Epithelial cells of the alveolar wall (600X)

Epithelial cells in the alveolar wall (pneumocyte type 1 and 2), the endothelial cells in the capillary wall and the intermediate common basal lamina consititutes the blood-air barrier.

The type 2 pneumocyte secretes a mixture of lipoproteins and phospholipids called surfactant. The surfactant reduces the surface tension and prevents the alveoli from collapsing at the end of expiration. The cell has a central round nucleus with clumps of condensed chromatin. Cytoplasm is pink with hints of granulation.

The alveolar macrophages have an irregular nucleus and often irregular granules in the cytoplasm. They originate from monocytes.