1. Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. It is a thin layer of simple, or single-layered, squamous cells called endothelial cells.
2. The esophageal lining is protected by a stratified squamous epithelium. Because this epithelium is normally not exposed to dryness or to abrasion, it is non-keratinized.
3. The respiratory or gas-exchange surface consists of millions of small sacs, or alveoli, lined by a simple squamous epithelium. This epithelium is exceedingly thin to facilitate diffusion of oxygen and carbon-di-oxide.