The respiratory epithelium changes dramatically in cellular composition as the conducting airways give way to the alveolar regions of the lung. This chapter will concentrate on the culture of cells that make up the epithelial monolayer of the peripheral gas exchange (pulmonary) region of the lung. The pulmonary epithelium is composed of alveolar type I and type II epithelial cells, which are present in a 1:2 ratio. Alveolar type II cells differentiate into type I cells as part of the normal physiological replacement or repair mechanism, but the two cell types (also known as pneumocytes) are functionally and morphologically distinct (see Table 1). Despite their greater numbers compared to type I cells, alveolar type II cells comprise less than 10% of the epithelial surface area, even when their apical microvilli are taken into account. The type II cells are cuboidal in shape and can be distinguished by their lamellar bodies, which are unique to these cells (1). Alveolar type I cells are squamous, contain few mitochondria or cellular inclusions, and make up over 90% of the epithelial surface (2).

	Type II cells	Type I cells
Description	Cuboidal cells containing	Squamous cells with an
	distinctive lamellar bodies	extended cytoplasm and
	and numerous cytoplasmic	few cytoplasmic inclusions.
	inclusions.
Approximate	Volume: 900 µm3	Volume: 1800 µm3
dimensions a	Surface area: 200 µm2	Surface area: 5100 µm2
	Perimeter: 50 µm	Perimeter: 250 µm
	Thickness: 1 µm	Thickness: 0.1 µm
Functions	Active ion and water transport.	Provides the thin blood-air
	Processing alveolar surfactant.	interface of the gas exchange
	Type I cell progenitor.	region.