Type II pneumocyte
Pneumocytus typus II
Also known as: type II alveolar cell, great alveolar cell, septal cell, AT2 cell
Cuboidal alveolar epithelial cells, typically at septal corners, that synthesise and secrete surfactant from lamellar bodies and act as the stem cells of the alveolar epithelium.
Overview
Type II cells are more numerous than type I cells (≈60% of alveolar epithelial cells) but cover only ≈5% of the surface. They have apical microvilli and characteristic lamellar bodies — membrane-bound organelles containing concentric layers of surfactant phospholipid — which are released by exocytosis, unfold into tubular myelin and spread as a surface film. They also take up and recycle used surfactant.
After injury, type II cells proliferate (type II hyperplasia) and differentiate into type I cells to re-epithelialise the alveolus. They pump Na+ from alveolar fluid via apical ENaC channels and basolateral Na+/K+-ATPase, driving alveolar fluid clearance, and secrete immune-modulating proteins (SP-A, SP-D) and cytokines. Surfactant production begins in the late canalicular stage (≈20–24 weeks) and is stimulated by glucocorticoids, thyroid hormones, β-adrenergic agonists and lung stretch, but inhibited by insulin.
- Location
At the junctions (corners) of alveolar walls.
Anatomy
- Histology
Cuboidal cells with a foamy/vacuolated cytoplasm (lamellar bodies), microvilli and abundant rough ER and Golgi; joined to type I cells by tight junctions.
Function
- Synthesise, store, secrete and recycle pulmonary surfactant.
- Progenitor cells regenerating type I and II cells.
- Clear alveolar fluid by active Na+ transport.
- Innate immune functions (SP-A and SP-D collectins).
Clinical significance
Neonatal respiratory distress syndrome
Immature type II cells in preterm infants produce too little surfactant, causing alveolar collapse. Antenatal corticosteroids accelerate type II cell maturation.
Surfactant protein gene defects
Inherited SP-B deficiency is lethal in neonates; SP-C and ABCA3 mutations cause childhood or adult interstitial lung disease.
Test yourself
Which cell secretes pulmonary surfactant?
- Type II pneumocyte
- Type I pneumocyte
- Club (Clara) cell only
- Alveolar macrophage
Answer: Type II pneumocyte
Cuboidal type II pneumocytes store surfactant (mainly dipalmitoylphosphatidylcholine plus surfactant proteins A–D) in lamellar bodies and secrete it by exocytosis. They are also progenitors that replace type I cells after injury. Club cells secrete some surfactant-like proteins in bronchioles but are not the main source.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.