Alveolar Cell Types:

1. Type I cells. Also called type I alveolar cells, type I pneumocytes, and squamous alveolar cells, these are squamous epithelial cells that make up 97% of the alveolar surfaces.

They are specialized to serve as very thin (often only 25 nm in width) gas-permeable components of the blood-air barrier. Their organelles leg, Golgi complex, endoplasmic reticulum, mitochondria) cluster around the nucleus.

Much of the cytoplasm is thus unobstructed by organelles, except for the abundant small pinocytotic vesicles that are involved in the turn over of pulmonary surfactant and the removal of small particles from the alveolar surfaces. They attach to neighboring epithelial cells by desmosomes and occluding junctions.

The latter reduce pleural effusion--leakage of tissue fluid into the alveolar lumen. Type I cells can be distinguished from the nearby capillary endothelial cells by their position bordering the alveolar lumen and by their slightly more rounded nuclei.

2. Type II cells. These cells, which are also called type II alveolar cells, type II pneumocytes, great alveolar cells, and alveolar septal cells, cover the remaining 3% of the alveolar surface. They are interspersed among the type I cells, to which they attach by desmosomes and occluding junctions.

Type II cells are roughly cuboidal with round nuclei; they occur most often in small groups at the angles where alveolar septal walls converge. At the electron microscope level, they contain many mitochondria and a well-developed Golgi complex, but they are mainly characterized by the presence of large (0.2-um), membrane-limited lamellar (mutlilamellar) bodies. These structures, which exhibit many closely apposed concentric or parallel membranes (lamellae), contain phospholipids, glycosaminoglycans, and proteins.

Type II cells are secretory cells. Their secretory product, pulmonary surfactant, is assembled and stored in the lamellar bodies, which also carry it to the apical cytoplasm. There, the bodies fuse with the apical plasma membrane and release surfactant onto the alveolar surface. 3. Alveolar marcrophages. Known also as dust cells, these large monocytc-derived repre sentatives of the mononuclear phagocyte system are found both on the surface of alveolar septa and in the interstitium. Macrophages are important in removing any debris that escapes the mucus and cilia in the conducting portion of the system.

They also phagocytose blood cells that enter the alveoli as a result of heart failure. These alveolar macrophagcs, which stain positively for iron pigment (hemosiderin), are thus designated heart failure cells.