for gas exchange, but cannot divide
- Type II pneumocyte - cuboidal, more numerous but covering little area. Secrete surfactant and act as the stem cell that regenerates type I cells after injury
- Alveolar macrophage (dust cell) - phagocytoses inhaled particles and debris
Surfactant lowers alveolar surface tension, preventing collapse at end-expiration and increasing compliance. It appears from about 24 weeks of gestation and reaches adequate levels by about 35 weeks, which is the basis of neonatal respiratory distress syndrome and of antenatal corticosteroid administration.
Macrophages laden with haemosiderin in pulmonary oedema are the “heart failure cells” seen in sputum cytology.